Tunable Metasurfaces: The Path to Fully Active Nanophotonics

Optics plays a key role in the development of science and technology, with evidence that the ancient Egyptians had produced lenses dating as far back as 2000 BC. As the field of optics has progressed from geometric, to diffractive, and now to nanoand quantum optics, the state-of-the-art applications have been influential in a range of disciplines, such as astronomy, medicine, and telecommunications, to name a few. Today, we rely on optical systems and devices more than ever, with a large percentage of the population carrying mobile devices that contain numerous optical elements or driving cars with an ever-increasing number of cameras and sensors. However, conventional optics depends on heavy and bulky components with limited functionalities, which restricts their usage in certain conditions. In the past few decades, the field of optics at the nanoscale has exploded with the advent of metamaterials, allowing the manipulation of light using subwavelength structures. Metamaterials consist of artificial nanoscale building blocks, so-called meta-atoms, that are arranged in ways that allow for properties that rival conventional optics, and, in some cases, even outperform their bulky counterparts. Through the careful design and engineering of the geometry of individual metaatoms arranged randomly or in specific well-determined locations, their interactions with electromagnetic (EM) waves can be tailored at will. This provides us with the chance to arbitrarily control the phase, amplitude, and polarization of light to suit our needs, with both high performance and a small footprint, paving the way for novel applications in various fields. Metasurfaces, the planar equivalent of metamaterials, have shown unprecedented functionality and performance, despite being almost 2D. This 2D nature allows for much simpler structures to be used, which is extremely favorable for the implementation of practical purposes. They have already been proved to be competitive with conventional materials in applications, such as holography, lensing, and structural coloration. Typically, metasurfaces are designed with a particular use or output in mind and are engineered in a way to achieve their goal with the maximum performance possible. As with all new technology in its infancy, research must start from the ground up, so a lot of focus has been given to produce metasurfaces that achieve a specific singular function or purpose. The obvious limitation of this is the lack of flexibility for the optical properties of the metasurface after it has been fabricated. The next logical step is toward metasurfaces that can be adjusted and reprogrammed to alter their optical properties at will, even after they have been physically created. This process has already been underway for a few years, developing hand in hand with advancements in materials science and increasingly more sophisticated nanofabrication techniques to produce tunable metasurfaces. In this review, we present the recent progress of tunable metasurfaces based on their degree of tunability, from switchable T. Badloe, J. Seong, Prof. J. Rho Department of Mechanical Engineering Pohang University of Science and Technology (POSTECH) Pohang 37673, Republic of Korea E-mail: jsrho@postech.ac.kr J. Lee, Prof. J. Rho Department of Chemical Engineering Pohang University of Science and Technology (POSTECH) Pohang 37673, Republic of Korea


Introduction
Optics plays a key role in the development of science and technology, with evidence that the ancient Egyptians had produced lenses dating as far back as 2000 BC. [1] As the field of optics has progressed from geometric, to diffractive, and now to nano-and quantum optics, the state-of-the-art applications have been influential in a range of disciplines, such as astronomy, medicine, and telecommunications, to name a few. Today, we rely on optical systems and devices more than ever, with a large percentage of the population carrying mobile devices that contain numerous optical elements or driving cars with an ever-increasing number of cameras and sensors. However, conventional optics depends on heavy and bulky components with limited functionalities, which restricts their usage in certain conditions. In the past few decades, the field of optics at the nanoscale has exploded with the advent of metamaterials, allowing the manipulation of light using subwavelength structures. [2,3] Metamaterials consist of artificial nanoscale building blocks, so-called meta-atoms, that are arranged in ways that allow for properties that rival conventional optics, and, in some cases, even outperform their bulky counterparts. [4,5] Through the careful design and engineering of the geometry of individual metaatoms arranged randomly or in specific well-determined locations, their interactions with electromagnetic (EM) waves can be tailored at will. This provides us with the chance to arbitrarily control the phase, amplitude, and polarization of light to suit our needs, with both high performance and a small footprint, paving the way for novel applications in various fields.
Metasurfaces, the planar equivalent of metamaterials, have shown unprecedented functionality and performance, despite being almost 2D. [6][7][8] This 2D nature allows for much simpler structures to be used, which is extremely favorable for the implementation of practical purposes. They have already been proved to be competitive with conventional materials in applications, such as holography, [9][10][11][12] lensing, [13][14][15] and structural coloration. [16][17][18] Typically, metasurfaces are designed with a particular use or output in mind and are engineered in a way to achieve their goal with the maximum performance possible. As with all new technology in its infancy, research must start from the ground up, so a lot of focus has been given to produce metasurfaces that achieve a specific singular function or purpose. The obvious limitation of this is the lack of flexibility for the optical properties of the metasurface after it has been fabricated. The next logical step is toward metasurfaces that can be adjusted and reprogrammed to alter their optical properties at will, even after they have been physically created. This process has already been underway for a few years, [19][20][21][22][23][24][25][26][27][28] developing hand in hand with advancements in materials science and increasingly more sophisticated nanofabrication techniques [29][30][31][32][33][34] to produce tunable metasurfaces.
In this review, we present the recent progress of tunable metasurfaces based on their degree of tunability, from switchable DOI: 10.1002/adpr.202000205 In the field of nanophotonics, metasurfaces have come to the forefront in realworld applications, owing to their accessible exotic optical properties that can be readily designed and fabricated using currently available techniques. The subwavelength dimensions and lightweight characteristics of metasurfaces are attractive qualities for the miniaturization of optical devices and are already exploited in devices that can rival, and sometimes even outperform, conventional bulky optics. Over the past decade, ample research has been undertaken to produce high-performance metasurfaces with exciting optical properties that cannot be found in nature or achieved with conventional optics. To open the path to widely used devices in our everyday lives, the next obvious step for metasurfaces is the development of tunability. Herein, the techniques and development of applications of tunable metasurfaces are presented through the incorporation of active materials and controllable external stimuli, and the uncovered tunable characteristics are critically analyzed. The review rounds up by proposing the future directions and prospects for actively tunable metasurfaces and their potential practical applications. metasurfaces, onto continuously tunable metasurfaces, finally arriving at individually addressable meta-atoms that can produce active, reprogrammable metasurfaces ( Figure 1). Various tuning mechanisms are reported and compared, with an overall focus on experimentally realized applications, to highlight the current status of the field. Where possible, we mainly focus on demonstrations in the visible and IR regimes; however, because the manipulation of nanoscale meta-atoms is still an arduous task, in the latter section, in particular, applications in the THz and GHz regimes are also presented where necessary. The sections are presented as follows. First, we introduce metasurfaces that can be tuned between two different sets of optical properties after fabrication. These metasurfaces have also been referred to as switchable rather than tunable, as the output can be switched between two states, i.e., A-B (Section 2) (Figure 1b). Second, we present metasurfaces that are continuously tunable between two states, i.e., A-AB-B (Section 3) (Figure 1c). Then, to round up our review of tunable metasurfaces, we discuss fully reprogrammable metasurfaces that are made up of individually addressable meta-atoms for complete tunability and reconfigurability (Section 4) ( Figure 1d). We conclude with an overview of the current state of the field, a comparison of the introduced tuning methods, and finally provide our views on the perspectives for the future directions of tunable metasurfaces and their applications in the real world (Section 5).

Switchable Metasurfaces
Metasurfaces are generally designed to exhibit the maximum performance for a given material and design by structuring the meta-atoms in a way that produces the desired optical response. By adding a controllable parameter to the design, the optical properties can be modulated after the metasurface has been fabricated. The most basic example of this is a switchable metasurface, where the optical properties can be changed between two distinct states. Here, we will introduce three specific ways of achieving that feat, namely, through the usage of phase-change materials (PCMs), metal oxides, and the different switching properties of liquid crystals (LCs) in combination metasurfaces.

Phase-Change Materials
PCMs have unique properties in that they undergo a phase transition under certain conditions. In general, PCMs have two stable phases that exhibit a large contrast in the optical and electrical properties, making them an ideal candidate for usage in switchable nanophotonics. Other exciting qualities are the rapid switching response times, [35] long lifetimes (>10 15 cycles), [36] and multiple stimuli that can be used to trigger the phase transition, such as electronically, [37] thermally, [38] and optically. [39] The exact Figure 1. Examples of metasurfaces from static to fully addressable at the meta-atom level. Schematic diagram of a metasurface with different adjustable properties in terms of color production. a) In a static metasurface, the output color is fixed. b) A switchable metasurface allows two colors to be swapped between through an external stimulus. c) A continuously tunable metasurface can change between two states while passing through the intermediate states with a controllable stimulus. Here, the color can be changed from yellow to red, going through orange. d) Metasurfaces with fully addressable meta-atoms can be changed at will to output new functions or properties. Here, an example of a display that can be changed between any images, with examples of artwork by Franz Marc. classification of what solid-state PCMs are depends on the field with an amorphous to crystalline transition being generally required, [40,41] but in nanophotonics, insulator-to-metal transitions (IMTs) such as that shown by vanadium dioxide (VO 2 ) have also qualified the material to be considered as a PCM. A key characteristic of the different PCMs is known as volatility, which is related to whether the external stimulus needs to be continuously applied (volatile) or not (nonvolatile) for the phase of the material to be maintained in a particular state. Common nonvolatile PCMs exhibit a stable crystalline and amorphous state, with well-known examples being chalcogenide glass-based materials, such as Ge-Sb-Te (GST) and Ag-In-Sb-Te (AIST). [42,43] GST can have several stoichiometries, causing differences in optical properties, which have been investigated deeply to identify new and potentially useful PCMs. [44][45][46] It generally exists in a stable amorphous phase below the glass-transition temperature (T g ) [47] at around 150-210 C [20] and exhibits a phase change to a crystalline phase between T g and the melting temperature (T m ), where the material become liquidus. A schematic representation of this amorphous to crystalline phase change is shown in Figure 2a-i. In the amorphous state, the atoms are bonded covalently, whereas in the crystalline state, the insufficiency of valence electrons for interatomic covalent bonding results in resonance bonding. [23] Over the phase transition, the optical properties of PCMs, such as the refractive index, change remarkably, particularly in the IR region. This is an extremely attractive quality for use in tunable metasurfaces.
The nonvolatile transition of GST has been applied successfully in a variety of different optical applications, such as perfect absorption, [48] electromagnetically induced transparency, [49] and beam steering, [50] with Ge 2 Sb 2 Te 5 (GST-225) and Ge 3 Sb 2 Te 6 (GST-326), in particular being utilized in commercial storage technologies. [43,44] As the optical properties of GST have the greatest contrast in the IR region, it is an extremely useful candidate for applications in thermal emission, as this is the regime that terrestrial objects are able to radiate at. With this in mind, an application of tunable thermal emission has been demonstrated that allowed for switchable spectral emission through the atmospheric window of 2.0-3.0 μm. [51] By designing the locations of the plasmonic resonances through the geometry of Au nanorods on top of a GST layer and back reflector, the resonant wavelength of emission was modulated and experimentally measured to produce a wavelength change of up to 425 nm (Figure 2b). The switching time between crystal and amorphous states was also calculated to be in the order of 300 ns. This idea has been extended to thermal camouflage. [52] Using the concepts of beam steering and lensing, an active plasmonic beam switching and bifocal zoom device at a wavelength of 3.1 μm has been demonstrated using GST-326. [53] By utilizing an active GST layer below two sets of gold (Au) meta-atoms, one that is resonant when the GST is in its amorphous phase, and the other in the crystalline phase, the beam direction can be actively switched for a particular handedness of circularly polarized incident light. This was demonstrated by thermally heating the device for 2 min with a hot plate at 180 C to induce the phase transition. In another demonstration of lensing, a broadband switchable lens in the mid-infrared (MIR) range (2.8-3.8 μm) was developed. [54] By combining the plasmonic response from Au meta-atoms on top of a multilayer structure, two types of metasurface were produced; the first focuses light in the amorphous state of GST with no focusing in the crystalline state, whereas the second has different focal lengths for each state. The focal length of the device can be tuned by changing the thickness of the GST layer. More recently, a diffraction-limited varifocal metalens has been demonstrated at the wavelength of 5.2 μm using Ge 2 Sb 2 Se 4 Te 1 (GSST) (Figure 2c). [55] This type of GST offers extended broadband transparency in the IR regime in both the amorphous and crystalline states. [56] Through an extensive parameter search of meta-atom designs, a four-level phase map was designed that produces a focal spot at a different focal length for the two states. To achieve meta-atoms that demonstrate the required phase delay for both states, a figure of merit based on the phase delay of the metaatoms for both the amorphous and crystalline states was set up and evaluated. In another example of phase modulation with GST, a metasurface that switches between a quarter-and halfwave plate with 99.9% polarization conversion at wavelengths between 10.3 and 10.9 μm has been proposed. [57] The metasurface, composed of catenary-shaped GST meta-atoms on barium fluoride (BaF 2 ), utilizes a combination of electric and magnetic dipole resonances to achieve this functionality, and could be useful in applications where the polarization of light needs to be controlled with a single device. Most of these examples of tunable metasurfaces using GST utilize a thermal stimulus to change the state of the whole device; however, to control individual meta-atoms and, therefore, the optical properties of the device more precisely, the phase change should be induced either optically [58] or electrically. [59] This adds a deal of complexity to experimentally realize. Despite these obstacles, local phase transitions in GST films using a high-repetition-rate laser have been demonstrated. [60] A writing resolution of 0.59 μm using a femtosecond pulse train was realized. This allowed for dynamically reconfigurable optical devices, to read, write, and erase patterns, as well as for holography and use as a resonant material. Further research into the phase changing properties of GST has revealed hybrid states that exist between the transitions. These hybrid states have been exploited to produce an optical encryption device. [61] The metasurface was designed to only display the encrypted holographic information under certain conditions that produce the required hybrid state of GST, between the standard amorphous and crystalline states. This work demonstrated that the intermittent states of GST during the phase change can also be exploited as an extra degree of freedom, potentially changing the two-step switchability into three. The nonvolatile nature of the phase transition of GST leads to low-power memory storage devices, because no energy is required to maintain the crystalline or amorphous state after the required transition has been induced.
Similar to chalcogenide glass-based PCMs, VO 2 is another PCM that has been used to great success in metasurfaces. [38,41,[62][63][64] However, rather than an amorphous to crystalline transition, it undergoes an IMT with an accompanying crystalline structure change from a monoclinic phase to a rutile phase. [65,66] A schematic of this transition is shown in Figure 2a-ii. The IMT transition occurs at around 68 C, which is considerably lower than the crystallization temperature of GST, making it a candidate for photonic applications at reasonable temperatures. The transition temperature can be controlled through doping with tungsten (W), with higher ratios of W to VO 2 bringing it down as low as room temperature (25 C), whereas doping with hydrogen (H) can control the transmissivity. [67,68] The IMT brings on changes of the optical properties that are reversible with a long lifetime, as long as the VO 2 can be protected from oxygenation to prevent the formation of V 2 O 5 , [69][70][71] and can be induced both thermally [62,72] and electrically. [73] These properties, alongside its volatile IMT, make VO 2 an attractive material for use in passively adaptive metamaterials and nanophotonic devices. Although the classification of VO 2 as a PCM is still under debate, [41] we consider its two distinct phases with controllable and switchable properties, along with the various research in the field using it as a PCM sufficient enough to include here.
As with GST, the optical properties of VO 2 also show the most drastic changes in the IR and THz regions, [74] making it an ideal material for applications in those regimes. Subsequently, numerous examples of IR absorbers have been designed, using multilayered films, [75] stacked structures, [76,77] and biomimetic meta-atoms. [62] Aside from switchable absorbers, using Au C-shaped split ring resonators together with a VO 2 connection, a thermally controllable metahologram has been demonstrated. [78] At a working wavelength of 0.8 THz, a complex amplitude and phase metahologram was experimentally released to produce an image of the letter H at low temperatures and G at high temperatures (Figure 2d). At temperatures near the IMT, a composite image of the H and G can be observed due to the incomplete interference of the incident light. This is similar to the intermediate state that was utilized for cryptographic holography discussed in the previous section for GST. Using a cavity coupled plasmonic system, with nanoholes in an Au layer, with dielectric spacers, and a functional VO 2 layer, adaptive IR camouflage has been demonstrated. [79] The change of properties of the VO 2 layer controls the cavity length of the system. It acts as a mirror in the metallic state, effectively cutting off the bottom dielectric layer from participating in the cavity. By designing the localized surface plasmon resonance (LSPR) to be generated in the MIR region, the resonance peaks can be modulated on and off through the phase transition of the VO 2 layer based on the temperature of the system. Despite having a fairly low modulation of optical properties in the visible regime, researchers have succeeded in utilizing VO 2 to realize structural color by combining an active VO 2 layer into plasmonic metasurfaces. [80,81] Using silver (Ag) nanostructures on top of a SiO 2 layer with a functional VO 2 layer below, switchable colors particularly from green to yellow have been demonstrated ( Figure 2e). [82] Over the IMT, the sudden change in optical properties of the VO 2 layer affects the location of the LSPR. Furthermore, extra degrees of freedom have been added to the on/off switching properties of VO 2 using hydrogen and electron doping along with palladium (Pd) nanoparticles ( Figure 2f ). [83] Both color printing with the help of Al/Al 2 O 3 nanorods and dynamic twofold optical encryption using the four distinct switchable states were demonstrated. The Pd nanoparticles are sensitive to hydrogenation, so by loading the device with hydrogen or oxygen, new states can be achieved alongside the phase transition of VO 2 .By depositing ultrathin metal coupons that have different work functions, the IMT temperature can be tuned using the electron-transfer process at the VO 2 / metal interface. This provides an interesting way to create switchable metasurfaces with more than two states. Other than modulating the phase using heat, optical switching of VO 2 has also been demonstrated to produce a metacanvas that can be written and erased at will using a low-power laser. [84] This opens the possibility of using VO 2 metasurfaces that can be rewritten to produce any type of a complex photonic device.

Metal Hydrogenation
Nobel metals such as Au and Ag have been used widely in plasmonic metasurfaces, due to their robustness and plasmonic resonances. As seen in the final example in the previous section, the hydrogenation and subsequent dehydrogenation (through oxygenation) of alternative metals are a method of creating switchable metasurfaces. When a metal is exposed to hydrogen, it can be changed into a metal hydride, which exhibits a dramatic difference in optical properties, as is moves from a metallic state to a dielectric one. Metal hydrides can be simply returned to their original metallic state by a reverse dehydrogenation process using oxygen. Metals, such as Pd [85][86][87] and yttrium (Y), [88] are effective at storing large amounts of hydrogen that can produce extreme optical changes through hydrogenation [89] ; however, magnesium (Mg) [90] is particularly interesting, as it can also provide exiting plasmonic responses with low loss in the visible region, as it transitions from Mg to MgH 2 . One drawback of this method compared with using PCMs is the long chemical reaction times, making switching speeds somewhat slower, although research into speeding up these processes has been undertaken.
In one of the pioneering pieces of work to combine the hydrogenation of metals with metasurfaces, a plasmonic color display, which could be erased and restored reversibly, was demonstrated ( Figure 3a). [90] Using periodically arranged Mg meta-atoms of different dimensions, a diverse range of colors were produced. To catalyze the dissociation of hydrogen molecules into hydrogen atoms, a Pd capping layer was used on the Mg meta-atoms. A titanium (Ti) layer was also introduced as a spacer between the Mg and the Pd to prevent it from merging into an undesired alloy. Interestingly, the hydrogenation process can be halted by pausing the injection of hydrogen, to freeze the Mg in an intermediate state. The same group has also realized scanning displays, whereby the hydrogenation can be controlled in a particular direction, [91] and structural coloration based on a Fabry-Perot-type (FP) cavity that allows the structures to produce either a blank or a colorful state through the hydrogenation and oxidation processes. [92] Not only limited to color printing, the hydrogenation of metals has also been applied to metaholograms. By designing two different meta-atoms that have different reaction speeds with hydrogen, a hologram that switches between four different states was developed (Figure 3b). [93] One meta-atom is made up of Mg/Pd, whereas the other is made up of Mg/Pd/Cr. This gives them different reaction kinetics over time as the meta-atom with a Cr cap hydrogenates at a slower rate. A similar idea also demonstrated switchable holograms using composite meta-atoms that can be freely hydrogenated. [94] These two concepts have also been combined to create dualfunctional metasurfaces that can be used to switch between color printing and holography applications in a single device (Figure 3c). [95] This further extends the possibilities of using the same metasurface for completely different applications through. Using an FP cavity, the hydrogenation of the Mg results in a change in the phase delay or creates resonance peaks in reflection. When in the wrong state, the color image or holographic image cannot be used. As a further application of metasurface holography, dynamic holography switching through orbital angular momentum (OAM) multiplexing has been produced ( Figure 3d). [96] By combining a unit cell of Au and Mg meta-atoms, whereas the Au meta-atoms are unaffected by the influx of hydrogen, the Mg meta-atoms hydrogenate and cause the outgoing vortex beam to switch between different OAM states with different topological charges. This was used to produce a unique encryption method by cascading two metasurfaces in series to produce high-security optical elements that could be useful in anticounterfeiting applications. By controlling the hydrogenation of the first metasurface, the OAM that the second metasurface experiences can be tuned, and therefore, the information that is encrypted in the second metasurface can be decoded and recovered.
With the advanced techniques that are being uncovered as ways to control the hydrogenation of metals, new methods of tuning metasurfaces continue to be revealed. With more work on increasing the speed of the transition of Mg to MgH 2 and the reverse oxygenation process, this metal hydrogenation process could provide a strong platform for tunable metasurfaces, because the chemical reaction is robust and can be reversed many times. A key stumbling block for this tuning mechanism is the conditions that are required for the H 2 and O 2 loading to take place, as experimental demonstrations have generally been conducted at around 80 C, which is a fairly high temperature for implementation in general devices.

Liquid Crystals
LCs are anisotropic particles that exist in an intermediate state between an LC and a solid crystal and possess noteworthy characteristics. Their well-known properties have already been used extensively in commercial displays for many years, and now in nanophotonics. [97] Due to their optical birefringence, i.e., the difference between the ordinary (n o ) and extraordinary refractive index (n e ), integrating LCs with metasurfaces provides an opportunity to actively tune the optical properties of a device after fabrication. This difference in refractive index is usually around 0.2-0.4. [98] LCs exist in several phases, namely, smectic, nematic, and isotropic, as shown in Figure 4a. Isotropic LCs have random orientations, whereas nematic LCs have fixed orientations, and smectic LCs have fixed orientations in well-defined planes. [99] LCs can be easily tuned with various stimuli, such as electrically, optically, [100,101] and thermally, [102,103] so the specific LCs used in a device can be chosen and designed accordingly. Under the  [90] Copyright 2017, The Authors, published by Springer Nature. b) Switchable holography using metal hydrogenation. The scattering intensity over time and holographic images over the hydrogenation and dehydrogenation of a metasurface. Four distinct states can be achieved with one metasurface. Adapted with permission. [93] Copyright 2018, American Association for the Advancement of Science. c) Dual-functional structural color display and metahologram with switchable properties. Hydrogenation and oxygenation are used to control the function of the metasurface from color printing to holography. Adapted with permission. [95] Copyright 2020, American Chemical Society. d) Switchable holography using OAM. Different OAM states are produced through the hydrogenation and oxygenation of a metasurface. Adapted with permission. [96] Copyright 2019, American Chemical Society.
www.advancedsciencenews.com www.adpr-journal.com influence of an external electric field, the crystals tend to align themselves in the same direction. [104] In combination with their optical birefringence, this leads to a useful tunable mechanism for metasurfaces. The LCs that are in contact with a surface can also be aligned in a desired direction through the application of a thin polymer coating, whereby the LCs orientate themselves in the same direction that the coating was applied, known as the rubbing direction. Although the long-range order that the smectic phases of LCs could offer potentially useful characteristics, to the best of our knowledge, there are yet to be any examples of smectic phase LCs incorporated into metasurface-based devices.
The numerous states of LCs [105] that are yet to be explored in nanophotonics could be a prosperous resource to expand the use of tunable metasurfaces. The phase of the LCs can be changed through an external stimulus, which results in a modulation of the optical properties.
The reorientation of nematic LC cells surrounding a nanostructure creates an effective change in the refractive index of the surrounding medium. This has been utilized in switchable applications, such as diffraction gratings [106,107] and lenses. [108,109] By combining nematic LCs with an all-dielectric Huygens metasurface, an electrically switchable transparent display at an operating wavelength of 669 nm has been introduced ( Figure 4b). [110] The Mie resonances of the metasurface interact differently depending on the orientation of the LCs, resulting in an absolute transmission modulation of 53% at a driving voltage of 20 V. Beam steering applications have also been demonstrated. [111] Using an computational inverse design method, beam steering at a working wavelength of 1550 nm of up to 114 , with diffraction efficiency into the AE1 orders of 78% between the on and off states, has been proposed ( Figure 4b). [112] To ensure that the grating structures exhibits the desired beam switching into In the nematic phase, the LCs are orientated in the same direction. In the smectic phase, the LCs are orientated in the same direction and have a degree of long-range order. In the isotropic phase, there is no order, and the LCs are orientated in random directions. b) Switchable transparent display. The display can be turned on and off via a voltage of around 20 V. Adapted with permission. [110] Copyright 2019, American Chemical Society. c) Switchable beam steering using LCs. The electric field intensity changes direction via an applied voltage to the LCs. Adapted with permission. [112] Copyright 2020, American Chemical Society. d) Schematic of beam switching between nematic and isotropic phase LCs. In the isotropic phase, the beam is shifted from the zeroth order to the first diffraction order at 12 from normal. Adapted with permission. [115] Copyright 2018, American Chemical Society. e) Switchable holography using LCs. Experimental results of a switchable hologram controlled by the polarization control of LCs. The incident linearly polarized light is converted into left-or right-circularly polarized light depending on the orientation of the LCs, which is controlled by an external voltage. Adapted with permission. [116] Copyright 2018, American Chemical Society. f ) Switchable holography using designer pressure-sensitive LCs. Schematic of the pressure-sensitive LC cells to produce metaholograms. Pressing the LC cell manipulates a compressible liquid, which changes the orientation of the LCs, modulating the handedness of the circularly polarized light to produce different holographic images. Adapted with permission. [117] Copyright 2020, Wiley-VCH GmbH.
www.advancedsciencenews.com www.adpr-journal.com the AE1 diffraction orders with high efficiency, an adjoint-based optimization method was used for the inverse design. The transition between the nematic and isotropic phases of LC cells can also be used to modulate the effective refractive index of the cell. The alignment of LCs disappears through the phase transition above the critical temperature producing the random isotropic phase. [113] Changes of the electric and magnetic resonances due to the phase transition of LCs by external temperature have been demonstrated experimentally. [114] Irrelevant to the anchoring of the LCs, a temperature-dependent refractive index change was shown to shift the electric dipole resonance by around 40 nm with a transmission contrast of around 500%. Using this concept, beam switching from 0 (zeroth order) to 12 (first order) by increasing the temperature of the device to 60 C has been shown ( Figure 4d). [115] The compact 5 μm-thick beam steering metadevice produced an efficiency of 50%.
LCs can also change the polarization state of incident light through their orientation, acting as a polarization rotator or phase retarder. The optical activity of twisted configuration of nematic LCs (TN-LC) can alter the polarization angle of linear polarized light. This property is extremely useful in applications that are polarization-sensitive. Using the polarization manipulation abilities of LCs, a switchable geometric phase hologram has been demonstrated ( Figure 4e). [116] The plasmonic metasurface was integrated was an LC cell that acts as a quarter-wave plate depending on the orientation of the LCs. When a voltage is applied to the LC cell, the linearly polarized incident light can be switched between right-and left-handed circularly polarized light, turning the holographic image on or off. This phase transition of the LCs can also be induced optically. Using a laser to optically stimulate the TN-LC cell, local heating occurs with a response time of around 100 ms. The pumping power of the laser is an important factor in the response time, as a higher pumping power can increase the heating rate and, therefore, lower the response time. [100] Recently, metasurfaces incorporated with LC cells that respond to various stimuli have been demonstrated ( Figure 4f ). [117] Multi-responsive metaholograms were realized by controlling by the electric field, temperature, and also the surface pressure of the device. The pressure-sensitive application works by reorientating the LCs in a compressive fluid to switch the polarization of the incident light. By designing the meta-atoms to exploit both the propagation and geometric phase simultaneously, different holographic images are displayed depending on the circular polarization of the incident light. This is controlled by the LC cell, whereby in one state, the retardation of the phase of the incident light through the LCs produces left-circularly polarized light, and right-circularly polarized light in the other. A similar concept has been demonstrated to create visual alarms using holographic gas sensors. [118] When the toxic gas enters the LC cell, the LCs are rearranged to change the handedness of the transmitted light to the metasurface. This allows for instantaneous visual alarms that can be easily fabricated and combined with existing equipment, such as safety goggles. The response of LCs to multiple stimuli proves that LCs could play an important role in not only electrically driven applications, but additionally applications that can be tuned through temperature, physical force, and passively as sensors.
Switchable metasurfaces hold an interesting place in applications of active nanophotonics. As two completely different states can be produced within a single device, they are attractive for applications is various fields. Switches are particularly useful when a device needs to only determine the presence or lack of a given material or substance without the need to know about how much or little of it there is, or for applications that require only two distinct states. As a proof of this concept, switchable metasurfaces using LCs to produce a single color display using an electrically tunable device have been demonstrated with the realization of a real-time video display. [119] Switchable metasurfaces have been proved to be effective in numerous applications as highlighted in this section. However, for active devices that will be needed in applications, such as imaging or full color displays, rather than simply switching between two states, multiple tunable responses in a single metasurface are much more desirable. In that light, in the next section, we will discuss continuously tunable metasurfaces, which expand on the concept of switchable metasurfaces by introducing intermediate states that have practical uses or produce interesting optical responses that can be tuned over a continuum of states.

Continuously Tunable Metasurfaces
The previous section focused on metasurfaces that can be switched between two distinct states. Of course, the next logical step is to increase the number of distinct states that can be achieved through actively tuning the metasurface, which can lead to a significant expansion of the functionality of the device. Continuously tunable metasurfaces have been developed over the past decade for numerous applications with various methods of modulating the optical properties of a whole metasurface at once. Here, we will present some of the recent breakthroughs using electrically driven solid-state devices, polarization control through LCs, and integration of the impressive 2D material, graphene. We will then discuss mechanically tunable metasurfaces using stretchable or bendable substrates, and an impressive new technique that takes inspiration from kirigami, the article cutting variant of origami, to produce structures that can be continuously deformed to tune their optical properties.

Electrical Bias
One technique of modulating the optical properties of a solidstate material is through an electrical bias to drive electrons and change the conductivity, and, therefore, the optical response. This is achieved through the field effect, which combines both the high-speed modulation of multiple meta-atoms at once and low power dissipation. This technique has been used for years in the semiconductor industry and can also be applied to active metasurfaces with the correct materials. The formation and depletion of charges have been shown to give rise to extremely large changes in the refractive index of the material. [120] A schematic of this process is shown in Figure 5a.

Semiconductors
A great benefit of using semiconductor materials in metasurfaces is that they can be processed using highly mature complementary metal-oxide-semiconductor (CMOS) technology. This is highly advantageous for the development of large-scale nanophotonic devices. Silicon (Si) is one of the most well-known and -understood materials for use in various fields, and the modulation of its optical properties has been researched deeply. [121] A numerical demonstration of p-and n-doped Si that was configured into a multijunction structure that allows the modulation of the carrier concentration across the whole volume has been proposed. [122] The asymmetrical zigzag shape of the nanobar shaped meta-atom array can be used to control the phase retardation over 240 with a transmission between 0.66 and 0.88. The asymmetry enables symmetry-protected bound states in the continuum to collapse into ultrahigh Q-factor Fano resonance modes. This shows an example of how Si could be used to provide a basis for electrically tunable metasurfaces. However, more commonly used semiconductors in metasurfaces belong to the category of transparent conductive oxides (TCOs).
TCOs are available with a range of different properties. [123][124][125][126] In one particular example of using TCOs for the continuous electrical tuning of metasurfaces, Ge surrounded by Al-doped ZnO (AZO) was used to produce a reflectance difference of over 40% with gate biases from þ4 to À4 V while also displaying a large phase modulation up to 270 . [127] The devices were modeled using an effective medium theory to calculate the phase shifts. However, this design suffers from low optical efficiency, with a maximum reflection of around 40%. This is because the device relies on critical coupling as its working mechanism. This could be overcome by optimizing the thickness of the gate-oxide layer or by using a thinner Si substrate with a back reflector to limit the loss from transmission through the Si. Engineering the AZO to have lower loss could also be a viable option. Indium tin oxide (ITO) is a particularly useful TCO for use in metasurfaces that Figure 5. Electrically tunable metasurfaces through semiconductors. a) Schematic of the electrical bias-driven field effect. The charge carriers accumulate at the interface due to the induced electrical bias, causing a change in the conductivity of the material and, in turn, modulating the optical properties. b) Tunable perfect absorbers. i) The electron density at the HfO 2 -ITO interface with an applied bias of 0À5 V. ii) The absorption spectra at 0 and 5 V bias, with the ENZ highlighted by the dashed line. Adapted with permission. [131] Copyright 2018, American Chemical Society. c) Tunable transmission using ITO. i) Simulated and ii) measured transmittance spectra of the active metasurface for different voltage biases and iii) the z-component of the electric field at the interface between the ITO and Al 2 O 3 layers, showing the charge accumulation at 0 and 8 V of applied bias. Adapted with permission. [133] Copyright 2020, Wiley-VCH GmbH. d) Structural color using WO 3 . The resonance peak in reflection is continuously modulated through the injection of Li ions for tunable i) red, ii) green, and iii) blue color production. Adapted with permission. [143] Copyright 2020, American Chemical Society.
www.advancedsciencenews.com www.adpr-journal.com operate in the visible and near-infrared (NIR) regions, as it exhibits high transparency for films that are around 300 nm thick. To achieve a large modulation, metasurfaces are usually designed to operate in the epsilon-near-zero (ENZ) region, where the real part of the permittivity of the conducting oxide is between À1 and 1. [128][129][130][131] This effect was deeply explored for p-polarized optical modes in extremely thin layers. [131] A multilayer stack that included unpatterned ITO was used to achieve perfect absorption tuning over a wavelength range of 32 nm with an applied voltage of 5 V (Figure 5b). The electron density for the ITO layers was varied to enable the perfect absorption, to control the ENZ wavelengths. This allows for the broadband bandwidth of the device to be controlled through the design of the ENZ wavelengths of the individual layers, the number of ENZ layers, and their thicknesses. Furthermore, to understand how the higher carrier density and the reduced mobility in the ITO layer affect the optical properties, it was shown that a multilayer model is needed to accurately reproduce experimentally observed changes. [132] The control of transmission through a hybrid plasmonic waveguide has also demonstrated. [133] A change of transmission of around 33% was produced with a 6 V gate bias, with a fast modulation speed of around 826 kHz (Figure 5c). The hybridization of the fundamental waveguide mode of the bottom Si layer with the surface plasmon polaritons from the Au grating allows for a strong field enhancement in the active ITO layer. Using arrays of plasmonic resonators in the form of a 1D grating, 180 phase modulation was achieved. [134] The abrupt phase change is witnessed when the resonators switch between under-and over-coupling regimes. In another example, ITO was integrated into a guided-mode resonance-based grating to numerically design real-time controlled phase and amplitude modulators. [135] The Si nanograting with a Si waveguiding core and Au back mirror produced an amplitude modulation of %0.8 with a wide phase change of %210 at the operating wavelength of 1.537 μm, by utilizing the field-effect tunability of ITO. A similar result has been achieved with all-dielectric metasurfaces, [136] where the electrical contacts were created by the periodic layers of ITO and hafnium oxide inside the gaps of the nanograting. In addition, by incorporating ITO into a semiconductor-insulatorsemiconductor structure, large phase modulation in both reflection and transmission has been proposed. [137] By sandwiching the ITO between a layer of Al 2 O 3 and a Si slab, beneath an array of silicon meta-atoms, phase modulations of 240 and 270 in reflection and transmission mode were shown. Another interesting direction is to combine ITO with the ionic conductance-mediated nucleation and growth of Ag nanoparticles, that has been proved to produce modulation with only a few millivolts of electrical bias. [138] In a particularly exciting demonstration of using ITO in continuously tunable metasurfaces, an all-solid-state spatial light modulator (SLM) has been demonstrated. [139] With potential applications in light detecting and ranging (LiDAR) [140] sensors and virtual reality (VR) and augmented reality (AR) displays, the SLM is able to completely modulate the amplitude, phase, and polarization of light, while being robust to mechanical impacts and vibrations. In terms of LiDAR, a scanning angle of 8 and a deflection efficiency of over 34% in the NIR regime are shown. As a proof-of-concept, a LiDAR device with a detection range of up to 4.7 m was experimentally demonstrated at a wavelength of 1.56 μm. Although these measured values of the performance are still not quite up to the level of consumer-level LiDAR sensors, they prove that tunable metasurfaces can be a key to unlock miniaturized devices that could soon open up new directions of research in terms of solid-state LiDAR applications that will be useful in a plethora of fields, including integration into self-driving cars and all types of AR and VR devices. As the proposed device is all solid state, it provides an obvious defense against some of the common problems with standard LiDAR devices, as there are no moving parts that can be broken or impacted through undesired vibrations. This kind of added robustness is a valuable quality for next-generation LiDAR and the future of active metasurfaces in real-life settings.
There have been concerted efforts in the discovery of new materials for use in tunable metasurfaces. Another family of semiconductors that can be used as an active layer to continuously tune the optical properties of metasurfaces are known as electrochromic oxides. [141] One example, in particular, is tungsten trioxide (WO 3 ). By injecting lithium (Li) ions into a WO 3 film, the optical properties can be tuned. A device that modulates the polarization-dependent gap plasmon resonance by as much as 58 nm at visible wavelengths has been demonstrated. [142] The refractive index was measured to change from 2.1 to 1.9 through the injection and removal of the Li ions. The nonvolatile responses with switching speeds in the order of 10 s using an electrochemical bias voltage of less than 2 V prove that WO 3 has significant potential for integration in metasurfaces that operate in the visible regime. In another demonstration of WO 3 in metasurfaces at visible wavelengths, tunable color in both reflection and transmission has been displayed. [143] The resonance wavelength shift was measured to be up to 107 nm under an electrochemical bias of 3.2 V (Figure 5e). Switching times of around 60 s were shown for the reflection-based device, whereas longer times of up to a few minutes were recorded for the transmission-based device. These response times could be improved by reducing the diffusion length of the Li ions and the WO 3 film. A similar technique has been demonstrated by injecting hydrogen ions into GdO x . [144] Another option is to use conductive polymers, such as poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). These organic polymers conduct electricity, so the refractive indices can be tuned by driving different voltages into the material. Applications in beam steering [145] and resonant wavelength modulation have been reported. [146][147][148][149] Finally, black phosphorus in the ENZ region has been explored as a potential material for tunable metasurfaces. [150] As new interesting tunable optical properties of materials are uncovered, their integration into engineered metasurfaces and photonic devices will continue to expand the field into new exciting directions.

Graphene
Graphene is a 2D material that caused a lot of excitement when it was discovered. It has remarkable electrical and optical properties that lend extremely well for integration in metasurfaces. [151] The active tunability of graphene comes from the Fermi energy through electrostatic gating and chemical doping. The exotic properties of graphene have been shown to strongly enhance www.advancedsciencenews.com www.adpr-journal.com light-matter interactions through the interaction with metaatoms for memory devices and to display large chiral responses in the THz regime. [152][153][154] The examples of electrically tunable slow light at THz frequencies and tunable cloaking have also been demonstrated. [155][156][157] In the MIR regime, graphene has been utilized for various applications. The tunability of graphene has been used to produce controllable chiroptical responses that could be useful in biochemical detection and information processing. [158] By varying the Fermi energy in the graphene layer from 0.7 to 0.9, the peak in the circular dichroism (CD) is shifted, while maintaining a response of over 10%. In addition, the broadband electrical tuning of plasmonic meta-atoms with graphene has been demonstrated. [159,160] To improve the graphene-light interactions through the plasmonic structures, the graphene was incorporated into the nanogap between end-to-end coupled metaatoms in a metal-insulator-metal structure. This is the region where the electric field is greatly enhanced. This allows for an improvement in the tuning of the plasmonic resonances, as they interact strongly with the single-layer graphene. The resonance location was measured to be tunable by around 1100 nm, which related to 18% of the resonant frequency (Figure 6a), with a circuit speed of around 3 GHz. These results were achieved through p-doping the graphene by immersing it in diluted nitric acid, and the charge neutrality point moved to a higher voltage, allowing for the dynamic voltage range to be increased by 80%. Furthermore, IR sources have also been introduced using graphene plasmonic resonators on a silicon nitride substrate. [161] The maximum thermal power was calculated to be 50 pW cm À1 at a wavelength of 7.1 μm that could be used as a thermal MIR source that is electronically controllable over 100 cm À1 of bandwidth. This is comparable to commercially available MIR light emitting diodes around the same wavelength, highlighting the potential of graphene-integrated metasurfaces. In another example of plasmonic resonators integrated with graphene, electrostatic tunability of the phase over 237 at a wavelength of 8.5 μm has been demonstrated. [162] This was achieved by modulating the Fermi energy of the graphene layer from 0 to %0.4 eV. For different wavelengths, the tunable response varies drastically. At longer wavelengths, there is a smoother transition of the phase over the tunable energies, but with a lower maximum tunability of 200 . This range can be attained for wavelengths from 8.50 to 8.75 μm, which would be sufficient for active metasurfaces that operate over that broadband regime. In addition, the control of the complex amplitude and phase of MIR light using graphene along with plasmonic meta-atoms has been numerically demonstrated. [163] Recently, an electrically focustunable metalens based on graphene with performance that is competitive with typical mechanical lenses has been proved. [164] This was achieved by designing a Fresnel lens made of five layers of graphene, using the focused-ion beam method to pattern the device. Transmission of over 80% was demonstrated at a wavelength of 405 nm. By controlling the applied voltage to the metalens, the focal length can be modulated with the focusing efficiency calculated to be over 60% (Figure 6b). Implementation of graphene in metasurfaces working at visible wavelengths could give new functionalities to existing devices and expand the number of degrees of freedom to provide tunability to plasmonic and dielectric platforms. For example, the integration of graphene with PCMs as discussed in Section 2.1 has been demonstrated using GSST. [165] The interesting characteristics of graphene and 2D materials, in general, could play a big part in tunable metasurfaces in the visible regime, as the methods of fabricating and manipulating them at the nanoscale improve.

Liquid Crystals
In Section 2, we discussed LCs for switchable metasurfaces, and two of the methods we briefly mentioned can achieve not only switching properties, but also complete control of the polarization of the incident light and the modulation of the local refractive index. The reorientation of nematic LC cells near the surface of a meta-atom creates an effective change in the refractive index of the surrounding medium due to their birefringence. This ability has been utilized to create structural colors that can be changed through an applied voltage of up to 10 V that causes a shift in the plasmon resonance by 110 nm within 90 ms (Figure 5d). [166] Using a highly birefringent (Hi-Bi) LC in contact with the Al surface, unpolarized light can couple to the plasmonic modes at the nanostructured interface. The resonance shift is proportional to the birefringence of LCs, so using Hi-Bi LCs increases the available tunability by giving a larger range of possible values of background refractive index. The TN-LC configuration changes to the isotropic phase, whereas the laser is turned on to rotate the LCs to produce different colors at different voltages, i.e., with a different effective background medium refractive index. The same group later demonstrated a full-color tunable metasurface with Hi-Bi LCs (Figure 6c). [167] By controlling the surface roughness of the Al nanostructures, the color can be controlled using only an applied voltage; however, to achieve this feat, a complex polarizer and analyzer system was introduced. Meanwhile, dynamic color tags were shown for the low bias voltages below 5 V (Figure 6d). [168] The TN-LC configuration diminishes with the driving voltage, and the polarization-dependent nanostructures interact with the modulated polarization of the incident light. Transmission resonance shifts greater than 100 nm were reported. There are numerous reports of other polarization-dependent metasurfaces [169][170][171][172][173][174] that could benefit from the integration of LCs or electrically controllable polarizers as a way to actively control the optical response and enhance their functionality.
The continuous tunability of metasurfaces through electrical bias has many benefits such as the well-understood CMOS processes for the fabrication of semiconductor devices and the mature field of electronics. The fast switching times and precise control of the input bias that are available to electronic systems are also exceptionally attractive properties; however, the integration with complex electronics limits the applications in the visible regime. New options for transparent electrodes or materials that show a large enough optical modulation at visible wavelengths need to be developed to allow for electrical tuning of optics in that regime. LCs are an interesting alternative, with their unique properties for tuning the phase and amplitude of light but are limited by their physical size and slower modulation speeds. A single LC cell is generally much larger than the diffraction www.advancedsciencenews.com www.adpr-journal.com limit, which hinders the miniaturization of metasurfaces to subdiffraction limit unit cells for high-resolution applications.

Mechanical Force
Another option for the tunability of metasurfaces is to physically deform them after fabrication. In this case, rather than changing the properties of the background medium or nanostructures themselves, the whole device can be stretched and bent, so that the periodicity of the nanoarrays or the gaps between meta-atoms are physically modified. This kind of functionality has been explored, because techniques to deposit or transfer metasurfaces to flexible substrates were developed. [175] For in depth reviews focused on mechanically tunable photonics and soft metamaterials, please see these article. [176,177] Another option for mechanically altering metasurfaces is through the use of the microelectromechanical Figure 6. Electrically tunable metasurfaces using graphene and LCs. a) Graphene-integrated metasurfaces for reflection modulation i) Measured reflection spectra after chemical doping of graphene for different gate voltages. ii) Measured reflectance peak wavelengths as a function of gate voltage for chemically doped and undoped graphene. Adapted with permission. [159] Copyright 2014, American Chemical Society. b) Continuously tunable focal length of a metalens based on graphene. Experimental results of i) the intensity variation at the focal point and ii) the spot size. iii) The focal spot distance between the two peaks at the focal length, with the lowest distance near 0 V, which gradually increases for positive and negative voltages. iv) The calculated focal length shift based on the focal spot distance. Reproduced under the terms of the Creative Commons Attribution 4.0 International License. [164] Copyright 2020, The Authors, published by Springer Nature. c) Continuously tunable structural color using LCs. i) Measured and simulated reflection spectra of the metasurface as a function of applied field and input polarization. ii) The coordinates of the tunable color on the CIE 1931 chromaticity diagram for 0 polarized incident light. Adapted with permission. [168] Copyright 2020, Wiley-VCH GmbH.
www.advancedsciencenews.com www.adpr-journal.com system (MEMS) technology but is generally limited in the THz and IR regions. We will not cover MEMS here, so for some comprehensive reviews of tunable metasurfaces and metamaterials using MEMS, please see these references. [178,179]

Stretching
An obvious method of changing the periodicity or gaps between meta-atoms that have already been fabricated is by simply moving them. Of course, this is not possible with rigid devices, but the advent of low-temperature deposition techniques along with a multitude of polymer-based substrates has allowed this to become a reality. [180][181][182][183] A schematic of this idea is shown in Figure 7a. The manipulation of the substrate can lead to the formation of complex shapes, and even 3D structures. [184] By utilizing a plasmonic grating between two Au microrods embedded in polydimethylsiloxane (PDMS), a dynamically tunable device has been demonstrated. [185] The external strain on the Figure 7. Mechanically tunable metasurfaces using stretchable substrates and kirigami. a) Schematic of stretchable metasurfaces. When strain is applied to the stretchable substrate, the meta-atoms are physical translated to new positions based on the amount of induced strain, allowing for the modulation of optical properties. The periodicity between the meta-atoms is changed from x and y to x 0 and y 0 . b) Tunable color printing using stretchable substrates. i) x-and ii) y-polarized images at different percentages of strain from a metasurface producing structural color. Adapted with permission. [187] Copyright 2020, American Chemical Society. c) All-soft metasurfaces using liquid gallium. i) Scanning electron microscope (SEM) images of relaxed and stretched metasurfaces. ii) Electric field intensity for relaxed and stretched metasurfaces. iii) Reflection spectra in the IR region for the metasurfaces; the reflection resonance peak can be shifted by stretching the metasurface. Adapted with permission. [190] Copyright 2020, Wiley-VCH GmbH. d) A metalens fabricated on stretchable PDMS. i) Modulation of the focal length of the metalens through the application of different levels of strain. ii) Measured electric field intensity at different levels of strain. iii) The focal length depending on the stretch ratio. Adapted with permission. [194] Copyright 2016, American Chemical Society. e) Chiral response of kirigami-based metasurfaces. i) Schematic illustrations of the metasurface under compression and in its relaxed state, as it transforms from a 2D to a 3D shape and ii) its CD response. Adapted with permission. [208] Copyright 2020, Wiley-VCH GmbH. f ) Manipulation of reflection through kirigami-based metasurfaces. i) Measured reversible reflection under repeated inflation and exhaustion of nitrogen gas. ii) Measured changes in the optical responses at two wavelengths when the pressure is tuned from 0 to 137 kPa in three cycles, showing good repeatability with a modulation contrast of %20%. Adapted with permission. [209] Copyright 2020, Chinese Laser Press.
www.advancedsciencenews.com www.adpr-journal.com PDMS makes the periodicity of the grating change, which inevitably changes the optical properties. Both the LSPR and Rayleigh-Wood anomalies can be simply modulated with this stretching mechanism, which induces a change in the reflection spectra by red shifting the resonances, as the periodicity of the meta-atom array increases. This has obvious applications for tunable coloration if the resonance is in the visible regime, because a resonance peak in reflection defines the production of a specific color. In one example of a stretchable plasmonic device, through applying forces in the x-or y-direction to Al meta-atoms that had been transferred to a PDMS substrate, the meta-atoms were physically moved from their original positions. [186] Horizontal stretching of the device caused a red shift in the resonance peak, whereas vertical stretching caused a blue shift. Relaxation from the external stresses brought the device back to its original green state. The elastic strain required to achieve this full visible spectrum modulation was less than 35%. In another example of dynamic structural coloration, an all-dielectric metasurface using titanium dioxide (TiO 2 ) meta-atoms embedded in a PDMS substrate has been demonstrated ( Figure 7b). [187] The meta-atoms were each a trapezoid with a corner of around 72 , producing rotational symmetry in the relaxed state. However, when the substrate was stretched in the x-direction, a related compression in the y-direction must occur, which breaks the symmetry, allowing for polarization-sensitive results. As the meta-atoms were made up of a high index dielectric material, electric and magnetic dipole resonances were induced and were modified accordingly due to the stretching in the orthogonal directions. The resonance wavelengths were affected not only by the periodicity change, but also by the near-field interactions of the electric dipole moments. By skillfully combining multiple optical phenomena that can be simultaneously modulated with the application of mechanical forces to the device, interesting polarization-sensitive functionalities can be designed or avoided. Outside of solid-state materials, soft materials, such as liquid metals, have also gained some attention, especially in applications of flexible devices. [188] The fabrication processes of these materials are generally easy and convenient though techniques, such as ultrasonication and patterning, can be achieved using the simple printing methods at room temperature. In particular, gallium is an extremely attractive material due to its optical and electrical properties. [189] Metasurfaces using this material with a stretchable PDMS substrate to create a resonance peak in the IR regime have been produced (Figure 7c). [190] The oxidation and substrate states were carefully controlled to allow for the deposition of thin films of liquid gallium. The films were then combined with standard nanofabrication techniques, such as lithography, to realize the meta-atoms to create metasurfaces with optical responses in the IR region. Stability for over 15 000 cycles of 25% strain was demonstrated to prove the robustness of stretchable metasurfaces based on gallium. However, liquid metals have some drawbacks that must be overcome, such as their tendency to oxidize over time and their interactions with moisture in the air.
By controlling the phase of incident light, static metasurfaces have been used in applications, such as metaholograms [191,192] and metalenses. [5,193] Usually, these devices can produce a single image, or be focused on a single plane; but, by incorporating well-designed meta-atoms with stretchable substrates, actively tunable applications have been demonstrated. In one example, a flat optical zoom lens was produced (Figure 7d). [194] At a wavelength of 632.8 nm, the anomalous refraction was adjusted from 11.4 to 14.9 by stretching the Au meta-atoms on a PDMS substrate by %30%. This allowed for a demonstration of a metalens that has a focal length that can be continuously changed from 150 to 250 μm, which could be used as a zoom lens. This kind of functionality is extremely attractive in consumer products, such as mobile and wearable devices. The same group then produced multiplexed holograms. [195] By stretching the substrate, different holographic images can be produced. Although the performance is hindered by the inherent ohmic losses of the plasmonic metaatoms, changing the material to dielectrics, such as Si or TiO 2 , could help to improve the quality of the reconstructed images, whereas a well-designed phase map could lead to practical applications in VR and displays. In a final example of the application of stretchable metasurfaces for continuous modulation of the optical properties, the stretching of a multilayer filter has also been proved as a way to achieve tunable radiative cooling. [196] This could be implemented with the existing high-performance radiative cooling devices [197,198] to give an added layer of tunability and flexibility to allow the user to control the amount of cooling at will. This is an extremely attractive quality for radiative cooling applications [199] and could open the path for nanophotonic-based energy devices to provide solutions for sustainable and clean energy.

Kirigami
Origami-based metamaterials that involve folding and manipulating 2D metasurfaces into 3D objects have been proved to produce devices with exotic optical properties, such as tunable chirality. [200] Kirigami, on the other hand, is the art of cutting and folding to create amazing new shapes and designs and has inspired a new form of metasurface that can be extended into elaborate new shapes and 3D metamaterials. [201][202][203][204] By carefully designing where the cuts or holes of a metasurface are, through additional folding, bending, or twisting of the structure, new shapes can be formed. An in-depth review about kirigami in photonics can be found here. [205] In one of the pioneering examples of nano-kirigami, giant optical chirality in 3D nanostructures was demonstrated. [206] This work was expanded upon to produce tunable metasurfaces and metamaterials. By changing the deformation direction of the planar metasurface, switching of the chirality and, therefore, reconfigurable toroidal CD have been proved. [207] In a demonstration of continuously tunable kirigami metasurfaces, Fano-enhanced CD has been shown ( Figure 7e). [208] By creating shapes with different symmetries, an extremely high CD of 0.61 was experimentally achieved when the metasurface was compressed, whereas chiral transmission was not observed when in the relaxed state. The change from 2D metasurfaces to 3D metamaterials opens up new ideas of out-of-plane chirality. This idea was used to demonstrate an on-chip device that could be combined with optofluidics to realize biosensing in chiral pharmaceuticals. In another realization of active metasurfaces using the art of kirigami, control of the reflection of NIR wavelengths from a spiral shape has been achieved (Figure 7f ). [209] The reflection of the 2D spirals was modulated by 137% through pneumatic pressure that produces a change into a 3D spiral. The repeatability of this functionality was proved for three cycles with a modulation contrast of %20%. This technique has also been achieved using thermal expansion rather than pneumatic pressure. [210] By embedding an array of cuts into stretchable sheets, the hierarchical patterns can be designed to form hinged squares. [211] This allows for a variety of bucklinginduced 3D deformation patterns to be triggered with specific stress-strain relationships. These kinds of adaptable metasurfaces could be used in soft material robotics and could be extremely useful in active sensing.
Although mechanically tunable metasurfaces show extreme promise for active photonics, they require the integration with precise machinery and components to allow for accurate control and are limited by the flexibility and lifetime of the polymer substrate. The selectivity of materials that can be deposited or moved onto these polymer substrates also needs to be improved. The modulation speeds are governed by the mechanical actuations, which will always be slower than other methods. The moving parts of MEMS and the stretching of polymer substrates can also lead to mechanical failure, so the development of substrates that can exhibit high strain and exhibit long lifetimes is paramount.

Individually Addressable Meta-Atoms
In the previously discussed implementations of tunable metasurfaces, the tuning stimulus affects the metasurface as a whole, allowing them to move between two states either directly or via intermediate states. However, complete freedom of the optical properties of a metasurface requires the independent modulation of individual meta-atoms. Again, factors, such as response time, become key obstacles to overcome in order to apply fully reprogrammable and actively tunable metasurfaces into real-world commercial devices. Here, we discuss the demonstrations of completely reconfigurable metasurfaces. As of yet, fabricating the diodes that are capable of individually addressing meta-atoms at the nanoscale is an extremely arduous task, so most of the work presented here functions in the GHz and THz regime where meta-atoms are large enough to be able to fabricate whole systems reliably.
An active field of research for tunable metasurfaces at the meta-atom level is in beam steering, which could hold the key for applications in LiDAR and active holography. Using individual active regions of ITO that are related to the single meta-atoms in a metasurface with 96 elements, a multifunctional device has been demonstrated. [212] The applications of both dynamic beam steering and a cylindrical metalens with a reconfigurable focal length in a single device were proved in the NIR region. The group then improved the performance of the device using an inverse design method. As the individual meta-atoms are designed to be tuned individually through the active ITO layer, an array-level optimization technique could be performed to create high-performance beam steering applications for any desired angle (Figure 8a). [213] The phase profile can be matched after the fabrication of the metasurface through electrical bias to change the phase and amplitude profile, rather than adapting the meta-atoms themselves. High-directivity beam steering that was nonideal meta-atoms was proved, even with a phase modulation range as small as 180 . With another example combining LCs with the individual meta-atoms by creating an anisotropic metasurface, an extra degree of freedom in the number of possible optical responses has also been demonstrated for beam steering. [214] The phase can be controlled for x-and y-linearly polarized light independently and simultaneously, and the metasurface was designed, so that the external stimuli can be encoded with low crosstalk between the two polarization states. Using a field-programmable gate array (FPGA) to control individual meta-atoms, 2-bit coding was achieved. Dynamic holography has been demonstrated in the microwave regime using tunable capacitors. [215] The design allows for multiplexed holography that shows different greyscale images under different polarization of incident light.
Using PIN diodes to control individual meta-atoms, completely programmable metasurfaces in the GHz regime have been realized. [216] An active 1-bit 16 Â 16 unit cell metasurface has been demonstrated to encode various different functions ( Figure 8b). [217] The conversion of linearly polarized light to circularly polarized light in different frequency bands was shown in the frequency domain, whereas in the spatial domain, the reflection phase can be encoded, leading to flexible beam steering and switching between different OAM beams. Other similar work has also been demonstrated, [218] and there are also examples of this functionality in the THz regime. [219] In another example of dynamic modulation using PIN diodes, a metasurface that is capable of producing different states of OAM that was connected to a steering logic board to control it in real time has been produced. Nearly uniform and equal transmission magnitudes with inverse phase states over a wide band and OAM states up to l ¼ AE2 were shown (Figure 7c). [220] An improvement on this 1-bit coding metasurface has been presented as a 2-bit space-time-coding metasurface that is able to break Lorentz reciprocity. [221] By inducing spatiotemporal phase gradients via the digital modulation of the phase of the individual meta-atoms, anomalous reflection and frequency conversions were realized. This kind of nonreciprocal system could be of great use in the fields of energy and nonreciprocal wireless and radar systems. These fully reconfigurable multiplexed metasurfaces have also been created using VO 2 as the actively tunable element, [222] and for fully reconfigurable metalenses, [223] further expanding the applications and tunable mechanisms.
Another interesting application of meta-atom reconfigurability is to produce holographic images. This feat has been achieved in the GHz regime for a 1-bit coding metasurface (Figure 7d). [224] By controlling the input voltage at each meta-atom, the unit cell can be turned on or off, resulting in a difference in phase of around 180 . This allows for the metasurface to produce any arbitrary image by controlling the individual meta-atoms. The switching speed of the PIN diode is measured to be 3 ns, so the integration of the control circuit is an important factor in the switching speed of the entire metasurface. Fully reprogrammable metasurfaces in the visible regime are inherently difficult to fabricate for experimentation. This is due to the nanoscale unit-cell size that requires functional diodes of similar dimensions for electrical tuning, or highly focused and precise control of a laser for optical tuning. Nevertheless, pixel level control of a metahologram in the visible regime has been reported. [225] However, rather than controlling the meta-atoms individually, an SLM is used to control the phase of the incident light and tune the pixels of the output hologram. Through the combination of the encoded phase profile of the metasurface and the control of the phase through the SLM, holographic images can be produced. Optical encryption was demonstrated with this setup, because only the correct phase profile of incident light will decrypt the holographic information accurately. However, as the meta-atoms themselves are smaller than the diffraction limit, each pixel was made up of a 4 Â 4 array to match the size of the incident laser beam. This fact again highlights the difficulty in producing nanoscale devices that are fully programmable at the meta-atom level that work in the visible light regime. Despite this, an example of a large-scale nanophotonic-phased array has been demonstrated to produce far-field patterns. [226] The static system produced to display a desired holographic image as a proof of concept consisted of 64 Â 64 optical nanoantennas on a silicon chip. However, the active metasurface was reduced to an 8 Â 8 array. Each pixel of the array included an independently tunable phase shifter with electrical controls connected in rows and columns. By applying different voltages at Figure 8. Individually addressable meta-atoms. a) Addressable meta-atoms for beam steering. Through the application of a voltage on a single metaatom, the complex dielectric permittivity of the active layer is altered to control the scattered light amplitude and phase of the element for beam steering. The repetition number (RN) refers to the number of adjacent meta-atoms that have the same phase. The inversely designed phase profiles outperform the stairstep designed ones for RN ¼ i) 3, ii) 4, iii) 5, and iv) 6, demonstrating steering angles of 17.4 , 12.9 , 10.3 , and 8.5 , respectively. Adapted with permission. [213] Copyright 2020, American Chemical Society. b) 1-bit programmable reflective metasurface. i) The optical properties of the two states of the meta-atoms. Schematic illustrations of the multiple functions, including ii) dynamic 2D beam scanning, iii) vortex-beam generation of OAM, and iv) directional beam splitting. Adapted with permission. [217] Copyright 2020, American Chemical Society. c) Addressable meta-atoms for OAM generation. Code distributions and measured radiation patterns for different OAM modes, l ¼ i) À2, ii) À1, iii) 0, iv) þ1, and v) þ2 of the fully reprogrammable metasurface. Adapted with permission. [220] Copyright 2020, Wiley-VCH GmbH. d) Addressable meta-atoms for holography. The metasurface is formed by an array of meta-atoms, each with a pin diode welded between two metallic loops and independently controlled by a direct current voltage to produce holographic images. Reproduced under the terms of the Creative Commons Attribution 4.0 International License. [224] Copyright 2017, The Authors, published by Springer Nature.
www.advancedsciencenews.com www.adpr-journal.com each pixel to achieve thermo-optic tuning, different patterns were achieved in the far field in the form of holographic images at a wavelength of 1.55 μm. This system is extremely tolerant to errors in fabrication and becomes more effective with more meta-atoms; therefore, this nanophotonic-phased array device could be expanded up to millions of pixels.
There have been numerous demonstrations of meta-atom level control for metasurfaces in the microwave and THz regimes, with far fewer in the visible and IR regions. The scale of the individual meta-atoms is the biggest stumbling block, whereas functional materials that have a large enough modulation of their optical properties in the visible regime are also lacking. In the example of metaholography at visible wavelengths, the unit cell size must be adjusted to meet the size limitations of the SLM, and this also rings true for using individual unit cells of LCs. The manipulation of individual meta-atoms to produce fully functional metasurfaces at visible wavelengths that can be reprogrammed after their fabrication is a highly sought-after commodity, but limitations in realizing such nanoscale circuitry prove to be a difficult hurdle to overcome. Developments in techniques to miniaturize the required modulation systems, such as LCs or SLMs, or the discovery of new functional materials with a large contrast of their optical properties at visible wavelengths are possible avenues to be explored. Graphene could prove to be a useful material for programmable meta-atoms, with a demonstration in the microwave regime having already been proved. [227]

Discussion and Outlook
As technology moves forward, the demand for high-quality optical devices that consumers can carry around or even wear is becoming more prevalent. Applications in AR and VR devices could be impactful in numerous sectors if safe, fully functional devices become available. With the advances in artificial intelligence (AI), autonomous vehicles, and robotics, compact devices that allow machines to see and sense their environment without the interjection of humans are paramount. Metasurfaces offer a thin, lightweight solution to all of the above, so fully reconfigurable metasurfaces could provide a platform for the next level of advanced technology.
Research on metasurfaces has improved their functionality, performance, and, along with the development of reconfigurable metasurfaces with actively tunable properties, has unlocked new fields of nanophotonics. New design techniques that combine multiple meta-atoms [228] or optical phenomena [229,230] in a single metasurface to produce metasurfaces that are polarization-or direction-sensitive could be useful for the integration of modulation methods that control the polarization of light. The materials that make up metasurfaces are a key ingredient to attain tunable functional devices. Metasurfaces based on semiconductors or TCOs have shown great promise due to their simplicity and readiness to be incorporated with mature CMOS fabrication processes to allow for reliable mass production. However, the intrinsic losses of materials at visible wavelengths, along with the unfortunate locations of their bandgaps, impart some restrictions on how they can be used. An interesting functional semiconductor material that could be useful for active metasurfaces is indium-gallium-zinc-oxide (IGZO). To date, IGZO has been widely used as a thin-film transistor in the backplane of flat-panel displays. Recently, in the field of nanophotonics, a FP-type solid-state tunable color filter that can be switched between different colors quickly and reliably, with a low energy consumption, has been demonstrated. [231] By doping with hydrogen, the carrier concentration can be modulated, causing a large shift in the refractive index. If a method of inducing an electrical bias across the IGZO layer to control the number of carriers actively, the switching mechanism could be extended to a fully continuously tunable one. The examples of the tunability of zinc oxide [232] and refractory metals, such as titanium nitride and zirconium nitride, [233] have also been demonstrated, providing various new materials to be explored. The development of new PCMs could open lots of new fields, with exciting new materials, such as antimony trisulfide (Sb 2 S 3 ), that have attractive properties in the visible regime. [234] An experimental demonstration of rewritable subdiffractional resolution color printing with Sb 2 S 3 using femtosecond laser pulses has already been demonstrated. [235] In other work, it has also been shown that the lifetime of the phase change of optically switched Sb 2 S 3 strongly depends on the thickness of the layer and the ambient temperature. [236] Interestingly, the pulse energy used to amorphize the material also has an effect on the lifetime of the phase change.
Another PCM that has been introduced recently is In 3 SbTe 2 (IST). [237] Direct writing and erasing of metallic crystalline IST meta-atoms were demonstrated in the IR regime. These new generation PCMs could help to push applications of tunable nanophotonics to the next level.
Other mechanisms for tunable metasurfaces that were not covered in this review have also been established. The electrically modulated mechanisms described in this review exploit free carrier modulation; however, another type of electro-optic metasurface based on III-V multiple-quantum-well structures has also been demonstrated. [238] These interesting new developments could offer new insights and bring multiple new functionalities to metasurfaces for use in the real world. Another rather simple but effective way of actively tuning metasurfaces through controlling the local refractive index of the background medium is to fill it with a different substance. Subsequently, high index liquids have been proposed to achieve active switching in metasurfaces. [239] The high index liquid can be easily injected and removed to modulate the local background index of the metasurface and, therefore, the optical response in the far field. Nonlinear responses of materials are another interesting tuning mechanism based on optical pumping. These responses come from materials that have a dielectric polarization that has a nonlinear dependence with the application of an external electric field, generally the electric field of light. This is usually observed at high intensities of light that can be generated through the use of lasers to optically excite the material. Metasurfaces are interesting for nonlinear physics due to the resonant field enhancements that generally increase the effective field strength that is confined in small gaps or structures. This can lead to an increase in the nonlinear susceptibility. Common nonlinear optical effects that have been used in metasurfaces include the secondharmonic generation (SHG), third-harmonic generation (THG), and high-harmonic generation (HHG). [240] These effects are frequency-mixing processes, where the frequency of the emitted photons is 2 (SHG) or 3 (THG) times the frequency of the www.advancedsciencenews.com www.adpr-journal.com pumped light. To achieve these nonlinear effects, materials that exhibit nonlinear responses must be included into the metasurface design, either as an active layer or as the meta-atom itself. We direct the reader to numerous reviews and book chapters dedicated to nonlinear effects in metasurfaces that can give a deeper insight into the fundamentals and mechanisms in nonlinear metasurfaces. [241][242][243][244][245] New mechanisms such as SHG in the backward direction using phase-matched stacked metasurfaces that extend the 2D metasurfaces into 2.5D [246] and the spin-orbit angular momentum interaction of light [247] have been reported. By designing the metasurface to produce both SHG and THG with comparable amplitudes, nonlinear bicolor holography using a single laser source has also been introduced. [248] These ideas could be stepping stones to further advances in nonlinear applications of metasurfaces with switchable properties. One important issue that needs to be overcome for the integration of nonlinear metasurfaces into real-world applications is the efficiency of the nonlinear optical processes. As this is based on the intricate design of the metasurface, from the constituent materials, meta-atoms geometry, and layout, to the integration of an optical pumping source, there remains plenty of potential for improvements in this area. For example, combining the nonlinear response of materials with other tuning mechanisms is one avenue that is being actively explored, such as in the nonlinear response of ENZ TCOs, such as AZO [249] and ITO, [250] the integration of graphene, [251][252][253] in kirigami-based metasurfaces, [254] and through the nonlinear responses of PCMs. [255] The switching mechanisms of metasurfaces here all have their inherent advantages and drawbacks. For switchable metasurfaces, PCMs are an obvious choice. The variety of options, along with the possibilities of doping and engineering new variations of wellunderstood PCMs, allow for a broad range of choices. While PCMs can be switched electrically and optically, thermal modulation has taken the lead in research due to the accessibility of simply heating and cooling a sample. However, this leads to fundamental problems due to the phase transition temperatures of GST-based PCMs being fairly high and the volatile nature of the transition of VO 2 , meaning that any external source must be continuously applied to it. On the other hand, these drawbacks could be seen as benefits for the use in high-temperature environments or for passive modulation due to the ambient temperature. The optical properties of these PCMs offer a straightforward modulation method over broad wavelength regimes, even down to the visible. The hydrogenation of metal oxides provides an interesting modulation option but is generally slow due to the nature of chemical reactions and needs specific environmental conditions to be used. On the other hand, the unique transitions that chemical reactions allow, such as the directionality, are properties that is hard to engineer with other methods. With work being done to speed up the reaction times, the hydrogenation and oxygenation of metal oxides could prove to be an essential technique for switchable metasurfaces. For LCs, their multiple modulation stimuli and well-understood fabrication methods are extremely useful for tunable metasurfaces. However, the difficulty of controlling individual LC cells at the micro-and nanoscale limits the development of fully functional LC-based devices. The thickness of LC cells is also another limitation, as it can aid to decrease the performance and overall usability, as it increases the minimum dimensions of the unit cell of the meta-atoms. As new types and phases of LCs are uncovered and new methods of controlling them are introduced, the possibility of meta-atom level controllability could be reached. A pixelated LC-mediated metasurface display has already been demonstrated; however, it is only able to display a single color and black. [119] Nevertheless, big steps toward fully active metasurfaces using LCs have been taken. Semiconductors are an obvious choice for tunable metasurfaces, owing to their well-understood fabrication conditions and optical and electrical properties. Problems arise from the breakdown voltages of certain materials that limit the amount of electrical bias that can be used and the bandgaps of the materials that limit the optical properties in the visible region. However, through careful consideration of the deposition parameters and fabrication environments, the optical properties and bandgaps of the deposited thin films can be controlled to enhance the refractive index or lower the extinction coefficient based on the morphology of the deposited material. [256,257] Mechanically tunable metasurfaces are an interesting option, although their working mechanisms require extremely precise physical manipulation in 2D and 3D, which is burdensome for the integration of flat metasurfaces into physical devices. However, with advances in MEMS technology and research into origami and kirigami-based metasurfaces, new novel tuning methods could be uncovered. Mechanically tunable metasurfaces will always be limited in terms of switching speed by the mechanical components that induce the stress or strain to the metasurface, which is generally slower than other methods. In addition, mechanically moving parts should be robust, with long lifetimes to produce devices that can be used extensively without requiring expensive repairs. For individually addressable meta-atoms, the method of applying a stimulus to a certain meta-atom without disturbing neighboring meta-atoms is the most crucial point. For metasurfaces with working wavelengths in the THz and GHz regimes, the larger meta-atoms can be electrically tuned through PIN diodes and through other standard electrical contacts. However, at shorter wavelengths, such as in the visible and near-IR regimes, this becomes much more difficult. The subwavelength meta-atoms in unit cells with dimensions in the order of hundreds of nm make the integration of electrodes for electrical tunability less than straightforward. TCOs can be used as transparent electrodes, but still face the same difficulties of miniaturization. Optical tunability could be a potential solution if the integration of the precise control of the required lasers can be achieved.
Outside of the demonstrations presented in this review, metasurfaces have applications in many other fields outside of optics and photonics, which could all benefit from research into fully active devices. Metasurfaces have been used in abundance in the biomedical field, where more stringent factors related to materials must be considered. [258,259] The toxicity and biodegradability of materials are important in the biomedical field, and extra limitations on how the modulation mechanisms can be achieved exist due to the interaction with human subjects. Therefore, tunable metasurfaces based on non-toxic liquids such as water could offer a potential solution, [190,[260][261][262][263] as could the use of organic materials such as chitosan for the active components. [264] For applications, such as invisibility cloaking, passively reconfigurable metasurfaces could be a key in hiding a target without any external intervention. [265,266] This kind of metasurface could be described as adaptive rather than active, as they sense and adapt to their surroundings automatically. Active metasurfaces are not only confined to EM waves, but are also being developed for acoustic waves with numerous devices already been proved. [267][268][269] The impact of fully active metasurfaces has the potential to be groundbreaking and push technology as we know it to whole new levels.
Although there are certainly situations where a highperformance single-functionality passive metasurface is more than sufficient, the extended flexibility and potential practical applications that active metasurfaces can offer are undeniable. Similar considerations simultaneously exist in traditional photography with conventional optics, where a photographer can choose between a faster prime lens with a larger aperture but a fixed focal length or a more flexible zoom lens at the expense of added bulk and a smaller aperture. After years of development, although the compromise between performance and functionality has been reduced significantly, the photographer must still choose the best tool for the job at hand. In the field of nanophotonics, at the present time, performance may need to be compromised to achieve active metasurfaces, but as the fundamentals of light-matter interactions at the nanoscale are uncovered, along with progress in nanofabrication techniques and materials science, high-performance active nanophotonics are set to open the door to applications of devices with unprecedented properties, whereas the research and development of passive metasurfaces will also continue to grow hand in hand. These advancements could see some rapid growth due to the amount of computational power available to research laboratories through the use of high-performance central processing units and graphics processing units, which has opened up exciting opportunities for powerful computational inverse design in the field of nanophotonics, [270] with topology optimization of meta-atoms and metasurfaces proving to be a valuable tool in the field. [271,272] Computationally efficient full wave simulation tools that quickly solve Maxwell's equations are a key component for computational design and also lend themselves well to generate valuable big data and large datasets that have been successfully used with machine learning algorithms and artificial intelligence (AI) with applications in nanophotonics and active metasurfaces. [273][274][275][276][277][278][279][280][281][282][283][284][285][286] In particular, the design of meta-atoms and complete devices with arbitrary and unintuitive geometries could expand our understanding of physics at the nanoscale, while producing devices with ever-increasing performance and multiple, tunable functionalities. The difficult task of optimizing designs for multiple optical responses in for multiple states of a material, such as ensuring that full 2π coverage can be realized in both the amorphous and crystalline states of a PCM, can also be achieved with relative ease. There will be many more fruitful avenues to pursue by incorporating the fundamental physics of metasurfaces with real-time processing using computer-driven systems [287,288] and devices to actively control individual meta-atoms with unprecedented speed and precision. Tunable metasurfaces that are completely designed and controlled by AI could be a reality in the near future.