A review of light‐controlled programmable metasurfaces for remote microwave control and hybrid signal processing

Programmable metamaterials and metasurfaces have gained great interest due to real‐time electromagnetic control and digital information processing capabilities. Different from traditional electrically‐controlled programmable metasurfaces, recently, light‐controlled programmable metasurfaces have been presented, on which the microwaves can be manipulated and modulated wirelessly in the space domain or time domain by incident light. More importantly, such the metasurface platform offers an efficient interface to link directly light and microwave signals, showing huge potential to develop wave‐based optoelectronic hybrid devices and relevant applications. Here, we review some of these recent developments, focusing particularly on the mechanisms of light‐controlled programmable metasurfaces and their fascinating functions from remote microwave control to hybrid signal processing. We survey related implementation methods based on the hybrid integrations of microwave digital metasurfaces and different photoresponsive components including photodiodes, photoresistors, infrared modules, and light sensors, as well as discuss their unique advantages. In the summary, the perspective on the challenges and future directions of light‐controlled programmable metasurfaces are presented.

two-dimensional (2D) and planar version, and thus metasurface emerge as the times require. 6 Metasurface consists of many subwavelength artificial elements arranged on a surface, which not only can control accurately electromagnetic (EM) waves, but also provides more opportunities for engineering applications due to 2D properties, and has aroused enormous interest and extensive attention in many fields.  In 2011, the generalized Snell's law was proposed by introducing abrupt changes at metasurface interface, 10 and provides an effective method to design metasurfaces, and in turn shows the strong manipulation capabilities of metasurfaces. Guided by the generalized Snell's law, numerous metasurfaces have been constructed to control amplitude, phase, polarization, frequency, and more other features of EM waves. With these ability and advantages, metasurfaces show great potential for applications in antennas, [16][17][18][19] polarization converters, [20][21][22] absorbers, 23,24 cloaks 25,26 and so on.
Recently, the design of active metasurfaces with tunability or/and reconfigurability has attracted great attention from related researchers all over the world, which offers an actual route to realize advanced multifunctional devices. [27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] The typical approach to implement active metasurfaces is to integrate sensitive materials such as semiconductor, phase change material or 2D material into metasurface element (meta-element), and then control dynamically it by external electrical, optical, or thermal stimuli. At microwave and millimeter-wave frequencies, the most common and effective way to construct active metasurfaces is to use varactors or positive-intrinsic-negative (PIN) diodes. 27,28 To further achieve the real-time control ability, programmable metamaterial and metasurface were proposed and demonstrated in 2014. 44 In programmable metasurface, different EM responses of meta-elements are characterized by digital "bits." In this digital manner, programmable metasurface can generate many obviously different functions on a single physical platform by changing simply the digital coding sequences in real time. [45][46][47][48][49][50][51][52][53][54][55][56][57][58] Therefore, programmable metasurface can be considered as a high-level active metasurface. In addition to the spatial EM control, programmable metasurfaces with rapid time modulation can be used to tailor EM spectrum, yielding the digital time-domain metasurfaces. [59][60][61][62][63][64][65][66] By combining with sensors and artificial intelligence, programmable metasurface can be extended to develop intelligent metasurface with self-adaptive ability. [67][68][69][70][71] More importantly, because of real-time switching capability, programmable metasurface can realize information processing while manipulating EM fields and waves, giving rise to a new concept of information metasurface. [72][73][74][75][76][77] Information metasurface bridges the physical world and digital world, which provides an efficient platform for controlling wave-information-matter interactions, thus enabling many attractive devices and systems, such as simplified-architecture transmitter, 64,77 real-time imager, 49,78 programmable radar, 75 and so on. [79][80][81][82][83] Electrical control is a popular way to achieve programmable metasurfaces. However, it needs to design additional direct-current (DC) bias networks on metasurface, which will affect its EM performance and increase overall design difficulty. Moreover, electrically-controlled programmable metasurfaces are typically focused only on controlling and processing single-domain signals (e.g., microwave signals), lacking of processing of multidomain signals across different frequency bands. In contrast to wired electrical control scheme, optical control is a non-contact remotely tuning method that provides more possibilities to release these limitations. [84][85][86][87][88][89][90][91][92][93][94][95] Figure 1 shows conceptually the light-controlled programmable metasurfaces, on which microwaves can be controlled and modulated by incident light in real time, offering a multiphysics interface to link light and microwave in space. [96][97][98][99][100][101][102][103][104][105][106][107][108][109][110] Light-microwave interaction is of fundamental important for some emerging fields including microwave photonics 111 and lightwave electronics. 112 Light-controlled programmable metasurfaces make it possible to achieve direct light-microwave-matter interactions on a single physical platform, which is helpful for developing and realizing advanced wave-based optoelectronic hybrid devices and applications. On one hand, light-controlled programmable metasurfaces can be used to realize optically-driven and optical-sensing EM devices for wireless tuning of functions, such as beam tuning, cloaking, absorbing, and nonlinear generation. [96][97][98][99][100][101][102][103][104][105][106][107][108] On the other hand, light-controlled programmable metasurfaces are able to realize optoelectronic mixed signal processing including wave-based information modulation and direct light-to-microwave conversion, which is the basis and key for hybrid wireless communications and quantum networks. [113][114][115][116][117][118][119] In this article, we review recent progress on light-controlled programmable metasurfaces, including two kinds of important applications of remote microwave control and hybrid signal processing. We first present the several practical implementations to achieve different light-controlled microwave scattering functions in space domain, such as beam tuning, external cloaking, vortex beam generation, and dynamic absorbing. Then, we introduce the currently exciting progress on the time-modulated light-controlled programmable metasurfaces that can be used to realize microwave spectra control, and further show their unique advantages for direct signal modulation and conversion in hybrid wireless communications. Finally, we give a summary and possible future research directions of broad interest. F I G U R E 1 Schematic overview of light-controlled programmable metasurfaces. Such the metasurface platforms are constructed by the hybrid integrations of digital EM metasurfaces and photoresponsive materials or components, which can be used to realize microwave remote control and hybrid signal processing for advanced wave-based optoelectronic hybrid devices and related applications.

REAL-TIME AND REMOTE CONTROL OF MICROWAVES
The first unique advantage of light-controlled programmable metasurfaces is that it can avoid effectively complex bias network design and adverse DC-EM signal crosstalk, achieving wireless and non-contact tuning of microwaves by light. To achieve optical control, the photoresponsive materials or components are essential. In this section, recent developments in light-controlled programmable metasurfaces based on different tuning techniques including photodiodes, photoresistors, infrared modules and light sensors are described.

Photodiode-enabled approach
As an exquisite candidate, photodiode can be adopted to construct light-controlled metamaterials and metasurfaces due to high-sensitivity photoelectric response and mature packaging. In 2018, a light-controlled programmable metasurface based on photodiodes and varactors was proposed, 96 as shown in Figure 2A. The light-controlled programmable metasurface contains digital meta-elements based on varactors and a photodiode series array used to receive light signal. When light is projected onto the photodiode array, it can convert various illumination intensities into different bias voltages based on photovoltaic effect, which are then used to control the varactor loaded in the meta-element for tuning its microwave reflection phase. The photodiodes were connected in series to provide enough bias. In this case, the phase distribution on the metasurface aperture can be controlled dynamically by light, thus realizing real-time manipulation of microwave reflection beams. As a demonstration, a light-controlled 1-bit digital meta-element was realized to achieve a 180 • phase difference. Using such the digital meta-elements, a light-controlled programmable metasurface was fabricated and measured, and the experimental setup is shown in Figure 2B. The metasurface consists of six columns along y-direction and every column can operate at "0" and "1" states by changing light intensities. In measurement, a light-emitting diode (LED) array was adopted to generate different coded light signals to driven the metasurface sample and the measured far-field scattering pattern at 3.75 GHz under two different coding sequences are given in Figure 2C,D, respectively. Measured results prove that the metasurface can well generate one beam and split beams under these two coded light illuminations, showing the effectiveness of the proposed light-controlled microwave scattering approach.
Recently, the more advanced version of photodiode-based light-controlled programmable metasurface was proposed and realized, in which the microwave phase distributions of metasurface can be controlled wirelessly in 2D directions by light patterns, enabling more interesting functions, 97 as seen in Figure 3A. The designed metasurface consists of multiple identical subarrays. The front of the subarray is 4 × 4 meta-elements based on varactors, and the back is the optical interrogation network (OIN) based on a photodiode series array. The side length of the meta-element is 10.0 mm and the thickness is 2.0 mm. The working principle of the light-controlled subarray is the same as that described in the above-mentioned work, that is, the varactor integrated in the meta-element is controlled by the bias generated by OIN, thus achieving the microwave reflection phase tuning. But in this design, the meta-element with carefully selected "MA46H120" varactor (from MACOM) has a wider bandwidth and a lager phase difference. Simulation results show that the meta-element can realize an accurate 180 • phase difference ranging from 5.2 to 7.6 GHz, and the maximum phase difference exceeds 270 • . More importantly, each subarray can be controlled independently by illumination and thus different EM functions, such as external cloaking, microwave illusion and dynamic vortex beam generation can be realized by receiving projected light patterns. To verify the approach, a light-controlled programmable metasurface sample composed of 6 × 6 subarrays was fabricated and its front and back views are shown in Figure 3B,C, respectively. To generate different light patterns to control remotely the sample, a light source with 6 × 6 LED spotlights was designed and fabricated. The distance between the light source and metasurface sample is 60 cm. Figure 3D illustrates the experimental setup in a microwave chamber, where each subarray can be controlled accurately and independently by the light source. Measured results show that the three different functions of external cloaking, illusion and vortex beam generation can be realized under the required illumination patterns, as shown in Figure 3E-G, respectively, which are in good agree with the simulation ones. Besides the above discussed reflective light-controlled programmable metasurfaces for microwave phase control, in 2018, a transmission-type light-controlled programmable metasurface was proposed and realized for microwave amplitude tuning, 98 as shown in Figure 4A. The designed digital meta-element is no grounded and has a circular-ring gap loaded with a varactor. The geometrical parameters of the meta-element were optimized as a = 18.0 mm, h = 2.0 mm, g = 0.4 mm, and r = 8.4 mm. To optically control the capacitance of varactor for realizing microwave transmission manipulation, a photodiode series array was designed to provide a DC reverse voltage under external illuminations. In such a case, the microwave transmission amplitude can be programmed by illumination light in real time. The simulated amplitudes of the transmission coefficient S 21 of the designed meta-element are presented in Figure 4B. It is obvious that the transmission responses can be controlled dynamically by switching the capacitances of the integrated varactor. In addition, the meta-element can produce two transmission peaks due to the dual-mode resonance characteristic. Different from the common phase coding in digital and programmable metasurfaces, in this work, the amplitude coding was performed. Specifically, the meta-element whose transmission amplitude below −13 dB is encoded as a "0" unit and one whose transmission amplitude above −1 dB is encoded as a "1" unit. As an experimental verification, a transmissive light-controlled programmable metasurface was fabricated, which consists of 15 × 15 digital meta-elements and 70 photodiodes located by the side of the metasurface, as shown in Figure 4C. The measured transmission coefficients of the metasurface sample are shown in Figure 4D. At 3.28 GHz, the digital meta-element acts as a "0" unit with zero light intensity and as a "1" unit with light intensity of 6000 lx. By contrast, at 5.96 GHz, the digital states are switched under these two light intensities. Therefore, the transmission states can be well controlled by visible light and their also depend on the working frequencies. It is noted that the photodiode-based light control scheme can also work at millimeter-wave band by using the photodiode to drive the millimeter-wave meta-element.

Photoresistor-enabled approach
Recently, a light-controlled programmable metasurface with a controllable modulation range of microwave reflection phase based on photoresistors and varactors was proposed and realized, 99 as schematically shown in Figure 5A. With this design, the EM scattering fields can be controlled dynamically by light for generating different functions, such as scan beams and orbital angular momentum (OAM) beams. Figure 5B presents the designed light-controlled digital meta-element consisting of a reflection phase unit and an OIN with two dielectric layers and three metal layers. The circuit diagram of the OIN is shown in Figure 5C, in which the photoresistor R 1 and two fixed resistors (R 2 and R 3 ) are connected in series. In such a case, by illuminating the photoresistor, its resistance can be tuned in real time and thus the voltage on the photoresistor will be changed accordingly. Then, the varactor connected with the photoresistor in parallel can be controlled by light for realizing phase tuning. In addition, all the OINs are connected in parallel and powered by the same voltage source, which could simplify the metasurface design from unit to array. Based on optimization, a 2-bit digital meta-element with four different states was realized at 6.35 GHz under light control, as shown in Figure 5D. As a demonstration, a light-controlled programmable metasurface with 10 × 10 such meta-elements was fabricated and tested to achieve microwave pencil beam scanning and OAM beam. Another similar approach was also proposed to realize the light-controlled programmable metasurface, in which a photoresistor and a voltage-driven module are connected to control each row of PIN-diode-loaded meta-elements for achieving beam deflection based on anomalous reflection. 100 In above two designs, the photoresistors were used to affect circuit voltage under light illuminations for controlling the integrated diodes in metasurfaces. Besides this working principle and microwave control, the photoresistors were also adopted as the tunable components for realizing the optically controllable transformation DC illusion device, 101 as schematically shown in Figure 5E. When external light source is turned off and on, the functions of the device can be switched dynamically between DC cloaking and illusion. Figure 5F shows the tested potential-distribution comparison along the observation line for light "off" and "on" states, which demonstrates markedly the different behaviors of the light-controlled DC device.

Scheme based on infrared transceiver
In addition to visible light, recently, infrared light was also adopted to control microwave metasurfaces for a longer control distance. In 2020, an infrared-controlled programmable metasurface based on designed infrared transceiver was proposed and realized, 102 as depicted schematically in Figure 6A. In this light control scheme, the infrared transmitter and receiver were used to send and capture digital control information, respectively, through the infrared rays. When receiving the control signal, the infrared receiver will instruct the connected field programmable gate array (FPGA) to output the driving voltages with pre-designed coding sequences for controlling the varactors integrated in the metasurface. Therefore, the microwave reflection phase can be tuned by the infrared ray in real time, thus realizing the beam splitting and beam scanning based on phase modulation. More recently, the infrared control method was further extended to implement the light-controlled programmable and scalable reconfigurable intelligent surfaces (RISs) for wireless communications. 103 The RIS was achieved based on the 1-bit digital metasurface, which consists of many building blocks and each block contains four independently phase-switchable patches based on varactors and a common microcontroller based on the photodiode. Each block responds only to its own pre-coded address from a sequence, and then extracts the information for generating control signals to change the reflection phase states of patches. With this approach, all the blocks can be controlled independently and remotely by receiving the infrared rays illuminating the entire RIS aperture, without any complex wire control circuitry connecting them together. That is to say, the microwave phase distribution of the RIS can be controlled in 2D direction for strong functional reconfigurability. Moreover, the size of the infrared-controlled RIS can be scaled freely by changing the number of building blocks, which further demonstrates the advantages of wireless light control schemes. As well as the phase of an EM wave, its amplitude is also an important property and many useful devices can be developed by amplitude modulation. In 2022, Liu et al. proposed an infrared-coded dual-polarized metasurface absorber for realizing remote-control of microwave amplitudes, 104 as illustrated in Figure 6B. The key of such the programmable metasurface absorber is introducing the infrared-coded remote-control system composed of an infrared transceiver and an intelligent DC voltage regulation module into the PIN-diode-based metasurface. The infrared receiver is used to identify the different 8-bit encoding signals emitted by the remote infrared encoder. Then, the coding control signals are transmitted to the intelligent DC voltage regulation module for driving the loaded PIN diodes. In this case, the microwave absorption properties in dual polarization states can be tuned independently by infrared ray in real time.

Scheme based on light sensor
Light-sensing technique has been generally applied in display, communication, and imaging by detecting the intensity, wavelength, and other characteristics of light. Therefore, more opportunities can be provided by using multifarious light sensors for developing light-controlled programmable metasurfaces. Recently, different from the previously discussed work using light intensity as control parameter, a trichromatic-color-sensing metasurface with reprogrammable EM functions was proposed and realized. 105 By integrating four light color sensors into the corner of the metasurface, it can detect the blue, green, and red components in incident visible light and then generates various microwave functions including dual-beam, four-beam, and radar-cross-section (RCS) reduction, as shown in Figure 7A. One advantage of light-sensing method is that it can be adopted to realize self-adaptive EM control by further introducing the sensing-feedback circuit into metasurface. In 2021, Yu et al. proposed and constructed a smart metasurface that can achieve microwave beam control automatically based on image recognition. 106 Figure 7B presents the designed smart metasurface platform, in which two independent cameras were integrated in the metasurface as light sensors for detecting the position of the objects in real time. In addition, a single-board computer was adopted for processing the images captured by the cameras. With this design, the location information of external objects can be sensed in real time, as shown in Figure 7C. In this case, the single-board computer can generate the driving voltages with required coding sequences to change the beam direction of the metasurface self-adaptively and rapidly, realizing automatic tracking of microwave beam ( Figure 7D). We have reviewed several different mechanisms for realizing the light-controlled programmable metasurfaces. For photodiode-enabled approach, the external light can provide the control signal and driving signal simultaneously based on photovoltaic effect. But for the scheme of combing light modules/sensors and voltage drive circuits, illuminating light just offers the control information to instruct the circuits to generate the corresponding driving signals. For both the methods, the microwaves can be controlled wirelessly by light in real time and remote fashion.

LIGHT-CONTROLLED TIME-DOMAIN METASURFACE AND THE APPLICATIONS
Besides the wireless control of microwave scatterings in space domain, the light-controlled programmable metasurface can be further constructed to realize microwave spectra control based on time modulation. In this section, we present the latest progress on light-controlled time-domain metasurface and its important and unique application for wave-based optoelectronic hybrid signal processing.

Light-controlled time-domain metasurface
In 2022, Zhang et al. proposed and implemented a light-controlled time-domain metasurface, from which the microwave reflection spectra can be tailored by time-varying light signals with periodic coding sequences, 107 as schematically shown in Figure 8A. This realization was achieved by integrating the digital metasurface with the designed high-speed photoelectric detection circuit composed of a photodiode and two cascaded transimpedance amplifiers. When light illuminating the photoelectric detection circuit, it can output corresponding voltage for controlling the varactors in metasurface, and thus the microwave reflection phase can be changed by light intensity in real time. It should be noted that for EM control in time domain, the incident light intensity is changed quickly with time. In this design, two different intensities of light were encoded as digital "0" and "1," under which a 180 • phase difference can be produced by metasurface. Therefore, when receiving the periodic light signal with a periodic coding sequence, the time-domain metasurface will generate harmonics based on phase modulation. As an experimental verification, a light-controlled time-domain metasurface sample was fabricated and measured. Figure 8B,C shows the tested microwave reflected harmonics under the designed two different time-coding sequences with modulation frequency of 200 kHz. It is clearly that the metasurface sample can generate the symmetrical spectrum and white-noise-like spectrum upon reflection under these two time-coding sequences, which indicates that the reflected harmonics of the metasurface can be well controlled by light signals. Moreover, the metasurface is polarization-insensitive due to the rotation symmetry property. In our designs, the switching speed of the time-domain metasurface mainly depends on the response time of the used varactor and the photoelectric detection circuit. Except for the visible light source, a laser source can be also used to modulate the metasurface, because the used photodiode can work under the wavelength ranging from 400 to 1100 nm. metasurface dispersion characteristics to implement the frequency division multiplexing (FDM), which improves greatly the information processing capability and efficiency. Benefiting from these properties, a dual-channel light-to-microwave wireless link was built up based on the hybrid metasurface transmitter, in which two different videos can be transferred from the optical transmitter to microwave receiving terminal simultaneously and independently ( Figure 9). It is important to note that unlike the microwave photonics technologies typically target fiber and on-chip methods, the light-controlled time-domain metasurface can realize hybrid signal processing based on wave modulations, which could stimulate attractive information-oriented applications. The fabricated light-controlled time-domain metasurface is shown in Figure 10A, which contains a full-polarization metasurface based on varactors and a high-speed photoelectric detection circuit based on a photodiode and two cascaded transimpedance amplifying circuits. The photoelectric detection circuit was designed to achieve a fast-switching time and a large voltage output range. With this hybrid integration design, the metasurface can generate a certain reflected microwave harmonic based on phase modulation under the illuminating of periodic light signal with a special intensity F I G U R E 9 Light-to-microwave transmitter based on light-controlled time-domain metasurface. A dual-channel light-to-microwave wireless link can be realized using the hybrid metasurface transmitter, in which two different videos can be transferred from the optical transmitter to microwave receiver simultaneously and independently (reproduced from Reference 108, with permission from Springer Nature).

Light-to-microwave transmitter for hybrid wireless communications
waveform. Thus, the digital information can be modulated on the light waveforms and then are mapped directly onto the reflected microwave frequencies, achieving the direct light-to-microwave signal conversion. Figure 10B illustrates the light waveforms for efficiently producing the microwave blue-shifted and red-shifted components at several different frequencies. It is clearly that the required light waveforms at different microwave frequencies are obviously different, which indicates that one light waveform can carry two digital signals simultaneously under microwave incidences of two corresponding frequencies, realizing the FDM. As demonstrations, a dual-channel hybrid wireless communication system was built, which mainly contains an optical transmitter, a metasurface signal converter, and a microwave receiver, as shown in Figure 10C. The optical transmitter and metasurface signal converter form the light-to-microwave hybrid transmitter. In experiments, two different videos ( Figure 10D) were first modulated onto the visible light signals, which were then converted directly to two BFSK signals. These two reflected BFSK signals were received and demodulated by the microwave receiver to recover the two videos independently and simultaneously, as shown in Figure 10E. The transmission rate of the hybrid communication system is 100 kbps, which mainly depends on the response speeds of the photoelectric detection circuit and the programmable metasurface as well as the used modulation mode. To improve communication rate of the hybrid system, an effective solution is to develop high-order modulation formats on the programmable metasurface, such as quadrature phase shift keying and quadrature amplitude modulation.
We have reviewed and discussed currently exciting advances on the light-controlled microwave functions in the time domain, including real-time spectra manipulation, wave-based information modulation and direct light-to-microwave conversion. These fascinating features further demonstrate the potential and advantages of light-controlled programmable metasurfaces.

SUMMARY AND OUTLOOK
In summary, the distinctive feature of programmable metasurfaces is that they can control EM fields/waves and process digital information simultaneously. During the past 9 years, programmable metasurfaces have exhibited unprecedent capabilities in many aspects, from real-time EM manipulation to advanced device realization and novel system development. As a special kind of programmable metasurfaces, light-controlled programmable metasurfaces can control microwave by spatial light, offering more possibilities to realize some unique functions and applications. In this article, we have provided a comprehensive review of the light-controlled programmable metasurfaces for remote microwave control and hybrid signal processing. We summarized different methods to construct the light-controlled programmable metasurfaces for wirelessly manipulating microwave reflections and transmissions in the space domain, and then presented the light-controlled time-domain metasurfaces for microwave spectra control and light/RF hybrid wireless communication.
Applying light to control and modulate microwave based on metasurface offers a platform to link light and microwave in free space, enabling a new degree of freedom in wave-information-matter interaction, which is helpful to further explore more physical phenomena and interesting applications. In view of the present trends of light-controlled programmable metasurfaces, we point out three promising research directions in this field based on our perspectives.

Designing direct light control scheme
In current demonstrated light control schemes, both the photosensitive components and electronic components are required. Although the joint control method is very effective and robust, it will add an additional photoelectric conversion process and metasurface performance mainly depends on varactors and PIN diodes. Thus, this approach can be considered as the indirect light control. To achieve the direct light control, novel tuning mechanisms are needed to be explored by adopting photosensitive material (such as photodiode, silicon, and graphene) itself as tunable element to interact with microwave metasurface. In this case, the EM response of metasurface can be completely manipulated by light through directly changing the characteristics of photosensitive materials under illumination, without involving any electrically-dependent materials and steps.

More wave-based optoelectronic information systems and better performance
Light-controlled programmable metasurfaces provide an actual route to control and process hybrid optoelectronic signals based on wave modulation. Besides the currently demonstrated metasurface-based light-to-microwave wireless communication system, in the future, more wave-based optoelectronic information systems could be developed by further investigating this feature, such as optical-sensing computing system, light-controlled radars and emerging quantum network system. In addition, the system performance including efficiency and communication rate should be further improved by exploring higher-speed physical devices and higher-order modulation methods. It is worth mentioning that the metasurface-based optoelectronic systems can use the advantages of both light and RF carriers to process signals, which shows great promising for current fifth-generation and future sixth-generation information applications. 115,120

Metasurface-enabled spatial light-microwave interaction
In the current demonstrations, many efforts have been only devoted to use light to control microwave propagation. As we pointed out in this article, light-controlled programmable metasurfaces build a bridge to directly link light and microwave. Therefore, many other light-microwave interaction functions can be further developed by referring to the ideas in microwave photonics, such as light-controlled microwave generation and amplification. In turn, how to use metasurface to realize microwave control of light is also an interesting and important topic. In sharp contrast to microwave photonics technology where interaction between light and microwave is achieved on fibers or circuits, metasurface enables free-space light-microwave interaction, which may open new avenues for developing advanced wave-based optoelectronic platforms and systems.