A Review: Active Tunable Terahertz Metamaterials

The diversity and practicability of terahertz metamaterials have experienced rapid development in the past decade due to the increasing demand for various devices. This topic has attracted significant interest from researchers. Among the key functional devices in terahertz metamaterial systems, the active control ability of terahertz metamaterials is highly valuable and captivating. This implies that the electromagnetic properties of metamaterials can be modulated over a wide dynamic range by external stimuli. This review categorizes the different types of tunable terahertz metamaterials based on the external stimuli to which they respond, namely, mechanical modulation, electrical modulation, magnetic modulation, and optical modulation. Mechanically modulated devices offer simple yet efficient modulation, while electrical and magnetic modulation provide effective active modulation through electrical mechanisms. Optical modulation, in contrast, focuses on incorporating various materials to achieve active modulation.


Introduction
Terahertz waves are generally defined as electromagnetic waves in the frequency range from 0.1 to 10 THz.With the development of high-power terahertz sources and high-sensitivity detectors, [1][2][3] terahertz technology has a great potential for broad applications in wireless communication, imaging, material spectrum, and biomedical detection. [4,5]However, efficient devices such as isolators, [6][7][8] modulators, [9][10][11] and polarizers [12,13] are in high demand for the further development of terahertz application systems.Conventional terahertz materials often suffer from issues such as efficiency, bandwidth, and tunability.As a result, recent research has focused on novel artificial materials known as metamaterials.
Metamaterials, particularly 2D materials, also named as metasurfaces, demonstrate novel properties in manipulating the phase, amplitude, and polarization of electromagnetic waves.Unlike traditional devices, metasurfaces are formed by subwavelength unit arrays.By strategically placing these unit cells to form the desired phase map, metasurfaces can achieve functions such as anomalous reflection/ refraction, [14] flat lenses, [15,16] holograms [17] and more, based on the generalized Snell's law. [18]However, most of these metamaterials are limited in bandwidth in the terahertz regime due to the incomplete frequency response of fixed structures and materials.
To overcome the limitations of passive and static systems, dynamic control of electromagnetic waves through switchable metamaterial designs is a viable solution.This topical review focuses on terahertz metamaterials with electromagnetic wave dynamic control, specifically the strategies and characteristics of active modulation.Four methods for realizing the active modulation of terahertz metamaterial devices have been identified: mechanical modulation, electrical modulation, magnetic modulation, and optical modulation.Section 2-5 of this article introduce the details of the basic regulatory mechanism, basic structural design, and best research results for each category, respectively.Figure 1 provides an overview of these four modulation mechanisms and their internal classification criteria.

Mechanically Tunable Terahertz Metamaterials
Mechanical control methods for achieving tunable terahertz metamaterials involve applying mechanical forces, such as stretching or straining, to change the geometry of the subwavelength unit arrays in the metamaterials and the spacing of adjacent unit cells.In the early days, one classical method for achieving mechanical tuning of the electromagnetic response was by fabricating cantilever legs on both sides of the metamaterial metal structure. [19]Another mechanical control method involves loading a dielectric or liquid onto the surface of metamaterials. [20,21]ith the development of new polymer materials, lithography technology, and microelectromechanical system (MEMS) technology, two new mechanically tunable terahertz metamaterials have been reported.One approach is based on flexible materials as the substrate to achieve mechanical deformation control, while the other approach utilizes MEMS technology.These advancements have allowed for more precise and efficient mechanical control of terahertz metamaterials.

Mechanically Tunable Terahertz Metamaterials Based on Flexible Substrates
Flexible electronic technology involves attaching inorganic or mechanical components to a flexible substrate to create circuits.Unlike traditional silicon-based electrons, flexible electrons refer to thin-film electronic devices that can withstand bending, folding, twisting, compressing, stretching, and transforming into various shapes while still maintaining efficient optoelectronic performance, reliability, and integration.Commonly used flexible substrate materials include polymer compounds such as polyvinyl alcohol (PVA), polyimide (PI), polydimethylsiloxane (PDMS), and polyethylene glycol naphthalene dicarboxylate (PEN).Achieving tunable metamaterials through flexible substrates involves manufacturing a superatomic array on a flexible substrate and mechanically controlling the substrate to change the superatomic structure and achieve tunability.This approach allows for the manipulation of the metamaterial's properties by altering the geometry and arrangement of the superatomic units through mechanical deformation of the flexible substrate.By utilizing flexible substrates, tunable metamaterials can be designed to adapt to different shapes and deformations, expanding their potential applications in various fields.
In 2008, Tao et al. fabricated an ultrathin plane terahertz metamaterial sample on an independent PI substrate, which naturally rolled into a cylinder. [22]In the next year, Azad et al. demonstrated that reducing the thickness of the PI substrate, thereby reducing the spacing between adjacent open ring resonator layers, generated a negative dose-response. [23]In 2012, an optical pressure sensor designed by Ramuz et al. was reported that demonstrated the stretchability and transparency of PDMS substrates. [24]In the same year, Lee et al. incorporated stretchable wrinkles into PDMS substrates to adjust the resonance frequency of their terahertz metamaterials, and the experimental results demonstrated the reversibility of stretching smooth the wrinkles. [25]In addition, PEN has also been utilized to fabricate terahertz metamaterials as substrates. [26,27]These reports not only made breakthroughs in this field at the earliest but also confirmed the mechanical durability, flexibility, reversibility, and enormous potential of these flexible materials in the preparation of metamaterials.In 2013, Li et al. focused on the recoverability of the tuning mechanism.By mechanically stretching the terahertz metamaterial samples with a PDMS substrate, the resonance frequency was adjusted to 8.3% and fully restored during the relaxation process, which also enabled repeated deformation measurements without degradation. [28]The two designed metamaterial unit structures are shown in Figure 2a,b.The THz-TDS system was used for measurement in the experiment, with a 65 μm discrete step size, and the stretching experiment was conducted at Δl 0 (≈1% of the initial sample length).The results are shown in Figure 3, which shows the tunable results.As shown in Figure 4, to characterize the reversibility and repeatability of the mechanically tunable metamaterials, the stretching and releasing processes were repeated multiple times.Each metamaterial sample was stretched to 10% with a strain step of 1% and then relaxed back to its initial position with the same discrete step.After completing the stretching cycle shown in Figure 4a,b, the resonance frequency of the two structures returned to near the initial value with only a small hysteresis loop, indicating that the resonator and substrate had almost perfectly recovered to their initial positions.In the same year, they reported another fourfold symmetric design of flexible metamaterials that was able to be applied to biaxial strain sensing, as shown in Figure 2c,d. [29]By applying tensile strain in two orthogonal directions, the resonance frequency in a single structural unit was independently adjusted.
Most of the tunable metamaterials discussed previously rely on the shift in resonance frequency of the structure.
The plasmon-induced transparency (PIT) phenomenon, in contrast, involves the active control of coupling effects in elastic media.PIT occurs when two different modes, namely, the light mode and the dark mode, interfere with each other to eliminate system damping, resulting in a sharp transparent window. [30]his phenomenon bears a striking resemblance to electromagnetically induced transparency (EIT) and can be studied in a similar manner.35] Wang et al. demonstrated the active PIT phenomenon by manipulating the single/double layer metamaterial they designed through stretching the PDMS substrate. [36]The metamaterial structure they used was an adjustable open resonant ring.In the case of the single-layer metamaterial, the electric resonance of the U-shaped resonator and the electric dipole resonance of the metal strip were excited, resulting in a transparent window at a frequency of 1.43 THz through bright-bright coupling.In contrast, the double-layer metamaterials excited the dipole resonance of the upper layer as a bright mode, and through near-field coupling, the magnetic resonance of the lower layer pair was induced to form a dark mode.Through the coupling between the bright and dark modes, a PIT peak was generated at 1.04 THz, which could be modulated in combination with the mechanical stretching of the PDMS substrate in the structure.

Mechanically Tunable Terahertz Metamaterials by MEMS
MEMS refers to miniaturized devices or combinations of devices that integrate electronic functions with mechanical, optical, or Reproduced with permission. [28]Copyright 2013, AIP publishing.The unit structure in both views c,d); their structural parameters are a ¼ 78 μm, s ¼ 47 μm, e ¼ 9 μm, Reproduced with permission. [29]Copyright 2013, Optica Publishing Group.The structure stretches from 0 to 10 times the step size Δl 0 .Reproduced with permission. [28]Copyright 2013, AIP publishing.Reproduced with permission. [28]Copyright 2013, AIP publishing.
other functions into a comprehensive integrated system.These devices utilize microstructures to achieve intelligent capabilities within a very small space, and their mechanical structures can be altered by external stimuli.MEMS is an emerging discipline that encompasses various technical disciplines, such as precision machinery, microelectronic material science, micromachining, systems and control, as well as fundamental disciplines, such as physics, chemistry, mechanics, and biology. [37]Unlike traditional macroscopic mechanical systems, MEMS devices typically have sizes ranging from millimeters to microns.The extremely high machining accuracy at this scale enables the fulfillment of requirements for various devices operating in the terahertz frequency band.
In 2017, Cong et al. [38] demonstrated a tunable metalinsulator-metal (MIM) metamaterial loaded with MEMS, which achieved the dynamic regulation of the resonance of the active control of the cantilever by applying different voltages on the MEMS to control the cantilever angle.The designed unit structure is shown in Figure 5a.They adopted two states, "On" and "Off", to represent different cantilever control conditions.In the experiment, the amplitudes under the reflection mode were measured, and the bipolar resonance at 1.34 THz was obtained under the "Off" state.After switching to "On" mode, a resonance at 0.74 THz is obtained.In addition, the suspension angle of the cantilever was gradually changed by applying different voltages in the experiment to adjust the amplitude and resonance frequency of the reflection spectrum.
Fu et al. from Nanyang Technological University utilized MEMS to achieve a switchable magnetic metamaterial by adjusting the distance between two half-square open rings, which could switch the magnetic response of the metamaterial, broaden the resonant frequency, and tune the range of the metamaterial by realizing the asymmetry of the unit structure. [39]In a clever design, they separated the asymmetrical opening rings.One was fixed to the substrate, and the other was etched onto the silicon wafer of the insulator to create the support structure and MEMS, thus achieving distance adjustment in the experiment, as shown in Figure 6c.
In 2020, Huang et al. considered a metamaterial as an analog of EIT in classical oscillating systems; proposed an actively tunable EIT metamaterial to control terahertz waves, which consisted of movable gold bars and fixed line pairs; and changed the distance g between gold bars and line pairs by MEMS technology to controllably adjust the EIT phenomenon and manipulate terahertz waves as a dynamic filter [40] The metamaterials they designed and the experimental results are shown in Figure 7.

Electrically Tunable Terahertz Metamaterials
Electrically tunable metamaterials have garnered significant research interest.From the perspectives of electronics and semiconductor technology, integrating unit cell structures with various electrosensitive materials, such as varactor/PIN diodes, semiconductors, graphene, transparent conductive oxides (TCOs), and liquid crystals (LCs), enables the realization of tunable metamaterial devices.On" state and c) in the "Off" state, corresponding to the cantilever angle β = 0°and β = 2°, the spacer substrate is SiO 2 , its thickness t = 5 μm, the metal is aluminum, and the surface thickness of the structure is 500 nm (the reflector thickness is 1 μm).The detailed geometric parameters are shown in the illustration: g = 4 μm, w = 6 μm, d = 4 μm, l 1 = 100 μm, and l 2 = 50 μm.The squared period of the cell is 104 μm.The d) reflection amplitude and e) phase spectra measured in the "On" and "Off" states, where the illustration shows the gradual modulation with different voltages applied.Reproduced with permission. [38]Copyright 2017, Wiley-VCH.

Varactor/PIN Diodes
In the realm of microwaves, varactor/PIN diodes can be integrated with metamaterial unit cell structures to construct electrically tunable metamaterials, where electromagnetic characteristics, such as capacitance, can be significantly adjusted by applying different voltages.M. K. Emara et al. proposed a metamaterial reflector unit cell based on dual-coupled resonators, demonstrating its real-time reconfigurable beamforming capabilities in the X-band (8-12 GHz). [41]As illustrated in Figure 8, the design of reconfigurable unit cells is based on a DC bias voltage connected along one direction to adjacent cells.The DC bias voltage is also exposed to incident radiation, thereby enforcing a linear variation in the working polarization.Three metamaterial reflectors were designed and fabricated to validate the proposed concept: the first based on a varactordiode-equipped split-ring resonator (SRR), the second based on a PIN diode-equipped double-ring resonator (DRR), and  and c) electron microscope unit structure.Reproduced with permission. [39]Copyright 2011, Wiley-VCH.and d) experiment.Reproduced with permission. [40]Copyright 2020, Springer Nature.
the third based on a coupled SRR-DRR configuration with both varactor and PIN diodes for simultaneous amplitude and phase control.The experiments demonstrated that metamaterial reflectors based on coupled resonator unit cells can achieve multifunctional beam transformations, including beam steering with amplitude control and multibeam modes.Similar concepts have been employed to realize multifunctional dynamic microwave devices with different functionalities, such as tunable metamaterial absorbers [42] and tunable polarization converters. [43]2.Semiconductors However, due to the lack of suitable varactor diodes, the aforementioned design approach cannot be efficiently applied in the terahertz frequency range.Fortunately, semiconductors can serve as alternative active elements for designing tunable metamaterial unit cell structures in both the terahertz and infrared regimes.This is because their conductivity (closely related to their optical responses) can be easily tuned through carrier doping.Additionally, this method offers advantages such as a high modulation speed, wide bandwidth, large dynamic range, and compatibility with complementary metal-oxide-semiconductor (CMOS) technology, demonstrating significant potential for practical applications.Metamaterials integrated with semiconductors can exhibit dynamic electromagnetic characteristics, influencing charge carriers in the semiconductor through electrical biasing.
H. T. Chen et al. conducted the pioneering experimental realization of terahertz electrically tunable metamaterials. [44]The metamaterial structure proposed comprises of an array of electrically connected gold split-ring resonators arranged on a thin ntype gallium arsenide (GaAs) layer grown on a semi-insulating GaAs wafer.By changing the external voltage applied across the Schottky diode formed at the metal-semiconductor interface, the carrier density in the n-doped GaAs layer can be significantly modulated.This modulation, in turn, effectively controls the transmission of terahertz waves through the metamaterial structure.The investigation revealed a remarkable 50% transmission modulation at 0.72 THz, which was driven by the voltage-modulated conductivity of the doped GaAs layer.By systematically optimizing the design of top-tier split ring resonators, the same research group has further enhanced the performance of the designed terahertz metamaterial structure.This optimization enables real-time active modulation of both the amplitude (55%) and phase (0.56 rad) of the transmitted beam at a frequency of 0.81 THz. [45]. Goran et al. introduced a terahertz electro-optic modulator consisting of a metal-semiconductor-metal cavity array with subwavelength thickness. [46]The metamaterial device works by depleting carriers in the doped semiconductor layer buried beneath the Schottky junction.Combining the highly confined terahertz field with the reverse-biased Schottky junction in the pass-through region enables modulation performance expected to match or even surpass existing devices.The study revealed that the performance of modulators designed for the high end of the terahertz spectrum may degrade, with the insertion loss reaching 54% and the reflectance modulation decreasing to 46% at 7 THz.
H. T. Ling et al. introduced InGaZnO (IGZO) Schottky barrier diodes (SBDs) for the first time to reconfigure electric fieldcoupled inductor-capacitor metamaterials to modulate terahertz waves. [47]The SBDs are designed to bridge the capacitors of the resonator, allowing modulation of the average conductivity within the capacitor gap by applying a bias voltage while keeping the capacitance nearly constant.The structure and results are given in Figure 9.A device with a 14 400 metamaterial unit cell was fabricated and characterized using frequency-domain spectroscopy.The measured transmission exhibited continuous modulation from À14.2 to À9.4 dB at a frequency of 0.39 THz, corresponding to a modulation depth of 14.3%.This work paves a new path for realizing tunable terahertz metamaterials utilizing industrially compatible thin-film technologies.

2D Materials
Graphene is a two-dimensional material consisting of only one atomic layer.Due to its extensive electrically tunable conductivity, it is an ideal material for realizing tunable terahertz metamaterial Reproduced with permission. [41]Copyright 2023, Institute of Electrical and Electronics Engineers, Inc.
devices.[50] Y. C. Fan et al. achieved tunable metamaterial responses in the terahertz regime using a graphene cut-wire array. [51]By optimizing the geometric shape of the graphene cut wires, a maximum absorption enhancement of 50% was achieved.Two graphene metamaterial structures with different graphene fill factors but the same absorption enhancement were studied using the thin-slab retrieval method.The findings revealed that 1) the collision frequency of graphene determines the damping frequency of the graphene metamaterial, and 2) the resonance amplitude (κ/Γ) can be quantitatively used to describe absorption.A systematic study of graphene fill factors indicates significant progress in enhancing the Q factor of graphene metamaterials through the preparation of high-quality graphene.Research on the terahertz absorption enhancement and tunability of graphene flakes via electric resonance excitons may have potential applications in terahertz graphene devices. [52]owever, because graphene is only a single atomic layer, its interaction with electromagnetic waves is weaker than that of bulk materials.The absolute optical response of these patterned graphene structures is much weaker in practical applications.To overcome this limitation, researchers have combined electrictunable graphene with appropriately designed metamaterials to create graphene metamaterials.These metamaterials exhibit both high electrical tunability and strong optical response.In 2012, S. H. Lee et al. experimentally demonstrated, for the first time, the achievement of significant sustained switching and linear modulation of terahertz waves in a 2D metamaterial incorporating an integrated electrically controlled graphene layer.
The strong resonance of the graphene layer can greatly enhance the wave-matter interaction in the electrically controllable graphene layer.The thickness of the embedded single layer of graphene is six orders of magnitude smaller than the wavelength (<λ/1 000 000), which is an atomic layer.When combined with metamaterials, this layer can control the amplitude (up to 47%) and phase (32.2°) of the transmitted wave at a frequency of 0.86 THz.Motivated by the aforementioned study, numerous other graphene metamaterial structures have been proposed to perform different functionalities in the THz frequency range, [53][54][55] including amplitude modulators, [56] polarization controllers, [57][58][59] and phase modulators. [60]s shown in Figure 10, A. D. Squires et al. proposed a duallayer metamaterial structure in which the entire device pattern is overlaid onto 2D materials, graphene, and gold. [48]This is the first experimental demonstration for terahertz communication devices where graphene is patterned into the microstructures of the entire device.The dual-layer structure addresses issues associated with a gold microstructure lacking tunability, as well as problems with pure graphene microstructures or graphene patches exhibiting poor performance.Therefore, the experimentally developed graphene/gold dual-layer terahertz frequencyselective absorber demonstrates tuning capabilities of more than 16 dB at a bias voltage of only 6 V while maintaining a Q factor of 19.The device also exhibits over 95% broadband tuning at the same low 6 V bias.The graphene/gold dual-layer concept and fabrication approach can be applied to numerous terahertz metamaterial devices theoretically modeled in the literature.Copyright 2021, Optica Publishing Group.

Transparent Conductive Oxides (TCOs)
Transparent conductive oxides (TCOs), such as indium tin oxide (ITO), aluminum-doped zinc oxide (AZO), and gallium-doped zinc oxide (GZO), are wide bandgap oxide semiconductors capable of heavy doping up to 1022 cm À3 , approaching the carrier concentration of metals. [61]Additionally, these materials exhibit metal-like optical characteristics in the near-infrared range.Similar to semiconductors, the carrier density can be altered by an external electrical bias, leading to tunable optical characteristics in the terahertz frequency range, which is a challenge with traditional semiconductors.In comparison to traditional metals, another notable advantage of TCOs is their lower loss due to their lower carrier density.Therefore, TCOs have potential as alternatives to plasmonic materials in the terahertz frequency range, representing a new class of active materials for modulating metamaterial operation responses.
In 2016, W. Z. Xu et al. experimentally realized an electrically tunable metamaterial through the integration of amorphous-In-Ga-Zn-O (a-IGZO) thin-film transistors into unit cells. [49]igure 11 depicts the electrically modulated terahertz metamaterial with a-IGZO thin-film transistors.The resonant mode at Reproduced with permission. [48]Copyright 2022, Springer Nature.e) Close-up view of the sample.f ) Cross section of the a-IGZO thin-film transistor.Reproduced with permission. [49]Copyright 2016, Springer Nature.0.75 THz is primarily attributed to the electric dipole response to the external terahertz field.The characteristics of this resonance can be effectively modulated by electrically adjusting the conductivity of the active IGZO layer.In the experiment, a relative intensity change of 4 dB can be achieved with a positive voltage of 24 V. Considering the appealing characteristics of a-IGZO and the high performance of oxide thin-film transistors, metamaterial structures based on transparent oxide thin-film transistors integrated at the metamaterial unit cell level may hold promise for exploring the terahertz and other frequency ranges.

Liquid Crystals
Liquid crystals (LCs) are anisotropic materials composed of dispersed elongated molecules, known as mesogens, oriented in a master solvent.These molecules can either be oriented in a specific direction or remain disordered, resembling a crystalline structure.LCs typically exhibit a rod-shaped molecular structure, where the long axis forms a dipole moment p = qd due to charge separation (q).The dipole moment is proportional to the distance (d) between the charges.Here, d refers to the displacement vector from the negative to the positive charge.When an external electric field is applied, it induces a torque (τ) in the liquid crystal, given by the equation τ = p Â E. [61] As a result, the torque aligns the liquid crystal along the field direction, thereby changing the refractive index nðVÞ ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi ε LC ðVÞμ p , where ε LC ðVÞ is the voltagedependent dielectric constant of the liquid crystal.
Due to the presence of rod-shaped molecules, when the induced dipole direction shifts from being parallel to the long axis of the molecules (the optic axis) to being perpendicular, these particles collectively exhibit noticeable polarization, resulting in optical anisotropy.For a nematic liquid crystal with a parallel-plate structure subjected to an external AC voltage, the LC mesogens rotate along the direction of the applied electric field, inducing dipole moment alignment.This occurs when the applied voltage exceeds a critical voltage known as the Freedericksz threshold voltage (V c ). [61] The rotation angle of the mesogens relies on the voltage used.Under a sufficiently high voltage, the alignment of mesogens leads to a refractive index, either n e or n ∥ , perpendicular to the optic axis for a normally incident plane wave.Likewise, a plane wave polarized along the optic axis encounters a refractive index, n e or n ∥ , parallel to the optic axis.The birefringence (Δn) of the LC is expressed as Δn = n e Àn o = n ∥ Àn ⊥ .64][65][66] G. S. Deng et al. introduced a tunable liquid crystal-based metamaterial absorber with broadband absorption. [67]igure 12 illustrates the unit cell structure of the designed liquid crystal-based absorber, which is a sandwich-like configuration.The upper and lower substrates are made of quartz glass, with copper imprinted on both inner surfaces.The upper copper layer serves as the pattern layer, while the lower copper layer functions as the reflective layer.In this configuration, the two copper layers also act as the top and bottom electrodes.For unbiased and fully biased states, the absorption bandwidths with rates exceeding 90% are 4.6 and 3.9 GHz, corresponding to relative bandwidths of 3.5% and 3.3%, respectively.As the bias voltage increases from 0 to saturation, the central resonance frequency shifts from 130.0 to 119.9 GHz, achieving a tunability of 7.8%.The experimental results validated a tunability of 6.7%.Comparative analysis reveals that the performance of the proposed LC-based broadband tunable metamaterial absorber surpasses that of several recently reported absorbers, indicating that the proposed absorber has significant potential for applications in electromagnetic shielding, sensing, and stealth technology.
In 2023, G. S. Deng et al. proposed a configuration for reorienting liquid crystal molecules in the terahertz frequency range using an all-electric control approach. [68]By introducing crossshaped electrodes and applying different bias voltages at adjacent Reproduced with permission. [67]Copyright 2021, Optica Publishing Group.
prong locations, the orientation of liquid crystal molecules can be dynamically controlled, effectively adjusting the dielectric constants of the respective liquid crystal materials.They experimentally tuned the resonance frequency from 105.8 to 102 GHz, achieving a maximum frequency tuning rate of 3.6%.Importantly, the initial direction of the LC molecules is set using the lateral electric field generated by the cross-shaped electrodes, eliminating the need for a polyimide layer.Furthermore, the response of the designed structure is enhanced by an order of magnitude compared to that of traditional LC metamaterials with a polyimide layer.

Terahertz Nonreciprocal Magneto-optical Modulators
The magneto-optic (MO) effect and MO effect are traditionally studied in the microwave and communication bands, which are famous due to the Faraday MO effect.MO materials and the MO effect can be used to form special devices, such as isolators and phase shifters.In the THz band, there are also many studies on THz MO isolators.For instance, Shalaby et al. [8] first developed a THz isolator using the traditional Faraday MO rotation effect in the bulk permanent magnet SrFe 12 O 19 at 300 K, but due to the large thickness of the MO materials, the insertion loss is very large.In 2016, Tamagnone et al. [69] proposed a circularly polarized (CP) THz isolator by combining graphene with a reflective structure.The isolation power of this device can reach 20 dB with a large insertion loss of 7.5 dB at 2.9 THz, and a strongly biased magnetic field (MF) of 7 T is needed.Magnetic circular dichroism and Faraday rotation were subsequently reported for continuous and patterned graphene. [70]When the temperature is 250 K and the external MF is 7 T, the Verdet constant of the graphene is large.However, due to the thickness of graphene, the MO effect and the Faraday MO rotation angle are still limited, which requires extreme MF or temperature. [71]oreover, recent studies have demonstrated that MO materials as well as MO devices [72][73][74] also have strong potential in manipulating the optical field and studying the physical mechanics of topological photonics due to their MO tunability, nonreciprocity, and MO charity.In 2020, J. Qin et al. [75] proposed a metasurface based on an MO material, in which the MO effect can be enhanced, and the circular dichroism of the device can be tuned with 2.5°modulation.Furthermore, by breaking the timereversal symmetry by the MO microstructure, Z. Wang et al. [76] first experimentally observed the chiral edge states in an MO photonic crystal, realizing the intriguing phenomenon of oneway transmission and an anti-scattering topological photonic insulator.All of these studies proved that the MO effect can be obviously enhanced by combining MO materials with microstructures; therefore, many novel functions and phenomena have been realized.
In particular, a particular gyrotropic MO material, InSb, has attracted much interest in the THz band.The cyclotron resonance frequency of InSb is in the THz band with a considerable MO effect.In the past few years, InSb has shown unique properties in terms of MO amplitude and polarization manipulation, nonreciprocal transmission and observation of unique phenomena in the topological photonics regime.For instance, Lin et al. [7] reported a THz reflective isolator using InSb under a transverse MF (i.e., Voigt configuration), and the reflective isolator can realize 35 dB isolation with a 6.2 dB loss at 2.9 THz with a specific incident angle.Our previous work [12] also experimentally demonstrated THz nonreciprocal circular dichroism of InSb, and by combining InSb with metallic gratings, the giant tunable Faraday effect of over a 90°rotation angle and nonsymmetric one-way transmission under a weak magnetic field of 0.17 T can be realized.Moreover, D. Wang et al. [77] reported the topologically photonic properties of a transverse magnetized InSb-plasmonic grating hybrid structure in the THz band and determined the photonic Weyl points and Fermi arc by analyzing a dispersion relation map with different bias MFs.Therefore, the novel properties of InSb are promising for manipulating both the wave front and chiral states with strong magnetic tunability and nonreciprocity.
Therefore, this approach is promising for designing THz functional devices with InSb, especially when it is combined with metasurfaces.Herein, we review recent studies on THz MO devices based on InSb and introduce our work on InSb-based THz devices.
MO effects, referring to changes in the polarization state of light upon interaction with magnetic matter, are one of the most basic phenomena in solid-state physics.In 1846, Faraday discovered the first magneto-optical phenomenon in which the plane of linearly polarized light is rotated after passing through a piece of glass exposed to an external magnetic field.In the terahertz band, there are also many MO materials, such as InSb, HgTe, YIG, and graphene, [78][79][80] which show gyrotropy properties near the cyclotron frequency under a relatively small external magnetic field; these materials can be used as candidates for THz MO materials.Here, we take InSb as an example to introduce the MO property of these materials in the THz regime.When we applied an external magnetic field along the z direction, the gyroelectric material InSb showed strong gyrotropy near the cyclotron frequency ω c .ω c is proportional to the external magnetic field through ω c = eB/m*, where B is the intensity of the magnetic field, m* is the effective mass of the carrier and m = 0.015m e for InSb, me is the mass of the electron, and e is the electron charge.The dielectric function of InSb becomes a nonreciprocal tensor expressed as: [12] ε ¼ where there are three different tensor elements in the Equation ( 1) can be written as: [12] ε where ε ∞ is the high-frequency limit permittivity, ε ∞ = 15.68;ω is the circular frequency of the incident THz wave; γ is the collision frequency of carriers, γ = e/(μm*) = 0.1π THz; ω p is the plasma frequency written as ω p = (N e 2 /ε 0 m*)1/2; N is the intrinsic carrier density; and ε 0 is the free-space permittivity.Furthermore, Re(ε y ) = ÀIm(ε yz ) and Re(ε yz ) = Im(ε y ), where Re() represents the real part and Im() represents the imaginary part.Moreover, the dielectric property of InSb strongly depends on N, and N strongly depends on the temperature T, which follows [78,81] Nðcm À3 Þ ¼ 2.9 Â 10 11 ð2400 À TÞ 3=4 ð1 þ 2.7 Â 10 À4 TÞT 15 Â exp½À10 6 ð0.129 À 1.5 Â 10 À4 TÞ=ð8.617 The dielectric tensor of InSb shows strong dispersion and gyrotropy properties, and it is strongly dependent on the external magnetic field and temperature in the THz regime.If a THz wave propagates along the z direction in this MO medium, the wave equation can be written as follows: [81] Àβ 2 where, in the Voigt MO configuration, K, E, and B are orthogonal to each other.Thus, two independent solutions for left-and right-handed circularly polarized waves can be obtained via Equation ( 5): Then, we can theoretically calculate the transmittance of the left circularly polarized (LCP) or right circularly polarized (RCP) wave through InSb as follows: here, we assume that the thickness of InSb is d = 500 μm, and we can calculate the transmittance map, which is defined as the transmittance in different MFs and frequencies, as shown in Figure 13a,b for RCP and LCP, respectively.The chiral transmission properties of InSb strongly vary with the MF.When B = 0 T, both orthogonal states are forbidden by InSb from 0.4 to 1.5 THz.In contrast, with increasing MFs, the transmittances for the LCP and RCP states are completely different.The RCP state shows a cyclotron resonance at ω c with a Lorentz spectral line, but the LCP state shows a Drude spectral line with the plasma frequency ω p .Therefore, these results indicate that InSb is transparent for the LCP state but forbidden for the RCP state within the cyclotron resonance band, which indicates strong nonreciprocity.

Terahertz MO Modulators
Because the MO effect can be tuned through MFs, the MO material can be used as an MO modulator.First, ferrofluids have MO effects in the THz band and are magnetically tunable.However, traditional MO effects in the THz band are severely lacking.Thus, Fan et al. [11] proposed an MO-dielectric hybrid structure to enhance the MO effect by combining ferrofluid with an artificial microstructure.In this work, they filled the surface of a Si metasurface (photonic crystal) with ferrofluid to form a ferrofluid-filled photonic crystal (FFPC), as shown in Figure 14a.
Here, the external MF was applied with the Voigt configuration (transverse).The measured transmission spectra of the THz waves are shown in Figure 14b.With the growth of the MFs, the resonance peaks gradually split from one to two peaks and became more divergent in the frequency band.As a result, at 1.04 THz, the transmittance gradually increased from 0.011 to 0.4 when the MFs increased from 0 to 0.15 T. The FFPC Reproduced with permission. [12]Copyright 2019, Optica Publishing Group.
therefore realized magnetic amplitude modulation, i.e., magnetic-induced THz wave transparency with a 150 GHz bandwidth.Moreover, at the spilt peaks (0.9 and 1.14 THz), both the resonance strength and the peak depth were still modulated by the MFs.
A THz MO modulator based on InSb has also been proposed.T. Li et al. [10] observed and studied the transverse MO effect (Voigt configuration) of the gyroelectric semiconductor InSb in the terahertz (THz) regime both theoretically and experimentally, as shown in Figure 14c,d

Terahertz MO Wavefront Controller
The manipulation of the wave front of THz waves is highly desired in THz communication, imaging, and sensing.However, most conventional THz devices have low efficiency and narrow bandwidths and are difficult to manipulate due to the lack of effective materials in the THz regime.By applying the generalized Snell's law, [18] the metasurfaces show strong potential in the THz wave-front manipulation area; once the units are placed appropriately to form the desired phase map, the metasurfaces can attain many functions.However, most of these metasurfaces have limited bandwidths in the THz regime due to the incomplete frequency response of these fixed structures and materials. [83,84]n particular, a unique metasurface that consists of unique geometric phase optical elements and typically serves as a half-wave plate with a spatial axis, also known as a Pancharatnam-Berry (PB) metasurface, shows the intriguing phenomenon of arbitrary manipulation of CP waves.The desired phase can be obtained by just rotating the axis of the PB units (a rotation angle equivalent to half of the desired phase).Moreover, this process is frequency-irrelevant, which means that once the PB phase condition is fulfilled, the function can be realized, making PB phase-based devices good candidates for designing broadband devices.On the basis of the PB phase, many functional devices have been proposed for various functions, such as anomalous reflection or deflection, [85,86] observation of the giant photonic spin Hall (PSH) effect [87,88] and generation of special beams, such as Bessel beams. [89]Although PB metasurfaces have strong potential for manipulating CP waves, there is still a lack of research on active PB metasurfaces.Research on active PB metasurfaces is therefore important for more flexible functions, and active devices require dynamic control over THz waves.The experimental transmission spectra of the FFPC under different external magnetic fields.Reproduced with permission. [11]Copyright 2013, AIP publishing.c) Schematic diagram of the THz-TDS system of InSb.Reproduced with permission. [10]Copyright 2020, Optica Publishing Group.d) Experimentally measured THz time-domain pulses of InSb under different EMFs at T = 80 K.
To date, some research has focused on active P-B metasurfaces.For example, T. Kim et al. proposed gate-controlled active THz devices by combining graphene with metallic PB metasurfaces.By adjusting the voltage, these devices can realize amplitude modulation for both anomalous refraction and focusing.Although the refraction and focusing efficiency can be actively improved, they are still limited by the modulation depth (28%) and efficiency (30%).There are also active THz PB devices focused on all-optical tuning; for instance, J. Li et al. [85] proposed an all-optical active P-B metasurface to modulate the amplitude of reflected THz CP waves.Although this device shows effective and chiral modulation of CP waves, this device still suffers from a narrow bandwidth and can only work 1.09 THz.Overall, research on THz-active devices is still immature, and the combination of MO materials and PB metasurfaces is therefore promising for realizing these functions.
In 2020, by combining the gyroelectric semiconductor InSb with a PB metasurface, [90] a gyrotropic PB meta-surface (GPBM) was developed to actively manipulate THz photonic spin states.The metasurfaces showed two important effects: the photonic spin Hall (PSH) effect and the photonic spin filter (PSF) effect.First, this work studied the active broadband PSH effect, which also manipulated the CP states and depended on the deflection from the "ON" state to the "OFF" state.This intriguing effect could be achieved with a broad frequency band ranging from 1.02 to 1.7 THz, a large sweeping angle ranging from 36.6°to 83.5°and a maximum efficiency of over 70%.When the external MF varied from 0 to 2 T, the PSH effect gradually changed from the "ON" state to the "OFF" state, and the efficiency decreased to 0. In other words, when B = 0 T, the proposed device deflected with AE1st diffractive orders; when B = 2 T, the device reflected normally.Moreover, the diffractive efficiency can be modulated by the MFs from 0 to 2 T, making this device flexible and functional.Second, at a higher frequency, this device showed a PSF effect.When B = 1 T, the LCP state could be reflected with no polarization conversion, but for the RCP state, it had been absorbed.The isolation power of these two CP states could reach 24 dB at 2.4 THz.
Tan et al. [91] developed another PB metasurface based on the MO meta-atom (MMA), as shown in Figure 16a,b.A split-ringshaped metal was fixed on the dielectric SiO 2 substrate, and the defect of the split ring was filled with InSb to form a total ring.The back of the substrate was a metal that reflected the light.When there was no MF, the InSb showed metallic properties because the THz wave could not transmit but rather was reflected, in which case the total structure served as a complete metallic ring.However, when the MF was applied, the THz wave could transmit InSb as a result of the MO effect, thus making the structure act like a metallic split ring, which could typically serve as a half-wave plate.The proposed MMA could therefore be converted from the "OFF" state to the "ON" state (i.e., no phase shift unit to the PB unit).On the basis of the proposed MMA, three kinds of THz functional devices were developed with an MO PB metasurface (MPBM): an active THz deflector, a Bessel beam generator and a vortex beam generator.These proposed MPBMs are efficient and can be turned by external MFs from  [12] Copyright 2019, Optica Publishing Group.
the "OFF" state to the "ON" state.An active THz deflector can also realize active beam deflection for THz CP waves with different deflection angles ranging from 28 to 44.8°and different THz chiral states.The active vortex and Bessel beams are also realized, as shown in 4.4(c)-(f ), and further, by analyzing the superchiral effect, we find that the optical chirality can be manipulated in the MMA units.As a result, these GPBMs can enhance the optical chirality of the chiral electromagnetic field when an EMF is applied.

Terahertz MO Isolators
In recent years, based on MO materials, several THz isolators have been experimentally developed.MO materials include ferrite, the gyroelectric semiconductor InSb, and graphene.Most of them applied a longitudinal MF (i.e., Faraday configuration) in the THz band and realized THz one-way transmission.For instance, Shalaby et al. [8] demonstrated a nonreciprocal terahertz Faraday isolator operating on a bandwidth exceeding one decade of frequency, a necessary requirement to achieve isolation with (few-cycle) pulses generated by broadband sources.The traditional Faraday MO rotation effect was isolated in the bulk permanent magnet SrFe 12 O 19 at 300 K, but due to the large thickness of the MO materials, the insertion loss is very large.This was the first stand-alone complete terahertz isolator with bulk MO material under an external MF.
In 2016, Tamagnone et al. [69] reported a THz isolator based on graphene under a strongly biased magnetic field (MF) of 7 T, which exhibited an isolation of ≈20 dB but an insertion loss of 7.5 dB at 2.9 THz.The unique properties of InSb may provide more effective applications in THz nonreciprocal functional devices.
In 2018, Lin et al. [7] explored the nonreciprocal reflectance of InSb in an applied magnetic field in the Voigt geometry.They found a very high asymmetry in the p-polarized reflectance for positive and negative incidence angles, which corresponds to the forward and backward propagation directions in the THz optical isolator.At optimal incidence angles (60°-75°) and applied magnetic fields (0.2-0.35 T), the isolator performance at room temperature exceeds 35 dB.However, it has only a narrow bandwidth, strict incident angle, and polarization angle.
However, these MO isolators are difficult to realize and have large insertion losses.Thus, in 2020, Tan et al. [92] proposed an efficient THz MO isolator based on both theory and experiments.They first investigated the nonreciprocity of bulk InSb under different MFs with the Voigt configuration, and the analytical and experimental results showed that InSb could isolate the chiral state under these conditions with 20 dB of isolation.However, the bulk InSb could only be isolated to the chiral state (i.e., transmit one CP wave and forbidden the orthogonal one) and could not be isolated to the LP state due to the chiral mirror symmetry.This work developed a dielectric/InSb hybrid metasurface to artificially increase birefringence, thus breaking the chiral mirrorreversal symmetry.Under these conditions, the dielectric/InSb hybrid structure exhibited both time-reversal symmetry and mirror-reversal symmetry.One-way transmission for the LP state was therefore realized, as shown in Figure 17a.Then, the isolation effect was experimentally demonstrated using a THz time-domain polarization spectroscopy (THz-TDPS) system.Figure 17b shows the experimental results of the total power transmittance for the proposed structure with LP incidence.The LP state was isolated, and the isolation power is shown in Figure 17c.When B = 0.17 T and f = 0.45 THz, the isolation power reached 30 dB.Moreover, the insertion loss was less than 2 dB in the broad range of the 0.38-0.85THz band.These results were significantly better than those of previous experimental reports on THz isolators.This mechanism and the results demonstrated the intriguing properties of magnetized InSb, provided an effective way to manipulate both the nonreciprocal and asymmetric transmission of THz spin-state transmission in a magnetized semiconductor, and promoted the development of high-performance THz isolators with lower loss and higher isolation under a weak MF.The MPBM can shift from the "OFF" state to the "ON" state when the EMF changes from 0 to 2 T. (c) Schematic view of the MPBM for generating vortex beams and e) the amplitude distributions of the E-field on the x-y cutting plane at z = 300 μm when an LCP Gaussian beam of 1.3 THz is reflected by the MPBM with topological charge L = 1 and f ) L = 2. Reproduced with permission. [91]Copyright 2021, Optica Publishing Group.

Tunable Terahertz Metamaterials by Light Modulation
In this section, terahertz metamaterials/metasurfaces are modulated in combination with other materials, such as graphene, semiconductor materials, and photoinduced phase change materials.The principle of achieving light modulation is to generate photogenerated charge carriers via an external laser, thereby changing the characteristics of the relevant materials (impedance, conductivity, etc.) and further changing the electromagnetic response of the terahertz transmission wave to obtain tunable results.Next, we will use the types of materials as the classification standard to expand the description.

Graphene
The 2010 Nobel Prize in Physics was awarded to Novoselov et al. for the first time in 2004 to obtain a single layer of graphite flakes (graphene) by mechanical stripping. [96]Graphene is a singlelayer structure of graphite, consisting of a thin sheet of carbon atoms, which has a regular hexagonal structure of carbon atoms connected by a σ bond, each carbon atom through a π orbital and an extranuclear electron together to form a delocalized large π bond. [94]101][102] It is worth mentioning that graphene has high electron mobility at room temperature [103] and can achieve a tunable optical response by changing the Fermi level or etching patterns, and at the same time, graphene can easily combine with other metamaterials. [104]This provides graphene as a solution for modulated terahertz metamaterials: the addition of graphene materials improves the tunability of the overall device, the metamaterial structure can improve the interaction between terahertz waves and the thin layers of graphene atoms, and the two are unified and mutually reinforcing.
The most widely used terahertz metamaterial devices with graphene are absorbers, which can be divided into single-band metamaterial absorbers and multiband terahertz metamaterial absorbers according to the number of operating frequency bands.After the first emergence of metamaterials in 2000, [105] scholars soon began to use this new material to improve traditional electromagnetic devices; metamaterials in the field of electromagnetic absorbers such as a shark can not only achieve strong absorption of incident waves [106] but also have a thinner custom structure.
In 2008, Landy et al. first used metamaterials to improve absorbers and created a "sandwich" structure composed of two metamaterial resonators. [104]As shown in Figure 18, the absorber used the finite-difference time-domain (FDTD) solver of CST to simulate a single peak ( f = 11.48GHz) with nearly 100% perfect absorption.In the actual experiment, an absorption rate of 88% (@11.5 GHz) was obtained by using FR-4 as the substrate material.Their work laid a very good theoretical and experimental foundation for later scholars to design metamaterial structures to improve absorbers.
In the same year, Tao et al. followed up with the first step in the terahertz field; their metamaterial was similar to the structure designed by Landy et al., but using polyimide as the substrate material and replacing the metal material with GaAs wafers, the electromagnetic metamaterial absorber achieved an Figure 17.Sketch picture of the proposed magnetoplasmon/dielectric metasurface.a) When an LP wave is incident to the metasurface, with the MF, the LP state can be converted to the RCP state; however, the reflected backward RCP wave cannot transmit through the InSb/dielectric metasurface.b) The experimental total power isolation between the forward and reverse waves.c) The difference in the theoretically calculated total power transmittance (in dB) between forward and reverse propagation for LP incidence, which indicates the isolation property of this device.Reproduced with permission. [92]opyright 2021, Wiley-VCH.absorption rate of 70% (@1.3 THz) in experiments. [105]Soon after, Tao et al. further improved their metamaterial-based absorber; [106] the thickness of the absorber was only 16 μm, and a high absorption rate of 97% (@1.6 THz) was obtained in the experiment.At the same time, the material has high flexibility, and the incidence angle of transverse electrical radiation and magnetic radiation is wide.Therefore, there is sufficient space for scholars to broaden the absorption spectrum of the absorber and increase the tunable link.
In 2013, Amin et al. used graphene materials to design an absorber for the terahertz band, using a three-layer structure to extend the 90% absorption bandwidth to 7 THz. [107]In the same year, Sailing He et al. combined graphene into metamaterial structures for the first time and designed a wideband terahertz absorber with a multilayer pyramid structure of graphene-dielectric material installed on its metal sheet [108] to block transmission with a homogeneous metal film as the base.Finally, they achieved high absorption with a very large bandwidth from 8 THz to more than 100 THz, and they found that the resulting absorption spectrum could be reduced to lower frequencies by increasing the cell size.
Since the rapid development of metamaterial absorbers, a variety of frequency bands, a variety of materials, and a variety of designs, we still focus on the terahertz band.Deng et al. proposed a polarization-sensitive, tunable metamaterial absorber composed of a metal ground, a SiO 2 dielectric spacer, a graphene layer and a metal pattern layer at terahertz frequencies. [109]hen the Fermi level of graphene was fixed at 0.7 eV, the absorption rates of graphene for X-polarized light waves at 6.42 and 8.37 THz were 98.5% and 99.1%, respectively, while the absorption rate of Y-polarized light at 7.22 THz was ≈99%.The absorption spectra of graphene at different Fermi levels and different incident angles are also shown, and the related tuning functions are discussed, which are of great significance for the realization of tunable terahertz metamaterial devices combined with graphene.Liu et al. proposed a wideband tunable metamaterial absorber consisting of a single layer of annular graphene, a dielectric substrate and a metal ground floor. [110]The unit structure is shown in Figure 19a,b.The top layer is graphene with annular grooves, and four joints are left to connect the graphene for easy control.The substrate was SiO 2 , and the ground metal plate was gold.The absorber showed an absorption frequency of 7-9.25 THz, a width of ≈2.25 THz, and an absorption efficiency of 90%.They then tried to change the parameters, including changing the Fermi energy level of the graphene layer, and eventually designed a double-ring graphene absorber to enhance the absorption and extend the bandwidth and obtained an even greater bandwidth of 3.2 THz.
In the same year, Mou et al. designed a tunable terahertz absorber [111] with a similar idea, except that the surface structure they chose was composed of a graphene concentric double ring array, as shown in Figure 19d,e, for which the graphene Fermi level was fixed at 0.5 eV.The structure exhibits a high absorption rate of more than 90% in the band at 1.18-1.64THz and has two distinct absorption peaks at 1.26 and 1.54 THz.As shown in Figure 19f, when the Fermi energy changes from 0.35 to 0.65 eV, the wide absorption band changes from 0.98-1.36 to 1.36-1.94THz, and the absorption rate remains above 80%.At the same time, it is also proven that the surface conductivity of graphene is almost linear with its Fermi level in the terahertz band.The graphene-based tunable wideband terahertz metamaterial absorber designed by Zhou et al. is also a graphene-SiO 2metal layer structure, [112] which adjusts the absorption rate from 0.4 to 0.9 in the range of 0.5-1 THz by controlling the Fermi level of graphene, regardless of the angle and polarization.It is worth mentioning that they achieved a high light transmittance of 70% in a transparent window at 1.65 THz, and the device can meet the two requirements of both an absorber and a light transmitter.
Sakib et al. designed and analyzed a tunable dual-wideband metamaterial terahertz absorber [113] based on single-layer continuous-structure graphene, which utilizes the variable conductivity of graphene to provide controllable absorption strength and bandwidth, as shown in Figure 20a-c.The absorber produces two wide-band absorptions for different graphene Fermi levels: 0.1-3.1 THz and 6.25-8.55THz.In 2023, Bahareh et al. designed a tunable polarimetric independent triband absorber [114] consisting of patterned graphene in the first layer of the unit structure, containing a graphene ring in the center of the frame surrounded by four graphene wheels and four triangular graphene in the corners.The absorber achieves 98.64%, 99.97%, and 99.98% perfect absorption peaks at 8.17, 9.74 and 11.95 THz, respectively, as shown in Figure 20i.

Semiconductor Material
The application of semiconductor materials in optoelectronic devices is well known, and it is logical to load semiconductor materials into metamaterials.Due to the photoconductivity of the semiconductor, by embedding the semiconductor in the unit structure of the metamaterial, the incident laser power is changed to regulate the concentration of the photogenerated  [111] Copyright 2018, Optica Publishing Group.and e) a cell structure consisting of a graphene layer and a gold mirror separated by silica, where the geometric parameters are a =   [114] Copyright 2023, Nature Springer.carrier, triggering different resonance modes and thus tuning the resonant frequency of the terahertz wave, which is the photoexcited carrier regulatory mechanism.
Padilla et al. used optical pumping to modulate metamaterial devices.They prepared a copper SRR array on a gallium arsenide (GaAs) substrate and stimulated GaAs to cause changes in its conductivity through optical pumping. [115]This behavior further inhibited the resonance of the metal SRR, resulting in the modulation of the transmittance in the terahertz band.Shen et al. used photoconductive silicon as a mediator to switch two modes of a resonator to achieve a wirelessly tunable metamaterial switch from 0.76 to 0.96 THz, as shown in Figure 21 [116] Gu et al. used a similar idea to design a composite structure metamaterial of photoconductive silicon and aluminum, using infrared laser excitation to change the silicon conductivity, change the open/closed loop state of the resonant ring, and achieve maximum modulation of the refractive index.It is worth mentioning that their work demonstrated for the first time that at room temperature, by integrating photoactive silicon into a functional unit, active control of terahertz waves in classical EIT metamaterials was achieved. [117]hang et al. chose to prepare niobium nitride (NbN) with open resonant rings on a 1 mm thick magnesium oxide (MgO) substrate and changed the inherent conductivity of the NbN through changes in the incident strong terahertz field.They observed a large amplitude modulation, which is shown in Figure 22 [118] They also made a keen discovery of the effect of temperature on the sensitivity of metamaterials.In the same paper, thermal control experiments of metamaterials were also carried out.Deng et al. used a subwavelength indium antimonide (InSb) grating structure, which was prepared by molecular beam epitaxy of a 2 μm thick InSb layer grown on a semi-insulated GaAs (SI-GaAs) substrate. [74]They experimentally demonstrated tunable terahertz plasma responses of subwavelength InSb gratings by optically tuning carrier concentrations and that the THZ resonant frequencies of this structure can have a wide tuning range.Seren et al. proposed making tunable terahertz metamaterials from an array of gallium indium (InAs) plasma disks, which exhibit a strong nonlinear response due to electric-field-induced scattering, resulting in a reduction in carrier mobility that dampens the plasma response. [119]ndium tin oxide (ITO) is a transparent conductive oxide material that is often used as a transparent conductive electrode in optoelectronic devices.Although the band structure of ITO does not clearly exhibit a band gap or a band gap similar to that of traditional semiconductors, it still has semiconductor properties to a certain extent.At room temperature, ITO exhibits high conductivity, mainly due to the free electron contribution, and the doped indium (In) ions in tin oxide (SnO2) introduce additional free electrons, resulting in N-type semiconductor properties.Xiao et al. demonstrated a tunable terahertz absorber based on an ITO metamaterial, which is shown in Figure 23 [120] They used femtosecond laser direct etching to prepare crossshaped metasurfaces on ITO with different arm lengths, and the thickness of the intermediate dielectric layer was only   [74] Copyright 2013, Wiley-VCH.c) Top view.d) Side view.e) A 405 nm continuous laser is shone at the InSb grating at an incidence angle of 45°, and the electric and magnetic configurations of the vertically incident TE and TM waves are indicated in (c).H: magnetic field; E: electric field.The metamaterial designed by Seren et al. [119] Schematic diagram of an InAs disk array on a metamaterial.f ) Semi-insulated GaAs.Copyright 2020, Optica Publishing Group.60 pm.The absorption peak frequency can be continuously adjusted from 0.92 THz to 1.04 THz between the TE and TM polarizations.

Photoinduced Phase Change Materials
Vanadium dioxide (VO 2 ) is a metal oxide with phase transition properties that has attracted much attention in the field of optoelectronics.In 1959, Morin first observed a unique and reversible insulation-metal phase transition (IMT) of VO 2 at 340 K, accompanied by a transition (structural phase transition, SPT) of the crystal structure of VO 2 from a low-temperature insulative state (monoclinic phase) to a high-temperature metallic state (tetragonal rutile phase). [121]At the same time, the physical properties of VO 2 also change.The phase transition of VO 2 is affected by a variety of pathways, including temperature changes, applied electric fields, [122] applied magnetic fields, [123] ion implantation, [124] and light modulation. [125]Here, we focus on light modulation.
Liu et al. prepared a 200 nm thick gold SRR structure on the surface of VO 2 , reduced the carrier transport barrier by pumping a terahertz source to stimulate the local electric field, and induced the phase transition of VO 2 . [126]As shown in Figure 24b, as the local electric field intensity increases from 0.3 to 3.3 MV cm À1 , the transmittance of the metamaterial changes by ≈13% at 0.42 THz.Zhang et al. designed a polarization-angle-insensitive wideband tunable terahertz metamaterial absorber by using a multilayer design to separate a wheeled VO 2 periodic array from a Dirac semimetallic (DS) backplane with a PI dielectric layer. [127]y changing the conductivity of VO 2 , the Fermi energy of DS, the dielectric constant of the dielectric substrate and the structural parameters, the absorption properties can be flexibly adjusted from 4.3% to nearly 100%.
GST is one of the most frequently used and mature chorionic phase change materials.In the 1980s, Yamada et al. of Panasonic, Japan, successively studied GeSbTe phase change alloy materials with different components for the development of data storage technology. [128]Yamada et al. proved through experiments in 1987 that with the difference in the metering ratio, this series of phase change materials have different crystallization temperatures and crystallization rates, and GST has a good balance between crystallization temperature and crystallization rate, so it is widely used.The switching of the optical properties of the chalcogenide PCM results from the transition between two states: amorphous and polycrystalline.Guo et al. proposed a tunable terahertz metamaterial consisting of a GST particle array and a Si layer, whose two absorption peaks (86% @1.98 THz and 51% @5.88 THz) shifted with the crystallization of GST. [129]Unfortunately, however, the application of this material in the field of terahertz metamaterials is still very limited and needs to be developed.

Conclusions
In this review, we provide a comprehensive overview of the progress made in tunable terahertz metamaterials from the early 21st century to the present.In the introduction section, we highlight the development of terahertz technology and metamaterials, emphasizing their immense potential and significance in scientific research.The active control of terahertz devices, specifically the tuning of terahertz metamaterials, has garnered significant interest among researchers.While numerous creative and feasible designs have been published by enthusiastic researchers, a comprehensive summary and compilation of this work is currently lacking.Therefore, we aim to fill this gap by reviewing the tunable terahertz metamaterials that have been explored by researchers.
Throughout our study, we classify tunable terahertz metamaterials into four categories based on different tuning mechanisms: mechanical regulation, electrical regulation, magnetic regulation, and optical regulation.Each category is described in detail in Chapters 2, 3, 4, and 5. Mechanical tuning through physical means is the most straightforward approach, although it is constrained by the current state of fabrication processes.In Chapter 2, we provide a brief explanation of the mechanical tuning mechanism and introduce representative articles in the field, focusing on two specific mechanical tuning mechanisms: flexible substrate materials and MEMS.The use of flexible substrate materials for stretching demonstrates the remarkable feature of recoverability, which is crucial for the long-term utilization of metamaterials.MEMS,in contrast, has gained significant attention in the field of electricity and presents an intriguing concept for integration with terahertz metamaterials.Imagine that  [126] Copyright 2012, Springer Nature.b) Experiment showing the field-dependent nonlinear transmission spectrum of SRRs at 324 K on VO 2 .
the miniature robotic arm delicately alters the structure of each metamaterial unit, synchronizing their movements at the micron scale.
Chapter 3 delves into electrical regulation as a tuning mode, classifying it based on different electronic control materials.We explore five main electronic control materials, namely, diodes, semiconductors, graphene, TCOs, and LCs, loaded onto terahertz metamaterials.Representative articles are provided to support each material's effectiveness in tuning.The design concepts behind these articles are clever and astonishing.
Moving on to Chapter 4, we delve into the equally fascinating field of magneto-optic modulation.We conduct a detailed mathematical analysis of the magneto-optic modulation mechanism and describe three main types of research devices: modulators, wavefront controllers, and isolators.Representative articles are also provided in this chapter, showing the remarkable advancements in this area.
Chapter 5 explores light modulation, similar to Chapter 3, where we classify different materials, such as graphene, semiconductors, or photoinduced phase change materials, loaded onto terahertz metamaterials.These materials can be tuned by altering the Fermi level of graphene, utilizing the carrier concentration in semiconductors, or leveraging the properties of photoinduced phase change materials.While these methods share similarities, each method represents a unique manifestation of ingenuity.
Ultimately, this topical review serves as a valuable guide for researchers investigating terahertz metamaterial devices, extending beyond the realm of tunable mechanisms.We hope that both we and readers can gain a deeper understanding of the remarkable achievements in this field.

Figure 1 .
Figure 1.Organizational structure of this review.

Figure 3 .
Figure 3. a,c) Transmission magnitude of experiments and simulations under the type I1 design.b,d) Transmission magnitude of the experiments and simulations under the type I2 design.The structure stretches from 0 to 10 times the step size Δl 0 .Reproduced with permission.[28]Copyright 2013, AIP publishing.

Figure 4 .
Figure 4. Functional relationship between resonance frequency and strain.a,b) are the first stretching cycles of the I1 and I2 structures, respectively.c, d) I2 structures under different stretching periods.Reproduced with permission.[28]Copyright 2013, AIP publishing.

Figure 5 .
Figure 5. Structure and experimental verification of metamaterial unit-loaded MEMS by Cong et al. a) Schematic model of the unit structure b) in the "On" state and c) in the "Off" state, corresponding to the cantilever angle β = 0°and β = 2°, the spacer substrate is SiO 2 , its thickness t = 5 μm, the metal is aluminum, and the surface thickness of the structure is 500 nm (the reflector thickness is 1 μm).The detailed geometric parameters are shown in the illustration: g = 4 μm, w = 6 μm, d = 4 μm, l 1 = 100 μm, and l 2 = 50 μm.The squared period of the cell is 104 μm.The d) reflection amplitude ande) phase spectra measured in the "On" and "Off" states, where the illustration shows the gradual modulation with different voltages applied.Reproduced with permission.[38]Copyright 2017, Wiley-VCH.

Figure 6 .
Figure 6.Metamaterials designed by Fu et al. a) Single asymmetric open ring resonator and b) coupled asymmetric open ring resonator at different spacings TE (top row), TM (bottom row) polarization incident transmission spectrumand c) electron microscope unit structure.Reproduced with permission.[39]Copyright 2011, Wiley-VCH.

Figure 7 .
Figure 7. a) The metamaterial designed by Huang et al.The lower right picture shows the overlapping structure of the chip and b) the unit structure of the EIT metamaterial, where the geometric parameters are Q x ¼ 42.5 μm, Q y ¼ 10 μm, D x ¼ 10 μm, D y ¼ 48.5 μm, T Si ¼ 3.5 μm, T Cr ¼ 5 nm and T Au ¼ 200 nm: The periods in each cell in the x and y directions are 120 μm.c) Device EIT transmittance spectra obtained by simulationand d) experiment.Reproduced with permission.[40]Copyright 2020, Springer Nature.

Figure 8 .
Figure8.a) Illustration of the proposed unit cell based on an SRR with tunable capacitance (e.g., using a varactor diode) and a DRR with tunable resistance (e.g., using a PIN diode).b) Dual-beam reflection from the normal incident plane wave based on the metamaterial structures.Reproduced with permission.[41]Copyright 2023, Institute of Electrical and Electronics Engineers, Inc.

Figure 9 .
Figure 9. a) Illustration of the tunable metamaterial structure.b) Results of the S-parameters.c) H-field and E-field distributions and d) surface current density vector of the designed metamaterial unit cell.Reproduced with permission.[47]Copyright 2021, Optica Publishing Group.

Figure 10 .
Figure 10.a) Unit cell dimensions and array structure of the designed metamaterial structure.b) Cross section of the graphene/gold bilayer superimposed metamaterial structure.c) Illustration of the frequency-selective absorber.d) Fabricated metamaterial device sample.Reproduced with permission.[48]Copyright 2022, Springer Nature.

Figure 11 .
Figure 11.Electrically modulated terahertz metamaterial with a-IGZO thin-film transistors.a) Illustration of the metamaterial structure.b) Illustration of two microstructure layers in one unit cell.c) Transmission curve of the metamaterial without an applied bias.d) Photograph of a fully fabricated sample.e)Close-up view of the sample.f ) Cross section of the a-IGZO thin-film transistor.Reproduced with permission.[49]Copyright 2016, Springer Nature.

Figure 12 .
Figure 12. a) Illustration of the unit cell structure.b) Top view of the Cu microstructure.Side view of the unit cell c) without and d) with bias voltage.Reproduced with permission.[67]Copyright 2021, Optica Publishing Group.

Figure 13 .
Figure 13.a) Simulated carrier density of InSb at different temperatures; maps of the real part of b) ε L and c) ε R of longitudinally magnetized InSb in the THz regime under different magnetic fields from 0 to 0.2 T; maps of theoretical transmittance d) TL and e) TR of longitudinally magnetized InSb in the THz regime under different magnetic fields from 0 to 0.2 T; f ) map of the theoretical transmittance difference between the LCP and the RCP (TL-TR).Reproduced with permission.[12]Copyright 2019, Optica Publishing Group.
. At first, T. Li et al. calculated the photonic band structure, and experimental measurements showed that a unique circularly polarized magneto plasmon mode and a linearly polarized transverse magnetic mode can be manipulated by a weak MF.Moreover, these results indicate that InSb under transverse MFs can serve as a THz tunable high-pass filter and an MO modulator.The cutoff frequency of the filter could be broadly adjusted from 0.4 to 0.8 THz when the magnetic field varied from 0 to 0.22 T, and a modulation depth of 20 dB could be obtained.By combining InSb with the microstructure, InSb can have more intriguing properties for modulation, for example, for polarization modulation.In 2018, Q. Mu et al. proposed and fabricated an InSb MO microstructure for polarization conversion based on the MO effects of InSb by combining InSb with several orthogonal artificial metallic gratings Figure 15.This work revealed the MO enhancement mechanisms in the MO microstructure and achieved broadband (>0.6 THz), near-perfect (ER > 30 dB), and highly efficient (reaching 70%) orthogonal linear polarization conversion, which could be modulated by a weak magnetic field (0.15 T) in the experiment.Moreover, MO modulation with a modulation depth of 95.8% can be achieved by this device under a weak magnetic field of 0.15 T, as shown in Figure 17.

Figure 14 .
Figure 14.a) Schematic diagram of FFPC.b)The experimental transmission spectra of the FFPC under different external magnetic fields.Reproduced with permission.[11]Copyright 2013, AIP publishing.c) Schematic diagram of the THz-TDS system of InSb.Reproduced with permission.[10]Copyright 2020, Optica Publishing Group.d) Experimentally measured THz time-domain pulses of InSb under different EMFs at T = 80 K.

Figure 15 .
Figure 15.a) 3D schematic diagram of the InSb plasmonics in the experimental configuration; microscope image of grating 1 and grating 2; b) side view of the InSb plasmonics.Experimental results of the InSb plasmonics: c) amplitude transmission spectra under different magnetic fields; d) spectra of the extinction ratio.Reproduced with permission.[12]Copyright 2019, Optica Publishing Group.

Figure 16 .
Figure 16.a) Sketch map and b) top view of the MMA.c) Schematic view of the GPBM for actively generating the Bessel beam and the d) amplitude distribution of the x-y cutting plane at z = 300 μm when an incident LCP Gaussian beam of 1.3 THz is reflected by the MPBM.The MPBM can shift from the "OFF" state to the "ON" state when the EMF changes from 0 to 2 T. (c) Schematic view of the MPBM for generating vortex beams and e) the amplitude distributions of the E-field on the x-y cutting plane at z = 300 μm when an LCP Gaussian beam of 1.3 THz is reflected by the MPBM with topological charge L = 1 and f ) L = 2. Reproduced with permission.[91]Copyright 2021, Optica Publishing Group.

Figure 18 .
Figure 18.Absorber structure proposed by Landy et al.Reproduced with permission.[104]Copyright 2008, American Physical Society.a) Electric resonator, b) cutting metal wire, and c) unit structure in the propagation direction, where the geometric parameters are a 1 = 4.2, a 2 = 12, W = 3.9, G = 0.606, t = 0.6, L = 1.7, and H = 11.8 μm and in the z direction at a distance between the two metal structures of 0.65 μm.Tao et al. modified the terahertz metamaterial absorber structure.Reproduced with permission.[105]Copyright 2008, Optica Publishing Group.d) Perspective view of the resonant ring unit e) absorber.f ) Photograph under a microscope, where the geometric parameters are a = 36, b = 25.9, c = 10.8, g = 1.4,w = 3 μm, and t 1 = t 2 = 8 μm.

Figure 19 .
Figure 19.a) Top view of the absorber unit structure proposed by Liu et al.Adapted under the terms of the CC BY license.[110]Copyright 2018, The Authors.AIP publishing.b), where the geometric parameters areP = 3 μm, R 1 = 1 μm, G 1 = 0.15 μm, W 1 = 0.4 μm, t g = 1 nm, t d =5 μm, and t m = 0.1 μm.c) Absorption spectra of metamaterial absorbents at different Fermi levels.d) Schematic diagram of a wideband terahertz absorber designed by Mou et al.Reproduced with permission.[111]Copyright 2018, Optica Publishing Group.and e) a cell structure consisting of a graphene layer and a gold mirror separated by silica, where the geometric parameters are a = 5.5 μm, b = 4 μm, c = 2.5 μm, d = 2.2 μm, t = 28 μm, and L = 15 μm.f ) The X-axis for the frequency of the absorber, and the Y-axis is the graphene Fermi level.
Figure 19.a) Top view of the absorber unit structure proposed by Liu et al.Adapted under the terms of the CC BY license.[110]Copyright 2018, The Authors.AIP publishing.b), where the geometric parameters areP = 3 μm, R 1 = 1 μm, G 1 = 0.15 μm, W 1 = 0.4 μm, t g = 1 nm, t d =5 μm, and t m = 0.1 μm.c) Absorption spectra of metamaterial absorbents at different Fermi levels.d) Schematic diagram of a wideband terahertz absorber designed by Mou et al.Reproduced with permission.[111]Copyright 2018, Optica Publishing Group.and e) a cell structure consisting of a graphene layer and a gold mirror separated by silica, where the geometric parameters are a = 5.5 μm, b = 4 μm, c = 2.5 μm, d = 2.2 μm, t = 28 μm, and L = 15 μm.f ) The X-axis for the frequency of the absorber, and the Y-axis is the graphene Fermi level.

Figure 21 .
Figure 21.The metamaterial designed by Shen et al.Reproduced with permission. [116]Copyright 2017, American Physical Society.a) Experimental pumpprobe configuration diagram and cell structure diagram of the metamaterial device used to measure terahertz transmission, where the geometric parameters are a = 50, d = 36, l = 4, w = 2 and g = 2 μm.The photoconductive silicon (red) is placed in the gap between the two sides of the gold ELC resonator (yellow), and the substrate is sapphire (blue).b) Curve of the experimental measurement results with the pumped light energy flow.The metamaterial designed by Gu et al.Reproduced with permission. [117]Copyright 2012, Springer Nature.c) Unit structure, where the geometric parameters are L = 85, l = 29, s = 7, w = 5, h = 495, g = 5, P x = 80 and P y = 120 μm, and d) EIT metamaterial laser pumping -terahertz wave detection and measurement.

Figure 22 .
Figure 22.The metamaterial designed by Zhang et al.Adapted with permission.[118]Copyright 2013, AIP publishing.a) Unit structure of the metamaterial, in which the geometric parameters are g = t = 5 μm, w = 10 μm, a = 50 μm and p = 60 μm.b) Amplitude transmission spectrum at 4.5 K ambient temperature at different incident field intensities, where E 0 = 30 kV cm À1 .Simulation diagram of the transmission measurement of the metamaterial designed by Deng et al.Adapted with permission.[74]Copyright 2013, Wiley-VCH.c) Top view.d) Side view.e) A 405 nm continuous laser is shone at the InSb grating at an incidence angle of 45°, and the electric and magnetic configurations of the vertically incident TE and TM waves are indicated in (c).H: magnetic field; E: electric field.The metamaterial designed by Seren et al.[119] Schematic diagram of an InAs disk array on a metamaterial.f ) Semi-insulated GaAs.g) Photograph of the metamaterial prepared with an InAs layer thickness of 2 μm, Si-GaAs substrate thickness of 500 μm, disk diameter D = 70 μm, and period P = 100 μm.

Figure 23 .
Figure 23.Metamaterial absorbers designed by Xiao et al. a) Structural diagram of the absorber and b) unit structure, where the geometric parameters are as follows: the substrate period P = 150 μm, the ITO arms are 100 and 120 μm, the arm width is 10 μm, and the thicknesses of the ITO surface and ITO bottom layer are both 200 nm.With a PET layer thickness of h = 60 μm and an arm angle of θ = 90°, c) time-domain results and d) TE and TM wave absorption spectra from 0.5 to 2.0 THz were obtained by THZ-TDS.Reproduced with permission.[120]Copyright 2020, Optica Publishing Group.

Figure 24 .
Figure 24.a) Optical image of metamaterial SRR with a gap of 1.5 mm deposited on VO 2 /sapphire by metamaterial designed by Liu et al.Reproduced with permission.[126]Copyright 2012, Springer Nature.b) Experiment showing the field-dependent nonlinear transmission spectrum of SRRs at 324 K on VO 2 .