A Review on Potential Mechanically Resistant Materials for Optical Multifunctional Surfaces: Bioinspired Surfaces with Advanced Properties

Mimicking naturethrough nanostructuring allows the creation of multifunctional surfaces withremarkable properties, such as anti‐reflectivity, high optical transmittanceand controlled wettability, enabling anti‐icing or anti‐fogging behaviors. These multifunctionalsurfaces have gained significant interest in civil and military domains. However,integrating them into real‐life applications faces challenges related tocost, high‐throughput large‐scale compatible nanofabrication techniques, andtheir mechanical resistance. While sub‐wavelength patterning improves the optical performance,it often comes at the cost of compromising the mechanical resistance. As opticalperformance improves, mechanical resistance tends to deteriorate, and viceversa. To address this challenge, taking inspiration from the lotus leafstructure, where patterns are covered by a thin 2D wax film, covering thepatterns of structured surfaces with a protective layer can be a viablesolution. This protective layer should enhance the mechanical resistance of thesurface without compromising its multifunctional capabilities. This reviewhighlights the most suitable materials that can be employed as protectivecoatings and their potential to enhance the resistance of structured surfaces.The fundamental concept behind the creation of multifunctional optical surfacesis discussed, followed by a comprehensive examination of mechanical tests thatcan be utilized to characterize their mechanical behavior. This review aims topave the way for the development of durable multifunctional optical surfaces,making them more amenable to industrial applications.


Introduction
Nature has always been a rich source of inspiration for scientists seeking novel solutions.The discovery of bio-inspired structures has led to the development of various smart materials with diverse designs.Biomimetic has become a common field of research aiming to create multifunctional surfaces inspired by the patterns found in plants, insects, and animals. [1]These natural surfaces possess a remarkable ability to adapt to environmental changes due to their multifunctional aspects.
Advancements in scanning electron microscopy have allowed researchers to closely observe and understand the geometries of surfaces such as lotus leaves, butterfly wings, and moth eyes surfaces, [2] as shown in Figure 1.For instance, the hierarchical micro-and nanostructured surfaces of the lotus leaves illustrated in Figure 1b,c, make them water repellent with self-cleaning and antibacterial characteristics. [3]Similarly, studies on the corneas of insects reveal their hexagonal sub-wavelength Figure 1.a) Shows a water-drop on the surfaces of natural lotus leaf, (b,c) represent the SEM images of the lotus leaf surface.Reproduced under the terms of the CC BY 2.0 license. [8]Copyright 2023, Authors, Published by the Beilstein Journal of Nanotechnology.d) A butterfly wings.e,f) SEM images of the wing.Reproduced under the terms of the CC BY 4.0 license. [9]Copyright 2023, Authors, Published by Nature.g) Shows the head of a moth with particular black eye. []h,i) SEM images of a moth-eye. [10]otuberances on their surfaces, as shown in Figure 1h,i, providing the mosquito eye not only superhydrophobicity but also an anti-reflective effect, thanks to their graded refractive index profile. [4]Taking cues from nature, researchers have successfully created artificial multifunctional surfaces using polymers, [5] coatings, [6] or bulk materials [7] tailored for specific applications.
In recent times, optical systems and optoelectronic devices have demanded high optical transmission surfaces with controlled wetting performance, particularly anti-fogging and antiicing capabilities.In such applications as infra-red cameras, solar cells, aircraft, and in military equipments, reflection of light is a critical concern due to transmission losses.Consequently, researchers are actively exploring ways to create multifunctional anti-reflective surfaces. [11]Multi-layer coatings have been investigated for anti-reflection, wherein different materials with suitable refractive indices and optimized thickness are deposited.While these coatings can effectively reduce reflectivity compared to single or double layers, [12] they still have limitations in bandwidth and polarization dependence.Additionally, they become costly for broad-band applications, requiring a large number of coatings. [7]To overcome the limitations of multi-layer coatings, researchers have turned to micro-and nanopatterning structures inspired by nature.These patterned surfaces are widely soughtafter for various applications.The choice of morphology and dimension of the surface structure play a crucial role, depending on the intended application.For example, periodic artificial moth eye structures and randomly micro or nanopatterned surfaces are commonly used for broadband operation, while sub-wavelength structures perform well for narrow-band applications. [13]Hence, the fabrication technique of this structure plays a major role in the effectiveness of these textured surfaces, and depending on the targeted spectral range, a diverse array of materials have been structured for optical windows.For the visible and near infrared ranges, glass and sapphire are the most commonly used, while for the mid-and far-infrared regions, the researchers usually rely on more suitable materials such as silicon, germanium, zinc sulfide (ZnS), AMTIR, and GASIR.Although this method generates high performance optical multifunctional surfaces, the micro-or nanopatterning of each material is not trivial.
Over the years, different technologies have been developed for fabricating micro-and nanopatterned surfaces, categorized as bottom-up and top-down technologies.The former involves deposition techniques [14] like sol-gel, spin or dip coating, chemical vapor deposition (CVD), and atomic layer deposition (ALD).The latter relies on etching techniques, [15] which can be wet or dry, masked, or mask-less, and may involve a combination of steps. [16]eanwhile, several structures have been formed by chemical and plasma etching in different ways to find out the most advanced nanofabrication technique.As it is so challenging to control the resolution of the final structure, a combination of different fabrication steps is essential.The lithography process has been reportedly employed as a patterning technique while etching with a mask.This fabrication step is crucial creating patterns that can be later transferred through dry or wet etching to the substrate or by following the process of metallization and lift-off before etching.
In fact, the top-down approach has attracted much interest for optical component, [17][18][19] since the graded refractive index can be directly obtained from the bulk material itself without the need to integrate a second one with a different refractive index.
Despite the progress in creating optical multifunctional biomimetic surfaces, the mechanical robustness of these surfaces remains a challenge, especially in harsh environments where raindrops, hail, and sand exposure can cause patterns to fracture and buckle.As a result, the multifunctional aspect of this material will be lost due to the significant change in the surface geometry.Unfortunately, relevant studies on the mechanical properties of patterned surfaces for optical windows are still lacking.However, a hard coating can be deposited on the top of these surfaces as a solution in order to protect the patterns.
Hard coatings have been widely investigated in order to improve the mechanical properties of substrate materials. [20]Various deposition techniques are currently used to lay down a hard material on a substrate such, as ALD, CVD or plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), and radio frequency magnetron sputtering.The deposition of a hard thin film has been used as an efficient method to enhance the wear resistance, hardness, and mechanical robustness of bulk materials. [21]Many researchers have explored the use of hard coatings to protect non-patterned surfaces and enhance their wear resistance, hardness, and mechanical durability. [22,23]However, their use on patterned surfaces is still limited.For such surfaces, it is challenging to find the suitable hard material that can be optimally deposited and would be thin enough not to alter the multifunctional operation of these structures, but at the same time can protect the patterns in harsh environments.
This review aims to not only focus on hard coating materials but also on hard and transparent materials with controlled wetting properties in different wavelength ranges.The goal is to provide a road map for selecting the ideal material to protect structured surfaces based on the desired application and spectral range.The review will cover theoretical aspects, mechanical properties, mechanical testing, hard and transparent materials, and controlled wetting properties, with an emphasis on the challenges and future prospects of achieving optical multifunctional bio-inspired surfaces with high resistance.

The Physics behind Optical Multifunctional Surfaces
Researchers have extensively explored the concept of multifunctionality, recognizing its potential to significantly improve the performance of surface materials in a wide array of applications.Optical multifunctional surfaces, in particular, exhibit the ability to transmit light efficiently while also possessing controlled wetting properties.Such surfaces offer a host of advanced characteristics, making them highly valuable in various practical applications where minimizing light reflectance, self-cleaning capabilities, anti-icing, and anti-fogging properties are of utmost importance.Consequently, gaining a deep understanding of the underlying physics governing these surfaces is crucial for attaining optimal multifunctional performance.

Basic Concept of Anti-Reflection
The propagation of light is a well-established concept that has been extensively studied over time.When an electromagnetic wave of a specific wavelength () interacts with a surface, the interaction depends on the refractive index (n) of the surface material.The refractive index is defined as the ratio between the speed of light in the medium and the speed of light in a vacuum.According to Maxwell's equations, as light travels from one medium to another, various interactions occur at the interface due to the change in refractive index.As a result, the incident light can be divided into different waves, leading to reflection, transmission, diffraction, or scattering.These interactions depend on factors such as the nature of the material, the morphology of the surface, and the polarization of light.
Reflection is an optical phenomenon that occurs when a portion of the incident light reflects at the interface between two media and returns along the original path at normal incidence.Fresnel's theory provides an explanation for the propagation of light and quantifies reflection in terms of reflectance, R. Reflectance is the fraction of incident light intensity that is reflected at the interface between two media with different refractive indices, n 0 and n s , when the incident light is perpendicular to the interface.We chose the notation n s to refer to the substrate with light incoming from the air, n 0 (case of two infinite media).Fresnel's equation is used to calculate the reflectance.
Indeed, Equation (1) indicates that to minimize reflection, the difference between the refractive indices of the media through which light propagates should be kept minimal (Δ(n) = 0).
Reducing reflectivity has become a focal point in various optical applications.Optoelectronic devices, military equipment, and optical systems greatly benefit from increased light transmission as it enhances their overall performance and efficiency.Undesired reflected light can lead to a reduction in optical power and the lifespan of these devices. [24]Prior to the discovery of structured anti-reflective surfaces in nature, anti-reflective coatings were primarily employed to diminish surface reflectivity.Various strategies were developed for this purpose, as described below.

Single-Layer
The concept of anti-reflective coatings was initially explored by adding a single thin film on top of the surface material, as depicted in Figure 2a.This system involves two interfaces: air-thin film and thin film-substrate, each with different refractive indices.As a result, according to Fresnel's theory, an interference phenomenon occurs, leading to the generation of two reflected waves, one from the air-thin film interface with a reflection coefficient in amplitude r 01 and the other from the thin film-substrate interface with a reflection coefficient in amplitude r 1s .For the sake of simplicity, we consider here only normal reflectance.However, it has to be mentioned that, for real applications, different incident angles have to be considered.Therefore, one has to take into account the two polarization states (p,s).For the single where at normal incident r 01 and r 1s are given by: with n 0 , n, and n s are the refractive indices of the air, the thin film material, and the substrate respectively.The reflectivity for the single layer can be expressed as: This relation is only valid when the phase difference between the two reflected waves is equal to , which is the condition for destructive interference and achieving anti-reflectivity.To achieve anti-reflectivity using a single-layer thin film, two conditions related to the characteristics of the film material need to be satisfied: 1) The refractive index of the thin film n should be lower than the refractive index of the substrate n s that is, (n < n s ).
2) The optical thickness of the thin film (d) should be such that the phase shift of the reflected wave is equal to  upon reflection from both interfaces (air-thin film and thin film substrate).This condition ensures that the reflected waves interfere destructively, leading to a reduction in the overall reflectivity.
Following the first condition, having a low refractive index n 1 lower than n s , the reflectance of the thin film can only decrease as compared with the substrate case.Additionally, when the optical thickness of the film is one-quarter of the incident wavelength , destructive interference occurs, leading to the cancellation of the two reflected waves.It is worth noting here that this cancellation is wavelength-dependent making broadband anti-reflection coating difficult to achieve for a single layer.
In other words, optimizing the layer thickness is still possible, but in a limited wavelength range because of the refractive index dispersion. [25]Likely in the transparency region, the change in the refractive index can be however neglected, allowing for the one-or two-layer approaches to be of practical use.Moreover, finding a low refractive index material is challenging, and even more so for a low refractive index substrate.For instance, glass substrates typically have a refractive index of 1.5.Depositing a single layer with a refractive index (n) equal to the square root of the glass refractive index n = √ n glass is highly challenging due to the lack of suitable materials with a refractive index close to 1.2226. [26]Although magnesium fluorides (MgF 2 ) [26] with a refractive index of 1.38 are commonly used as it possess a relatively low refractive index, they still do not provide sufficient reduction in reflectivity.This limitation applies to bulk materials; however, porous materials can be used as an alternative.The porosification of silicon via electrochemistry is one such approach, but its generalization remains challenging.Further details will be discussed later in this article.
The aforementioned limitations of a single layer hinder the achievement of zero reflectance over a large spectral range or incident angle.Consequently, alternative approaches to reduce reflection have been proposed.

Multi-Layers
The multi-layer structure is a generalized form of the singlelayer principle.To enhance the performance of AR coatings and achieve zero reflectance, duplicating the number of layers or creating a multi-layered structure has shown better results compared to single-layer configurations. [26]By increasing the number of layers, more interfaces are introduced into the system with a lower refractive index difference at each interface.As a consequence, successive reflections occur from the interaction of light at each interface, as depicted in Figure 2b.The total reflectance of the surface is the sum of all reflected waves.The transfer matrix method is a well-known approach used to calculate the reflection and transmission of the multi-layer system. [27]Zhou et al. [28] have extensively explained the theoretical calculation of the multilayer design transmission using this method.In essence, the reflectance, and transmission of the multi-layer system are obtained by determining the amplitude of the incident wave A 0 and the amplitude of all the reflected waves A ′ 0 .By utilizing the transfer matrix method and the continuity equations, [29] the amplitudes of the incident and reflected light at each interface are calculated, taking into account the direction of the propagated light and the phase difference between the interfaces.Consequently, the expressions for A 0 and A ′ 0 are derived, and the reflectance is defined as Equation (7).
Optimizing the refractive index and thickness of each film enables achieving high anti-reflective performance in multi-layered structures.Simulation is a valuable tool for complex structures where broadband wavelength operations are required and multiple layers are involved.Different optimized algorithms can be used to compute the perfect matching film with the desired refractive index and thickness.While multi-layer structures exhibit superior anti-reflective performance compared to singlelayer systems over a wide wavelength range, they also have some limitations.The effectiveness of multi-layer designs in reducing reflectivity can be influenced by the incident angle and polarization of light, making it challenging to achieve destructive interference in certain cases.Additionally, the large number of thin films needed for broadband applications complicates the deposition process and can lead to issues such as thermal mismatch and poor adhesion, resulting in undesirable outcomes and higher fabrication costs.
An alternative solution that bypasses the need to find dielectric materials with specific refractive indices is to adopt bioinspired structures.By mimicking nature and incorporating structured designs onto the surface of the substrate material, promising results have been achieved not only in terms of optical properties but also in creating superhydrophobic or superhydrophilic surfaces.These biomimetic approaches offer a cost-effective and efficient route to achieving multifunctional surfaces with the desired optical and wetting characteristics.

Structured Surfaces
The discovery of the optical performance of natural surfaces, especially the moth eye, has led to the development of surface structuring as an alternative approach creating artificial antireflective surfaces with superior performance.The goal is to achieve broadband antireflective properties that work irrespective of light polarization.Various strategies have been explored in the realm of surface structuring, with nanostructured coatings being employed to reduce substrate reflectivity. [26]However, substrate surface structuring itself has shown remarkable antireflective characteristics without the need for additional dielectric materials.
To structure the substrate surface, different techniques have been employed.Patterns can be created using wet or dry etching methods in conjunction with an etching mask, designed using lithography.The desired anti-reflection properties are dependent on the morphology and dimensions of the surface structure. [30]epending on the intended optical multifunctionality, various macro, micro, and nanopatterns have been fabricated on square or hexagonal lattices, leading to different interactions of incident light with each structure.As a result, anti-reflective structured surfaces can be classified into three categories (Figure 3): geometrical optics-compatible surface structures, scattering surface structures, and sub-wavelength structures.For the sake of clarity and referring to the visible range, geometrical optics, scattering, and sub-wavelength structures are called macro, micro, and nano structures, respectively.It is obvious that the reference scale is the operating wavelength.
Geometrical Optics Compatible Structures: The interaction of light with macro-structures with pattern dimensions significantly larger than the incident light wavelength  can be explained through geometrical optics.In this approach, two parameters are crucial: the height (z) of the pattern and the periodicity (p) of the structure.The macro-structure periodicity is far larger than the incident wavelength  and the pattern height (z) should be much larger than  with sharp tops to achieve antireflection.In this regime, the light behaves as optical rays.The bigger the pattern height, the better the antireflective effect (Figure 3b).However, achieving zero reflectance with macro-structures is challenging due to the large spacing between the patterns and the scattering effect, which increases the backward reflected light.Moreover, the very high height required makes this approach incompatible with thin-film technology.
Scattering Structures: Moving from macro-to microstructures, researchers have aimed to reduce the dimension scale of the structure to achieve zero reflectance.In this case, the periodicity p of the structure is on the same scale as the incident wave-length , resulting in a micro-structured surface.Regarding the height, it has to be high enough to obtain an anti-reflective performance.Achieving the desired dimensions can be challenging and requires careful parameter optimization during fabrication.
At the micro-structure scale, the flat zones on the surface of the substrate are less prominent compared to the macro-structure design, leading to a lower reflectance. [31]However, these surfaces suffer from scattering, which limits their ability to achieve high direct transmission and introduces a certain degree of haze (Figure 3c).The percentage of haze represents the amount of scattered light relative to the total incident light.The haze factor depends on the incident wavelength and the roughness of the surface. [32]Typically, haze is quantified by measuring the percentage of light scattered in various directions using specialized instruments such as haze meters.Alternatively, haze can be estimated by considering the ratio of the diffuse transmission of light (T diff ) to the total transmitted light T total , and haze is calculated using the formula A high degree of haze reduces the clarity and transparency of the surface.These scattering structures are not suitable for many optical applications, especially in imaging and military equipment where high direct light transmission and clear vision are essential.However, in photovoltaic applications, microstructures can be very useful due to their multi-directional transmission, and the introduced haze may not be a hindrance but rather an advantage for light trapping and energy absorption.

Sub-Wavelength Structures:
To achieve a fully transmitted surface in the visible range, as well as in the near and far infrared domains, researchers have turned to nanoscale structuring to minimize the distance (p) between two patterns and eliminate flat areas on the structured surfaces.This approach allows for the reduction of reflectance to zero by maximizing transmission.Nanostructured surfaces with a periodicity (p) much smaller than the incident wavelength of light are referred to as subwavelength structures.
For the design of a sub-wavelength structure, two rules must be followed to achieve a high performance anti-reflective surface, which are related to the spectral range of the incident light ( min −  max ).According to the grating relations, the periodicity p of the structure has to be smaller than  min , and the maximum periodicity p max should not exceed a certain value.For the hexagonal lattice, p max is given by [33] p max = 1.15 *  min ∕(n + n 0 sin ) ( 9 )   where n and n 0 are the refractive indices of the material and the incident medium, respectively, and  is the angle of the incident light.
Regarding the height of the structured patterns, a greater height is advantageous as it allows the incident light to effectively pass through the substrate, as it is interacting with a multilayer thin film with a specific refractive index gradually changing along the direction of the propagating light.This results in the creation of a gradient refractive index, as illustrated in Figure 3d.The reduction of reflection in sub-wavelength structures can be explained using the effective medium approximation, where the effective refractive index n eff (z) of the material at each depth z (z-direction) is given by [34] n eff (z) 2 where n eff , n s , and n 0 are the effective, substrate, and air refractive indices, respectively, and f(z) is the filling factor of the nanostructure.The filling factor profile varies according to the dimension and the lattice of the structured surface.Whether the filling factor profile is linear, cubic, quintic, parabolic, or exponential, the effective refractive index profile changes and thus the anti-reflective performance changes.
Simulation techniques like the finite difference time domain (FDTD) method play a crucial role in the design and optimization of structured surfaces for optimal optical performance.FDTD simulations allow researchers to predict and analyze the optical properties of various nanostructured surfaces without the need for time-consuming and expensive fabrication processes.
It is important to mention that these structures might not display optimal direct transmission, specially when they are fabricated using colloidal lithography.Researchers have proved that the direct transmission of a sub-wavelength structure with a periodicity of 1 μm shows a falloff in direct transmission from 1.5 to 25 μm which is attributed to the scattering phenomenon. [35,36]he main reason behind this scattering is the presence of surface defects with dimensions larger than  during the self-assembly process, and this was also validated by FDTD simulation using the supercell approach.However, this problem can be mitigated by preventing the creation of defects on the surface.Researchers have shown that through the assisted self-organization of colloidal nanoparticles, defects can be minimized, leading to reduced transmission losses. [37]To achieve high-performance antireflective surface with very high direct transmission, it is crucial to minimize flat areas and avoid optical phenomena such as scattering and diffraction.
Porous Surfaces: In addition to the aforementioned AR structured surfaces, porous materials offer another approach to achieve anti-reflective (AR) properties and have shown promise in various applications. [38]This method involves creating pores in the surface of a material, and the antireflective effect is achieved through the interaction of light with the porous structure.However, achieving antireflectivity with porous materials depends on several factors, such as the original properties of the material, the thickness of the porous film and the size of the pores.
It is worth noting that not all porous materials are antireflective.For the antireflective effect to be significant, the size of the pores should be much smaller than the wavelength of light, which leads to the creation of a gradient refractive index profile due to the presence of air within the material.Several techniques can be used to fabricate porous materials, including chemical etching, [39] electrochemical methods, [40] and sol-gel processing. [41]Each technique has its advantages, depending on the specific application and the choice of material.
Among porous materials, porous silicon (PSi) is the most commonly used material as an AR structure. [42]Porous silicon forms a sponge-like material with a very low refractive index, resulting in a significant reduction in reflectance to less than 3% in the visible region. [41]However, the porosity level and the morphology of the pores have a significant impact on the elastic behavior of porous silicon. [43]urther research is needed to study the efficiency of PSi as an antireflective surface with high mechanical resistance and its suitability for various optoelectronic applications, particularly in harsh environments.By exploring and understanding the properties of porous materials, scientists and engineers can develop new and innovative solutions for enhancing the performance of optical devices and systems.

The Theory of Wetting Properties
The wetting behavior of a solid surface can be understood by examining the interactions between a liquid and the solid.When a drop of liquid comes into contact with a surface, it can either bead up or spread out, depending on the nature of the surface.Surface tension, also known as surface energy (), plays a crucial role in determining whether a surface is hydrophilic or hydrophobic.Surface energy is a measure of the contractile force resulting from adhesion and cohesive interactions between the liquid molecules and the surface.It is expressed in energy per unit area.The degree of wetting of a surface is determined by the spreading parameter, denoted as S. To calculate the spreading coefficient, the surface tension needs to be taken into account. [44]Each surface can be viewed as an interface between two media, each having its own specific surface energy.Therefore, the wetting phenomenon involves three media: air, liquid, and solid.Consequently, three interfacial tensions are considered to determine the wetting degree, expressed by the following equation (Equation (11)): Indeed, in the equation,  SV , ∼ SL , and  LV represent the surface energies at the solid-air, solid-iquid, and liquid-air interfaces, respectively.By using this equation, we can determine the level of wetting on the surface.
A positive value of S indicates that the surface is wetted and the liquid spreads out completely, forming a thin film on the surface.On the other hand, a negative value of S indicates that the liquid forms a droplet that takes the shape of a spherical cap on the surface with a specific contact angle ( E ).This contact angle characterizes the wetting properties of the surface and is influenced by the surface morphology. [45]

Flat Surfaces
Young's model, proposed by Thomas Young in 1805, [46] is a wellknown model used to calculate the contact angle ( E ) between a solid surface and the liquid-vapor interface on ideally flat and chemically homogeneous surfaces.This model serves as a basic law to describe the wetting behavior on smooth surfaces by relating the contact angle to the three surface tensions ( SV ,  SL , and  LV ) as shown in Figure 4.The model is based on the principle that each interface seeks to attain equilibrium by minimizing surface energy. [6]Therefore, Young's equation is expressed as follows: Based on Young's model, the nature of a flat surface can be studied by analyzing the value of the contact angle  E .If the contact angle is larger than 90°, it indicates that the surface is hydrophobic, meaning that the liquid droplet tends to bead up and does not spread out easily on the surface.On the other hand, if the contact angle is less than 90°, it means that the surface is hydrophilic, and the liquid droplet tends to spread out and wet the surface.
It is important to note that Young's model assumes an ideally flat and chemically homogeneous surface, neglecting the influence of surface roughness.In reality, the roughness of the surface does play a significant role in determining the contact angle.Surface roughness can affect the wetting behavior by providing more contact points for the liquid to adhere to the surface, leading to a change in the contact angle and wetting properties.Therefore, in practical applications, the impact of surface roughness should be considered when studying the wetting behavior of liquids on real surfaces.

Rough Surfaces
Until 1936, wetting properties were primarily studied based on the assumption of perfectly smooth surfaces.However, in reality, most surfaces are not perfectly smooth and have some degree of roughness that significantly influences the wetting behavior.To account for the impact of roughness on the contact angle, two models have been developed: the Wenzel model [47] and the Cassie-Baxter model. [48]enzel Model: When a liquid droplet contacts a rough surface, the contact angle is affected not only by the surface energy but also by the level of roughness of the surface.In 1936, Wenzel observed that the wetting properties are directly related to the roughness of the wetted surface.The Wenzel contact angle is expressed as: The Wenzel model describes the Wenzel contact angle  We for a drop of liquid in a Wenzel state, where r represents the roughness factor defined as the ratio between the actual surface area and the geometric surface area (or projected surface area).By substituting Equation (12) into Equation ( 13), the reformed equation of the Wenzel model can be expressed as follows: In the Wenzel state, the roughness factor, r is always greater than unity, whereas for a perfectly smooth surface, the actual and geometric surface areas are equal, resulting in r∼= 1, and the Wenzel equation reduces to Young's relation (Equation ( 12)).Including roughness as a parameter to calculate the contact angle modifies the inherent wetting properties of the material.In other words, the nature of the material is affected by its surface morphology.For a hydrophobic material where  E > 90°, the roughness factor in Equation ( 14) increases the value of where  E .Consequently, the material becomes more hydrophobic.Similarly, for a hydrophilic material ( E < 90°), increasing the surface roughness enhances its wetting properties, making the material even more hydrophilic.
Additionally, it is worth mentioning that in the Wenzel state, the contacted area is homogeneously wetted by the liquid, following the shape of the surface grooves.However, the Wenzel model is not valid for situations where the interface between the solid and the liquid is entrapped by air.
Cassie-Baxter Model: In the Cassie-Baxter model, introduced by Cassie and Baxter in 1994, it was observed that the value of the contact angle is influenced by another crucial factor, which is the surface heterogeneity.When a rough surface exhibits chemical heterogeneity, the system will have different surface tensions that impact the exact value of the contact angle.On such surfaces, the liquid drop exhibits different contact angles due to varying levels of heterogeneity.The apparent contact angle, according to the Cassie-Baxter model, is given as [49] cos where f 1 and f 2 represent the surface area fractions that correspond to the different contact angles  1 and  2 , respectively, due to the varying degree of heterogeneity in the composite geometric area.This expression can also be applied in the case of a planar but heterogeneous surface.
It should be emphasized that in real situations, when a drop of liquid contacts a rough heterogeneous surface in a Cassie-Baxter state, the drop contacts the rough grooves only on the top.This is because, on heterogeneous surfaces, air pockets are located between the liquid and the solid.Instead of having a solid-liquid interface, an air state will be present as a third medium between the solid and the liquid. [50]Thus, a gradient-type multi-state surface is formed.
The contact angle between the liquid and the air section  2 is then equal to 180°( 2 = 180°) since at this interface, the state is considered as non-wetting area.Therefore, knowing that the sum of the surface area fractions f 1 and f 2 is equal to unity (f 1 + f 2 = 1), the Cassie-Baxter equation is written as The expression above, with f denoting the surface area fraction on the air-solid composite and  E representing the contact angle on a flat solid surface, allows us to calculate the contact angle on a heterogeneous surface in a Cassie-Baxter state.In contrast to the Wenzel model, the apparent contact angle  CB is inversely proportional to the solid-liquid contact area, which means that when more air pockets are trapped beneath the grooves the contact angle will increase.It is important to note that these two models have different mechanisms, and a transition from a Cassie-Baxter state to a Wenzel state or a coexistence of both states is possible depending on the level of roughness. [51]lthough these models provide a strong foundation for understanding the behavior of the wetting phenomenon, in practical situations, there are many more complex factors to consider.Other vital factors such as the shape of the drop, the surface free energy, the work of adhesion, and the determination of the equilibrium state of the drop need to be taken into consideration to study the wetting properties of a solid surface accurately. [52]ence, in complex cases where multiple factors come into play, these two models may not be sufficient, and other modifications should be carefully applied to adjust the value of the contact angle. [53]

Achieving Superhydrophobicity or Superhydrophilicity
Over the past decades, the wetting behavior of surfaces with advanced properties has gained significant interest in various fields, particularly in optical systems.In addition to the contact angle, another crucial parameter that must be considered when studying the interaction between a liquid drop and a solid surface is the contact angle hysteresis (CAH).
The contact angle hysteresis (CAH) is the difference between the advancing contact angle ( a ), measured at the front of a drop on a tilted surface, and the receding contact angle ( r ), measured at the rear. [54]Another way to measure the CAH, can be done by increasing and then decreasing the volume of the same drop of liquid, which corresponds to the advancing and receding angles, respectively.Here, the contact area is not affected until the droplet starts to advance or recede, and thus,  a and  r can be determined. [55]A larger CAH indicates stronger adhesion of the droplet to the surface.
For superhydrophobic surfaces, where the droplet barely adheres, the contact angle is greater than 150°with a low CAH, typically less than 10°.Superhydrophobicity has been observed in nature on surfaces such as lotus leaves, rice leaves, rose petals, water strider legs, and butterfly wings. [56][59][60] These surfaces can be achieved by combining controlled roughness with low surface energy or by creating micro or nanopatterns on the surface.
On the other hand, achieving superhydrophilic surfaces is more complex.Superhydrophilic surfaces are highly wettable, characterized by high surface energy and a very small contact angle, often close to 0°.Simply increasing the roughness of a surface is not sufficient to achieve total wetting with a contact angle of zero, especially if the surface is already hydrophobic.Complete wetting is impossible.A surface is considered superhydrophilic if the measured contact angle is less than 5°.To create such superwetting surfaces, the key is to enhance the surface energy.Researchers have taken inspiration from natural surfaces, such as shark skin, moss epidermis, and sponges, to develop strategies for achieving superhydrophilic properties.Similar to the fabrication techniques used for the creation of superhydrophobic surfaces, increasing the roughness of a hydrophilic surface through etching or chemical modification of the surface or integrating a hydrophilic material using different deposition techniques, [61,62] can impact the superhydrophilic properties of the surface.
In recent years, various fabrication techniques have been developed to convert a superhydrophobic surface into a superhydrophilic surface and vice versa. [63]One such technique, developed by Agnieszka Telecka et al. [59] involved using blockcopolymer (BCP) nanolithography combined with reactive ion etching (RIE) to create patterned silicon surfaces with enhanced wetting properties.This technique produced surfaces with nanoholes, nanopillars, and conical nanopillars, exhibiting superhydrophilic properties.However, when these surfaces were coated with a hydrophobic perfluorodecyltrichlorosilane (FDTS) monolayer, they transformed into superhydrophobic surfaces.as represented in Figure 5 for a straight side wall nanopillars surface structure.
Nanostructuring primarily concentrates on the alteration of physical surface geometry to attain superwetting properties, whereas chemical modification involves the manipulation of surface chemistry through the introduction of specific chemical groups via methods such as chemical coating, nanoparticle incorporation, and surface treatments.Chemical modification plays a crucial role in tailoring the surface properties of materials.Through the introduction of specific chemical groups, accomplished via techniques like chemical coating (including The contact angle of a drop of water on the superhydrophobic and superhydrophilic states of the same sidewall nanopillars structured surface.Reproduced with permission. [60]Copyright 2023, The Royal Society of Chemistry. methods such as chemical vapor deposition, dip or spin coating, and sol-gel processes), functionalization with nanoparticles, or surface treatments like UV irradiation or plasma treatment, chemical modification plays a pivotal role in customizing surface properties. [64]However, it is important to note that the scope of our current review primarily centers on structured surfaces, where surface roughness and geometric patterns are the keys to achieving superwetting properties.The combination of both nanostructuring and chemical modification techniques also offers a viable strategy for fine-tuning and controlling wetting properties, depending on the targeted properties and applications.However, it has to be noted that the chemical modification has to be mechanically resistant, to friction as an example.
Numerous successful fabrication techniques have been developed for creating both superhydrophilic and superhydrophobic surfaces, and they have been extensively reviewed in various works due to the high demand for these surfaces in many applications. [53,65,66]The unique properties of superhydrophobicity and superhydrophilicity are particularly useful for selfcleaning purposes.On superhydrophobic surfaces, droplets can easily roll off and clean the surface, while superhydrophilic surfaces, allow thin films of water to run off, facilitating cleaning.These surfaces find applications in a wide range of fields, including self-cleaning, anti-icing, anti-fogging (especially for optical equipment), oil-water separation, and anti-bacterial coatings for various biological operations. [67]

On the Mechanical Properties of Optical Multifunctional Surfaces
In the previous sections, we have discussed the optical and wetting properties of the bio-inspired surfaces.In this section, we will focus on the mechanical properties of optical multifunctional structured surfaces, particularly in the context of high-transmission optical windows.Additionally, we will explore the main challenges that arise in the performance of these structures when they are exposed to harsh environments.

The Required Mechanical Properties
The sustainability of multifunctional bio-inspired optical surfaces relies on their potential high mechanical resistance.Combining anti-reflective properties across a wide wavelength range with super-wetting properties, such as anti-fogging, anti-icing, and self-cleaning, using a single material is crucial for various applications in optoelectronics, photovoltaics, and defense systems.
These structured surfaces, known as sub-wavelength structures, typically have a high aspect ratio, which is the ratio between the height and the width of the pattern.For instance, achieving a conical-structured silicon wafer with an aspect ratio between 2 and 6 and a periodicity smaller than 1 μm on a hexagonal lattice can result in high transmission between 96% and 98% in the mid-wave infrared range (MWIR).However, the use of such surfaces is limited in outdoor environments or in military applications where the surface is exposed to weather conditions like raindrops and hailstorms and becomes vulnerable to severe abrasion caused by sand grains and dust that can impact their multifunctional performance.Such environmental factors can alter the patterns, which in turn affect the optical and wetting performance of the surface. [68]The variation of the structure, geometry, and shape of the pattern causes detrimental effects on the optical and wetting performance.As a result, the gradient refractive index profile and the contact angle of the surface are changed.Consequently, the robustness of the patterns becomes essential for maintaining the high performance and longevity of these optical multifunctional surfaces.
[70][71][72] However, sub-wavelength optical windows present unique challenges in studying their mechanical properties, which has led to relatively fewer works addressing this issue. [73]onetheless, there have been efforts to investigate the mechanical resistance of these multifunctional optically structured surfaces through various mechanical tests.
In the following sections, we will review and present an overview of the mechanical tests that have been conducted to assess the mechanical resistance of these multifunctional, optically structured surfaces.

Common Mechanical Tests on Multifunctional Surfaces
Indeed, the choice of materials for multifunctional optical windows depends on the specific application and the desired spectral range.Various materials, including glass, sapphire, zinc sulfide (ZnS), zinc selenide (ZnSe), silicon, and germanium, have been utilized for structuring these surfaces.
However, achieving a durable and mechanically robust multifunctional surface remains a significant challenge.Several parameters influence the mechanical resistance of these structured materials, making it essential to study their mechanical properties.Understanding the reasons behind their brittle behavior in harsh environmental conditions is crucial for developing more resilient and long-lasting multifunctional optical surfaces.
By comprehensively studying the mechanical resistance of these structured materials, researchers can identify potential weaknesses and develop strategies to improve their durability in harsh environments.This knowledge is vital for enhancing the performance and longevity of multifunctional optical windows, making them more suitable for a wide range of practical applications.

Sand and Rain Erosion Test
In 2005, Hobbs et al. [7] have reported the fabrication of different pattern designs, such as pyramidal, cylindrical holes, and sinusoidal structures on several optical windows.The optical performance of these surfaces has been studied, highlighting the importance of the mechanical performance of these surfaces, as they suggested that the hole pattern may be more resistant to abrasion as a result of the impact of sand and rain in real-life applications.
Later, Hobbs and colleagues conducted sand and rain erosion tests on microstructured ZnS surfaces. [74]Various pattern designs have been tested.The sand erosion tests were performed under two conditions, using large and small sand grains at different speeds and doses directed at normal incidence on the surface.In the first condition, the diameter of the sand particles ranged from 125 to 177 μm and being displaced at a speed of 168 m s −1 with three different dose levels: 10, 20, and 30 mg cm −2 .Whereas in the second condition, the diameter of the sand grains is reduced to a range from 38 to 53 μ2 where the speed has increased to 262 m s −1 and the three dose levels were 4, 8, and 12 mg cm −2 .The measurements show that the transmission of all the ZnS structures has decreased with the increase in the exposure dose of the sand particles over the infrared spectral range between 6 and 12 μm.This transmission loss was explained by the damage caused to the surface, as shown by the SEM images in Figure 6.Here, the light is traveling the surface without any gradient change in the refractive index, so a part of the incident light becomes reflected and scattered due to the damage caused to the surface morphology; hence, the transmission is decreased.Additionally, a rain erosion test has been used to understand the impact of rain drops on the microstructured ZnS surfaces.The speed, the size, the incident angle, and the time exposure of the water droplets defined the conditions that should be applied to the surface to test the rain impact.A significant transmission loss appeared for all the ZnS structures except the Motheye structure with a period of 2.7 micron and a height more than 3 micron where a minimal transmission loss appeared after 20 min of rain exposure.This study has demonstrated that the structured surfaces have a great potential over the anti-reflected thin films in terms of optical and mechanical performance in harsh environment.
In 2009, Hobbs conducted similar experiments on structured sapphire surfaces. [75]Sand erosion tests on sapphire resulted in a negligible transmission loss over the near infrared spectral range between 1300 and 1700 nm compared to the ZnS material, highlighting sapphire's superior mechanical properties, which are known to be harder than those of ZnS.However, the transmission of the structured sapphire has increased only slightly compared to the untreated surface.For the rain erosion tests, the transmission measurements show no change before and after the erosion.However, surface damage has appeared on the surfaces after the sand erosion and that may affect the wetting properties of the surface, which has not been mentioned in this study.
Recently, Chen and Zhang [76] studied the erosion resistance of hydrophobic micro and nanostructured titanium surfaces for aircraft applications.Their mechanical tests exposed the surfaces to high-speed sand particles, causing plastic deformation under specific impact angles.However, the optical properties were not included in this study as the dimensions of the patterns are not targeting a high transmission optical surface.Zambrano-Mera et al. [77] conducted a sand erosion test to study the effectiveness of a multi-layer anti-reflective TiO 2 / SiO 2 surface.The test was performed to analyze the effect of Zr-oxide doping and thermal annealing on the mechanical properties of the surface.After the test, the structure doped with 1 at% Zr and annealed at 400 °C has shown better mechanical robustness.
These studies emphasize the importance of erosion tests to simulate real-life harsh environments and the capability to do these tests on structured and non-structured surfaces.While erosion tests have provided promising results, they also reveal the brittle mechanical performance and low mechanical resistance of micro-and nanostructured optical multifunctional surfaces.Based on these results from sand erosion tests, it is found that the mechanical resistance of the structured material itself depends on its initial hardness.This is the reason why the damage to the microstructured sapphire after the same sand erosion test was less represented compared to the ZnS structure.Moreover, the geometrical form also plays a crucial role not only in the optical properties but also in the resistance of the structure.The  c,d) SEM images of the damaged area of the ZnS structured surfaces after exposure to sand conditions A and B, respectively.Adapted with permission. [74]th-eye structure in these studies has been shown to be more resistant to rain erosion than the hole-type structure.Owing to the importance of having an optical multifunctional surface, different optical materials can be structured aiming to enhance their optical performance, such as sapphire, silicon, germanium, and ZnS.Whether in civil or military applications, the choice of materials depends on the target spectral range.While sapphire has been shown to have more resistance to sand erosion, it does not transmit in the spectral range between 8000 and 13 000 nm as in ZnS for example.To summarize, the sand and rain erosion tests are useful to study the mechanical performance of these surfaces in real-life and outdoor applications.From the optical point of view, the sub-wavelength conical structure is the ideal geometrical form to enhance the transmission.The material can be chosen based on the demanded spectral range.However, a mechanical solution needs to be investigated in the future to protect these surfaces from the environmental conditions.

Nanoindentation
Instrumented nanoindentation is a valuable technique used to continuously measure the applied load and displacement on a specimen during the indentation process.This test is commonly employed to determine important mechanical properties such as hardness, Young's modulus, and elastic-plastic deformation of various materials, including substrates and thin films. [78]r rough surfaces, analyzing nanoindentation results can be challenging due to surface roughness. [79]In 2016, Scarratt et al. [80] studied the mechanical properties of a superhydrophobic Teflon surface with spontaneous wrinkling.They found the surface to be mechanically robust, but they acknowledged the potential errors in nanoindentation measurements for rough surfaces.To validate their results, they performed the same test but on a flat area and compared the hardness values of the flat and wrinkled surfaces, which were found to be very close, thus validating the accuracy of the adapted protocol and the results on the rough surface.Thus, before using the nanoindentation test on structured and rough surfaces, a well-defined and validated protocol should be developed to validate the accuracy of the results.
The hardness of porous materials depends on the level of porosity.Higher porosity, particularly at the sub-wavelength scale, can enhance anti-reflectivity but may lead to reduced hardness.In 2018, Fakiri et al., studied the mechanical behavior of porous silicon with different porosity levels using the nanindentation technique.The results show that the hardness and Young's modulus of the porous material decrease by increasing the porosity level.For a porosity larger than 60%, the material becomes mechanically unstable.A solution has been proposed in this study to enhance the hardness by applying controlled oxidization.As a result of the impact of the thin layer of SiO 2 , the porosity level is reduced and the hardness increases.87][88] These tests allow researchers to investigate the response of structured surfaces under mechanical stress and evaluate their mechanical robustness.Among different structure geometries, determining the Young's modulus and hardness of pillar-shaped structures is relatively easier compared to conical-shaped structures, mainly due to the presence of a large flat surface on the top of the pillar, which facilitates the measurement.However, the dimensions of such structures may not be suitable for high-transmission anti-reflective surfaces that require a gradient refractive index profile, making the mechanical behavior more complex.
Nanoindentation is a powerful technique to assess the mechanical properties of various materials, including structured surfaces.It helps researchers understand the impact of surface roughness, porosity, and patterning on the mechanical performance of multifunctional optical surfaces.Here, we are focusing on the optical multifunctional surfaces with high optical direct transmission.These surfaces are structured at a sub-wavelength scale with a high height.As the pattern reaches a high height, its reflectivity approaches zero.While achieving zero reflectivity is advantageous, it comes at the cost of increased brittleness.Thus, testing the mechanical behavior of these surfaces with nanoindentation is very important to understand the performance of these surfaces at the nanoscale.
Nanoindentation has been used in a few studies to investigate the mechanical behavior of sub-wavelength scale structured surfaces with high transmission and a high aspect ratio.One recent study by Doquet et al. [89] focused on the mechanical properties of nanopatterned silicon (Si) and germanium (Ge) surfaces with a high aspect ratio using the nanoindentation technique.The aim was to test the compression resistance of nanocones patterned optical surfaces.
Various shapes and dimensions of structured Si and Ge surfaces were tested (Table 1).The nanostructured Ge surface underwent a load-controlled nano-compression test with a circular flatended diamond indenter with a maximum load of 6500 μN.The resulting load-displacement curves displayed pop-in and pop-out events, which are related to the nature and morphology of the tested surface.
At the maximum load, a penetration depth of up to 2000 nm was observed; however, when the maximum load was applied, the displacement was at 1000 nm, which means after passing 20% of the 5000 nm cone height, it suddenly jumped from 1000 to 2000 nm at the same load, indicating the occurrence of fractures in the cones.SEM images confirmed the presence of broken cones.This suggests that applying the maximum load to these cones led to their fracture.
In the study of the square lattice Si structured surface, both displacement-controlled and load-controlled nano-compression tests were performed.In the displacement-controlled nanocompression test, three maximum penetration depths were tested: 75, 105, and 120 nm which correspond to 75, 82, and 85 μN maximum loads, respectively.No damage was observed on the surface based on SEM images.However, the small maximum displacement in these tests, relative to the 8 μm height of the cones, may explain the lack of significant changes on the surface and no plastic flow was taking place upon unloading.In the loadcontrolled nano-compression test, a higher maximum load was applied, and the displacement increased significantly.Despite the displacement shift observed in the curves, the SEM images did not show significant damage, which could be attributed to various factors, including the indenter geometry, the dimensions of the structure, and the pyramidal shape of the patterns.Moreover, at this maximum load, the maximum penetration depth was close to 1000 nm, which corresponds to 12.5% of the 8 μm height cones.This also can explain the very little damage observed on the SEM images.
In addition to the square lattice structures, a conical-patterned Si wafer with a height of 3.5 μm and a period of 1 μm on a hexagonal lattice was also subjected to the nano-compression test.The observed maximum displacement was approximately 46% of the cone height at a maximum load of 2000 μN and during unloading, the displacement remained constant with the decrease of the load, suggesting plastic deformation.SEM images revealed a significant number of broken cones, indicating plastic deformation of the surface.
Considering all of these outcomes, the Si nanopyramides show better fracture resistance compared to the Ge nanocones.While many Ge nanocones were broken under a compressive stress of 1.1 Gpa, no significant deformation was observed for the Si nanopyramides under 2 GPa.From an optical point of view, the Si-structured surface with a periodicity of 2.4 μm of a square lattice is not the ideal structure to attain high optical transmission.The subwavelength structure with a hexagonal lattice increases the optical transmission due to the reduction of the flat zones compared to the square lattice.This structure may also have better mechanical resistance, except for the conical structure with a sharp tip, since the compressive stress will be divided on many numbers of cones thus, the applied load will be less severe at the top of each pattern.
It is important to note that in the mentioned studies, the structures were claimed to be anti-reflective and light-transmitting windows, but the exact transmission values were not provided.However, to the best of our knowledge, both the pyramids and Load-displacement curves of structure Si substrate obtained through micro-compression test.Reproduced with permission. [73]Copyright 2023, Elsevier.
the truncated cones are not the ideal structures for attaining a high transmitted surface since no perfect gradient refractive index effect can be obtained.
In another study by Fauvel et al. [73] in 2022, a microcompression test was conducted using the nanoindentation technique on a nanostructured Si surface.The structure was made on a hexagonal lattice with an oriented (100) silicon substrate, revealing a conical shape of the patterns where the top of the cone is slightly truncated.Unfortunately, the dimensions of the structure were not mentioned for confidentiality reasons; however, in the numerical study, they indicated a maximum height for the cone of 2.5 μm and a truncation radius of 50 nm, which should be close to the real value of the structure dimension.The microcompression was performed on a single nanostructure, aiming to understand the behavior of a unique structured cone.A conical flat end with 900 nm radius indenter was used for the indentation; the load-displacement curve obtained is shown in Figure 7.The maximum penetration depth attained is up to 1 μm at a maximum applied load of 100 μN which is relatively low.The loaddisplacement curve showed no purely elastic deformation, with a hysteresis phenomenon observed during loading and unloading related to the contact between the indenter and the adjacent structures.This brittleness of the truncated cones could affect their optical and wetting performance.However, there is no optical characterization that has been mentioned in this work, and we do not know the transmission of these structures that do not reflect the ideal shape for high transmission due to the truncation at the top.
Both studies included numerical analyses using finite element methods to better compare theoretical and experimental results.Additionally, coated layers were deposited on these structures to enhance their mechanical properties, which were also measured by nano-compression and it also will be discussed in Section 4.However, the details of these numerical studies were not provided in this paper.
To summarize, while the patterning of the optical surface can increase the optical transmission, it also reduces the fracture resistance of the surface.The mechanical resistance of the subwavelength structure of the hexagonal lattice needs to be enhanced without altering the desired optical performance.

Nanoscratch
The nanoscratch technique, similar to nanoindentation, is used to study the mechanical properties and durability of surfaces and thin films.In this test, an indenter applies a vertical force, where the friction coefficient and deformation of the specimen can be determined.
The nanoscratch test is a valuable method for investigating how sub-wavelength structures respond to stress.This technique provides a comprehensive analysis of the elastic-plastic deformation of the surface through pre and post-scan comparisons.Additionally, the nanoscratch test provides a quantitative measure of the critical load at which plastic deformation occurs.This information is valuable for predicting the mechanical limits of the material and can guide the design of structures that need to withstand specific loads without undergoing irreversible changes.While the application of nanoscratch tests on sub-wavelength structured optical windows, such as Si, Ge, and sapphire, is still lacking, it has been utilized in several studies to study the mechanical properties of superhydrophobic surfaces, [80] porous structures, [81] nanowires, [90,91] and polymers. [92]he nanoscratch test conducted on the durable superhydrophobic Teflon surface with spontaneous wrinkling by Sacarrat et al. [80] aimed to assess the surface durability under high loads.The Teflon film with different thicknesses was deposited on both polyshrink and polyolefin substrates.The goal was to determine the applied force required to delaminate the film from the substrates.
In the nanoscratch test, a spherical diamond probe with a diameter of 5 μm was used to apply a vertical force to the surface.By varying the applied force, the researchers could observe the behavior of the Teflon film on both substrates.The results showed that a lower force was needed to delaminate the Teflon film on the polyolefin substrate compared to the polyshrink substrate.This difference in force requirements can be attributed to factors such as the thickness, elasticity of the layer, and adhesion of the film to the substrates.The nanoscratch test provided valuable insights into the durability and resistance of the structured surface.
In the study conducted by Vojkuvka et al. [81] on nanoporous anodized alumina (NAA), the mechanical properties of the material were investigated using nanoscratch tests.Three types of NAA were fabricated using different acid electrolytes and varying porosity levels.
During the scratch test, both Berkovich and conical probes with different cantilevers were used, each applying different levels of force to the surface.The results indicated that the mechanical behavior of the nanoporous material was influenced by two main factors: the porosity level and the type of acid electrolytes used in the fabrication process.The sample fabricated using phosphoric acid exhibited elastic deformation, meaning that the porous structure was able to deform without undergoing irreversible damage such as cracking.Even when subjected to a load force of 400 mN, no cracks were observed in the material.Concerning the effect of the porosity level, scratch tests were conducted on various samples with different porosity levels, each treated with a specific type of acid.The tests were performed using steel cantilevers with applied loads ranging from 20 to 100 mN.The observations regarding the effect of porosity levels were as follows.When the porosity level was low, there  [92] Copyright 2023, Authors, Published by PubMed Central (PMC).
was no significant difference in the friction coefficient among the samples treated with different acid electrolytes.The friction coefficient for these samples ranged from 0.06 to 0.19.In contrast, at a high porosity level, the friction coefficient for the first two types of samples was approximately 0.24, which was higher than the sample treated with phosphoric acid, where the friction coefficient remained around 0.17.This indicates that, in general, the friction coefficient increased with an increase in the porosity level.However, it is noteworthy that even with a high porosity level, the sample treated with phosphoric acid maintained a relatively low friction coefficient.This suggests that the type of acid used in the treatment had a notable influence on the friction coefficient, and in the case of phosphoric acid, it was effective in reducing friction, even in samples with high porosity levels.
In 2016, Yilbas et al. [90] conducted a study on the optical and mechanical properties of Si nanowires fabricated through metalassisted chemical etching.The main focus of this research was to investigate the effects of octadecyltrichlorosilane (OTS) deposition followed by n-octadecane coating on the surface properties, especially the wetting properties.To assess the optical properties, the researchers measured the reflectivity using UV-vis reflectivity tests.The results indicated that after the deposition of a 1.5 μm-thick layer of n-octadecane coating, there was a decrease in the reflectivity for wavelengths below 500 nm and above 550 nm.Regarding the mechanical properties, a nanoscratch test was performed using an applied contact load of 0.03 N and an end load of 5 N with a scratch length of 1 mm.However, the specific material of the probe used in the nanoscratch test was not mentioned in the study.The friction coefficient was measured both before and after the coating process.The results showed that both the reflectivity and the friction coefficient decreased after the coating of n-octadecane.
These findings suggest that the surface properties of Si nanowires can be modified by coating them with n-octadecane, leading to changes in both optical and mechanical characteristics.The decrease in reflectivity indicates alterations in the optical behavior of the nanowires, while the reduction in friction coefficient impacts their wetting properties and surface interactions.The n-octadecane coating functions as a lubricant, resulting in a low friction coefficient.This slippery layer also has an impact on the surface wetting properties, as confirmed by the water droplet contact angle test.After the deposition of the n-octadecane layer in the solid phase, the contact angle of the surface decreased from 165 for the OTS-coated Si nanowires to approximately 150°.This decrease in the contact angle coincided with the reduction in the friction coefficient.
In 2021, Jacobo et al. [92] conducted a study on the optical and mechanical properties of bioinspired anti-reflective flexible structured poly(methyl methacrylate) (PMMA) films.The objective of the research was to investigate how the fabrication process influenced the geometry, optical performance, and mechanical properties of the structured PMMA surfaces.They employed the rollto-roll thermal nanoimprint lithography technique to fabricate moth-eye-like structured PMMA films, where two main parameters were varied to examine their effects: the imprinting temperature and the web speed.In the experiment, the web speed was kept constant at 0.02 m∼min −1 , while the temperature ranged from 80 to 140 °C.The results are represented in Figure 8.The researchers analyzed the transmission using a UV-vis spectrometer with an integrating sphere.The results showed that increasing the temperature led to a rise in direct transmission, indicating a change in the structure dimensions during the fabrication process.As the temperature increased, the moth-eye-like structure assumed a conical shape with a height of 300 nm, leading to higher transmission as the aspect ratio increased.
A scratch test was conducted in this study with a spherical diamond probe with a radius of 10 μm.A 16-μm-long scratch was performed at a maximum load of 100 μm.Pre-and postscans were measured to assess the surface topography and deformation of the specimen.From the post-scan, they determined the residual plastic deformation after the scratch.The results showed that the structured PMMA films below the glass transition temperature exhibited elastic behavior, which was related to the surface dimensions.At 80∼°C, the bulk PMMA had a height smaller than 100 nm, which could be considered porous.The plastic deformation occurred when the temperature increased, corresponding to the height of the structure.To summarize, the PMMA films with the highest moth-eye structure height showed an enhancement in optical transmission, increasing from 92% to about 96% on one side of the specimen.However, these structures exhibited lower mechanical performance due to plastic deformation.
Understanding the mechanical behavior of sub-wavelength structured optical windows through nanoscratch tests and by following the same strategies for porous and nanowire structures needs further investigation.The nanoscratch serves as a powerful tool for validating the effectiveness of advanced solutions aimed at enhancing the mechanical resistance of structured surfaces particularly, the concept of depositing a hard coating to cover the patterns.By subjecting the coated surfaces to nanoscratch testing, researchers can assess the durability and protective capabilities of the applied hard coating.This test is a crucial step in advancing the development of these optical surfaces and fostering improvements in their mechanical properties.

Abrasion Test
The durability of the multifunctional sub-wavelength structured surfaces can be estimated by assessing their abrasion resistance.Several mechanical tests, as mentioned earlier, can serve as abrasion methods to evaluate the surface's ability to withstand wear and friction.These methods include the sandpaper abrasion test, [93] the tape-peel test, [94] the wiping test, [17] the abrasive pad test, [95] and the bending test. [89]he sandpaper abrasion test performed on the transparent superhydrophobic polydimethylsiloxane (PDMS) film with microprotrusions structured between microwalls showed promising results in terms of abrasion resistance and surface robustness. [93]he femtosecond laser-ablated template allowed for precise structuring of the microprotrusions.
During the abrasion test, sandpaper was applied to the structured PDMS surface, and the surface was subjected to 200 cycles of abrasion using a counterweight with controlled velocity.The SEM images revealed that the structured PDMS surface exhibited very little damage after the abrasion test.This indicates that the geometrical design of the surface, particularly the arrangement of microprotrusions between microwalls, contributed to its ability to withstand abrasion and maintain its integrity.However, the optical properties, particularly the transmission of the PDMS film, did not show a significant enhancement after the microstructuring process.It is worth noting that the flat PDMS film already possessed a high transparency of 91% in the visible wavelength range.As a result, the microstructuring did not lead to a notice-able change in transparency and the surface retained its optical properties even after the abrasion test.
Overall, the sandpaper abrasion test demonstrated the durability and robustness of the multifunctional structured PDMS film, making it a promising candidate for various applications where both surface properties and mechanical resistance are crucial.
The study by Michalska et al. [94] in 2021 demonstrated the fabrication of a bioinspired multifunctional glass substrate using regenerative secondary mask lithography to create glass nanopillars.The transmission of the structured glass surface was measured using an integrating sphere, and it achieved a high transmission of 97.5%, indicating its potential for use as a hightransmission nanostructured surface.The authors mentioned that the transmission increased with the aspect ratio, and for an aspect ratio greater than 5, a very high transmission could be achieved with a double-sided nanostructured glass substrate in the near-infrared range.To evaluate the mechanical stability of the glass nanopillars, a tape-peeling test was conducted on two different structure designs.One design had a low aspect ratio of 2.5 with a high solid fraction of 0.5, while the other had a high aspect ratio of 5.5 and a low solid fraction of 0.1.The tape-peeling test involved applying scotch tape to the surface and then peeling it off.After removing the tape, the nanopillars remained unaffected, indicating that they resisted the tape peeling test.However, due to the high adhesion of the scotch tape, large areas of the surface were covered by the residual adhesive.While the tapepeeling test provided some insight into the mechanical stability of the glass nanopillars, it cannot fully estimate their durability in harsh environments or real-life applications.Additional mechanical robustness tests are necessary to further verify the performance and durability of these nanostructured glass surfaces.These tests could include more rigorous abrasion tests, impact tests, or exposure to other environmental stressors to assess the surfaces' ability to withstand harsh conditions.Such comprehensive testing is crucial to ensure the practicality and reliability of multifunctional surfaces in real-world applications.
The study by Infante et al. [17] focused on determining the mechanical durability of superhydrophobic, anti-reflective nanostructured glass substrates using a wiping test.The researchers fabricated different nanopillar dimensions on the glass surface using the dry etching method.Reflectance and transmittance measurements showed that the structure with the highest nanopillar height, ≈200 nm, had the lowest reflection value (0.4%) and an average transmittance of 94.7%.However, the surface design resulted in remarkable forward scattering.
To evaluate the mechanical robustness of the surface, the researchers performed a wiping test using a fiber cloth with 5000 runs and a load of 9 N. Before conducting the test, the structured glass surface was treated with an ion exchange (IOX) chemical process.This treatment induced a compressive stress layer on the glass surface, [96] which is known to enhance the mechanical resistance of the structured glass.After the 5000 runs of the wiping test, the nanostructured surface remained intact, indicating good mechanical stability.However, the SEM images did not show any visible damage to the nanopillars or holes on top of the structures.Despite the wiping test not causing visible damage, the transmission and contact angle of the surface remained unchanged, suggesting that the surface retained its superhydrophobic and anti-reflective properties.Overall, the results The change of the water and ethylene glycol (oil) contact angles during the abrasion test cycles on the nanostructured glass (i) and the effect of the heat treatment on the water and oil contact angles of the abraded sample (ii).b) SEM images of the surface before and after 500 abrasion cycles.Reproduced with permission. [95]Copyright 2023, The Royal Society of Chemistry.
demonstrated that the ion-exchange-treated, superhydrophobic, anti-reflective nanostructured glass surface exhibited good mechanical durability and retained its optical and wetting performance even after extensive wiping, making it a promising candidate for various practical applications.
The study by Haghanifar et al. [95] focused on testing the mechanical durability of a structured glass surface with random nanostructures.The glass surface was fabricated using the reactive ion etching (RIE) method, resulting in transparent, superomniophobic (repelling both water and oil) nanostructures with heights ranging from 100 to 500 nm and a distance of 100 nm between each pillar.
To assess the mechanical durability, an abrasion test was performed using a SCOTCHBRITE abrasive pad with a pressure of 1225 N∼m −2 .After 400 cycles of abrasion, the water and contact angles on the surface decreased from 160°to less than 90°d ue to the impact of the abrasion on the surface topography, as shown in Figure 9.However, during the fabrication process, a re-entrant structure was created on the surface by depositing a silicon dioxide (SiO 2 ) layer through the PECVD technique, followed by fluorination.This step gave the surface a self-healing character, leading to a significant increase in the contact angle values after the abrasion test by heating the sample for 15 min at 95 °C.The heating treatment caused the re-entrant structures to recover their shape, making the surface self-healing.The fluorine molecules helped to lower the surface energy, [97] contributing to the self-healing property of the surface.
Although the optical properties of the surface were not characterized after the abrasion test, it was noted that the transmission of the glass after double-side structuring increased from 93.5% to 97%, which represents an absolute improvement of 3.5%.This increase in transmission suggests that the structured glass surface with random nanostructures maintained good optical performance even after the abrasion test.The results indicate that the structured glass surface with self-healing nanostructures shows promise for applications requiring both mechanical durability and optical performance.The self-healing ability of the surface could potentially extend the lifespan and functionality of multifunctional glass surfaces in harsh environments.
The study conducted by Doquet et al. [89] focused on investigating the mechanical behavior of nanostructured (001) Si wafers using a biaxial bending test.The structured Si surface of 1 mm thickness was fabricated with conical shape patterns on a square lattice, with a height of 4 μm and a period of 1.6 μm.The bending test was performed using a ball-on-ring device with a steel ball of 7 mm in diameter to bend both flat and nanostructured surfaces.
During the bending test, it was observed that there was no significant deviation between the smooth and structured surfaces during the loading phase.This lack of deviation is attributed to the small dimensions of the patterns, which represent just 0.4% of the entire thickness of the surface.However, the loaddisplacement curves showed that the nanostructured surfaces exhibited cracks and failures at loads of 296 and 418 N. In comparison, the smooth Si surface exhibited a fracture load of up to 2100 N in some samples.This suggests that the fabrication of the nanostructured patterns decreased the fracture resistance of the surface.The study highlighted that the mechanical resistance of structured surfaces is strongly influenced by the geometry of the structure and the dimensions of the patterns.Various mechanical tests, such as adhesion tests, pencil scratch tests, and atomic force microscopy (AFM), [98] can be used to evaluate the mechanical behavior of these surfaces.However, the effectiveness and durability of these surfaces depend on the specific geometry of the structure and the nature of the mechanical test technique used.
Overall, the research on the mechanical behavior of structured surfaces is ongoing, and there is still much work to be done to fully understand and enhance the durability and robustness of these surfaces.Further investigations and optimization of the structure designs and mechanical test methods are necessary to improve the performance and reliability of multifunctional structured surfaces for real-life applications.

Hard and Transparent Materials for Robust Multifuncional Sub-Wavelength Nanostructures
The mechanical robustness of optical multifunctional surfaces is crucial for maintaining their durability and performance in harsh environments.One of the challenges faced in this field is the brittle behavior and weak mechanical resistance of sub-wavelength structures, which can limit their practical applications.In some cases, the simplest approach to fabricate a hard and anti-reflective surface is to deposit a single layer of a hard material.As discussed in Section 2, the deposition of a single layer with a refractive index (n) equal to the square root of the refractive indices of the surrounding medium and the substrate can significantly reduce the surface reflectivity.For instance, to reduce the reflectivity of a silicon substrate, a material with a refractive index close to 1.8 would be needed.
However, while single-layer coatings can provide anti-reflective properties, they may not always offer sufficient mechanical durability and high optical transmission.Sub-wavelength structures can be more prone to damage and wear, particularly in harsh environments.To address this issue, researchers are continuously exploring new fabrication techniques and materials that can enhance the mechanical robustness of these surfaces.
The study conducted by Ozhan et al. [99] in 2021, where they deposited boron carbide (B 4 C) and boron nitride (BN) thin films on a silicon (Si) substrate, highlights the potential of using hard materials to enhance the mechanical properties of surfaces.The deposited single layers of BN and B 4 C improved the transmission of the Si surface, but their anti-reflective performance was limited to the mid-infrared region.This means that while they achieved good mechanical properties, the optical properties were not optimized for broadband and omnidirectional high transmission.
The limitations observed in this study and other works focusing on multi-layer structures are not uncommon. [99,100]Achieving both excellent optical and mechanical performance on multifunctional surfaces remains a complex challenge.
The ideal surfaces with the required optical performance are biomimetic sub-wavelength nanostructures.In the previous section, we focused on the different mechanical tests that can be used to study the mechanical properties of structured surfaces.Unfortunately, the erosion test, as well as the nanoindentation test, have shown mechanical instability of the patterned structures, leading to surface damage.This results in a significant change in the gradient refractive index profile, affecting their multifunctional performance.To address this challenge, researchers are exploring suitable solutions to enhance the me-chanical robustness and durability of these surfaces without compromising their optical and wetting properties.
Protecting the surface with hard coatings has been widely adapted as a method to enhance the mechanical robustness and durability of optical windows. [21,101]To find the optimal parameters for creating a hard surface with mechanical stability against harsh environments, numerous materials have been studied using various deposition techniques, such as plasma enhanced chemical vapor deposition (PECVD), physical vapor deposition (PVD), ALD.While many studies have reported on the mechanical properties of hard coatings on flat surfaces, only a few have investigated the efficiency of this method in protecting patterned surfaces from damage and fracture. [73,74,89]he main goal is to preserve the shape of the pattern and maintain the durability of the surface by depositing a thin layer of a hard and transparent material without altering the optical properties obtained after structuring.This problem involves considering various parameters, and one critical aspect is the selection of the coating material.

Materials
This article reviews materials that exhibit both high transmission across a wide wavelength ranges and high hardness and elastic modulus.Table 2 categorizes the potential hard and transparent materials in different wavelength range based on their optical, mechanical, and wetting properties.Specifically, we shed light on the main representative characteristics of these properties, namely the refractive index at a specific wavelength, the wavelength range, the knoop hardness and the Young's modulus for the mechanical properties, and the wetting behavior of each material.On one hand, the refractive index and the wavelength range indicate the potential use of these materials as antireflective coatings and determine the transmission band.On the other hand, hardness and elasticity are two crucial parameters that provide valuable information about how a material responds to external forces and how it can be used in various applications.Below, we present a categorization of hard coating materials based on their optical, mechanical, and wetting properties.

Nitrides
Coatings based on nitrides, such as titanium nitride (TiN), chromium nitride (CrN), and boron nitride (BN) are known for their high hardness.Among them, boron nitride has demonstrated good transmission properties in a large wavelength range from 0.5 to 6.5 μm, making it suitable for protecting optical surfaces, particularly in the mid-infrared region. [20,99,112]

Carbides
Carbide-based materials are attractive for hard coatings due to their high hardness.B 4 C [99] and silicon carbide (SiC) [142,143] have shown significant optical and mechanical properties.These materials offer a low refractive index, wide transmission range, high hardness, and Young's modulus, making them suitable for protecting structured optical surfaces.By adding a thin layer of B 4 C or SiC, an intermediate refractive index can be achieved, which can further improve optical transmission.

Oxides
Oxide materials are widely used as hard, thin films for optical applications.Al 2 O 3 is known for its hardness, and heat treatment after the fabrication process can further enhance its mechanical resistance. [73]Other oxide materials, such as yttrium oxide (Y 2 O 3 ), magnesium oxide (MgO), titanium dioxide (TiO 2 ), and zinc oxide (ZnO), are also used as protective coatings due to their favorable mechanical properties.Aluminum oxynitride (AlON) [105,106] has shown promising mechanical and optical properties that can effectively replace Al 2 O 3 films.The choice of material can depend on the desired application and the target wavelength region based on its transmission wavelength range.

Carbon-Based Materials
Carbon-based coatings, such as amorphous hydrogenated carbon (a-C:H) and diamond-like carbon (DLC), have garnered significant interest as protective films due to their outstanding mechanical properties.DLC, in particular, has been widely used as a hard coating due to its robustness, high hardness, high Young's modulus, and low friction coefficient. [153]For optical applications, DLC exhibits high transmission in the mid-infrared region, making it suitable for protecting surfaces within this range.However, one drawback of DLC is the internal stress that can occur during the fabrication process, affecting its adhesion to substrates. [154]Another carbon-based film, diamondlike nanocomposite (a-C:H:SiOx), [119] is also a potential protective coating for optical infrared surfaces.This material has excellent mechanical properties and high transmission in both the mid-infrared and visible regions, and it boasts a low internal stress, allowing for good adherence to substrates.Moreover, diamond is considered the hardest material in the world, known for its exceptional durability and wear resistance. [117]The deposition of diamond thin film can be done through different deposition techniques, in particularly, CVD.However, the quality of the film is related to different factors such as parameter growth and the adhesion between the substrate and the film, which influence the final properties of the thin film compared to the bulk material.

Other Materials
Apart from the common hard coatings mentioned earlier, other materials like silicon, zinc selenide (ZnSe), zinc sulfide (ZnS), germanium (Ge), gallium phosphide (GaP), and black phosphorus (BP) are known for their good mechanical properties and are also used as hard coatings.Silicon, for instance, is a hard material with a relatively high refractive index, making it suitable for optical surfaces in the mid-infrared range (3.5-5 μm) where it exhibits high transmission.However, its refractive index prevents using it as a thin film for anti-reflective coating.ZnS and ZnSe are also applied in hard coating applications, and they possess good optical properties with transparency in both the visible and infrared regions.Ge high hardness makes it a suitable material for hard coatings, and while it has a high refractive index, it is used as a transparent material in the mid-infrared region. [155]owever, its optical properties are sensitive to temperature, particularly the refractive index.BP is an excellent candidate for antireflective surfaces, but it is not suitable for high-transmission optical surfaces as it is opaque in the visible range.This 2D material has good mechanical properties and great optical performance including linear and non-linear optics.However, these properties depend on the thickness and the structure of the BP film. [156]aterials like potassium bromide (KBr) and potassium chloride (KCl) have a wide transmission range, including the ultraviolet, visible, and infrared regions.However, their use as protective coatings is limited due to their brittle behavior and high sensitivity to humidity, making them less suitable for harsh environments.Polymers such as polymethyl methacrylate (PMMA) and polydimethyl siloxane (PDMS) are soft materials, and they can also be used as transparent and protective coatings.Their mechanical properties, characterized by low hardness and low Young's modulus, may not be optimal for hard coating applications.However, some modifications can be applied to the surface of these polymers to increase their hardness.Such modifications can enhance their suitability as protective coatings for specific applications. [157,158]t is worth noting that the choice of material for hard coatings depends on the specific requirements of the application, including the desired optical properties, mechanical robustness, and environmental conditions in which the surface will be used.

Influential Parameters
The properties of hard and transparent coating materials can be significantly influenced by various parameters involved in the deposition process (Table 2).Some of the key parameters that can impact the properties of the fabricated films include: i) Deposition technique: Different deposition techniques, such as magnetron sputtering, CVD, and ALD, can result in coatings with varying properties.Each technique has its own advantages and limitations, and the choice of the technique can influence the film thickness, uniformity, adhesion, and mechanical properties. [159]i) Substrate: The choice of substrate material can also affect the adhesion and performance of the coating.Compatibility between the coating material and substrate is essential to ensure good adhesion and durability.Reproduced with permission. [162]Copyright 2023, John Wiley and Sons.b) Sputtered indium tin oxide (ITO) on black silicon.Reproduced with permission. [160,161]Copyright 2023, SPIE.
iii) Thickness of the layer: The thickness of the coating layer can influence its optical and mechanical properties.Thin coatings may have different properties compared to thicker coatings.iv) Surface roughness: The roughness of the substrate or the coating surface can affect the overall performance of the coating.Roughness can influence the wetting properties, optical scattering, and mechanical adhesion of the coating.v) Heat treatment: Post-deposition heat treatment can alter the structure and properties of the coating.It can lead to crystallization, phase changes, and changes in mechanical properties, making it an essential step for improving the coating's durability.vi) Additional surface modifications: Certain surface modifications, such as chemical treatments or surface functionalization, can alter the coating properties.For example, making a hydrophobic surface superhydrophobic can enhance its water-repellent properties.vii) Deposition parameters: Various deposition parameters, such as temperature, pressure, and gas flow rates, can impact the properties of the deposited films.
Due to the wide range of parameters that can influence the properties of the coating, researchers need to carefully optimize the deposition process to achieve the desired combination of optical transparency, mechanical robustness, and other functional characteristics.Understanding how each parameter affects the final coating is crucial for tailoring the material to specific applications and requirements.
There have been studies comparing different coating techniques, such as magnetron sputtering and iALD, and their impact on the optical properties.Here is a summary of the findings: i) Magnetron sputtering: Fuechsel et al. [160,161] reported the deposition of indium-doped tin oxide (ITO) on black silicon substrate using pulsed DC magnetron sputtering to create a 700 nm ITO film.The coating showed a gradient thickness, and the coating uniformity was not ideal.While the mechanical properties were not characterized in this study, the optical properties showed an increase in reflectance after coating, which could be attributed to the thickness of the coating and the refractive index mismatch between the ITO film and the structured black silicon substrate.It is important to note that the conformal deposition depends not only on the deposition technique but also on the dimension of the structured substrate.The sputtering technique is not the ideal process for conformal coating on structured surfaces, where the patterns are very dense, as shown in Figure 10b.ii) Atomic layer deposition (ALD): Otto et al. [162] used ALD to deposit a thin aluminum doped ZnO (ZnO:Al) film on a black silicon surface.ALD allows for a uniform conformal thickness of the layer that follows the geometrical shape of the structure (Figure 10a).While no mechanical characterization was made in this study, the reflectance of the surface was measured using an integrating sphere.The reflectivity has slightly increased in the visible range from 3% for the black silicon to ≈5% after coating, since the refractive index of ZnO is less than that of Si (n ZnO < n Si ).
Overall, the findings suggest that hard coatings, especially those with good conformal thickness and uniformity achieved through techniques like ALD, can enhance the mechanical resistance and durability of optical multifunctional structured surfaces without significantly altering their optical properties.However, further studies are needed to explore and optimize the use of various coating materials and techniques for different applications and environmental conditions.

Deposition on Structured Surfaces
In the study by Hobbs et al., [74] a thin film of Y 2 O 3 was deposited as a hard material to provide durability to the structured ZnS surface.While damage sites appeared on the structured ZnS surface due to sand erosion, the coated structures with a 300 nm Y 2 O 3 coating (P-holes and P-posts) showed better transmission properties compared to the uncoated surfaces.Indeed, the size of sand grains and the dose level during the sand erosion test can significantly impact the results obtained for coated and uncoated surfaces.The results of the transmission measurements before and after the erosion test with the first condition (see Section 3.2.1)are illustrated in Table 3 over the spectral range from 7.5 to 10 μm.The study mentioned that by reducing the size of sand grains and the dose level in the second erosion condition, Table 3.The average transmission of different structured ZnS surfaces with and without Y 2 O 3 coating before and after submission to the sand erosion test at condition A [74] over the spectral range from 7.5 to 10 μm. the transmission loss value for the coated and uncoated surfaces became very similar.However, the average value of the transmission was still higher compared to the results after the first sand erosion condition.This suggests that the coating provided some level of protection to the structured surfaces even under more severe erosion conditions.However, it is important to note that no SEM images were provided in the mentioned study to directly compare the surface damage with the transmission loss.The absence of SEM images can make it difficult to visually assess the extent of surface damage and understand how the coating impacted the structural integrity of the surfaces under different erosion conditions.
The study conducted by Doquet et al. [89] in 2022 explored not only the mechanical behavior of patterned Si and Ge surfaces but also the effect of protective hard and transparent coatings on the structured surfaces.Specifically, they investigated the use of alumina (Al 2 O 3 ) and diamond thin films as protective coatings on the structured Ge surface.
For the structured Ge surface, a 100-nm-thin film of Al 2 O 3 was deposited using the ALD technique.The coated surface was then subjected to a load-controlled nano-compression test at the same load (6.5 mN) as the uncoated Ge surface.The results showed a significant difference between the coated and uncoated structures.With the Al 2 O 3 coating, the maximum penetration depth of the indenter into the surface was reduced to less than 100 nm, whereas the uncoated Ge surface exhibited a maximum penetration depth of 2500 nm under the same load.The difference in the maximum penetration depth between the coated and uncoated surfaces can be attributed to the compressive stress applied to the structures.The truncated cone-like geometrical structure of the patterned surface allows for an average surface to be estimated at the top of the cone.Due to the conformal coating, the average surface area at the top of the cone increases, resulting in a lower compressive stress on the coated pattern compared to the uncoated Ge surface for the same applied load.The coating with Al 2 O 3 provided a compressive stress of 0.49 GPa on the coated Ge surface, showing minimal damage.
The use of a thin film of a hard material like Al 2 O 3 significantly enhanced the mechanical resistance of the structured, high-transmitting surface.However, the study did not provide information about the effect of this coating on the optical transmission and the wetting behavior of the surface.Further studies would be required to fully understand how the coating affects these properties and to optimize the coating thickness and other parameters for specific applications.
In this study, the Si-structured surface with 3.5 μm high cones and a period of 1μm was coated with a hard layer of polycrystalline diamond, ≈80 nm thick, using the microwave plasmaenhanced chemical vapor deposition (MPECVD) method.Both the patterned (uncoated) and the coated-patterned surfaces were subjected to a load-controlled nano-compression test with a maximum load of 2 mN.The load-displacement curves obtained from the test revealed multiple displacement jumps on the uncoated Si surface, indicating cone fractures.This was further supported by the SEM images, which showed many broken cones.In contrast, the diamond-coated cones exhibited only a few displacement jumps, and the maximum displacement was less than 250 nm.This suggests that the indenter successfully penetrated the diamond layer.To further investigate the mechanical properties, the applied load on the coated-patterned surface was increased to 12 mN.As a result, multiple displacement jumps were observed, and many broken cones were visible in the SEM images.However, the maximum penetration depth remained significantly lower compared to the uncoated surface.
Our hypothesis suggests that the diamond coating fills in the gaps between the cones, effectively reducing the distance between them and increasing the diameter of the cones at the base.This could lead to enhanced mechanical resistance of the surface after coating, resulting in a relatively low penetration depth of the indenter during the nano-compression test.To validate this hypothesis, further experiments can be conducted with different thicknesses of the diamond coating.By varying the thickness of the coating, researchers can determine the optimal thickness that provides the best mechanical stability without compromising the optical performance of the surface.
However, there is still a crucial aspect missing in the study: the optical characterization and wetting properties of the coatedpatterned surface.Without this information, the overall multifunctional performance of the surface remains uncertain.It is essential to investigate how the diamond coating affects the optical transmission and wetting behavior of the structured surface to fully understand the impact of the coating on its multifunctional properties.
The mechanical behavior of a single structured Si cone before and after coating was studied by Fauvel et al. [73] Their research showed that the mechanical response of an individual coated structure is consistent and reproducible, despite the presence of some visual differences in the height and truncation of the cone, as seen in the SEM image in Figure 11.
The coating process involved depositing a thin layer of Al 2 O 3 , ≈200 nm in thickness, using the ALD technique.The mechanical properties of the thin film on a flat Si surface were tested using nanoindentation with a Berkovich indenter.The results showed that heat treatment significantly improved the elastic modulus of the Al 2 O 3 film, increasing it from 146 to 240 GPa after annealing, as observed by XRD due to the crystallization of the coating.
On the structured nano-cones, the ALD deposition provided a uniform conformal coating that perfectly followed the shape of the patterns.To test the mechanical response of a single-coated cone, a micro-compression test was conducted with load control up to 2 mN, as discussed earlier.From the load-displacement curves, the effective protection provided by the thin film on the nanostructured cone was evident.The maximum penetration depth of the indenter was less than 300 nm at a maximum load  [73] Copyright 2023, Elsevier. of 2 mN, while the uncoated cone showed a maximum penetration depth of up to 500 nm at a maximum applied load of 0.1 mN.The absence of fractured and broken cones demonstrated the efficiency of the heated Al 2 O 3 film as a hard and protective layer.However, some fractures at the top of the coated cone were observed in the SEM images after applying the 2 mN load.
It is important to note that this study focused solely on the mechanical behavior of the surfaces after coating and did not report details about the optical and wetting properties.The optical transmission values before and after coating were not mentioned.
In summary, the deposition of hard thin films like Al 2 O 3 , Y 2 O 3 , and polycrystalline diamond has demonstrated promising results in improving the mechanical resistance of structured silicon surfaces.Further research is needed to determine the most suitable deposition technique (ALD or sputtering) that can preserve the required optical and mechanical performance.Additionally, other hard materials listed in Table 2 can also be explored to test their effectiveness as protective coatings.Enhancing the mechanical resistance of optical multifunctional surfaces provides durable surfaces and unlocks the full potential of these outstanding properties for real-life applications.However, to fully evaluate the multifunctional performance of such coatings, studies must also consider optical and wetting properties alongside mechanical behavior.

Roadmap, Challenges, and Potential Solutions
As already mentioned, the creation of optical multifunctional surfaces has gradually attracted the attention of a diverse array of applications, ranging from improving infrared imaging systems to boosting the performance of solar cells and military equipment.However, the operation of these surfaces in real-life applications relies on their ability to withstand abrasive and harsh environments.
The ultimate goal is to find the right balance between mechanical resistance and optical performance in order to create multifunctional surfaces with high transmission that are capable of operating in harsh environments for industrial purposes.
Based on the findings and the results discussed in this review paper, we build a detailed roadmap on the development of optical multifunctional surfaces with enhanced mechanical resistance, as illustrated in Figure 12.One should start by determining the desired spectral range to select the optical bulk material that one needs to improve its optical transmission.High transmission with an anti-reflective effect can be achieved through patterning at sub-wavelength scale.The impact of patterning on the optical and mechanical properties of the surface is indicated by the arrows on the left schematic view of the different structure geometry.From an optical point of view, the ideal structure for achieving a perfect gradient index with high transmission involves a subwavelength pattern configured in a hexagonal lattice with sharp tip conical shapes of significant height.However, these surfaces exhibit a highly brittle mechanical behavior, which hinders their applicability for industrial purposes.Based on the findings of the mechanical tests conducted on structured surfaces, it becomes evident that surfaces patterned in a hexagonal lattice configuration exhibit higher resistance to compressive stress.However, it is important to note that the sharp tip conical geometry of the pattern, while enhancing the mechanical resistance, is not the most robust shape.In contrast, pyramid-shaped or dome-patterned surfaces display superior mechanical durability, but they are associated with reduced optical transmission.In addition to the geometrical shape and size of the patterned surface, the initial hardness of the structured material plays a crucial role in the resistance of the surface after patterning.Diamond offers exceptional hardness, but the process of fabricating nanostructured diamond patterns remains a challenging approach.In contrast, silicon with lower hardness proves to be more adaptable to currently available etching techniques, making it a practical choice, particularly for achieving high transmission in the mid-infrared range through nanostructuring.
Hard coatings have been widely used as a powerful solution to enhance the mechanical resistance of flat surfaces.However, their influence on protecting structured surfaces requires further examination.Hard coatings like Al 2 O 3 or polycrystalline diamond have shown promise to enhance the mechanical resistance of such surfaces.But the effect of these coatings on the optical and wetting performance remains to be fully explored.The selection of an optimal material for the protection of structured surfaces hinges upon several critical parameters, including the refractive index, the transmission range, and the hardness of the material.When determining the suitability of a material, it is essential to consider the spectral range of interest and whether it falls within the visible, near, mid, or far-infrared region.All this parameters were considered in the building of Table 2 for many potential materials.The transmission range of the chosen material should correlate with the transmission range of the structured substrate.In terms of refractive index, a lower value is typically preferred.The patterning process creates a gradient refractive index, making it crucial to select a material with a refractive index as close to 1 as possible.Materials like DLC, diamond-like nanocomposites, Al 2 O 3 , AlON, and B 4 C exhibit appealing characteristics, such as high hardness, a low refractive index, and a broad transmission range.These attributes position them as the best candidates, particularly when targeting superior transmission from the visible to the mid-infrared spectrum.
The deposition technique of each material also plays a crucial role in the morphology and performance of the coating.The choice between conformal and non-conformal coatings can di-rectly impact the reflectivity of the surface.Not all coating technologies can effectively cover nanostructured surfaces.While sputter deposition is widely used for creating thin films, it can face challenges when dealing with non-flat nanostructured surfaces.The uniformity and conformity of sputter-deposited films may be compromised on structured surfaces with high roughness.The worst-case scenario arises when an air gap exists between the structured substrate and the coating, especially in the case of PVD deposition.This scenario can significantly affect the anti-reflective effect as well as the wetting properties.In contrast, techniques like ALD and plasma enhanced ALD (PEALD) are better suited for conformal coatings on nanostructured surfaces, as they offer precise and controlled layer-by-layer deposition, making it possible to cover complex geometries effectively.Conformal coating with hard material enhances the mechanical properties of the dome and conical patterns, with a little influence on the optical properties if the refractive index of the chosen material is close to one.However, for the sharp tip conical structure, conformal coating may not be the ideal solution for highly enhanced mechanical properties.
To fully unlock the potential of optical multifunctional surfaces for industrial applications, it is essential to highlight the critical areas that require further research and development.Different approaches can be tested to optimize the trade-off between the optical and mechanical properties of subwavelength optical surfaces.Covering the patterns with a conformal hard coating is a very promising method and has shown promising results with Al 2 O 3 and polycrystalline diamond, but other materials should also be tested, and the thickness of the layer has to be optimized.Another approach consists of creating buried structures with a hard coating and a very low refractive index.In this way, the sharp tip conical patterns will be fully covered by the material, which will better enhance the mechanical resistance.This approach needs further investigation in order to study the influence of the buried structure on the optical and wetting properties.
Exploring innovative approaches, such as using soft and elastic materials instead of hard coatings like PMMA or PDMS, is a promising area for future research.By enhancing the elasticity of the surface.These methods introduce new variables that need in-depth investigation.

Conclusion
In conclusion, the pursuit of developing mechanically robust optical multifunctional surfaces is a multifaceted and intricate endeavor.The creation of sub-wavelength patterns with high aspect ratios and superior light transmission poses a considerable challenge, and concurrently, ensuring the durability of such structures in harsh environments adds another layer of complexity.These surfaces have the potential to greatly enhance a variety of applications, as they hold promise in domains such as infrared imaging, solar cells, and military aircraft.This review has highlighted the importance of various mechanical tests in understanding the mechanical properties of multifunctional surfaces.A recommended approach involves starting with nanoindentation, coupled with nanoscratch testing, to explore the mechanical resistance of the patterns at the nanoscale and define the critical load at which the patterns are damaged.This progression allows for an in-depth investigation of their mechanical characteristics and facilitates the analysis of their elastic-plastic deformation.Furthermore, following these nanoscale assessments, the investigation extends to macro-scale mechanical testing, incorporating abrasion and sand rain erosion tests under standardized conditions.It was found that the structured surfaces with a conical shape reflected a very brittle mechanical behavior during the nanoindentation test.To address this issue, hard coatings like Al 2 O 3 or polycrystalline diamond have been used as protective layers to improve the mechanical resistance of these surfaces.This approach shows promising results for enhancing the mechanical performance of these surfaces.However, their impact on the desired optical transmission and wetting properties needs future investigation and characterization.It is essential to execute the mechanical tests both prior to and subsequent to the deposition of a hard coating to validate the effectiveness of the chosen hard material in protecting the surface.A wide range of materials with different mechanical, optical, and wetting properties have been reviewed in this article.A detailed roadmap has also been provided that serves as a design guideline for selecting a suitable material and a combination of process parameters to produce an optical multifunctional surface with the desired optical performance and mechanical resistance.Hard materials such as DLC, diamond-like nanocomposites, Al 2 O 3 , AlON, and B 4 C are the ideal choices to protect the structured patterns.It is worth noting that the deposition technique has a significant effect on the properties of the deposited material.Finding the optimal deposition parameters with the ideal thickness that can improve the mechanical resistance without altering the optical performance is a key challenge in order to achieve the desired multifunctional, durable surface.One of the key characteristics of the coating lies in its crystallinity; even hard materials like Al 2 O 3 may not exhibit the best mechanical behavior in an amorphous phase.
For industrial applications in harsh environments, the structure must have high mechanical resistance combined with high optical transmission.We believe that this review paves the way for further research and investigation to enhance the mechanical resistance of optical multifunctional surfaces.Several approaches can be explored, including the deposition of conformal coating, the creation of buried structures with a hard coating and low refractive index, or the use of soft and elastic material to enhance the elasticity of the surface.Another major future challenge is large-scale nanofabrication, which needs to be addressed to fully unlock the potential of optical multifunctional surfaces for industrial applications.By addressing these challenges, optical multifunctional surfaces can become invaluable assets in various technological fields.

Figure 2 .
Figure 2. Representation of the optical path of light in interaction with: a) a single thin film on n substrate and b) multi-layered structure.

Figure 3 .
Figure 3. a) Representation of the critical parameters of a structured surface.Representation of the optical path of light in interaction with: b) geometrical optics compatible structures (macro-structure) where geometrical optics applies; c) micro-structures where the transmission is induced by the scattering of light; d) subwavelength nano-structures considered as a homogeneous medium with a refractive index gradient profile in the structure symmetry axis direction that is, the optical path direction.

Figure 4 .
Figure 4.A drop of water on a flat surface.

Figure 5 .
Figure 5. a) SEM images of straight sidewall nanopillars.b)The contact angle of a drop of water on the superhydrophobic and superhydrophilic states of the same sidewall nanopillars structured surface.Reproduced with permission.[60]Copyright 2023, The Royal Society of Chemistry.

Figure 6 .
Figure 6.Transmission measurements of structured ZnS surface a) after exposition of sand erosion condition A and b) after exposition of sand erosion condition B.c,d) SEM images of the damaged area of the ZnS structured surfaces after exposure to sand conditions A and B, respectively.Adapted with permission.[74]

Figure 8 .
Figure 8.The optical and the mechanical performance of the motheye PMMA structure at different nanoimpriting temperature: a) nanoindentation loading-unloading curves; b) scratch test at 100 μN applied load; c) transmission spectra; and d) total reflactance spectra.Reproduced under the terms of the CC BY 4.0 license.[92]Copyright 2023, Authors, Published by PubMed Central (PMC).

Figure 9 .
Figure 9. a)The change of the water and ethylene glycol (oil) contact angles during the abrasion test cycles on the nanostructured glass (i) and the effect of the heat treatment on the water and oil contact angles of the abraded sample (ii).b) SEM images of the surface before and after 500 abrasion cycles.Reproduced with permission.[95]Copyright 2023, The Royal Society of Chemistry.

Figure 11 .
Figure 11.Scanning electron microscope (SEM) image of the fabricated nano-cones.No scale is provided for confidentiality reasons, as mentioned by Fauvel et al.Reproduced with permission.[73]Copyright 2023, Elsevier.

Figure 12 .
Figure 12.Roadmap for selecting the ideal materials with the perfect geometry to enhance the mechanical resistance of optical high transmitted multifunctional surfaces.

Table 1 .
[89]dimensions of the structures submitted to the nanocompression test investigated by Doquet et al.[89]Structures submitted to nano-compression test

Table 2 .
Optical and mechanical properties of selected hard and transparent materials listed in alphabetical order.