Recent Progress on Plasmonic and Dielectric Chiral Metasurfaces: Fundamentals, Design Strategies, and Implementation

Over the years, researchers have been exploring ways to artificially design chiral structures and materials, namely metamaterials and metasurfaces. They exhibit unique optical properties that can be used for various applications. However, metasurfaces comprise symmetry‐breaking structures that provide a more convenient solution for planar chiral optics regardless of whether they are plasmonic or dielectric. In general, plasmonic chiral metasurfaces are more suitable for applications requiring a high confinement level and substantial optical near‐field enhancement. In contrast, dielectric chiral metasurfaces are ideal for wide operating wavelength ranges and low losses. This review summarizes the recent progress on plasmonic and dielectric chiral metasurfaces. It includes the fundamental concepts, design strategies, and their implementation for applications in holographic displays, imaging and sensing, and detection. Moreover, an overview of chiral metasurfaces to generate the nonlinear effects, hosting bound states in the continuum, and the significant role of machine‐learning‐based design approaches are also discussed. Finally, some future developments are highlighted where chiral metasurfaces are expected to play a vital role.


Introduction
The chirality concept refers to the non-superimposable property of a molecule, ion, or crystal on its mirror image. Since the 1840s, when French chemist Louis Pasteur first identified chiral molecules, the concept of chirality has expanded to other Figure 1. Summary of the review paper. The artistic representation of the topics covered in the review paper includes plasmonics and dielectric-based chiral metasurfaces with fundamentals, design strategies, and their implementation for metaoptics. various applications. Section 2 includes the fundamentals to understand the chirality concept. Moreover, it explains the necessary conditions to incorporate chirality in any object. Next, we have categorized the chiral metasurfaces into plasmonic and dielectric chiral metasurfaces and discussed them based on their design strategies (intrinsic or extrinsic/achiral) in Sections 3 and 4, respectively. Moreover, the chiral metasurfaces assisted by highquality resonances originating from bound states in the continuum (BICs) for highly efficient manipulation and control on the polarization state of light are discussed in Section 5. In Section 6, we discuss the second harmonic generation (SHG)/third harmonic generation (THG) in chiral metasurfaces, as many studies have been conducted on nonlinear optics to improve their functionality. Furthermore, machine-learning and deep-learningbased chiral metasurfaces have been presented recently for efficient optimization of chiral metasurfaces. A brief review of the role of machine learning and deep learning-based chiral meta-surfaces is discussed in Section 7. After that, Section 8 demonstrates the application of chiral metasurfaces in different areas, such as chiral sensing, chiral imaging, and holographic displays. Finally, we conclude with some future perspectives in Section 9. The artistic summary of the review paper is depicted in Figure 1.

Fundamentals
The chiro-optical effects, such as optical activity (OA)/optical rotation (OR) and circular dichroism (CD)/circular conversion dichroism (CCD), are generally associated with chiral structures. In optics, a pair of geometric figure enclosing chirality can differentiate between two opposite spins of photons and produce chirooptical effects. OA/OR and CD/CCD are defined as the rotation of the plane of polarization for linearly polarized (LP) light and the difference between the transmittance/reflectance of LHCP and RHCP light, respectively. The demonstration of chiro-optical A) The chiro-optical effect is described in terms of optical activity (OA)/optical rotation (OR) and circular dichroism (CD)/circular conversion dichroism (CCD). OA/OR is the rotation of the plane of polarization of incident linearly polarized (LP) light. In contrast, the CD/CCD is the difference between the transmittance/reflectance of LHCP and RHCP light. B) Different chiral structures include intrinsic 3D chiral, intrinsic 2D chiral (isotropic and anisotropic), extrinsic 2D chiral, and cascaded structures to incorporate chirality. effects and different chiral geometries is depicted in Figure 2. Different structural geometries are used to achieve chiro-optical effects in metamaterials and metasurfaces, such as intrinsic 3D chiral, intrinsic 2D chiral, extrinsic 2D chiral, and cascaded chiral structures. Irrespective of the numerous applications of chiral metamaterials and metasurfaces, the practical realization of such systems faces quite a lot of challenges. These challenges include fabrication difficulties, materials compatibility, integration with existing devices, etc. [66,67] The fabrication of metamaterials is categorized into two types of approaches termed as top-down and bottom-up methods. The top-down techniques comprise electron-beam lithography (EBL), focused ion beam (FIB) milling, photolithography, reactive ion etching (RIE), etc. The top-down approach starts from a bulk structure, creating the desired designs specifically at the nanoscale using lithography, etching, and deposition. In contrast, the bottom-up techniques involve creating the desired geometry by starting from the individual meta-atom or molecule. These fabrication methods include chemical synthesis, atomic layer deposition (ALD), self-assembly, etc. These methods are well suited for the fabrication of metamaterials with complex geometries at the nanoscale. However, top-down techniques are more established and commonly adopted to achieve more precise control of the final geometry of metamaterials. Regardless of the approach adopted, the fabrication of metamaterials poses substantial challenges at the nanoscale, like the conventional lithography techniques cannot be used for sub-100 nm features. Therefore, specialized methods such as e-beam lithography or focused-ion beam milling are required, which are expensive and timeconsuming.
Moreover, the metamaterials require materials compatible with the fabrication techniques and contain specific properties such as high refractive index, magnetic susceptibility, high bandgap energy, etc. Meanwhile, the chosen material should withstand the high temperature required during deposition or annealing. As lithography techniques are typically slow, nanoimprinting lithography has been explored for large-scale fabrication of metamaterials. However, these methods cannot provide the required precision, especially for subwavelength-featured metaatom-based metamaterials at the nanoscale. Furthermore, integrating with the already existing devices is a significant challenge for the practical applicability of the fabricated metamaterials. For example, the three-dimensional (3D) chiral structures ( Figure 2B) based on metamaterials cannot be easily fabricated and integrated with the existing technology at the nanoscale. [66] Given the challenges mentioned above, to reduce the complexities in the practical implementation of chiral metamaterials, the concept of planar chiral metamaterials, termed chiral metasurfaces, is introduced. [68,69] The fundamental conditions for chirality in metasurfaces are the presence of geometric asymmetry and/or the use of chiral materials. It is important to note that chirality is a property that is independent of the size of the structure and the wavelength of the incident wave so that the same chiral structure can be used for different frequencies and at different scales. Additionally, the chirality effect can be enhanced by cascading multiple chiral structures. Cascaded chiral geometries use numerous chiral or achiral designs (incorporating chirality) to interact with the incident wave in a specific way. The cascading of these structures allows for greater control over the polarization and phase of the reflected or transmitted waves. To achieve such control, compactness, and fabrication ease, planar cascaded or two-dimensional (2D) chiral geometries are used to design chiral metasurfaces for diverse applications.
The mathematical description of chirality inclusion into the structures based on structural and optical chirality parameters is described here as follows:

Structural Chirality Parameters (SCP)
The electromagnetic (EM) response for linear and isotropic medium in the absence of magnetoelectric coupling can be stated in Equation (1) as whereẼ,D,B, andH termed as the electric field, electric field density, magnetic field, and magnetic-field intensity, whereas the permittivity and permeability of the material are represented by and μ, respectively. However, it is not wise to assume an isotropic medium due to the same properties in all directions. This means the medium has no inherent chiral properties and cannot create www.advancedsciencenews.com www.advopticalmat.de chiral effects. Therefore, Equation (1) for the general case of an anisotropic medium can be written as in Equation (2) [64,65] [ wherẽand̃are the tensors used to define magnetoelectric coupling. Moreover, the following additional constraints should be considered for inversion symmetry materials, known as nongyrotropic materials, as mentioned in Equation (3) [64] [ Meanwhile, contemplating the specific scenario of isotropic chirality medium, the EM field terms in Equation (2) will reduce to complex-scalar terms and can be expressed using Tellegen's relations, [64,66,67] Post's relations, [64] and Drude-Born-Fedorov (DBF) [68] relations as in Equation (4) Tellegen ′ s relations wherẽr,̃r, and̃in Tellegen's relations representing the material's relative permittivity, relative permeability, and chirality parameter, respectively. In Post's relation,̃p,̃p, and̃c termed as the permittivity, permeability, and coupling constant. Similarly, in DBF relations,̃D BF ,̃D BF , and̃represent the permittivity, permeability, and susceptibility in that case. In the particular case of chiral metamaterials, Tellegen's relations are widely used in the analysis because they provide a convenient way to calculate the chiral response of a material based on its electromagnetic properties and can be used to analyze the behavior of chiral metamaterials under different conditions, and can be helpful in the design and optimization of chiral metamaterials. The chirality parameter is the key for negative-refractive-index materials because it determines the handedness of the material and the effects on its refractive index. Finally, the solution of Maxwell's equations for chiral medium illuminated with single wavelength plane wave results in two eigen-waves as written in Equation (5) where k o denotes the wavenumber in a vacuum. However, for enantio-specific medium k − and k + represent the wavenumber for LHCP and RHCP light, respectively. Similarly, the refractive index can be expressed as in Equation (6) [69] n ± = √̃r̃r ±̃ (6) Here,̃is a complex term like the refractive-indexñ and the real and complex parts behave as the phase velocity and attenuation contributing toward the refractive index, respectively. In the case of nonzero real and complex parts of̃, lead to simultaneous measurement of OA and CD. The refractive index would be negative for both handedness of circularly polarized (CP) light when |̃| > √̃r̃r in Equation (6).

Optical Chirality Parameters (OCP)
Chiral materials have a nonsymmetrical arrangement of atoms, resulting in a difference in the way that light of different polarizations is transmitted or absorbed. This difference in light transmission and absorption leads to a dissymmetrical interaction between light and matter, known as the chiral light-matter interaction. Meanwhile, a setup was demonstrated in 2010 to measure the dissymmetry factor, which exceeds its value compared to perfectly impinged circularly polarized light 76, and the "opticalchirality" C can be described as in Equation (7 whereẼ andB are the complex amplitudes for electric and magnetic field components, respectively. It is worth mentioning that the enhancement of parameter C is essential for enantio-selective applications. Generally, the optical response of a molecule encapsulated chirality and impinged by monochromatic light can be expressed in the form of electric-dipolep and magnetic-dipole-momentm as in Equation (8) p =̃e eẼ − ĩe mB m = ĩe mẼ +̃m mB (8) Here,̃e e ,̃m m , and̃e m represent the electric polarizability, magnetic susceptibility, and electromagnetic polarizability, respectively. The real part in Equation (8) is associated with the physical quantities in the structure. Meanwhile, the excitation rate (A ± ) for both handedness of CP light is given by [70,71] and using Equation (8) in Equation (9) will lead to a general relation of optical-chirality C. The modified relation can be written as in Equation (10) and Equation (11) www.advancedsciencenews.com

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After introducing the time-average electric and magneticenergy density here as ⟨U E ⟩ t = 4 |Ẽ| 2 and ⟨U B ⟩ t = 1 4 |B| 2 and substituting in Equation (11) becomes where is the weighting factor between the magnetic-field energy density and electric-field energy density. Hence, the dissymmetry factor g of chiral light-matter interaction can be defined here as in Equation (13 If the magnetic energy density is negligible, [70] the dissymmetry factor can be written as in Equation (14 However, including the magnetic energy density, the full expression is written as in Equation (15) which describes that the dissymmetry factor is inversely proportional to the total EM energy density and directly proportional to the structuralchirality-parameter and the optical-chirality parameter C of the electromagnetic-field

Plasmonic Chiral Metasurfaces
Several natural systems have been adopted to detect chirality since the initial experiments in 1848. However, weak chiral response generation in such systems limits their potential applicability. After advancement in fabrication at the nanoscale, the research trend shifted toward artificially engineered chiral systems based on plasmonic nanostructures. Such nanostructures can generate much stronger chiro-optical responses compared to many natural systems. [22,72,73] Meanwhile, they can withstand superchiral fields to manipulate spin-dependent light-matter interactions. The progress in chiral plasmonics paves the way for several real-life applications such as imaging, displays, sensing, and detection. Many plasmonic-based chiral metasurfaces have been reported previously in the literature based on chiroptical effects introduced intrinsically and extrinsically into the structure. In replacement of 3D complex chiral geometries, Hentschel et al. demonstrated chiral oligomers based on the 3D arrangement of plasmonic "meta-atoms" for intrinsically introduced strong chiroptical effects ( Figure 3A). [74] The proposed design strategy using a cluster of individual plasmonics meta-atoms made it possible to spectrally shift the optical response to a wide range of wavelengths in the visible and infrared regime. A twolayer quadrumeric structure is designed, and configurational or constitutional chirality is created based on their arrangement. To avoid the multilayer fabrication complexities, Ogier et al. reported a monolayer chiral plasmonic oligomer, either comprising closely spaced same material or different material silver and gold nanodisks with different heights. [75] Due to the gap between structures introduces substantial field enhancement and optical chirality at the visible and infrared regimes.
Ye et al. presented a planar plasmonic chiral metasurface to attain strong chiroptical effects in CD ( Figure 3B). [76] The Lshaped gold nanoantennas are used as the building block of the metasurface with a square lattice working at the near-infrared regime. Furthermore, a near-infrared regime plasmonic chiral metasurface-based absorber is presented. [84] A double rectangular gold-based resonator was used to achieve the maximum reported value of 0.87 and 0.70 for chiral absorption and CD, respectively. Wang et al. also reported a plasmonic chiral metasurface containing a gold nanoslit array for chiroptical resonances at visible to near IR wavelengths ( Figure 3C). [77] They have highlighted controllable propagating surface plasmon resonances for tailored chiroptical responses by changing the lattice periodicity and the length of slits.
In 2020, Li et al. reported a plasmonic chiral metasurface integrated with graphene-silicon to provide full stokes polarimetry at near IR wavelengths ( Figure 3D). [78] The proposed on-chip polarimeter incorporates four metasurfaces to obtain the intensity, orientation, and ellipticity of the incident arbitrarily polarized light. Graphene provides good physical contact for a rugged metasurface and suppresses the dark noise introduced into the system. Meanwhile, Jiang et al. reported an ultrathin plasmonic metasurface integrated with MoSe 2 (photoactive material) based circular polarimeter ( Figure 3E). [79] The plasmonic metasurface comprises asymmetric n-shaped gold nanoantennas to detect and distinguish circularly polarized light with fast photoresponsive speed. So far, intrinsic chirality-based plasmonic metasurfaces are discussed, which can overshadow the chirality in applications such as chiral sensing and detection. Therefore, the symmetric metasurfaces based on extrinsic chirality introduced using external sources are also reported in the literature.
In 2015, Leon et al. reported that a diffractive metasurface comprises gold-based split ring resonators on a glass substrate cladded by the material with a refractive index matched to the glass ( Figure 3F). [80] The enhancement of extrinsic chiral response is demonstrated by near-field diffraction optics. The proposed phenomenon provides CD enhancement based on an oblique incident angle close to normal incidence. Moreover, the proposed extrinsic chiral metasurface behaves as an ultrathin CP spectral filter at near-infrared wavelengths with a tuning range of 200 nm. In 2016, Cao et al. demonstrated a symmetric metasurface to induce extrinsic chirality for an oblique light incident in the terahertz regime ( Figure 3G). [81] The metasurface designed by gold-based circular holes introduces surface plasmon polariton (SPP) modes at an off-normal incident wave, leading to 2D extrinsic chirality.
Similarly, Mao et al. reported a symmetric achiral metasurface comprised of gold nanorods provides extrinsic chirality ( Figure 3H). [82] The proposed metasurface behaves as a metamirror with polarization-preserving property and spin-selective absorption for oblique CP incident light working in nearinfrared. In 2020, Lai et al. reported a bilayer plasmonic Figure 3. Plasmonic chiral metasurfaces. Intrinsic chirality: A) plasmonics chiral oligomers demonstrated based on the multilayer arrangement of metaatoms for intrinsically introduced chiroptical effects. A two-layer quadrumeric structure creates configurational or constitutional chirality based on their arrangement in the visible and infrared regimes. Reproduced with permission. [74] Copyright 2012, American Chemical Society. B) A planar plasmonic chiral metasurface was presented to achieve strong chiroptical effects using L-shaped gold nanoantennas as the building blocks with a square lattice working at the near-infrared regime. Reproduced with permission. [76] Copyright 2017, American Physical Society. C) A plasmonic chiral metasurface was reported with a gold nanoslit array to generate chiroptical resonances at Vis-NIR wavelengths. Reproduced with permission. [77] Copyright 2016, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. D) A plasmonic chiral metasurface integrated with graphene-silicon demonstrated full stokes polarimetry at near IR wavelengths. The proposed on-chip polarimeter incorporates four metasurfaces to obtain the intensity, orientation, and ellipticity of the incident arbitrarily polarized light. Reproduced with permission. [78] Copyright 2020, American Chemical Society. E) An ultrathin plasmonic metasurface integrated with MoSe2 (photoactive material) and comprises asymmetric n-shaped gold nanoantennas demonstrated to detect and distinguish circularly polarized light with fast photoresponsive speed. Reproduced with permission. [79] Copyright 2020, The Royal Society of Chemistry. Extrinsic chirality: F) a diffractive metasurface comprises gold-based split ring resonators on a glass substrate cladded by the material with a refractive index matched to the glass reported to enhance extrinsic chiral response by near-field diffraction optics. The proposed phenomenon provides CD enhancement based on an oblique incident angle close to normal incidence. Reproduced with permission. [80] Copyright 2015, Springer Nature. G) Asymmetric metasurface induced extrinsic chirality for oblique incidences in the terahertz regime. The metasurface comprises gold-based circular holes that introduce surface plasmon polariton (SPP) modes at an off-normal incident wave, leading to 2D extrinsic chirality. Reproduced with permission. [81] Copyright 2016, Optical www.advancedsciencenews.com www.advopticalmat.de metasurface for enhanced CD ( Figure 3I). [83] The proposed copper-based split ring apertures, asymmetrically arranged in bilayer geometry and rotated by 90°, exhibit intrinsic chirality. Still, oblique angle illumination enhanced the CD response at the designed frequencies in the microwave regime.

Dielectric Chiral Metasurfaces
Meanwhile, another area of research interest has emerged, named dielectric-based chiral metasurfaces. Even though plasmonic metasurfaces showed significant contributions toward the generation of chiro-optical responses, yet their potential applicability in real-life applications is limited due to the ohmic losses and the possibility to excite only electric dipolar resonances. Owing to such limitations, there has been a shift in metaoptics toward all-dielectric implementation. With low losses, the high refractive index-based all-dielectric structures can simultaneously excite electric and magnetic dipolar resonances, leading to enhanced light-matter interactions. Many all-dielectric chiral metamaterials or metasurfaces are designed and demonstrated in the literature. [35,51,85,86] Such metasurfaces include chiral dielectric structures or chirality-induced achiral geometries-the combination of achiral geometries adopted to improve the detection and discrimination of chiral molecules in biosensing applications.
Tanaka et al. demonstrated a bilayer all-dielectric chiral metasurface for 3D intrinsic chirality based on the excitation of electric and magnetic dipolar resonances ( Figure 4A). [87] A siliconbased fourfold symmetric structure arranged in a bilayer geometry provides a high CD and optical activity compared to previously discussed plasmonic chiral metasurfaces. Although multilayer structures provide better chirality to achieve fabrication ease, researchers presented monolayer all-dielectric chiral metasurfaces. Zhu et al. reported a planar gammadion dielectric nanostructure-based chiral metasurface for giant intrinsic chirality ( Figure 4B). [88] The designed TiO 2 -based metasurfaces provide ≈80% of CD and circular birefringence exceeding 1000 000°m m −1 in visible regime at the wavelength of ≈540 nm in transmission mode. Similarly, Hu et al. reported a planar silicon-based z-shaped structure to design an intrinsic chiral metasurface near the IR regime ( Figure 4C). [85] The reported value of CP dichroism efficiency and the extinction ratio at the wavelength of 1500 nm reaches 97% and 345:1, respectively. The proposed geometry provides easy fabrication, high dichroism, and compatibility with semiconductor devices. Likewise, another work reported by Ma et al. uses the same structural geometry, whereas germanium is the design material. [89] In 2020, Gómez presented high refractive index-based slotted dielectric nanodisks for enhanced chiroptical responses ( Figure 4D). [90] The used amorphous silicon-based nanodisks with a slanted aperture in the center optimized in the visible regime exhibits enhanced electric and magnetic fields in the gap region to eventually provide a significant value of CD. In 2021, an-other work on intrinsic dielectric chiral metasurfaces was demonstrated by Li et al. working in the terahertz regime ( Figure 4E). [91] The building block contains two specially placed L-shaped pillars to satisfy the chirality conditions. The theoretical and experimental reported CD values reached 69.4% and 43%, respectively.
Like plasmonic extrinsic chiral metasurfaces, researchers have reported dielectric-based extrinsic chiral metasurfaces and a couple of recently published works are discussed here. In 2022, Peng et al. reported an all-dielectric metasurface exhibiting extrinsic chirality for sensing applications at the terahertz regime ( Figure 4F). [92] The proposed metasurface has dual operating modes of normal and oblique incidences, whereas the designed metasurface acts as a chiral and an achiral metadevice, respectively. At an oblique incident angle, CD's reported efficiency exceeds 70%. Meanwhile, another work based on dielectric metasurface for extrinsic chirality has recently been presented by Liu et al. (Figure 4G). [93] The metasurface comprises quadrumeric silicon-based crossed slots nanodisks to enhance the chiroptical responses due to the bound states in the continuum.

Chirality Empowered by Bound States in the Continuum (BICs)
The highly efficient manipulation and control of the polarization state of the emitted light is one of the main targets of modern optics. So far, chiral optics based on chiral materials and optical cavities have been explored to show control of circularly polarized light. In the literature, it has been demonstrated that chirality can be included in the metasurfaces without breaking the timereversal symmetry into the subwavelength structures. However, all the conventional approaches used to engineer CP light suffer from a limited degree of polarization, large radiation angles, and incoherent broadband emission limits the practical implementation.
To circumvent these challenges, researchers used the concept of bound states in the continuum (BICs). [100] BICs are the localized states of light created due to destructive interference and confine the light in the continuum for an infinitely long time.
In reality, BICs appear as the quasi-BICs, which can contain light for a finite time but still provide a high quality factor (Q-factor). Such unique characteristics lead to simultaneous confinement and radiation of light to significantly enhance the performance of nanophotonics devices in terms of ultrafast control with unidirectional emission and record high harmonic generation. Recently, numerous chiral metasurfaces assisted by BICs lead to quasi-BICs have been demonstrated in the literature. [94][95][96][97][98][99][100][101][102][103][104] Wu et al. demonstrated a honeycomb-shaped gold hole structure to design a plasmonic metasurface to generate strong circular dichroism triggered by near-field perturbation without breaking the rotational and mirror symmetry ( Figure 5A). [94] Meanwhile, a rectangular hole structure is also demonstrated to prove the generation of strong chirality without breaking Society of America. H) Asymmetric achiral metasurface comprises gold nanorods demonstrated to provide extrinsic chirality for oblique incidences. Meanwhile, it behaves as a chiral metamirror with polarization-preserving property and spin-selective absorption working in near-infrared. Reproduced with permission. [82] Copyright 2019, American Chemical Society. I) A bilayer plasmonic metasurface reported for angle-enhanced CD based on copper split ring apertures asymmetrically arranged in bilayer geometry working in the microwave regime. Reproduced with permission. [83] Copyright 2020, Optical Society of America. intrinsic chirality in terms of high CD and optical activity based on the excitation of electric and magnetic dipolar resonances in a silicon-based fourfold symmetric structure. Reproduced with permission. [87] Copyright 2020, American Chemical Society. B) A planar gammadion dielectric nanostructures based chiral metasurface reported for giant intrinsic chirality providing CD value of ≈80% and circular birefringence exceeding 1000 000°mm −1 in visible regime at the wavelength of ≈540 nm. Reproduced with permission. [88] Copyright 2017, Springer Nature. C) A planar silicon-based z-shaped structure was reported to design an intrinsic chiral metasurface in a near IR regime at the wavelength of 1500 nm. The proposed geometry provides easy fabrication, high dichroism, and compatibility with semiconductor devices. Reproduced with permission. [85] Copyright 2017, Springer Nature. D) A high refractive index slotted dielectric nanodisks-based metasurface was reported for enhanced chiroptical responses. The amorphous silicon-based metasurface optimized in the visible regime exhibits enhanced electric and magnetic fields in the gap region to eventually provide a significant value of CD. Reproduced with permission. [90] Copyright 2020, American Physical Society. E) An intrinsic dielectric chiral metasurface based on a building block comprising two specially placed L-shaped pillars works in the terahertz regime. The theoretical and experimental reported value of CD for the designed dielectric chiral metasurface reaches 69.4% and 43%, respectively. Reproduced with permission. [91] Copyright 2021, Optical Society of America. geometric symmetry. Most recently, Tang et al. numerically demonstrated a 3D plasmonic metasurface to overcome the fundamental trade-off between the CD and the Q-factor in previously presented BICs-based chiral metasurfaces ( Figure 5B). [95] The proposed metasurface used an integrated unit comprising a vertically twisted split ring resonator and a wall to introduce a new degree of freedom that can decouple the CD and Q-factor. Moreover, the independent manipulation of CD and the Q-factor is illustrated by changing the height of the wall and the angle of the vertically twisted split ring resonator.
Apart from BICs in plasmonic chiral metasurfaces, few groups have presented BICs-assisted dielectric chiral metasurfaces. Gorkunov et al. presented a dielectric metasurface with strong circular dichroism and maximum Q-factor assisted by BICs and shaping into quasi-BICs ( Figure 5C). [96] A dimer of dielectric bars is used as the building block to numerically prove the maximum chirality generation in visible regime using quasi-BIC resonance. Another dielectric chiral metasurface was reported by Kim et al. to generate high-Q chiro-optical resonances based on quasi-BICs ( Figure 5D). [97] The proposed metasurface simultaneously breaks in-plane and mirror symmetries providing near-unity CD with a high Q-factor of several orders of magnitude. They showed the tuning of chiro-optical responses based on structural parameters defining inversion and mirror asymmetry. Another dielectric planar chiral metasurface demonstrated by Shi et al. to achieve maximum and tunable chiro-optical responses assisted by BICs ( Figure 5E). [98] It can achieve maximum chirality and ultrahigh Q-factor either by breaking in-plane symmetry or changing the incident angle of the used double-sided scythe-shaped structure.  Figure 5F). [99] They have also shown the simultaneous modification and control of chiral emission and spin-dependent photoluminescence and lasing without injecting any spin. Besides highly efficient manipulation of chiro-optical effects, BICs can be vital in generating nonlinear effects such as SHG and THG. [100]

Chiral Metasurfaces For SHG/THG
In previous decades, many studies have been conducted on nonlinear optics to improve their functionality and expand their information capacity. [60] In bulky nonlinear materials, there is a limitation in matching phase conditions for nonlinear processes. On the contrary, metasurfaces have the leverage to achieve the maximum output for nonlinear signals due to subwavelength features along the propagation direction of light. So far, several nonlinear metasurfaces have been reported to be designed with plasmonic and dielectric materials. Although using nonlinear materials is mandatory for nonlinear responses in metasurfaces, chiral meta-surfaces based on chiral elements can produce nonlinear effects even in linear materials.
Collins et al. demonstrated chiral plasmonic nanostructure for the second harmonic generation concerning optical rotation instead of circular dichroism (Figure 6A). [105] The obtained SHG in the optical rotation is due to the intrinsic structural chirality, which can be introduced by anisotropy in the structure or both of them. The gold nanohelices were used for SHG at 400 nm, whereas the 800 nm pulsed light illuminated the sample. In contrast, a planar g-shaped gold nanostructure was demonstrated by Valev et al. for asymmetric SHG, and the measurements were performed with a femtosecond laser system at the wavelength of 800 nm. [106] Zhang et al. presented a gammadion-shaped multilayer planar chiral metasurface supporting magnetic resonances for THG in circular dichroism ( Figure 6B). [107] The structure is investigated theoretically to generate high-intensity THG signals with opposite handedness in the IR regime. Kim et al. reported a polaritonic reflective chiral metasurface for spin-dependent nonlinear optical responses ( Figure 6C). [108] Trisceli C3-symmetric and gammadion C4-symmetric plasmonic chiral nanoresonators were used to make a hybrid metasurface for SHG and THG, respectively, based on the incident spin of CP light. The optimized hybrid metasurface results in a significantly high nonlinear harmonic signal and giant nonlinear CD for both spins of the input pump. The SHG and THG frequency conversion efficiency achieved through this approach exceeds 10 −4 % with near unity SHG-CD and THG-CD in mid-IR region.
Meanwhile, a few groups have worked on dielectric-based chiral metasurfaces to significantly increase harmonic generation's efficiency. Kim et al. reported a dielectric chiral metasurface based on Z-shaped lithium niobate nanoantennas supported by the gold substrate for giant CD and highly efficient SHG at the shorter wavelengths in the UV regime, which is crucial to obtain (Figure 7A). [109] For a peak pump intensity of 5 GW cm −2 , the achieved efficiency for SHG in the blue UV region is 10 −3 %, and SHG-CD reaches 1.8. Moreover, Vabishchevich et al. demonstrated a fano-resonance-based metasurface using broken symmetry semiconductor material for enhanced SHG ( Figure 7B). [110] A GaAs-based metasurface is designed with high Q-factor resonances which enormously enhance the local fields inside the L-shaped nanoresonators resulting in multifold enhancement of nonlinear optical responses in terms of SHG.

Design Strategies of Chiral Metasurfaces Using Machine Learning
Machine learning and deep learning can play a vital role in efficiently optimizing chiral structures. The primary motivation behind all the chiral metasurfaces presented above is to achieve enhanced chiroptical responses, irrespective of the complexity of the chosen chiral system. Hence, achieving such optical Extrinsic chirality: F) An all-dielectric metasurface exhibiting extrinsic chirality at the terahertz regime and operating in dual modes of normal and oblique incidences. At an oblique incident angle, CD's reported efficiency exceeds 70%. Reproduced with permission. [92] Copyright 2022, Wiley-VCH GmbH. G) An achiral dielectric metasurface comprising quadrumeric silicon-based crossed slots nanodisks for extrinsic chirality is reported to enhance the chiroptical responses due to the bound states in the continuum. Reproduced with permission. [93] Copyright the Owner Societies 2023, published by Royal Society of Chemistry.  . Plasmonic chiral metasurfaces for nonlinear optical responses. A) A chiral plasmonic gold nanohelix-based metasurface for second harmonic generation at 400 nm for optical rotation instead of circular dichroism. Reproduced with permission. [105] Copyright 2018, American Chemical Society. B) A gammadion-shaped multilayer planar chiral metasurface was investigated theoretically, supporting magnetic resonances for THG in circular dichroism in the IR regime. Reproduced with permission. [107] Copyright 2016, Author(s) Creative Commons Attribution (CC BY) license published by AIP Publishing. C) A hybrid metasurface for SHG and THG based on the spin of incident CP light comprises a combination of Trisceli C3-symmetric and gammadion C4-symmetric plasmonic chiral nanoresonators. The reported hybrid chiral metasurface results in a significantly high nonlinear harmonic signal and giant nonlinear CD for each state of circularly polarized light. Reproduced with permission. [108] Copyright 2020, American Chemical Society.

Figure 7.
All-dielectric chiral metasurfaces for nonlinear optics. A) A Z-shaped dielectric chiral metasurface supported by the gold substrate for giant CD and highly efficient SHG at the shorter wavelengths in the UV regime. The achieved efficiency for SHG in the blue UV region is 10 −3 %, and SHG-CD reaches up to 1.8. Reproduced with permission. [109] Copyright 2020, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. B) A GaAs L-shaped based chiral metasurface demonstrated high Q-factor resonances which strongly enhanced the local fields inside nanoresonators, resulting in multifold enhancement of nonlinear optical responses in terms of SHG. Reproduced with permission. [110] Copyright 2018, American Chemical Society.
responses requires rigorous optimization of symmetry-breaking nanoscale geometries. However, most chiral structures comprise complex geometries with multiple design parameters bringing the trade-off between the required chiroptical responses and the computational resources necessary to achieve them. Meanwhile, researchers either end up with highly optimized or underoptimized metasurface with a lot of patience and low productivity.
Owing to such limitations, researchers came up with algorithmic optimization approaches to ease the design process of metasurfaces. In general metasurface design, such approaches comprise genetic algorithms, [111][112][113] adjoint state method, [114,115] topology optimization, [115][116][117][118][119] etc. Later, these methods were improved to lower the simulation time and utilized in the design and optimization of metasurfaces. Yet the key issue is obtaining an optical response from the known geometry of a metasurface is time-consuming and computationally expensive. Recently, a novel approach adopted to overcome such limitations is the machine and deep-learning-based optimization techniques for metasurfaces. Irrespective of case-by-case optimization, the latter approach differs from traditional methods and requires extensive data to complete the learning process and converge to a correct representation. After completing the learning process, this model can predict the necessary optical responses with low-computational power from an extensive range of input Figure 8. Machine-learning-based approaches for plasmonic chiral metasurfaces. A) A deep-learning bidirectional neural network with a partial stacking strategy to optimize the chiroptical response of a chiral plasmonic structure. The optimized geometry operating in the terahertz (THz) region contains two gold split ring resonators stacked with specific separation and angle. Reproduced with permission. [53] Copyright 2018, American Chemical Society. B) A deep-learning-based virtual proximal policy optimization method is applied to design corner-stacked gold nanorod-based metasurface. Reproduced with permission. [120] Copyright 2022, Optica Publishing Group. C) A self-consistent platform based on Bayesian optimization and deep convolution neural network algorithms to optimize single-layer metallic nanostructure for highly efficient chiral optical responses. Reproduced with permission. [56] Copyright 2019, American Physical Society. D) A probabilistic graphic model in both forward and inverse is used to predict the design and chiro-optical response. The proposed model is applied to optimize a planar two-level split ring resonator for giant chiro-optical responses in the terahertz regime and predicts the highly nonlinear relationship between the metasurface geometry and the chiro-optical response. Reproduced with permission. [121] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. E) A deep neural network is presented to predict CD response in higher-order diffractive plasmonic metasurfaces, whereas the traditional coupled wave analysis is adopted for the learning process. The deep-learning model performs much faster and more accurately than several other methods. Reproduced with permission. [57] Copyright 2020, Zilong Tao (Figure 8A). [53] The presented approach works well in forward-predicting optical responses and can inversely retrieve the design parameters. The optimized geometry operating in the terahertz (THz) region contains two gold split ring resonators stacked with specific separation and angle. Similarly, another corner-stacked Figure 9. Dielectric chiral metasurfaces optimized by machine-learning algorithms. A) Utilized the combination of two machine-learning models, namely evolutionary algorithm and neural network, to adjust and optimize the spectral properties of dielectric resonator's based metasurface and also provides a frequency-dependent modification of CD responses. Reproduced with permission. [52] Copyright 2022, Mey et al. published by Wiley-VCH GmbH. B) The deep-learning-based optimization system used the reverse design strategy based on a target-driven conditional generative network (TCGN) to predict the optimal parameters of an all-silicon chiral structure working in the terahertz regime for the required CD response. Reproduced with permission. [122] Copyright 2022, Hou et al. published by Elsevier B.V.
gold nanorod-based metasurface is presented ( Figure 8B). [120] A deep-learning-based virtual proximal policy optimization method is applied to design the metasurface, and the Markov decision process is adopted to train the model. The optimized plasmonic metasurface provides an absolute circular dichroism value of ≈0.4 at various wavelengths in the near-infrared regime. Li et al. reported a self-consistent platform based on Bayesian optimization and deep convolution neural network algorithms to optimize single-layer metallic nanostructure for highly efficient chiral optical responses. The proposed self-learning framework achieved a CD efficiency of 82% in the far field ( Figure 8C). [56] Generally, a sizeable parametric space is associated with the free-form planar metasurface design. Therefore, a new machinelearning-based method is adopted to design a plasmonic chiral metasurface to overcome the limited parameter space optimization. Ma et al. have adopted a probabilistic graphic model which operates firstly by clustering similar design geometries into groups and then utilizing the variational autoencoder (VAE) structure to optimize the design and chiro-optical response in that group. The VAE assists in distributing the system into a latent space that learns the overall characteristics of the group and predicts the new geometries within the distribution ( Figure 8D). [121] The proposed model optimizes a planar two-level split ring resonator for giant chiro-optical responses in the terahertz regime.
In contrast to typical design standards, this model working forward and inverse, predicts the highly nonlinear relationship between the metasurface geometry and the chiro-optical response. A deep neural network was presented by Tao et al. to predict CD response in higher-order diffractive plasmonic metasurfaces ( Figure 8E). [57] A genetic algorithm is implemented to optimize metasurface properties, whereas the traditional coupled wave analysis is adopted for learning. The used deep-learning model performed much faster and more accurately than several other methods in predicting the chiro-optical responses.
A few recently reported works are discussed for machinelearning-based dielectric chiral metasurfaces. For rapid and ef-ficient optimization, Mey et al. demonstrated the utilization of two machine-learning approaches named evolutionary algorithm and neural network (Figure 9A). [52] The dielectric resonator's spectral properties were adjusted to the spectrum of transition metal dichalcogenide exciton resonances which are popular materials [123,124] for nanophotonic and optoelectronic applications. Moreover, the proposed framework provides a frequencydependent modification of CD responses. More recently, Hou et al. reported the deep-learning-based optimization of all-silicon chiral metasurfaces in the terahertz regime. The proposed system used the reverse design strategy based on a target-driven conditional generative network (TCGN) to predict the optimal parameters of chiral structure for the required CD response ( Figure 9B). [122] The method was applied to multiple chiral systems to achieve CD values exceeding 36%.

Applications
The interaction of electromagnetic waves with nanostructures introduces phase discontinuity throughout the metasurfaces. [22,125] Conventionally, metasurfaces can include specific information that cannot be reconfigured once the device is designed and fabricated. To subjugate this limitation, the researchers presented the idea of multiplexing metasurfaces, which can include multiple optical responses based on the polarization state of light. [126,127] However, polarization-insensitive structures and a lot of crosstalk between different polarization-dependent information limit the ability of multiplexed metasurfaces. Birefringent metasurface with superpixel metaatom has been proposed to demonstrate polarization multiplexing, [128,129] but it includes high-diffraction and cross-talk in the far field.

Chiral Metasurfaces for Polarization-Sensitive Metaholographic Displays
Metaholographic displays are one of the exciting applications of metasurfaces to realize computer-generated holograms (CGHs). [143][144][145][146][147][148] It requires rigorous optimization to attain complete control of polarization, phase, and amplitude for highquality holograms. Any image can be embedded in the metasurface using a phase profile which can be retrieved by Gerchberg-Saxton (GS) algorithm. [149] A lot of work has been done on conventional and chiral metaholographic displays. Montelongo et al. demonstrated an interleaved plasmonic metasurface based on silver nanoantennas for polarization-dependent holography ( Figure 10A). [150] Consequently, due to the nanoantenna's orientation, the designed metasurface reproduces different holograms based on the state of incident linearly polarized light. Meanwhile, using a similar design strategy, Montelongo et al. have also reported another work for color holograms using plasmonic nanoparticles. [151] Multiple arrays of nanoparticles are combined to reproduce polarization and wavelength-controlled holograms.
Likewise, Chen et al. reported gold-based nanoantennas to reproduce polarization-controlled holographic images ( Figure 10B). [152] A reflective metasurface is designed to work on the illumination of linearly polarized light in a visible regime. The maximum reported efficiency is ≈18% which decreases at the oblique incident angle of light. Based on intrinsic plasmonic chiral geometry, Chen et al. experimentally demonstrate a spin-controlled transmissive metasurface with a high CD ( Figure 10C). [153] To achieve chiral holography, two enantiomers of stepped nanoaperture are designed and merged in a single metasurface. Moreover, spin-dependent hybrid-order Poincaré sphere beams are realized. Many groups have reported dielectricbased chiral metasurfaces for highly efficient holography, like plasmonic chiral holography. Khorasaninejad et al. reported binary holograms at broadband visible to near-infrared wavelengths using subwavelength microgratings ( Figure 10D). [154] Moreover, nanofins geometry is adopted to design interleaved metasurfaces using geometric phase techniques for chiral holographic displays. With similar results, a new approach is adopted by Mueller et al. to design a metasurface using the merger of propagation and geometric phase modulation techniques to spin-dependent holography profiles. [155] In contrast to previous approaches demonstrated, in this work, the designed metasurface incorporates the ability to work on either circular or elliptical polarizations to characterize the chiral holograms. To achieve spin-decoupled chiral holography more efficiently, Chen et al. reported a phase chiral meta-atom-based metasurface ( Figure 10E). [156] Instead of using the rotation of conventional meta-atoms to use the geometric phase modulation techniques, the proposed metasurface used the symmetrybreaking structures, which do not need the rotation of meta-atoms to limit its functionalities to manipulate the CP light. The proposed metasurface also holds the property of broadband chiral holography reproduction in the near-infrared regime. A similar kind of work was also recently reported by Taimoor et al. to design a metasurface using phase chiral meta-atoms, including reduced complexity of design approach integrated with liquid crystal cell for dynamic chiral holographic displays at broadband visible wavelengths. [159] In view of broadband chiral holography, Wang et al. demonstrated a dielectric metasurface with nanoarc geometry to support different EM resonances for phase retardation at localized areas ( Figure 10F). [157] They experimentally incorporated the detour phase principle to achieve multiple functionalities such as chiral beam-splitting, broadband, and spin-selective holography at the visible and near-infrared regimes. Not much work has been reported on chiral holographic displays at whole visible wavelengths, including shorter wavelengths, due to the complex fabrication of chiral structures or the unavailability of loss-less materials. Khaliq et al. recently reported a broadband chiral holography with high efficiency at the whole visible regime, including the fabrication ease and using a loss-less hydrogenated amorphous silicon material ( Figure 10G). [158] The proposed planar dielectric metasurface, almost working in the visible regime, is designed using a building block comprising a pair of simple nanofins working as half-wave plates. The chiral holographic display is validated experimentally, and higher-order multipolar dielectric resonances investigate the inclusion of nonlinearity.

Chiral Metasurfaces for Sensing
Chiral metasurfaces have potential applications in sensing due to their ability to exhibit circular dichroism and the differential absorption of left-and right-circularly polarized light. [140] This property makes chiral metasurfaces useful for sensing specific chiral compounds, such as certain biomolecules with unique circular dichroism signatures. Numerous naturally occurring biomolecules, like sugar, the building block of proteins, amino acids, and nucleotides, are inherently chiral. [10][11][12][13]160] A pair of chiral molecules with opposite handedness is also termed as enantiomers. [161,162] Instead of exhibiting the same physical and chemical properties, enantiomers show different toxicity to cells. In pharmaceutical, pesticides, and medical industries, sensing and detecting enantiomers in small quantities is crucial in eradicating their unwanted side effects. [163][164][165][166][167][168] Several chiral metasurface-based sensors are being developed, including plasmonic sensors, [27,169,170] dielectricbased sensors, [171][172][173][174][175][176] surface-enhanced Raman scattering sensors, [177][178][179] and microcavity sensors. [180][181][182] These sensors have potential applications in biomedicine, environmental monitoring, and chemical sensing. Overall, chiral metasurfaces represent a promising new tool for developing advanced sensors with high sensitivity and specificity. Recently, much work has been proposed using chiral and cascaded achiral structure-based metasurfaces with enhanced CD for sensing and detection purposes. [183][184][185] Zhao et al. reported a plasmonic multilayer chiral metasurface with strong CD to detect and sense chiral molecules at the zeptomole level (Figure 11A). [162] The proposed system can detect Figure 10. Metasurface-based chiral holographic displays. A) Based on the orientation of sliver nanoantennas, an interleaved plasmonic metasurface is designed for polarization-dependent holography based on the state of incident linearly polarized light. Reproduced with permission. [150] Copyright 2013, American Chemical Society. B) A plasmonic reflective metasurface is demonstrated using gold nanoantennas to reproduce polarization-controlled holographic images working on linearly polarized light illumination in the visible regime. Reproduced with permission. [152] Copyright 2014, American Chemical Society. C) A spin-controlled transmissive metasurface is experimentally demonstrated using intrinsic plasmonic chiral geometry with high CD. Two enantiomers of stepped nanoaperture are merged in a metasurface to achieve chiral holography and spin-controlled hybrid-order Poincaré sphere beams. Reproduced with permission. [153] Copyright 2018, Springer Nature. D) A dielectric metasurface is reported based on subwavelength microgratings for binary holograms at broadband visible to near-infrared wavelengths. Meanwhile, a nanofin geometry is adopted for chiral holographic displays. Reproduced with permission. [154] Copyright 2016, The Authors Non-Commercial License 4.0 (CC BY-NC) published by AAAS. E) A phase chiral meta-atom-based dielectric metasurface was reported using the symmetry-breaking structures, which do not need meta-atoms rotation to limit their functionalities to manipulate the CP light. Reproduced with permission. [156] Copyright 2021, American Chemical Society. F) A dielectric metasurface is designed and demonstrated experimentally with nanoarc geometry to support different EM resonances for phase retardation at localized areas. The detour phase modulation technique achieves multiple functionalities, such as chiral beam-splitting, broadband, and spin-selective holography. Reproduced with permission. [157] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. G) A broadband chiral holography is reported using a loss-less hydrogenated amorphous silicon-based metasurface with high efficiency at the whole visible regime, including fabrication ease. The proposed planar dielectric metasurface experimentally demonstrated the chiral holography almost at the entire visible regime. Reproduced with permission. [158] Copyright 2022, Wiley-VCH GmbH. Figure 11. Chiral sensing and detection using plasmonic metasurfaces. A) A multilayer plasmonic chiral metasurface with strong CD is reported to detect and sense chiral molecules at the zeptomole level, which is smaller in several orders of magnitude than conventional CD spectroscopy. The designed nanophotonic platform can sense the chiral molecules even with weak circular dichroism at visible and near-infrared wavelengths. Reproduced with permission. [162] Copyright 2017, Springer Nature. B) A racemic plasmonic metasurface comprises an equal amount of gammadion enantiomers with no intrinsic CD but high optical chirality, including electric field enhancements for efficient sensing and detection. They have adopted the molecular thermal evaporation method for molecular delivery to the sensors. Reproduced with permission. [161] Copyright 2018, American Chemical Society. C) A multiresonant metasurface in an infrared regime is presented to detect multiple analytes in a heterogenous biosample with 1000-fold intensity enhancement. Reproduced with permission. [186] Copyright 2018, Springer Nature. D) A hybrid metasurface based on gold nanoantennas and graphene is demonstrated in the mid-infrared regime for highly sensitive protein's quantification and identification. They have used passive and active tuning to overlap the protein's vibrational modes with the metasurface resonances spectrally for efficient sensing. Reproduced with permission. [160] Copyright 2019, American Chemical Society. smaller molecules of several orders of magnitude compared to conventional CD spectroscopy. The designed nanophotonic platform can sense the chiral molecules even with weak circular dichroism at visible and near-infrared wavelengths. To detect the chiral molecules ideally, Guirado et al. demonstrated that racemic plasmonic metasurface made a nonchiral superstructure with no intrinsic CD but high optical chirality, including electric field enhancements in the near-fields ( Figure 11B). [161] The racemic structure comprises equal amounts of gammadion enantiomers, suppressing the CD of systems to avoid shadowing the chirality of molecules for efficient sensing and detection. Moreover, they have used the molecular thermal evaporation method for molecular delivery to the sensors, which was rarely used in past studies.
Rodrigo et al. presented a multiresonant metasurface working in an infrared regime to detect multiple analytes in a heterogenous biosample ( Figure 11C). [186] The proposed biosensor provides a 1000-fold intensity enhancement that can simultaneously detect vibrational fingerprints of various biomolecules. The designed sensor can be applied in real-time sensing, specifically in pharmaceutical and bioanalytical applications. Li et al. demonstrated a hybrid metasurface based on gold nanoantennas and graphene to design a mid-infrared biosensor for highly sensitive monolayer protein quantification and identification ( Figure 11D). [160] Additionally, they have used passive and active tuning of the designed metasurface-based sensor to overlap the protein's vibrational modes with the metasurface resonances spectrally. Figure 12. Dielectric metasurfaces for chiral sensing and detection. A) A dielectric achiral metasurface was demonstrated for chiral sensing and separation to obtain local enhancement of 138-fold and 15-fold for C and g values, respectively. Reproduced with permission. [172] Copyright 2018, American Chemical Society. B) A metasurface based on lossless titanium oxide achiral dimers geometry is presented at the UV wavelengths to obtain the 80-fold field enhancement of optical chirality with 50-fold CD enhancement. Reproduced with permission. [187] Copyright 2019, American Chemical Society. C) A dielectric metasurface based on diamond nanodisks is demonstrated at UV wavelengths to enhance the C as high as 1130-fold, with the averaged C enhancement exceeding 100-fold. Reproduced with permission. [188] Copyright 2020, American Chemical Society. D) A planar dielectric metasurface presented based on Si nanodisk geometry. For the measurement of differential absorption of CP light for optically active molecules, fluorescence-detected (FD) CD spectroscopy is utilized, which allows the system to deconvolute the absorption in the nanodisks from molecules for efficient sensing and detection. Reproduced with permission. [189] Copyright 2020, American Chemical Society. E) An all-dielectric metasurface comprises an array of high-index tetramer clusters exhibiting ultrahigh-Q toroidal dipole resonance due to BICs at terahertz frequencies. Two toroidal dipole resonances assisted by BICs are identified and investigated to achieve ultrahigh sensitivity levels. Reproduced with permission. [190] Copyright 2020, Yulin Wang et al., published by De Gruyter, Berlin/Boston. F) A dielectric metasurface demonstrated based on crescent-shaped meta-atoms to control light-matter interaction. The metasurface supports quasi-BICs and higher-order resonances to enhance and tune the electromagnetic fields for sensing purposes significantly. Reproduced with permission. [191] Copyright 2021, The Authors published by Wiley-VCH GmbH.
For highly efficient chiral sensing and detection, dielectricbased metasurfaces were also demonstrated. Solomon et al. presented a dielectric achiral metasurface for chiral sensing and separation (Figure 12A). [172] The metasurface contains high-index dielectric disks to obtain local enhancement of 138and 15-fold for C (chirality factor) and g (dissymmetry factor) values, respectively. Meanwhile, the obtained volumetric enhancement values are 30-and 4.2-fold. For surface-enhanced CD spectroscopy of molecules at ultraviolet regime, Yao et al. demonstrated a dielectric metasurface based on titanium oxide achiral dimers geometry ( Figure 12B). [187] The designed metasurface is completely lossless at the wavelength of 360 nm with an 80-fold field enhancement of optical chirality with 50-fold CD enhancement. The device can provide highly efficient chiral sensing and separation using an achiral metasurface in an ultraviolet regime.
Another work for CD spectroscopy at UV, reported by Hu et al., demonstrated a dielectric diamond-based metasurface to enhance the C factor by three orders of magnitude ( Figure 12C). [188] Due to Mie resonances enabling into the nanodisk lattice, activates the high Q resonances that eventually enhance the C as high as 1130-fold. At 40 nm from the surface, even the averaged C enhancement exceeds 100-fold. Solomon et al. reported a planar dielectric metasurface based on Si nanodisk geometry. The differential absorption of CP light was indirectly measured for optically active molecules by fluorescence-detected (FD) CD spectroscopy ( Figure 12D). [189] The used fluorescent technique in the proposed system allowed it to deconvolute the absorption in the nanodisks from molecules for efficient sensing and detection. For high-Q sensing, dielectric metasurfaces governed BICs and quasi-BICs are also presented in the literature. Wang et al. reported an all-dielectric metasurface assisted by BICs to achieve ultrahigh sensitivity for sensing at terahertz frequencies ( Figure 12E). [190] The building block of the proposed metasurface is based on a high-index tetramer cluster of cylinders creating asymmetry. The proposed structure introduces two modes of toroidal dipole (TD) resonances which can be controlled by asymmetric factors and turned into ultrahigh-Q resonances. Another all-dielectric metasurface based on quasi-BICs is presented based on crescent-shaped meta-atoms for sensing purposes ( Figure 12F). [191] Apart from quasi-BIC resonances, the proposed metasurface supports higher-order resonances with tunable electromagnetic fields to enhance the light-matter interaction for sensing. The metasurface is proposed for real-time and in-situ biosensing due to its high sensitivity and potential for microfluidic integration properties. The performance comparison of chiral metasurfaces for sensing applications is depicted in Table 1.

Chiral Imaging
Traditionally, curved and bulky dielectric lenses or reflectors are required to focus the EM waves, limiting the miniaturization of optical systems. Since the advancement in metaoptics, scientists and engineers have found novel planar lens designs. Moreover, light's spectral and polarization properties can additionally provide valuable information in imaging. [192,193] Even the human eye can perceive spectral properties due to the visible colors but still blinds to the polarization states. For polarization imaging, modern systems require cascading lenses, including several other optical components such as wave plates, polarizers, and beamsplitters. Polarized imaging and spectroscopy have significant applications from healthcare to sensing and detection. Hence, a dispersive optical system based on metasurfaces with polarization control leads to an efficient miniaturized imaging system. A simple metalens design requires the phase profile L (x,y) which can translate the incident planar wavefronts into spherical ones. [194,195] The mathematical expression to find the phase profile can be written as where x, y, , and f denotes the displacement in the x-axis, displacement in the y-axis, free-space wavelength, and the lens's focusing, respectively. Initially, the planar metalens concept was experimentally illustrated using V-shaped antennas. [196][197][198] Furthermore, much work has been done on metalenses, including achromatic metalens systems. [199][200][201][202] For plasmonic-based chiral imaging, He et al. reported a chiral metalens realized theoretically and experimentally using subwavelength helical surface arrays working in a mid-infrared regime ( Figure 13A). [203] The proposed metalens was designed using the rotation of the helical surface in an azimuthal direction to achieve the complete phase manipulation for chiral imaging using a single element. Due to the small efficiency achieved using the plasmonic metalenses for chiral imaging, dielectric metalenses were reported. Groever et al. demonstrated dielectric-based, highly efficient chiral metalens for imaging purposes ( Figure 13B). [204] Instead of using the conventional geometric phase modulation techniques in plasmonics, they have adopted a combined approach of propagation and geometric phase modulation to design a highly compact chiral imaging system. Additionally, for multispectral chiral imaging, Khorasaninejad et al. represented planar dielectric metalens with engineered dispersive response to reproduce two helicity-dependent images ( Figure 13C). [131] The designed metalens based on titanium dioxide (TiO 2 ) based nanofins with polarization chirality properties achieved multispectral chiral imaging in the visible regime. Figure 13. Metasurface-based chiral imaging. A) A plasmonic chiral metalens designed by subwavelength helical surface arrays working in a mid-infrared regime for chiral imaging using single-element phase manipulation. Reproduced with permission. [203] Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. B) A dielectric-based, highly efficient compact chiral metalens is designed using a combination of propagation and geometric phase modulation techniques for chiral imaging applications. Reproduced with permission. [204] Copyright 2018, Springer Nature. C) A planar multispectral dielectric chiral metalens is presented with an engineered dispersive response to achieve chiral imaging at the whole visible regime. The designed metalens experimentally performed the multispectral chiral imaging in the visible regime. Reproduced with permission. [131] Copyright 2016, American Chemical Society.

Conclusion/Future Prospective
For many years, scientists have studied the impact of chirality on various optical phenomena, including chiro-optical activity, circular birefringence, and circular dichroism. The discovery of chiral materials in the late 20th century and the development of chiral metamaterials in the early 21st century have led to new possibilities for controlling and manipulating chiral light. Furthermore, chiral metasurfaces have unique benefits over chiral metamaterials due to compact size, improved functionality, fabrication ease, and cost-effectiveness making them a promising platform for various applications in optics and photonics.
Here, we reviewed the recent progress on chiroptical effects in plasmonic and dielectric metasurfaces for various phenomena in optics and photonics based on fundamental principles, adopted design strategies, and their implementation. Additionally, a brief overview of BIC-based and nonlinear metasurfaces is provided, which can have potential applications in optical switching, optical frequency conversion, optical modulation, nonlinear frequency generation, optical sensing, and optical imaging. A few machine-learning-based plasmonic and dielectric metasurfaces are discussed for smart and innovative chiral design strategies. Although the mentioned metasurfaces in this review showed sig-nificant potential to enhance the chiroptical responses based on chosen material and design strategies, several challenges and opportunities remain to work in future chiral metasurfaces.
Firstly, for intelligent and efficient manufacturing of chiral metasurfaces, machine learning-based chiral metasurface design can take into account manufacturing errors. Machine learningbased algorithms can simulate the manufacturing process during design and can estimate the expected manufacturing error. This information can then be used to optimize the design of the metastructure to account for these errors, resulting in a final design that is more robust to manufacturing inaccuracies. Additionally, artificial intelligence can use machine learning techniques to learn from past manufacturing experiences and adjust the design process accordingly to improve the chances of successful fabrication. [205] However, the extent to which manufacturing errors are considered during AI-supported metastructure design depends on the specific AI algorithms used and the information available during the design process.
Secondly, developing chiral sensing techniques at UV wavelengths is an important and rapidly evolving field that holds great promise for various applications, such as detecting and characterizing chiral drugs and analyzing chiral molecules and materials in the environment and the development of new chiral www.advancedsciencenews.com www.advopticalmat.de materials. [206][207][208] There are several challenges associated with UV sensing based on chiral metasurfaces. Some key challenges include materials limitations, fabrication complexity and cost, wavelength dependence, and integration challenges. Currently, the available materials used for chiral metasurfaces in UV sensing applications have limited optical and mechanical properties; require specialized fabrication techniques for high precision. Such methods can cost a lot and limit their widespread use and commercialization. In addition, the optical response of chiral metasurfaces is highly dependent on the wavelength of light, which can affect the overall performance of the metasurface in UV sensing applications that require a broadband operation. Moreover, integrating chiral metasurfaces with other optical components, such as photodetectors and optical modulators, is also a challenge, which can affect the overall performance and efficiency of the system.
Thirdly, chiral metasurfaces can have great potential for use in augmented reality (AR) and virtual reality (VR) holographic displays. [209,210] The key benefits of using chiral metasurfaces in AR and VR holographic displays include high-resolution images, compact and lightweight devices, energy efficiency, and cost-effectiveness. Moreover, integrating active or dynamic chiral metasurfaces with AR and VR technologies can provide real-time imaging and holographic displays for various healthcare, security, and advertisement applications. [211] Furthermore, the form factor is the biggest issue in VR/AR adoption; chiral metasurfaces can provide compactness or minimize aberrations with additional functionalities in optical systems requiring high magnification.
To summarize, these approaches may lead to efficient, effective, and compact chiral metadevices for real-time patient monitoring, smart sensing, and detection, especially in UV, efficient multichannel data transmission based on multiplexed information. Further, integrating chiral metasurfaces with lab-on-chip devices, catering to manufacturing inaccuracies based on machine learning algorithms, can lead to numerous promising real-life applications, particularly in the healthcare and display industry for chiral bioimaging and highly efficient smart displays, respectively. Furthermore, integrating chiral metasurfaces with headmounted displays (HMDs) will lead to the miniaturization of next-generation optical systems, including additional functionalities such as high magnification, high resolution, and high efficiency.