Research Progress in Tunable Metamaterial Absorbers

Since the initial discovery of metamaterial absorbers (MAs), they generate significant interest across the electromagnetic device industry due to their capacity to attain subwavelength magnitudes, adaptable design, and almost complete absorption. In recent years, there has been a growing focus on incorporating active materials into MAs, making it a current focal point of investigation. In the past few years, MAs have seen the integration of various active materials and configurations thanks to continuous exploration and innovation. Herein, comprehensive and systematic research on different tuning methods of MAs is reviewed, highlighting innovative materials and unique designs to accurately control the electromagnetic absorption characteristics of MAs. Key parameters, such as the operating frequency and relative tuning range, are summarized and compared to provide guidance for optimization design. Finally, the challenges of tunable MAs and their future prospects are also summarized.


Introduction
Metamaterials refer to artificially designed subwavelength array structures that exhibit exceptional electromagnetic and optical properties that natural materials do not possess.[3][4] In 2008, Landy et al. [5] first demonstrated metamaterial absorbers (MAs) in the microwave range using 2D metamaterial electric ring resonators, where electric and magnetic resonances matched the impedance of free space, thereby eliminating reflections at the target frequency.This pioneering study has motivated further research to expand the applications of MAs to other frequency ranges and various applications.Compared to conventional absorbers, MAs have the benefit of being thin thickness and small volume, without compromising on high absorption capacity.And, MAs can be designed with multiband, broadband, polarization, and tunable characteristics for different applications, all of which can be achieved by optimizing the structure of metamaterials or adjusting material properties.
[8] The practical applications of MAs have been confirmed in diverse fields such as solar energy collection, [9][10][11] biosensing, [12][13][14][15] thermal photovoltaics, [16,17] optoelectronic detection, [18][19][20] the construction of imaging devices, [21][22][23][24] and absorption filtering. [25]With the extensive and comprehensive research on metamaterials, creating passive devices that can meet research needs has become difficult.28] To change the absorption frequency or operating bandwidth, it is necessary to integrate multiple devices or stack multiple resonant layers, [29,30] which conflicts with the miniaturization of electronic devices.44][45] In recent years, considerable investment has resulted in significant progress in tunable metamaterials (MAs) research.This article provides an overview of recent advancements in key material properties and structural design for achieving tunable MAs.Initially, the article introduces the development status of tunable MAs, which includes various modulation methods such as optical excitation, thermal radiation, electrical modulation, and others.Next, a detailed introduction was given, including the application of photoconductive semiconductor silicon, the use of vanadium dioxide (VO 2 ), indium antimonide (InSb), and germanium antimony tellurium (GST) phase-change materials in thermal radiation.Additionally, the individual application and combined designs of 2D materials such as graphene, black phosphorus (BP), and molybdenum disulfide (MoS 2 ) were discussed for their employment in electrical modulation.A comprehensive overview of tunable MAs was provided by comparing their key attributes, such as external excitation, operating frequency, tunability, and switching speed.This comparison offers valuable guidance for designing tunable MAs.Lastly, we summarized the challenges of tunable MAs and our outlook for future development in tunable MAs.

The Development Status of Tunable MAs
Tunable MAs are currently a hot topic of research.Reasonable adjustment of various parameters of metamaterials has a crucial impact on the absorption performance of absorbers.Dynamically adjusting these parameters effectively provides the possibility of manipulating MAs absorption.[44][45] A common method involves utilizing active photonic materials such as graphene, silicon (Si), vanadium dioxide (VO 2 ), and indium antimonide (InSb).7][38][39][40][41]

Optical Excitation
Semiconductor photoconductive silicon (Si) is widely used for optical excitation due to its excellent optoelectronic properties, low cost, high quantum efficiency, and good compatibility.By altering the energy of the incident optical pump beam, the carrier density of the silicon structure undergoes a change.If the energy of the pump beam exceeds that of the bandgap of the structure, it can generate an excess carrier density when it shines onto the surface of the structure.In this instance, the conductive state of the structure will change.In general, the pump beam used is 800 nm near-infrared (NIR) light.
Cheng et al. [46] numerically simulated a terahertz (THz) band optical switch broadband MA structure based on planar patterned photoconductive silicon (Si).The designed MA is a photoconductive silicon array with a planar-square-ring-shaped (PSRS) structure placed on a ground plane separated by a dielectric substrate, as shown in Figure 1a.The conductivity of the PSRS silicon array can be controlled by an external optical pump beam.By altering the conductivity of the Si array, the structure depicted in Figure 1b can achieve switching absorption of 2.8-99.9%.Furthermore, the relative bandwidth of 90% continuous absorption can be adjusted from 16.2% to 86.4%.Additionally, the proposed structure attains a high modulation depth of 97.2%, and the frequency tuning bandwidth of 90% continuous absorption is %81.25%.
Similarly, Zhao et al. [47] proposed an all-silicon metamaterial perfect absorber (MPA).The design consists of a single layer of uniformly distributed "H-shaped" metamaterials imprinted on a silicon substrate, as shown in Figure 1c,d.In addition to static characterization, the dynamic response under optical excitation was also studied.The MPA exhibited almost perfect absorption at 1 THz in static measurements, with an absorption bandwidth of 0.9 THz at 90%, as shown in Figure 1e.They used effective medium theory and finite difference time domain simulation to analyze the physical properties of perfect absorption, and attributed the broadband response to the overlap of different resonant modes.Figure 1f depicts that excitation with %100 fs 800 nm pulse causes resonance mode shift and modulation of bandwidth and absorbance when the pump fluence is raised from 0 to 4000 μJ cm À2 .The results can be explained by the multilayer carrier dynamics caused by the depth of pump light penetration in metamaterial structures.
As research is ongoing, the integration of MAs in the field of biosensing has emerged as a significant topic of discussion.Jing et al. [48] presented a multifunctional MA based on doped silicon.By introducing resonance and impedance matching into the absorber, achieved ultra-wideband absorption of over 90% in the range of 0.8-10 THz was achieved, as demonstrated in Figure 1g.Precise amplitude control within the 0.1-1.2THz range was accomplished by altering the pump luminous flux, as shown in Figure 1h.Furthermore, by applying the principle of regulating the concentration of doped silicon carriers using light, it was possible to achieve a sensor with a sensitivity of up to 500 GHz RIU À1 .This was accomplished through the replacement of medium-doped silicon material with highly doped silicon material and the combination of an absorber with microfluidic control technology.
Resonant perfect absorbers are currently a highly sought-after tool in the field of metamaterials.However, overcoming the inherent localization, narrow bandwidth, and static response of resonant metamaterials remains a difficult task.To address this challenge, Seren et al. [49] proposed a tunable perfect absorber with multiple optically modulated absorption bands, as demonstrated in Figure 1i.They achieved up to 97% and 92% of the maximum internal absorption in liquid crystal (LC) and dipole resonance modes, respectively, with modulation depths of 38% and 91%, as depicted in Figures 1j,k.
Due to the use of all dielectric silicon, optical excitation tunable MAs can be easily processed and manufactured using standard lithography techniques.Silicon with low resistivity is acknowledged as a high loss material in the THz region, thus rendering it ideal for the production of tunable absorbers with superior functionality.Moreover, the benefits of shorter excitation time and faster running time of light provide significant advantages.However, there are also limitations to optical excitation absorbers.First, the carrier concentration of optical excitation all dielectric silicon decreases continuously with an increase in silicon wafer depth, making it impossible to obtain a structure with uniform carrier concentration.Second, there are significant  [46] Copyright 2021, Elsevier.c) Schematic of all-dielectric broadband terahertz (THz) metamaterial perfect absorber (MPA).Inset: effective medium theory (EMT) model of the MPA, in which the metamaterial (MM) layer is considered as a homogeneous thin film.d) The scanning electron microscope (SEM) image of the all-silicon MPA.Inset: false color SEM view of the unit cell.The green part is the etched silicon and the purple part is the silicon substrate.e) The reflectance and absorbance spectrum of the fabricated MPA, comparing EMT and finite difference time domain with experiment.f ) Optical-pump THz-probe (OPTP) spectra of the all-dielectric MPA for different pump fluences.Inset: incident direction of the beams, with the optical pump beam incident at a normal angle and the THz probe beam is at a 30°angle of incidence.(c-f ) Reproduced with permission. [47]Copyright 2019, America Chemical Society.g) The absorption, reflectance, and transmittance of MA.Inset: structural schematic diagram of all dielectric tunable MA. h) Absorption spectra under pump light regulation of different energy densities.(g,h) limitations in practice due to the relatively large equipment for incident pump light.

Thermal Radiation
Thermal radiation primarily alters the electromagnetic properties of phase-change materials by changing their temperature, which is different from optical excitation.Common materials for thermal radiation tunable MAs include vanadium dioxide (VO 2 ), indium antimonide (InSb), and GST.
VO 2 is a phase-change material that can completely transition from an insulating state to a metallic state at a temperature of 340 K, and this phase change is reversible.As the temperature elevates, the gate structure of VO 2 steadily changes from a monoclinic structure to a polygonal structure.This leads to an increase in the strength of electromagnetic characteristics during temperature rise.MAs that are constructed of VO 2 can accomplish tunable absorption frequency by regulating temperature.
Zhang et al. [50] designed an ultra-wideband THz MPA, in which the absorption layer consists of four T-shaped structures and achieved over 99% perfect absorption within the frequency of 6.6-8.9THz.They designed three active tunable THz MAs based on VO 2 , respectively, achieving broadband mobility and conversion between broadband and multiband, as shown in Figure 2a.The simulation findings demonstrate that all three structures have the potential to expand, shrink, or shift the original absorber's absorption bandwidth, leading to ultra-bandwidth absorption into multiband absorption.
Zhao et al. [51] designed a broadband switchable THz MA that is different from the aforementioned structure, that utilizes VO 2 phase transition.It is a stacked structure composed of VO 2 periodic array, dielectric layer, VO 2 film, Au periodic array, dielectric layer, and Au reflection layer, as demonstrated in Figure 2b.When the temperature rises above the phase transition temperature of VO 2 , the absorption band of 0.76-0.86THz can be transformed into an absorption band of 1.12-1.25 THz at room temperature.Similarly, Yang et al. [52] proposed a dual-band tunable infrared MA based on VO 2 as its basis, depicted in Figure 2c.By changing the temperature of the VO 2 resonant structure to tune the absorption frequency and intensity, two absorption peaks appeared at 62.9 and 69.7 THz, individually, with absorption rates of 99% and 99.9%, respectively, as demonstrated in Figure 2d,e.When the temperature falls below the critical temperature of 68 °C, the VO 2 film functions as an insulator.Compared to metallic VO 2 , both absorption band width and intensity decrease.
MPAs based on vanadium dioxide (VO 2 ) have significant potential application value in sensing gas molecules.However, the tuning mechanism through temperature manipulation lacks compatibility with electronic devices.Xu et al. [53] formed an MEMS-based MPA for CO 2 -sensing applications by integrating H-shaped MPA-and MEMS-based microheaters, as shown in Figure 2f.Graphical design of microheaters using mathematical fractal theory to provide high heating temperature and high surface temperature uniformity, as depicted in Figure 2h,i.The microheater generates heat when a DC bias voltage is applied and directs it toward the upper H-shaped MPA, as shown in Figure 2j.By utilizing the insulator to metal transition properties of VO 2 materials enable accurate tuning of the VO 2 based H-shaped MPA can be achieved at a wavelength of 2.70 μm, as illustrated in Figure 2g.It exhibits perfect absorption within the CO 2 gas absorption spectrum and is suitable as a highly sensitive CO 2 gas sensor.
Due to its temperature-dependent dielectric constant, high electron migration rate, and small bandgap in the THz frequency band, InSb is widely used in tunable THz MAs.Jing et al. [54] designed and studied a dual-band THz MA consisting of patterned InSb films, polytetrafluoroethylene dielectric layers, and metal plates, as shown in Figure 3a.The numerical simulation results show that, without an external magnetic field, MA reaches an absorption peak of 94% at 0.434 and 0.692 THz, respectively, at a temperature of 300 K. Figure 3b illustrates that with a temperature change from 295 to 320 K, gradually changing the resonant frequency changes gradually from 0.413 to 0.529 THz, and from 0.654 to 0.863 THz.Furthermore, when an external magnetic field is applied, the absorption efficiency of InSb-based MA will alter.With the magnetic field variation from 0.2 to 1 THz, the double-resonance frequency gradually changes from 0.4816 to 0.423 THz, and from 0.7804 to 0.716 THz.Similarly, Luo et al. [55] proposed a temperature tunable MA based on an all-dielectric indium antimonide (InSb) resonant structure, as shown in Figure 3c.When the temperature is 285 K, the absorption rate of this MA is as high as 99.9% at 1.43 THz, and the Q factor is about 26.9, as shown in Figure 3d.
Li et al. [56] proposed a dual-band THz perfect absorber based on a vertical-square-open-ring structure InSb array all-dielectric metamaterial, as shown in Figure 3e.The simulation results demonstrated that absorption efficiencies of 99.9% and 99.8% were achieved at 1.265 and 1.436 THz, respectively, both at room temperature (T = 295 K).By adjusting the temperature, the absorption characteristics of MA undergo change, thereby enabling it to function as a temperature sensor with sensitivities of approximately 5.9 and 6.4 GHz K À1 , respectively, as shown in Figure 3f.In addition, the MA can also be utilized as an effective refractive index sensor at T = 295 K, with sensitivities of approximately 1.3 and 1.0 THz RIU À1 , respectively, as shown in Figure 3g.
In addition to VO 2 and InSb, certain other phase-change materials can modify their electrical properties in different crystalline states.These phase-change materials have also been proven to effectively regulate the absorption performance of MA.GST, a phase-change material, is regarded as a tunable medium material.By controlling the refractive index change of GST at temperature, a multi-gradient temperature controllable optical functional absorption structure can be achieved.GST can undergo both thermal radiation and optical-excitation-induced phase transitions.
Metal-dielectric-layered hyperbolic metamaterials (HMMs) have generated significant interest due to their extraordinary optical properties and straightforward design.Behera et al. [27] demonstrated a reconfigurable HMM perfect absorber in the NIR region by utilizing alternating layers of gold (Au) and Ge 2 Sb 2 Te 5 (GST225), as illustrated in Figure 4a,b.The results indicate that by transitioning the GST225 state from amorphous to crystalline, its absorption peak can be redshifted by 500 nm, as shown in Figure 4c.The almost perfect absorption rate is omnidirectional and polarization independent.Additionally,  [50] Copyright 2020, The Authors, published by IEEE.b) The absorption curve of this VO 2 MA at temperatures of 50 and 80 °C.Illustration: schematic diagram of VO 2 MA for this design.Reproduced with permission. [51]Copyright 2017, Elsevier.c) Schematic diagram of a switchable broadband absorber.d) At 300 K, VO 2 can switch the absorption spectrum of the absorber.e) At 350 K, VO 2 can switch the absorption spectrum of the absorber.(c-e) Reproduced with permission. [52]Copyright 2018, Elsevier.f ) Schematic diagram and tuning mechanism of micro-electro-mechanical system (MEMS)-based MPA design.g) Schematic diagram of MEMS-based MPA for CO 2 -gas-molecular-sensing function.The absorption rate of MEMS-based MPA with different applied DC voltages in h) transverse electric (TE) and i) transverse magnetic (TM) modes.j) The relationship between applied DC voltage and absorption intensity at a wavelength of 2.70 μm.(f-j) Reproduced with permission. [53]opyright 2022, RSC Pub.
the absorptance peak can be reversibly switched in just 5 ns by reamorphizing the GST225, enabling a dynamically reconfigurable HMM absorber.
Mou et al. [57] demonstrated through experiments a large-scale, broadband, polarization-independent, and tunable MA that is appropriate for visible and NIR light, as shown in Figure 4d.They prepared a centimeter-scale metamaterial sample of metal-insulator-metal (MIM) structure with nanoscale accuracy, in which the phase-change material Ge 2 Sb 2 Te 5 (GST) was used as the insulation spacer for the MIM structure, as shown in Figure 4f,g, h.In the case of two different resonance mechanisms working together, the proposed device exhibits high absorption (>80%) in a wideband (480-1020 nm), as shown in Figure 4e.Altering the phase state of the GST layer via thermal tuning means causes a shift of about 470 nm to significantly expand the operating bandwidth of MA.
By combining the phase-change material GST with the Fabry-Perot resonant cavity, this study investigates the changes in optical parameters during the thermal-induced phase-change process of GST are studied, and a tunable metamaterial absorption device with specific wavelength band is achieved.The effect of external excitation parameters on the phase transition of GST was also explored through thermal radiation treatment.The study findings demonstrated that upon thermal excitation, both  [54] Copyright 2022, Elsevier.c) Comparison of absorption spectra of pure InSb materials, proposed MA and perfect electric conductor (PEC) substrates with Au while external ambient temperature of T = 285 K. d) The absorption spectra of the designed MA at various temperatures (T ).(c,d) Reproduced with permission. [55]Copyright 2020, Elsevier.e) The designed all-media MA periodic array based on InSb.f ) The resonant frequencies f 1 (black square) and f 2 (red circle), and the corresponding linear fit curve as a function of T; g) the corresponding linear fit curve as a function of the surrounding analyte n. (e-g) Reproduced with permission. [56]Copyright 2022, Elsevier.
refractive index and extinction coefficient of GST increased in unison with the temperature rise.However, the refractive index plateaued, while the extinction coefficient continued to escalate.
Compared to tunable MAs excited by light, tunable absorbers utilizing thermal radiation do not necessitate large light devices for tuning and have simpler excitation conditions.Additionally, an assortment of thermal radiation phase-change materials exists, allowing for the selection of appropriate materials for varying applications in different bands.These advantages suggest that thermal radiation tunable MAs have prospects for developing higher-performance and simpler absorbers.However, the method of thermal radiation is not without its limitations.Phase-change materials, for instance, will only change their state when they attain their respective phase-change temperature.Additionally, temperature control requires a lengthy amount of time, rendering real-time control akin to "switching" impossible.

Electrical Modulation
Surface plasmon (SP) polaritons are surface electromagnetic waves that exist at the interface between metals and dielectrics.
In addition to bulk metals, 2D materials such as graphene, transition metal disulfide compounds (TMDCs), hexagonal boron nitride, and BP can also support SP. 2D materials are used as metamaterials for absorbers and can be used to provide flexible tunability.Frequency-selective surfaces have the capability to improve the interaction between THz waves and atomic layers of 2D materials.At the same time, the tunable properties of 2D materials can also control the resonant response of frequency-selective surfaces.Therefore, 2D materials and frequency-selective surfaces interact and reinforce each other.Usually, 2D material absorbers consist of subwavelength structural units, which modulate electromagnetic waves via a certain arrangement.
Graphene is a single-layer 2D structure composed of carbon molecules, which has enormous application prospects in the domains of optics, hardware, and materials science.The surface carrier concentration of graphene can be regulated via chemical doping and the application of bias voltage.When electromagnetic waves are present on the surface of graphene, SP polaritons are excited and resonated with them, resulting in the generation of strong electromagnetic activity.Copyright 2021, The Authors, published by Royal Society of Chemistry.d) Schematic of the broadband and tunable metamaterial absorber (MA), which consists of semi-ellipsoidal shaped aluminum (Al) nanoparticles array in hexagonal lattice and a continuous Al mirror separated by an ultrathin GST film.e) Measured absorption spectra of the broadband MAs fabricated with five different annealing temperatures.f ) Fabrication process of the proposed broadband and tunable MA. g) Tiltedview and h) side-view SEM images of the fabricated absorber samples and part of the aluminum oxide (AAO) mask left on it.(d-h) Reproduced with permission. [57]Copyright 2020, RSC Pub.
Feng et al. [58] designed a broadband THz graphene MA based on SP resonance, which is polarization independent and angle insensitive, and conducted full wave simulation, as shown in Figure 5a.The results showed that the designed MA achieved over 90% absorption in the range of 1.10-1.86THz, and MAs exhibited over 99% high absorption in the range of 1.23-1.68THz.In addition, as shown in Figure 5b, by changing the Fermi level of graphene from 0 to 0.7 eV, the absorption rate can be adjusted from 1% to 99%.In the simulation, when the incident angle of TM waves changes from 0°to 60°, the average absorption rate remains above 80%.By utilizing the same principle, Xiao et al. [59] designed a polarization-independent broadband MA with a large modulation range and simulated its absorption characteristics in the mid-infrared band, as shown in Figure 5d.When the Fermi level is E f = 0, two absorption peaks were observed at wavelength λ = 12.4 and 13.3 μm.Almost all incident light from 12.1 to 13.5 μm is absorbed by MA.As shown in Figure 5e, as E f increases, resonance shifts toward shorter wavelengths.When E f increases from 0 to 0.6 eV, the tuning ranges of the two resonant states reach 20.1% and 25.5% of the central wavelength, respectively.K. T. Lin et al. [60] proposed, simulated, and experimentally validated a 3D structured graphene metamaterial (SGM) for solar-selective absorption, as shown in Figure 5f.It utilizes the wavelength selectivity of metal groove like structures, the broadband non dispersion characteristics of ultrathin graphene metamaterial films, and excellent thermal conductivity.As shown in Figure 5h, the coupled light in the nanostructure will form a cavity resonance as a standing wave inside the groove.Due to the conformal coating of graphene metamaterials coated on the sidewall, light is absorbed as it propagates along the groove sidewall.The SGM absorber has excellent solar selectivity and omnidirectional absorption, flexible tunable wavelengthselective absorption, excellent photothermal performance, and high thermal stability, as shown in Figure 5g.The solar energy conversion efficiency to thermal and water vapor energies has been demonstrated at 90.1% and 96.2%, as shown in Figure 5i,j.The excellent performance of the SGM absorbers indicates their enormous potential for the practical applications of thermal solar energy collection and treatment.
To enhance the tunability of graphene-based MAs, combining with other tuning elements such as VO 2 and BP to form a hybrid scheme is an important and efficient method.Wang et al. [61] devised a THz MA, which is based on graphene and vanadium dioxide (VO 2 ).Through varying the Fermi level of graphene and the state of VO 2 , the MA can achieve mutual exchange between single band, dual band, and broadband, as shown in Figure 5c.When the Fermi level of graphene is at f = 0.9 eV and VO 2 is in a metallic-like state, MA demonstrates single-band absorption.In contrast, when VO 2 is in an insulated state, MA displays dual-band absorption of 0-0.84 and 3.4-4.64THz, respectively.In the case of that f = 0.1 eV and VO 2 is in a conductive state, MAs achieve broadband absorption of 0-6 THz through mutual coupling between graphene and VO 2 .
Liu et al. [62] proposed a dual tunable MA based on a VO 2 graphene hybrid structure, as shown in Figure 5k.The design provides a narrowband MAs when VO 2 is in a metallic-like state.Moreover, when VO 2 is in a dielectric state, the design serves as a broadband MA (with an absorption rate above 90% within the 0.335-1.275THz), as shown in Figure 5l.By adjusting the Fermi level of graphene from 0.1 to 0.7 eV, the absorption rate of the proposed MAs can be dynamically adjusted from 45.3% to 94.5% at 0.4 THz and from 31% to 96.3% at 1.0 THz, and the absorption bandwidth gradually widens, as shown in Figure 5m.Due to the unique dielectric metal transition characteristics of VO 2 , adjustable absorption frequency and intensity can be achieved through external stimuli.
Recently, BP has been considered another candidate for MA applications due to its metallic behavior within the IR and THz ranges.BP is a 2D material with excellent optical properties and an important component of multilayer hyperbolic metamaterials.As a direct bandgap 2D semiconductor material, BP bridges the characteristics of graphene with zero or near-zero bandgap and TMDCs with wide bandgap.Similar to graphene, the carrier density of BP can be adjusted through chemical doping or applying bias voltage, providing the possibility of modulating the absorption of BP-based MA.Electronic doping directly determines BP's surface conductivity, which subsequently impacts the effective dielectric constant and absorption properties of the entire MA composed of BP.
The incorporation of BP with graphene in MAs appears to be a promising option.Liang et al. [63] proposed a high-performance absorber with graphene-BP heterostructure, which resulted in significant anisotropic absorption in the THz range, as shown in Figure 6a.The absorption peak intensity can attain its maximum value (99.6%) by modifying the width and period of the nanoribbons and the Fermi level of graphene, as shown in Figure 6b.When the distance between graphene and BP nanoribbons is zero, the anisotropy is strongest, as shown in Figure 6c.These unique properties were not found in individual monolayer BP or monolayer graphene.This type of structure provides new ideas for high-performance 2D material plasma devices.Similarly, Cai et al. [64] proposed an infrared dual-band absorber based on periodic elliptical graphene-BP pairs, as shown in Figure 6e.This study integrates the benefits of graphene and BP to enable them to concurrently present robust anisotropic plasmon response.As shown in Figure 6f, for TE incidence, the two absorption peaks are observed at 8.8 and 14.1 μm.When TM waves are incident, the maximum absorption wavelengths shift to 9.5 and 15.4 μm, respectively, with the corresponding maximum polarization extinction ratios reaching 23 and 25 dB.Furthermore, Zhu et al. [65] proposed a tunable ultra-wideband absorber composed of multilayer BP/dielectric structures stacked on a gold mirror.This MAs can achieve almost perfect absorption efficiency in the wavelength range of 16-28 μm, as shown in Figure 6d.Meanwhile, thanks to the unique and universal properties of BP materials, absorption efficiency and bandwidth can be actively controlled through carrier doping or geometric parameters.When electron doping increases from 3 Â 10 13 to 6 Â 10 13 cm À2 , the absorption spectrum shifts toward shorter wavelengths.
Different from the method of combining BP with graphene mentioned before, Tang et al. [66] theoretically proposed a BP-Ag composite MA consisting of a continuous monolayer of BP sheets sandwiched between a circular silver ring and a dielectric layer stacked on a silver substrate, as shown in Figure 6g.The anisotropic properties of BP, ideal enhanced absorption in both the armchair and z-directions, resulting in  [58] Copyright 2021, The Authors, published by Optica Publishing Group.c) Simulated absorption of mixed graphene and VO 2 MA with different VO 2 conductivity (10 and 20 000 S m À1 ).The illustration shows a schematic diagram of mixed MA.Reproduced with permission. [61]Copyright 2021, Elsevier.d) Schematic diagram of the proposed graphene-loaded MA.Two subunits are arranged diagonally with different patch sizes.e) In the φ = 0, the absorption spectrum at the normal incidence point with different E F shows a large blueshift of the peak as E F increases.(d,e) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [59]Copyright 2019, The Authors, published by Springer Nature.f ) Schematic representation of the 3D structural graphene metamaterial (SGM) absorber.Inset: the structure of the graphene metamaterial.g) Measured reflectance and absorbance spectra of SGM absorbers having various hole widths and periods: that is, w/ p = 0.52/0.8μm (red line) and w/p = 0.57/1 μm (blue line).The red-dashed line and the blue-dashed line are the simulated absorbance spectra of the H2 and H3 SGM absorbers, respectively.h) Top-view SEM images of the SGM absorber at high magnification.i,j) Measured i) absorbance spectra and j) Raman spectra of SGM absorbers before, after 12 h, and after 24 h of heating at 100 °C in air.Inset: thermal images of SGM absorber under sunlight exposure in an open environment.(f-j) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https:// creativecommons.org/licenses/by/4.0). [60]Copyright 2020, The Authors, published by Nature Portfolio.k) Schematic diagram of VO 2 -graphene-mixed structure MA. l) The absorption rate of VO 2 as a metal (solid blue line) or dielectric (dashed red line).m) The effect of graphene with different Fermi levels on absorption efficiency.(k-m) Reproduced with permission. [62]Copyright 2021, Elsevier.(a-c) Reproduced with permission. [63]opyright 2020, IOP Publishing Ltd.d) The relationship between the absorption spectrum and wavelength of multilayer MA under different electron concentrations.Inset: schematic diagram of multilayer BP-stacking structure.Reproduced with permission. [65]Copyright 2019, IOP Publishing on behalf of the Japan Society of Applied Physics (JSAP).e) The structure of MA in elliptical graphene-BP pairs.Inset: structural schematic diagram of graphene and BP.f ) Absorption spectra between monolayer graphene, monolayer BP, and graphene BP pairs.(e,f ) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [64]Copyright 2019, The Authors, published by Springer Nature.g) The schematic diagram of BP-Ag composite MA, consisting of a continuous single layer of BP sheets sandwiched between a circular silver ring and a dielectric layer stacked on a silver substrate.Inset: structural schematic diagram of BP in the armchair direction and z direction.The absorption curves of h,i) under different electron doping affect polarization along the h) armchair direction and i) z direction.(g-i) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [66]Copyright 2020, The Authors, published by IEEE.j) Schematic diagram of hyperbolic MA stacked by BP/ dielectric layers.k,l) Under vertical incidence, in the proposed hyperbolic metamaterial, different electron-doped BP (n range from 1 Â 10 13 to 9 Â 10 13 cm À2 ), the anisotropic absorption spectrum of electric field E along the k) x and l) y directions.Inset: top view of the lattice structure of BP. (j-l) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [67]Copyright 2019, The Authors, published by IEEE.m) Schematic diagram of the proposed 2D BP sandwich structure MA. n) The absorption spectra of metamaterials with different thicknesses at different dielectric constants.o) Schematic diagram of transmission and reflection of a four-layer sandwich structure.(m-o) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [68]Copyright 2017, The Authors, published by Optica Publishing Group.
resonance absorption peaks at different wavelengths.When the circular ring and single-layer BP follow the direction of the armchair, they have a perfect absorption of nearly 99.8% at 16.5 μm, and when they follow the z direction, the absorption rate of 98.36% at 20.12 μm.As shown in Figure 6h,i, MAs were tuned by changing the electron doping of BP.With an increase in the electron doping amount from 7 Â 10 13 to 11 Â 10 13 cm À2 , the absorption spectra showed a slight blueshift in both polarization directions.As the effective mass of BP decreases, the resonance frequency increases.
In addition to the single-layer structure composed of BP/dielectric layers, multilayer stacked structures have also received great attention.Xiao et al. [67] designed a tunable MA with stacked cell patterns of BP/dielectric layers composed of multiple layers, as shown in Figure 6j.As shown in Figure 6k, in the x-polarization direction, when the electrondoping concentration increases from 1 Â 10 13 to 9 Â 10 13 cm À2 , the absorption peak of the structure gradually increases from 37.78% to 100%, reaches its peak at a concentration of 5 Â 10 13 cm À2 , and then gradually decreases to 81.58%.In the y-polarization direction, the concentration reaches its peak at 9 Â 10 13 cm À2 , as shown in Figure 6l.Similarly, Wang et al. [68] proposed a design aimed at infrared MA.The design features 2D BP metamaterials between dielectric layers with a multilayer sandwich MA structure installed on a total reflection gold mirror to create a Fabry-Perot resonant cavity for the enhancement of light-matter interaction, as shown in Figure 6m.Adjust the bandgap from 2 to 0.3 eV, with the bandgaps of 1.3 eV for double layer and 1.07 eV for three-layer BP, as shown in Figure 6n.As the bandgap decreases, the absorption peak position shows a blueshift, and the absorption peaks approach complete absorption of 100% in bilayer and three-layer BP metamaterials.
Molybdenum disulfide (MoS 2 ) is a representative 2D TMD material with unique optoelectronic and physical properties.However, the atomic thickness of single-layer MoS 2 is extremely thin, resulting in weak light absorption, which dramatically restricts its further application in the field of optoelectronic devices.To address this issue, numerous studies have been carried out to boost light absorption and strengthen the interaction between light and matter within single-layer MoS 2 .These include MoS 2 into metamaterial structures or using patterned MoS 2 .It is worth mentioning that gate voltage can dynamically regulate the carrier density of single-layer MoS 2 .This unique characteristic makes MoS 2 very advantageous for designing tunable absorbers based on metamaterials.Altering the carrier density via an external gate voltage or bias voltage changes MoS 2 's equivalent dielectric constant and surface conductivity, in turn modulating the absorption of MoS 2 -based MA.
Multiband MAs have always been a research hot spot, and tunable multiband MAs utilizing MoS 2 has attracted considerable attention in recent years.Zheng et al. [69] designed a six-band tunable THz MA, which consists of a MoS 2 or BP monolayer as a loss layer, a dielectric substrate, and a metal-insulator phase-change material VO 2 , as shown in Figure 7a.By simultaneously adjusting the thickness, conductivity, or carrier concentration of VO 2 , the absorption spectrum can be dynamically adjusted, as shown in Figure 7b,c.Under TE polarization with MoS 2 or BP as the loss layer, almost all absorption peaks at six frequencies is above 90% (up to 99.89%).Subsequently, the impact of VO 2 transformation between insulators and metals on MA was analyzed, which helped to increase the absorption rate of MA from 9% of MoS 2 and 18% of BP to almost perfect absorption, as shown in Figure 7d,e.Furthermore, the absorption characteristics of MA can be tuned by adjusting the carrier concentration and VO 2 height in addition to utilizing the phase transition characteristics of VO 2 .
Cai et al. [70] proposed a three-band THz MA with a MoS 2and graphene-layered structure, as shown in Figure 7f.Three absorption peaks occur at 0.6 THz (99.7%), 1.5 THz (95.4%), and 2.5 THz (99.5%).Altering the electromagnetic properties of MoS 2 and graphene enables tuning of the peak absorption frequency within a specific range.As shown in Figure 7g, their absorption characteristics were studied by stacking double-layer MoS 2 , single-layer MoS 2 and graphene, single-layer MoS 2 , and double-layer graphene.The stacked structures of different 2D materials offer innovative approaches for the design of THz MAs.
Similarly, Ge et al. [71] proposed an independently tunable double-layered molybdenum disulfide (MoS 2 ) MPA, as shown in Figure 7h.The absorber demonstrates three absorption peaks at frequencies of 17.55, 25.6, and 38.03 THz, with corresponding values of 99.92%, 99.83%, and 99.68%.To better distinguish the different structures in this design, the combination of MoS 2 nanorings (top MoS 2 layer), circular and cross-shaped single-layer MoS 2 layers (bottom MoS 2 layer), and the entire structure including top and bottom MoS 2 are referred to as MRS, MCD, and MRCD, respectively.By altering the density of the relevant MoS 2 layer's carrier, Figure 7i,j demonstrates that three absorption peaks can be tuned independently across a broad range of spectra, leading to a mid-infrared absorber with a high modulation depth.
Zhong et al. [72] proposed a tunable THz broadband absorber based on ring cross array patterned single-layer MoS 2 , as shown in Figure 7k.The combination of MoS 2 microring and MoS 2 microcross exhibits excellent broadband absorption characteristics, and continuous tuning can be achieved by adjusting the carrier concentration of MoS 2 by controlling the bias voltage, as shown in Figure 7l.Within the extensive absorption frequency range of 1.2-2.67THz, the absorption rate exceeds 90%, the absorption bandwidth, and relative absorption bandwidth are 1.47 THz and 76.0%, respectively.At the same time, the absorption effect of MoS 2 when replaced by other transition metal chalcogenides (TMDCs) (MoSe 2 , WS 2 , and WSe 2 ) was also studied, as shown in Figure 7m.As a result of the variances in the effective mass of the charge carriers, TMDCs display notable distinctions in the absorbance spectra.By replacing MoS 2 with MoSe 2 , wider absorbance can also be obtained.
Electrically modulated tunable MAs can control carrier concentration of 2D materials (including graphene, BP, MoS 2 ), through the application of a bias voltage.This method boasts faster response speeds than optical excitation and thermal radiation, and results in a greater selectivity of 2D materials.Consequently, this method offers distinct advantages in the design of tunable MAs.However, the majority of electrically modulated 2D material designs are restricted to numerical and theoretical calculations, with limited experimental verification and practical applications.(a-e) Reproduced with permission. [69]opyright 2022, Elsevier.f ) Schematic diagram of MoS 2 -and graphene-layered structure.g) Different absorption characteristic curves were obtained through different stacking and combination of MoS 2 and graphene.Inset: top view of MoS 2 -and graphene-layered structure.(f,g) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [70]Copyright 2023, The Authors, published by MDPI.h) Schematic diagram of a three-band perfect MA based on MoS 2 .The illustration shows the MoS 2 structural layer.i) The absorption spectrum of the MRCD structure as a function of the carrier density of the MRS structure.j) The absorption spectrum of the MRCD structure as a function of the carrier density of the MCD structure.(h-j) Reproduced with permission. [71]Copyright 2022, Elsevier.k) Schematic diagram of the designed ring cross array MoS 2 broadband tunable MA. l) Absorption spectra of the absorber under the conditions of uncoated monolayer MoS 2 , unloaded bias voltage (n = 1.105Â 10 10 cm À2 ) and different carrier concentrations from 1.0 Â 10 14 to 1.2 Â 10 15 cm À2 .The absorptivity at the three absorption peak frequencies under the most optimal condition (n = 1.0 Â 10 15 cm À2 ) are 99.96% (1.35 THz), 99.47% (1.7 THz), and 98.74% (2.5 THz), respectively.m) Absorption spectra of the absorber when MoS 2 was replaced by other transition metal disulfide compounds (MoSe 2 , WS 2 , and WSe 2 ).(k-m) Reproduced with permission. [72]Copyright 2021, Elsevier.

Other Modulation Methods
Furthermore, apart from the modulation techniques of optical excitation, thermal radiation, and electrical modulation mentioned previously, other modulation methods exist.In particular, Liu et al. [2] designed a tunable MA based on resonance strontium titanate artificial medium atoms.The absorber is composed of artificially engineered medium containing strontium titanate atoms as resonance and tunable units, which are arranged periodically to form a dielectric cube.This cube is embedded in the matrix material acrylonitrile butadiene styrene, and placed on a copper substrate.The dielectric atom strontium titanate is strongly coupled with the incident electric and magnetic fields, resulting in polarization insensitivity and wide incident angle absorption spectra.At room temperature of 9.56 GHz, the simulated and experimental absorption rates are 99% and 96%, respectively.As the temperature increases, the absorption frequency of the absorber gradually blueshifts.However, the absorption intensity at the operating frequency remains stable.

Design Principles and Methods of Absorbers
[75][76][77] By designing the metamaterial structure reasonably, the impedance of the absorber is matched with the impedance of the free space, thereby obtaining the best absorption effect. [78,79][82][83][84] Representing periodic patterns through the resistance-inductance-capacitance (RLC) circuit model, where the equivalent impedance (Z u ) can be described in Equation (1) [85][86][87] where R, C, and L are the equivalent resistance, capacitance, and inductance of periodic patterns, respectively.For dielectric spacers supported by a grounding plane, the equivalent impedance (Z t ) is expressed as [88] Z where Z 0 is the impedance of free space (377 Ω); μ r and ε r and μ 0 and ε 0 are the magnetic permeability and dielectric constant of the dielectric spacer and vacuum, respectively; ω is the angular frequency of the incident wave; and t is the thickness of the dielectric layer.Then, the input impedance (Z in ) of MA is given by Equation (3) When the metamaterial is not at the top of the structure, it is assumed that there is a metamaterial sandwiched between two distinct media.When two beams of light are perpendicular to the metamaterial from opposite directions, the scattering field can be expressed as [89] S where α and φ represent the relative intensity and phase difference of two beams, respectively; T 1 (T 2 ) and R 1 (R 2 ) are the transmittance and reflectance of two light beams, respectively, which are where n 1 and n 2 represent the refractive indices of the two media, respectively.Coherent absorption A coh can be written as Therefore, the conditions for achieving CPA are φ = 2 Nπ (N is an integer) and α = 1.

Optical Excitation
The most commonly used material in photoexcitation is semiconductor photoconductive silicon (Si), which has excellent optoelectronic properties, low cost, high quantum efficiency, and good compatibility.By changing the energy of the incident pump light, the carrier density of the silicon structure is changed.When the pump beam is incident on the surface of the structure, an excess carrier density can be generated provided that its energy surpasses the structure's bandgap energy.In this instance, the conductive state of the structure will change.In general, the pump beam used is 800 nm NIR light.
According to Beer Lambert's law, for an 800 nm pump beam, the pump flux decays exponentially along the propagation direction, with ðzÞ ¼ f 0 e Àαz .In this equation, f 0 is the fluence on the surface of silicon (z = 0), α (%1020 cm À1 ) is the absorption coefficient of the pump beam, and z is the depth in silicon.It is assumed that all photons that are absorbed will be converted into charge carriers while in silicon.The distribution of carrier density along the z-direction in silicon can be calculated using the following Formula (9) [47] n d ðzÞ ¼ where E p (=hν) is the photon energy of the pump beam, h is the Planck constant, and ν is the frequency of the pump beam.
According to the Drude model, the plasma frequency (ω p ) is given by w p ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi n d e 2 =ε 0 m eff p , where n d is the carrier density, e is the charge of the electron, ε 0 is the permittivity of free space, and m eff (=0.37m 0 , m 0 : mass of the electron) is the effective mass of the carrier in silicon.The relative impedance Z is obtained using the inversion method of S parameter.The most effective absorption for MAs is achieved when it matches the impedance of free space.Thus, the absorption rate of the structure is as follows

. Thermal Radiation
VO 2 : At room temperature, the VO 2 film operates in an insulated state.Upon heating, the temperature exceeds the phase transition temperature, resulting in structural transformation and the VO 2 in MPA becomes a metallic phase.The complex dielectric properties of VO 2 thin films in metallic and insulating phases can be described by the classic Drude-Lorentz model, which is represented by the following equation [104] εðωÞ Among the variables in this equation, ε ∞ is the constant contribution of high-frequency electronic transitions to the real part of the dielectric constant, ω n is the carrier density parameter, ω c is the collision frequency, S i is the strength, and Γ i is the line width.
In the third term of the aforementioned equation, due to the interband transition of bound electrons affected by temperature, the intensity S i and resonance frequency ω i and scattering rate Γ i /ω i is a temperature-dependent parameter.
The magnetic permeability and dielectric constant of MAs can be described as μðωÞ ¼ μ 0 μ r ðωÞ (13) where ϵ 0 and μ 0 are the dielectric constant and magnetic permeability of free space, respectively, μ 0 ¼ 4π Â 10 À7 Hm À1 and ε 0 ¼ 10 7 =ð4πc 2 Þ Fm À1 .In this case, the absorption rate can be written as where Z ¼ ffiffiffiffiffiffiffi μ=ε p is the impedance of the magnetic dielectric material, Z 0 ¼ ffiffiffiffiffiffiffiffiffiffiffi μ 0 =ε 0 p is the impedance in free space, and Z r ¼ ffiffiffiffiffiffiffiffiffiffi ffi μ r =ε r p is the relative impedance of the magnetic dielectric material.The effective refractive index and impedance coefficient of metamaterials can be obtained by the S-parameter inversion method. [105]eff ¼ Therefore, when Z r ¼ 1, the absorption reaches its maximum, which means ε r ¼ μ r .Therefore, we can adjust the structure to ε r and μ r is close to each other, thus achieving good absorption effect.
InSb: The dielectric constant ε(ω) of InSb can be described according to the Drude model [106,107] N ¼ 5.76 Â 10 20 T 1.5 exp À 0.13 where ε ∞ , ω, and γ represent the dielectric constant, angular frequency, and damping constant of the high-frequency body, respectively; ω p represents the plasma frequency, which is related to the intrinsic carrier density N, electron charge e, vacuum permittivity ε 0 , and the effective mass m* of the free carrier.The intrinsic carrier density N is strongly affected by temperature T. k B is Boltzmann's constant.Here, m* = 0.015m e , where m e is the mass of the electron.For InSb, γ = 0.1π THz, ε ∞ = 15.68.When the temperature is fixed, the intrinsic carrier density N can be obtained, and the plasma frequency ω p can be calculated.Thus, the dielectric constant of InSb is temperature dependent.The properties of InSb also change in the presence of an external magnetic field B. When a magnetic field B is applied in the x direction, the available resistivity of InSb is replaced by the dielectric constant ε(ω), which can be expressed as [108,109] ε ¼ The cyclotron frequency ω c is related to the applied magnetic field B, the electron charge e, and the effective mass m* of the free carrier.Thus, the permittivity of InSb may be adjusted by altering the applied magnetic field B.
The absorption mechanism of MAs based on InSb can also be acquired by utilizing the impedance matching theory.The relative impedance Z can be obtained from the S-parameter inversion method as follows When the transmission T(ω) = 0, the absorption rate of MAs can be expressed as

Electrical Modulation
Ideal MAs based on MoS 2 can be dynamically tuned by varying the carrier density of monolayer MoS 2 , making it a desired feature for practical applications.The conductivity of a single-layer MoS 2 can be described using a Drude-like model [110,111] σðωÞ where e is the electron charge, m* = 0.53m e indicates the effective mass of the carrier, and m e is the mass of the electron, τ is the carrier relaxation time, and n is the carrier density, τ = 0.17 ps.
According to the aforementioned formula, it can be seen that the conductivity has a linear relationship with the carrier density.Therefore, the conductivity of monolayer MoS 2 can be controlled efficiently by changing the carrier density.When the MoS 2 structure is located between two insulating layers, its effective refractive index can be expressed as [112] n ε 1 and ε 2 is the relative dielectric constant of the insulation layer above and below the single-layer MoS 2 , respectively.k 0 is the wave vector in free space.
To characterize the tunability and reconfigurability of MAs, the relative tuning range (W RTR ) criterion is introduced, which can be calculated by the following equation [113][114][115][116] where f max , f min , and f c is the maximum, minimum, and center frequency of %10 dB reflection or 90% absorption, respectively; W RTR represents the ability of MA to change operating frequency; and a larger value indicates wide range tunability.Under the guidance of the tuning mechanism and theory mentioned previously, it is possible to achieve tunable MAs with optical excitation, thermal radiation, and electrical modulation.The subsequent sections will elaborate on the preparation methods and process flow of MAs.

Preparation Method and Process Flow of Absorber
The development and practical application of metamaterials has always been constrained by the preparation of absorbers.Material selection is determined during the MAs design phase.Therefore, it is crucial to consider experimental processing conditions and the true electromagnetic characteristics of the materials in the design.According to the selection of materials in the design, the preparation methods for metallic MAs and dielectric MAs are different.
For MAs in the microwave band, the unit size is relatively large, usually in the millimeter range.Employing printed circuit board process, microwave and millimeter wave MAs with complex structures and high performance can be prepared.In addition, the printed circuit board process can also achieve the design and preparation of multilayer structures, further improving the performance of MAs.Therefore, the printed circuit board process is one of the preferred options for preparing metamaterials in the microwave and millimeter wave bands.In 2019, Feng et al. [117] Each metamaterial unit consists of a rotationally symmetric open resonant ring (metal clad copper layer, conductivity 5.8 Â 10 7 S m À1 , thickness 0.018 mm), flame retardant 4 (FR4) dielectric substrate (thickness h = 0.8 mm), and a pure metal floor.The MAs were prepared using a printed circuit board process, as shown in Figure 8a-d.
In the THz and high-frequency bands, metamaterials require smaller line widths and higher accuracy to meet higherfrequency demands.In this case, the printed circuit board (PCB) process is no longer able to meet the requirements due to its large machining accuracy and line width limitations.Conversely, photolithography technology can attain smaller line widths, achieving higher machining accuracy.The lithography process uses photoresist and lithography machines for manufacturing, which can produce smaller and more complex metamaterial unit structures.Photolithography technology, in addition, results in higher machining accuracy and better repeatability, leading to better performance and stability of metamaterials.As such, using photolithography technology is a superior choice for producing metamaterials in the THz and high-frequency bands.
In 2021, Wang et al. [118] designed a metal-type planar device with a square ring structure that enables three-band narrow-band absorption.As shown in Figure 8e-g, the experimental structure layer uses metal aluminum, and the dielectric layer uses polyethylene terephthalate (PET) film with a thickness of 13 μm.In the preparation process, first of all, the PET-aluminum film is fixed on the quartz substrate with ethanol to facilitate subsequent preparation.Great care should be taken during the fixing process to avoid damaging the surface aluminum film, and prevent  [118] Copyright 2021, The Authors, published by MDPI.e) Photoetching process preparation flowchart.f ) Optical microscope images.g) Simulated absorption, reflection, and transmission spectra.h-j) MA prepared based on photolithography and etching process.(h-j) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [120]Copyright 2019, The Authors, published by IEEE.(h) Photoetching process preparation flowchart.(i) SEM image of the structure.(j) The absorption spectrum of MA. k-m) MA prepared based on 3D printing.Reproduced with permission. [121]Copyright 2018, American Physical Society.bubbles from forming between the film and the quartz substrate.In the second step, a 60 Â 60 cycle structure is prepared on the photoresist using the photolithography process.The third step involves transferring the pattern from the photoresist to the aluminum film using an aluminum etch solution with a ratio of HPO 4 :CH 3 COOH: HNO 3 :H 2 O = 16:1:1:3.Finally, the photoresist is eliminated and an aluminum film is deposited onto the opposite side of the PET as the bottom metal layer.The graphics produced by the corrosion process exhibit great precision; however, the bond between the metal layer and substrate is weak, and the metal structure can be displaced or detached with ease when subjected to external forces.
By combining photolithography with other processes, more diverse structures can often be prepared.In 2013, Wilbert et al. [119] prepared a narrowband MAs using photolithography stripping process and electron beam (EB) metal deposition coating method.A layer of metallic copper is first deposited on a silicon substrate.Second, by spin coating a layer of polyimide film on the copper plating surface, the rotation speed and time can be controlled to control the film thickness.Afterward, the reverse pattern of the target pattern is produced on the surface of the polyimide by UV lithography.The next step is to deposit a 200 nm copper film.Before depositing the copper film, a connecting layer of several nanometers can be precoated to improve the connection strength between copper and polyimide.Lastly, using acetone, excess metal is peeled off.The stripping method requires higher process requirements compared to the corrosion method, but the metal layer produced by the stripping process is more tightly connected to the substrate.In 2018, Yuan et al. [120] used photolithography technology and inductively coupled plasma (ICP)-etching process to prepare ultra-wideband THz absorbers on doped silicon materials.Figure 8h-j shows the preparation process of MAs using ICP etching in dry etching.Due to the double-layer structure, it requires twice photolithography and etching processes to be fully prepared.
With the continuous progress of preparation technology, the application of 3D printing technology to the preparation of metamaterials has become a hot topic in recent years.In 2021, Shen et al. [121] proposed the preparation of ultra-wideband THz MAs based on 3D printing technology.Figure 8k-m presents a schematic diagram of a simple three-step manufacturing process for this MAs.First, an EB deposition method was used to deposit 10 nm thick Cr as a connecting layer on a clean quartz substrate, followed by a 200 nm thick Au layer.The connecting layer can enhance the adhesion between the quartz substrate and the metal Au layer.Second, the resin material is printed layer by layer on Cr/Au layers using surface projection photocuring 3D printing technology.Finally, an Au layer with a thickness of 200 nm was deposited on the printed structural surface by utilizing the EB deposition method.

Conclusion and Future Prospects
In summary, the research on MAs has generated considerable interest in the whole field of electromagnetic devices due to their subwavelength size, flexible design, and nearly perfect absorption.Meanwhile, by integrating active materials into the design of MAs, traditional passive devices can have tunable absorption performance.In this review, various materials and design methods have been emphasized to achieve tunable absorption of MAs across the frequency range from microwave to visible light.Table 1 summarizes different methods for achieving tunable MAs, and compares available data to provide representative reference reports, including external excitation, operating frequency, W RTR , different bandwidths, and switching speeds.
For light excitation, using photoconductive semiconductor silicon, its response speed is faster.Simply irradiate the pump light source onto the surface of the metamaterial and change the carrier density of the silicon structure by changing the incident pump light energy.When the energy of the pump light exceeds the bandgap energy of the structure, an excess carrier density can be generated.However, there are some limitations, as the carrier concentration of optical excitation all dielectric silicon decreases continuously with the increase of silicon wafer depth, making it impossible to obtain a structure with uniform carrier concentration.In addition, the equipment for incident pump light is relatively large, which has significant limitations in practical applications.Thermal radiation primarily alters the electromagnetic properties of phase-change materials by changing their temperatures.Thermal radiation does not require a large incident light apparatus as a tuning element, and the excitation conditions are more straightforward.At the same time, there are various types of thermal radiation phase-change materials, and suitable materials can be selected for various bands and applications.At the same time, there are also some limitations to the method of thermal radiation, such as the fact that phase-change materials only change their state when they reach their own phase-change temperature, and the time required for temperature control is long, making it impossible to achieve real-time control similar to a "switch".Electrically tunable MAs are capable of controlling the carrier concentration of 2D materials (including graphene, BP, and MoS 2 ) by applying bias voltage.The response speed of electrical modulation is faster, and the selectivity of 2D materials is more, making it have obvious advantages in the design of tunable MAs.However, most available designs based on electrically modulated 2D materials are restricted to numerical and theoretical calculations, with limited experimental verification and practical applications.Based on the previous analysis, it is evident that each method has its own advantages and disadvantages, as shown in Figure 9.It is essential for researchers to assess expenses, manufacturing, and applications to make appropriate choices.
In the coming years, with the deepening of research, tunable MAs will be designed more comprehensively.The difficulties and shortcomings discovered in research process will be addressed sequentially.It is anticipated that, supported by forthcoming technology, MAs will attain intelligent tuning.Improve methods progressively, such as implementing light excitation, thermal radiation, electrical modulation, and other forms of modulation, to streamline the design while reducing structural complexity and operational costs, as well as to broaden the range of applications for MAs.Researchers' sustained efforts will facilitate further enhancements and radical advancements in tunable MAs, enabling their extensive implementation throughout various fields.

Figure 1 .
Figure 1.a) The periodic structure of optical switch broadband metamaterial absorber (MA) and b) its absorption spectrum in the absence and presence of light (σ si = 1 S m À1 , σ si = 1.5 Â 10 5 S m À1 ).(a,b) Reproduced with permission.[46]Copyright 2021, Elsevier.c) Schematic of all-dielectric broadband terahertz (THz) metamaterial perfect absorber (MPA).Inset: effective medium theory (EMT) model of the MPA, in which the metamaterial (MM) layer is considered as a homogeneous thin film.d) The scanning electron microscope (SEM) image of the all-silicon MPA.Inset: false color SEM view of the unit cell.The green part is the etched silicon and the purple part is the silicon substrate.e) The reflectance and absorbance spectrum of the fabricated MPA, comparing EMT and finite difference time domain with experiment.f ) Optical-pump THz-probe (OPTP) spectra of the all-dielectric MPA for different pump fluences.Inset: incident direction of the beams, with the optical pump beam incident at a normal angle and the THz probe beam is at a 30°angle of incidence.(c-f ) Reproduced with permission.[47]Copyright 2019, America Chemical Society.g) The absorption, reflectance, and transmittance of MA.Inset: structural schematic diagram of all dielectric tunable MA. h) Absorption spectra under pump light regulation of different energy densities.(g,h) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[48]Copyright 2022, The Authors, published by Chinese Laser Press.i) Schematic representation of the fabricated tunable perfect absorber.Illustration: An optical microscope image of the unit cell.OPTP time domain spectroscopy measurement results for the j) absorption spectrum of the fabricated device for various optical pump powers and k) the change in absorption intensity as a function of pump power at 0.7 and 1.1 THz.(i-k) Reproduced with permission.[49]Copyright 2014, Advanced Optical Materials.

Figure 2 .
Figure 2. a) Schematic diagram of the relationship between the conductivity and temperature of the VO 2 MA designed.Inset: schematic diagram of the designed multilayer VO 2 structure.Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[50]Copyright 2020, The Authors, published by IEEE.b) The absorption curve of this VO 2 MA at temperatures of 50 and 80 °C.Illustration: schematic diagram of VO 2 MA for this design.Reproduced with permission.[51]Copyright 2017, Elsevier.c) Schematic diagram of a switchable broadband absorber.d) At 300 K, VO 2 can switch the absorption spectrum of the absorber.e) At 350 K, VO 2 can switch the absorption spectrum of the absorber.(c-e) Reproduced with permission.[52]Copyright 2018, Elsevier.f ) Schematic diagram and tuning mechanism of micro-electro-mechanical system (MEMS)-based MPA design.g) Schematic diagram of MEMS-based MPA for CO 2 -gas-molecular-sensing function.The absorption rate of MEMS-based MPA with different applied DC voltages in h) transverse electric (TE) and i) transverse magnetic (TM) modes.j) The relationship between applied DC voltage and absorption intensity at a wavelength of 2.70 μm.(f-j) Reproduced with permission.[53]Copyright 2022, RSC Pub.

Figure 3 .
Figure 3. a) The periodic structure and unit structure diagram of the proposed InSb-based MA. b) Simulated absorption spectra of InSb thin-film structure.Inset: top view of InSb metamaterial structure.(a,b) Reproduced with permission.[54]Copyright 2022, Elsevier.c) Comparison of absorption spectra of pure InSb materials, proposed MA and perfect electric conductor (PEC) substrates with Au while external ambient temperature of T = 285 K. d) The absorption spectra of the designed MA at various temperatures (T ).(c,d) Reproduced with permission.[55]Copyright 2020, Elsevier.e) The designed all-media MA periodic array based on InSb.f ) The resonant frequencies f 1 (black square) and f 2 (red circle), and the corresponding linear fit curve as a function of T; g) the corresponding linear fit curve as a function of the surrounding analyte n. (e-g) Reproduced with permission.[56]Copyright 2022, Elsevier.

Figure 4 .
Figure 4. a) Scheme of the reconfigurable phase-change hyperbolic metamaterial (HMM) absorber, where the structure is composed of Au-GST225 (germanium antimony tellurium [GST]) stacked layers.b) Focused ion beam image of the cross section of the HMM absorber.c) Absorptance spectra of the crystalline-HMM under the various laser energies of 0, 8.4, 18.1, 29.0, 34.3, 40.8, 48.3, and 55.2 mJ.(a-c) Reproduced under the terms of the CC-BY-NC Creative Commons Attribution 3.0 International license (https://creativecommons.org/licenses/by/3.0).[27]Copyright 2021, The Authors, published by Royal Society of Chemistry.d) Schematic of the broadband and tunable metamaterial absorber (MA), which consists of semi-ellipsoidal shaped aluminum (Al) nanoparticles array in hexagonal lattice and a continuous Al mirror separated by an ultrathin GST film.e) Measured absorption spectra of the broadband MAs fabricated with five different annealing temperatures.f ) Fabrication process of the proposed broadband and tunable MA. g) Tiltedview and h) side-view SEM images of the fabricated absorber samples and part of the aluminum oxide (AAO) mask left on it.(d-h) Reproduced with permission.[57]Copyright 2020, RSC Pub.

Figure 5 .
Figure 5. a) Broadband THz graphene MA. b) The absorption spectra of the absorber at different Fermi levels of graphene from 0 to 0.7 eV.(a,b) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/ 4.0).[58]Copyright 2021, The Authors, published by Optica Publishing Group.c) Simulated absorption of mixed graphene and VO 2 MA with different VO 2 conductivity (10 and 20 000 S m À1 ).The illustration shows a schematic diagram of mixed MA.Reproduced with permission.[61]Copyright 2021, Elsevier.d) Schematic diagram of the proposed graphene-loaded MA.Two subunits are arranged diagonally with different patch sizes.e) In the φ = 0, the absorption spectrum at the normal incidence point with different E F shows a large blueshift of the peak as E F increases.(d,e) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[59]Copyright 2019, The Authors, published by Springer Nature.f ) Schematic representation of the 3D structural graphene metamaterial (SGM) absorber.Inset: the structure of the graphene metamaterial.g) Measured reflectance and absorbance spectra of SGM absorbers having various hole widths and periods: that is, w/ p = 0.52/0.8μm (red line) and w/p = 0.57/1 μm (blue line).The red-dashed line and the blue-dashed line are the simulated absorbance spectra of the H2 and H3 SGM absorbers, respectively.h) Top-view SEM images of the SGM absorber at high magnification.i,j) Measured i) absorbance spectra and j) Raman spectra of SGM absorbers before, after 12 h, and after 24 h of heating at 100 °C in air.Inset: thermal images of SGM absorber under sunlight exposure in an open environment.(f-j) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https:// creativecommons.org/licenses/by/4.0).[60]Copyright 2020, The Authors, published by Nature Portfolio.k) Schematic diagram of VO 2 -graphene-mixed structure MA. l) The absorption rate of VO 2 as a metal (solid blue line) or dielectric (dashed red line).m) The effect of graphene with different Fermi levels on absorption efficiency.(k-m) Reproduced with permission.[62]Copyright 2021, Elsevier.

Figure 6 .
Figure 6.a) Schematic diagram of graphene-black phosphorus (BP) heterostructures.b) Simulated absorption spectra of graphene-BP structures along the armchair and z directions.Inset: schematic cross section of the structure.c) Absorption spectra at different Fermi levels.(a-c)Reproduced with permission.[63]Copyright 2020, IOP Publishing Ltd.d) The relationship between the absorption spectrum and wavelength of multilayer MA under different electron concentrations.Inset: schematic diagram of multilayer BP-stacking structure.Reproduced with permission.[65]Copyright 2019, IOP Publishing on behalf of the Japan Society of Applied Physics (JSAP).e) The structure of MA in elliptical graphene-BP pairs.Inset: structural schematic diagram of graphene and BP.f ) Absorption spectra between monolayer graphene, monolayer BP, and graphene BP pairs.(e,f ) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[64]Copyright 2019, The Authors, published by Springer Nature.g) The schematic diagram of BP-Ag composite MA, consisting of a continuous single layer of BP sheets sandwiched between a circular silver ring and a dielectric layer stacked on a silver substrate.Inset: structural schematic diagram of BP in the armchair direction and z direction.The absorption curves of h,i) under different electron doping affect polarization along the h) armchair direction and i) z direction.(g-i) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[66]Copyright 2020, The Authors, published by IEEE.j) Schematic diagram of hyperbolic MA stacked by BP/ dielectric layers.k,l) Under vertical incidence, in the proposed hyperbolic metamaterial, different electron-doped BP (n range from 1 Â 10 13 to 9 Â 10 13 cm À2 ), the anisotropic absorption spectrum of electric field E along the k) x and l) y directions.Inset: top view of the lattice structure of BP. (j-l) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[67]Copyright 2019, The Authors, published by IEEE.m) Schematic diagram of the proposed 2D BP sandwich structure MA. n) The absorption spectra of metamaterials with different thicknesses at different dielectric constants.o) Schematic diagram of transmission and reflection of a four-layer sandwich structure.(m-o) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[68]Copyright 2017, The Authors, published by Optica Publishing Group.

Figure 7 .
Figure 7. a) Structural schematic diagram of MA composed of 2D materials.Absorption spectra of b) MoS 2 and c) BP σ = 1 Â 10 14 cm À2 (black line) and σ = 1 Â 10 13 cm À2 (red line), n = 2000 S cm À1 under TE polarization.d) The absorption spectra of MA calculated by MoS 2 and e) BP under TE polarization, n = 2000 S cmÀ1 (black line) and n = 10 S cm À1 (red line), with carrier concentration of σ = 1 Â 10 14 cm À2 .(a-e) Reproduced with permission.[69]Copyright 2022, Elsevier.f ) Schematic diagram of MoS 2 -and graphene-layered structure.g) Different absorption characteristic curves were obtained through different stacking and combination of MoS 2 and graphene.Inset: top view of MoS 2 -and graphene-layered structure.(f,g) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[70]Copyright 2023, The Authors, published by MDPI.h) Schematic diagram of a three-band perfect MA based on MoS 2 .The illustration shows the MoS 2 structural layer.i) The absorption spectrum of the MRCD structure as a function of the carrier density of the MRS structure.j) The absorption spectrum of the MRCD structure as a function of the carrier density of the MCD structure.(h-j) Reproduced with permission.[71]Copyright 2022, Elsevier.k) Schematic diagram of the designed ring cross array MoS 2 broadband tunable MA. l) Absorption spectra of the absorber under the conditions of uncoated monolayer MoS 2 , unloaded bias voltage (n = 1.105Â 10 10 cm À2 ) and different carrier concentrations from 1.0 Â 10 14 to 1.2 Â 10 15 cm À2 .The absorptivity at the three absorption peak frequencies under the most optimal condition (n = 1.0 Â 10 15 cm À2 ) are 99.96% (1.35 THz), 99.47% (1.7 THz), and 98.74% (2.5 THz), respectively.m) Absorption spectra of the absorber when MoS 2 was replaced by other transition metal disulfide compounds (MoSe 2 , WS 2 , and WSe 2 ).(k-m) Reproduced with permission.[72]Copyright 2021, Elsevier.

Figure 8 .
Figure 8. a-d) MA prepared based on printed circuit board technology.Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [117]Copyright 2015, The Authors, published by Hindawi.a) Schematic diagram of unit structure.b) 40 Â 40 (200 mm Â 200 mm) unit arrangement.Illustration: 2 Â physical diagram of two-cycle structure.(c,d) Navy Research Laboratory arch is used to measure the absorption of MA at different incident angles.c) Front view and d) side view of the experimental device.Inset: an enlarged view of MA in the measurement.e-g) MA prepared based on photolithography-etching process.Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[118]Copyright 2021, The Authors, published by MDPI.e) Photoetching process preparation flowchart.f ) Optical microscope images.g) Simulated absorption, reflection, and transmission spectra.h-j) MA prepared based on photolithography and etching process.(h-j) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[120]Copyright 2019, The Authors, published by IEEE.(h) Photoetching process preparation flowchart.(i) SEM image of the structure.(j) The absorption spectrum of MA. k-m) MA prepared based on 3D printing.Reproduced with permission.[121]Copyright 2018, American Physical Society.(k) 3D printing preparation flowchart.(l) Schematic diagram of 3D printing technology based on projection microstereolithography. (m) Experimental and simulated absorption spectra of ultra-wideband THz absorbers.Inset: SEM images of ultrawideband THz MA prepared.
Figure 8. a-d) MA prepared based on printed circuit board technology.Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0). [117]Copyright 2015, The Authors, published by Hindawi.a) Schematic diagram of unit structure.b) 40 Â 40 (200 mm Â 200 mm) unit arrangement.Illustration: 2 Â physical diagram of two-cycle structure.(c,d) Navy Research Laboratory arch is used to measure the absorption of MA at different incident angles.c) Front view and d) side view of the experimental device.Inset: an enlarged view of MA in the measurement.e-g) MA prepared based on photolithography-etching process.Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[118]Copyright 2021, The Authors, published by MDPI.e) Photoetching process preparation flowchart.f ) Optical microscope images.g) Simulated absorption, reflection, and transmission spectra.h-j) MA prepared based on photolithography and etching process.(h-j) Reproduced under the terms of the CC-BY Creative Commons Attribution 4.0 International license (https://creativecommons.org/licenses/by/4.0).[120]Copyright 2019, The Authors, published by IEEE.(h) Photoetching process preparation flowchart.(i) SEM image of the structure.(j) The absorption spectrum of MA. k-m) MA prepared based on 3D printing.Reproduced with permission.[121]Copyright 2018, American Physical Society.(k) 3D printing preparation flowchart.(l) Schematic diagram of 3D printing technology based on projection microstereolithography. (m) Experimental and simulated absorption spectra of ultra-wideband THz absorbers.Inset: SEM images of ultrawideband THz MA prepared.

Table 1 .
Summary of various approaches used to create tunable MAs.