Review on Metasurfaces: An Alternative Approach to Advanced Devices and Instruments

This paper reviews the-state-of-the-art of electromagnetic (EM) metasurfaces and emergent applications in advanced integrated devices and instruments from the design method to physical implementation. The design method includes the analytical coupled mode theory model and commonly used building blocks to construct functional metasurfaces. The modeling approach creates a common design basis of metasurface devices for optical beam steering, focusing, modulation, lasing, and detection. The proof of concept of metasurfaces has been established and is translating to practical applications. Previous studies demonstrated promising applications of metasurfaces including but not limited to optical imaging instruments, biochemical sensing devices, and multifunctional microoptoelectromechanical systems (MOEMS). Signi ﬁ cant performance improvement of devices and instruments has been achieved due to the implementation of specially tailored metasurfaces. This review provides an alternative for researchers to step forward on the way of advancing devices and instruments by the deployment of metasurfaces.


Introduction
Metamaterials, consisting of subwavelength unit cells, represent a type of artificially engineered materials with effective properties, including permittivity [1], permeability [2], chirality [3], and other physical properties [4]. In metamaterials, the shape, geometry, and constituent elements of the subwavelength unit cells and the array fashion jointly determine the effective properties of these artificial materials. Ideal metamaterials are the three-dimensional (3D) array of unit cells, exhibiting bulk, effective electromagnetic (EM) responses [5]. However, due to the difficulties in constructing truly 3D metamaterials and the high insertion loss in the bulk metamaterials, the two-dimensional arrays of subwavelength unit cells, namely metasurfaces, have been proposed to efficiently manipulate the propagation of EM waves [6][7][8]. Following the pioneer work of infrared (IR) metasurfaces [6,7], emerging interests have been ignited aiming to control the wave propagation across the EM spectrum with ultrathin engineered metasurface devices.
Metasurfaces can be optimized to efficiently manipulate the wavefront by carefully engineering the amplitude and phase response of the meta-atoms through the unit cell structure design. Extraordinary effects, including anomalous deflection and reflection [1,9], high-efficiency beam focusing [10], polarization conversion [11], and orbital angular momentum (OAM) generation [12,13], among others [14], have been demonstrated by metasurfaces across the EM spectral from microwave and terahertz to IR and visible regimes. In addition, metasurfaces are capable of generating significant near-field effects by the resonantly local field confinement, thereby initiating giant nonlinearities [15]. The high degrees of freedom in controlling the metasurface properties enable unprecedented functionalities, such as reciprocity breaking in the magnetic-free conditions by using spatial and temporal modulation [16,17] and unidirectional propagation of EM energy by engineering the distributed loss and gain [18]. These functions have led to breakthroughs in the systems of light detection and ranging (LiDAR) [19], advanced imaging [20], biological and chemical sensing [21], communication [22,23], energy management [24,25], light emission [26,27], and augmented reality/virtual reality (AR/VR) [28].
Due to the versatile design space to achieve desired optical properties, metasurfaces are increasingly outstanding with the development of micro/nano fabrication, micro/ nanoelectromechanical systems (MEMS/NENS), and microsystems. The early metamaterials and metasurfaces were demonstrated in the microwave regime since the wellestablished printed circuit board (PCB). With the development of micro-and nanotechnologies, the subwavelength unit cells were fabricated and implemented by using advanced micro/nanoscale fabrication techniques, leading to artificially designed metasurfaces, which improved the performance of MEMS, microsystems, and optoelectronic devices. For instance, the large-scale optical metalenses have been developed as the deep ultraviolet (deep-UV) photolithography processes are available [29]. On the other hand, the unique functionalities of metasurfaces, such as perfect absorption, have enabled novel micro/nanodevices, such as near-zero power IR detectors [30], facilitating the development of microsystems. Therefore, the emerging metasurface technique allows us to interact with EM waves in new fashions, enabling microsystems with improved performance and novel functions. The metasurface technique is a potential factor in creating the virtuous circle of advanced microsystem devices and instruments. In this review, we start with fundamental theories of metasurfaces, describing the codesign framework of metasurface devices. Then, we present latest progresses on the integrated metasurface devices and instruments. Finally, we conclude with challenges in metasurface devices and outlook of the future direction.

Design and Implementation Methods of Metasurfaces
Metasurfaces manipulate the propagation of EM waves, or light, through the engineering of the local amplitude and phase response of each meta-atom, which acts like a subwavelength antenna. The interference of meta-atoms collectively determines the scattering characteristics of the metasurfaces, analogous to a phased-array antenna, enabling a wide design space to achieve various functions, as shown in Figure 1. When designing a metasurface, we first calculate the amplitude and phase responses of meta-atoms with varied parameters at designated frequencies using analytical models and finite element simulation to build a lookup table.
Then, the assembly of meta-atoms is designed based on the interactions in the metasurfaces for specific applications. Finally, the full-wave simulation of the metasurface device will be performed to optimize the metasurface by considering the local and nonlocal coupling among meta-atoms.
2.1. Modelling Metasurfaces. To achieve full control over the propagation of EM waves, independent manipulation of amplitude and phase responses, as well as the 2π phase coverage, is desired by varying the design parameters in metaatoms. The Pancharatnam-Berry (PB) phase is an efficient approach to control the phase response by the inherent geometric phase property in the circularly polarized EM waves, which are widely discussed elsewhere [34]. Another approach to manipulate the phase response is to exploit dispersive, resonance modes of meta-atoms, including the plasmonic resonance [35], dipolar or multipolar resonance [36], and waveguide modes [37]. Owing to the flexibility in dynamically tuning the resonant modes, we focus on the resonance-based metasurfaces in this section. The resonating behavior of the metasurface may be modeled theoretically by various approaches, including the equivalent circuit model [38], the Lorentz-like effective medium model [39], and the coupled mode theory (CMT) [40]. Among these approaches, CMT provides concise and accurate description on the resonant behavior and unveil the effects of coupling between distinct resonant modes, attracting increasing attentions in the metasurface community.
According to CMT, a single-mode resonator (Figure 2(a)) may be modeled by [41] in which a 1 represents the mode amplitude of the resonant mode, ω 01 is the resonant frequency, and γ 1 is the decay rate due to both intrinsic (γ 0 ) and radiative (γ r ) losses in the resonator. For the excitation with a specific frequency (ω), we can obtain the mode amplitude and transmission/reflection response by solving Equation (1) in the frequency domain [41,42]. Without loss of generality, we assume γ 1 to be 0:005 × ω 01 and use the normalized frequency difference dω = ðω − ω 01 Þ/ω 01 in the calculation. As shown in Figure 2(b), the phase of the transmission coefficient is bounded by -90°a nd 90°, and the amplitude varies significantly due to the resonance, indicating that the amplitude and phase of the metasurface are coupled leading to a limited design space.
The limitation of amplitude and phase response may be broken by taking multiple resonators or resonant modes into the systems [42]. Without loss of generality, we may consider a resonator exhibiting two distinct resonant modes (Figure 2(b)), the response of which may be modeled by [43,44] d dt in which a i represents the mode amplitude of the resonant modes, ω 0i is the resonant frequency, γ i is the decay rate, and κ ij = κ ji ði, j ∈ f1, 2g and i ≠ jÞ are the coupling factor between the modes. As shown in Figure 2(c), distinct resonant modes can be achieved by varied approaches, including multipolar resonances, waveguide modes, and the Fabry-Perot mode in the reflection configuration [45]. Herein, waveguide modes refer to the eigenmodes of the meta-atom due to the longitudinal multiple reflections. For a coupled resonator in the ideal condition (two orthogonal modes with matched resonant frequencies and decay rates), a 360°phase coverage may be achieved without amplitude variations [42], as shown in Figure 2(d), allowing it to be a qualified building block of Huygens' metasurfaces. CMT provides a lumped-parameter description of the resonating metasurface and can be exploited to design the unit cell structure through quasinormal modes (QNM) expansion [46].
After obtaining the response of constituent meta-atoms, the overall metasurface array will be designed for various applications. The overall response of a metasurface is governed by Huygens' principle, where every point on a wave front is a source of wavelets emitting waves with the same speed as the source wave to form the new wave front [47]. On the metasurface, waves reflected and transmitted by each meta-atom generate different amplitude modulation and phase shift, thereby leading to an arbitrarily desired wave front due to the interreference effect. The response of the designed meta-atoms and metasurface should be designed and simulated by using numerical approaches, including finite difference time domain (FDTD) simulation, finite element analysis (FEA), and finite integral techniques (FIT) [48]. Recently, inverse design approaches enabled by deep learning attract increasing interests due to the capability of efficiently identifying the global optimal solution for the metasurface design [49].

Building
Blocks. Meta-atoms of Huygens' metasurfaces are implemented by metals or dielectrics with judiciously designed geometries, which support coupled modes and generate desired phase distribution for wave front manipulation. In early efforts (Figure 2(e)), the electrical dipoles induced by the cut wires and magnetic dipoles induced by the split ring resonators were combined to generate the 360°phase variations in the transmission to steer the wave front with over 80% efficiency [32]. A similar approach is stacking multiple layers of metasurfaces, as shown in Figure 2(f), to achieve efficient wave manipulation via the collective interlayer mode coupling [50]. In dielectric metasurfaces, the low-loss nanoparticles support both transverse electric (TE) and transverse magnetic (TM) modes governed by Mie resonances, giving rise to the full-range phase coverage when the two modes overlap with each other [51], as shown in Figure 2(g). The amplitude and phase are independently controlled by combining the PB phase and scattering strength control in dielectric metasurfaces [52]. The emerging bound states in the continuum (BIC) provide another paradigm to achieve the Huygens' condition through controlling the coupling between distinct modes [53]. In addition, high aspect ratio dielectric scatters that support waveguide modes lead to the high-efficiency phase control, even for large incident angles (Figure 2(h)) [37]. These examples demonstrate that meta-atoms possessing multiple modes can serve as building blocks of metasurfaces for transmissive wavefront manipulation.
Metasurfaces can be configured in the reflection mode as well by adding a ground plane at the back of the subwavelength meta-atoms. In the reflection mode, the metasurfaces block the transmitted waves and manipulate the reflected waves. Such metasurfaces are considered a resonator coupled to one port, and the full range of phase coverage may be achieved by tailoring the loss factors [56]. When the intrinsic loss is larger than the radiative loss, meaning that the metasurface is overdamped, the phase of the reflection spectrum may cover the full 360°variations [41]. By tailoring the thickness of the low-loss spacer between the metasurface and the ground plane, the relation between losses may be modified to control the phase response. Both metallic (Figure 2(i)) [54] and dielectric ( Figure 2(j)) [55] subwavelength structures serve as the meta-atoms of the high-efficiency reflective metasurfaces. These basic building blocks in either transmission or reflection configurations or their variations form a toolbox, from which the designers may choose and optimize targeting to specific functions and applications.
In addition to manipulating the propagation of EM waves, metasurfaces provide a platform to engineer or enhance the near-field interactions. The metasurface may tailor the effective surface impedance to achieve perfect absorption [57][58][59][60] and superabsorption [61], thereby improving the conversion of EM energy to other types of energy, including heat [62] and electric potentials [63]. The electric and magnetic field is confined in the vicinity of subwavelength meta-atoms, arising significant field enhancement with ratios ranging from tens to hundreds of times. Therefore, improved nonlinear response and lasing effects are facilitated by near-field enhancement [14,31,64]. Metasurfaces with engineered near-field properties enable the enhanced performance and novel functions of devices and instruments, including MEMS and optoelectronic microsystems [65]. In this review, we mainly focused on the metasurfaces for imaging, bio/chemical sensing, and optoelectromechanical systems. Metasurfaces for other applications, such as wireless communication [66], energy harvesting [67], and thermal management [68], can be found elsewhere.
Aberration correction of lenses remains a challenge and adds complication to various imaging instruments. Defined by the phenomenon that rays emitted from a point object do not meet all at the same image point, aberrations are simultaneously caused by monochromatic and chromatic effects [82]. When operating at a single wavelength, metalenses are naturally able to perform aberration-free focusing under normal illumination [83], and elementary corrections are needed for their adoption in practical imaging applications requiring a large field-of-view. Methods for monochromatic aberration correction of metalenses include incorporating a curved surface [82] and doublet lens design [84,85] based on ray tracing analysis, and the former may be impractical due to manufacturing complexities. Arbabi et al. proposed a miniaturized optical planar camera composed of a metalens doublet and an image sensor, featuring a fisheye type photography with an angle-of-view larger than 60°× 60°, a small footprint of 1:6 × 1:6 × 1:7 mm 3 , and a nearly diffraction-limited resolution, operating at the wavelength of 850 nm [84], as shown in Figure 3(a). Further miniaturization of the camera and the potential for multicolor and hyperspectral imaging will be enabled by doublets designed for different frequencies and fabricated side by side on a single chip.
Metasurfaces are capable of mitigating the chromatic abbreviation through the flexible phase profile design. Apochromatic or superachromatic lenses typically refer to lenses that are corrected for chromatic aberrations at three or four wavelengths, respectively. Unlike their traditional counterparts, metalenses suffer from large phase dispersion. However, judiciously designed metalenses provide an achromatic performance at not only several discretized wavelengths [86][87][88] but also a continuous bandwidth [89][90][91] with a single layer of metasurface, as well as a metalens-cascading method [92]. Due to the lack of phase dispersion compensation capabilities, simultaneously achieving a large numerical aperture, polarization-insensitive operation, and broadband achromatic focusing remains a great challenge. The integration of a bandpass color filter and a multiwavelength achromatic metalens may be a practical solution, as shown in Figure 3(b). Redgreen-blue (RGB) light beams were selected after the broadband incident light passing through a bandpass filter composed of distributed Bragg reflectors (DBRs) and defect dielectric layers deposited on the backside of the substrate, and then deflected to the same focal spot by the metalens fabricated on the front side [93]. Moreover, simultaneously correcting monochromatic and chromatic aberrations is critical for practical applications, and doublet metalens design has been theoretically demonstrated as an efficient approach [94]. In the midinfrared regime, Ou et al. proposed an implementation scheme of broadband and achromatic metasurfaces based on the birefringent silicon nanopillars to achieve polarization-sensitive/insensitive varifocal metalens and optical vortex generation in the midinfrared regimes [95,96]. The principle incorporating the Jones matrix in the metasurface phase profile design represents a general approach to enable broadband and arbitrary optical beam manipulation for the imaging and detection applications. 4 Advanced Devices & Instrumentation Metalens also provides a pathway for the effective correction of existing imaging instruments. Figure 3(c) exhibits an endoscope in which a metalens replaced a conventional microlens to eliminate spherical aberrations and astigmatism, and the tailored chromatic dispersion was found to be helpful in achieving a larger imaging depth [97]. The fiber was responsible for delivery and subsequent collection of the light and sending the endoscope to hard-to-reach destinations. A higher image quality with the metalens compared to the conventional ball lens configuration was observed. Another metalens-based tomographic imaging instrument was proposed by redesigning the phase profile for spherical incident waves [98]. Exploiting metasurfaces to control other properties of light will enable more types of tomography and facilitate biochemical research and disease diagnosis.
Metasurface can arbitrarily design the focal spots, giving rise to novel functions, such as in-sensor computation. Three-dimensional depth sensing based on metalenses has been developed over the last few years [99][100][101]. For example, inspired by the light-field imaging theory, Lin et al. utilized an array containing 60 × 60 broadband achromatic metalenses to acquire a subimage array, and reconstruction of the scene was realized through arbitrarily rendering the images with different focusing distance based on these subimages [101], as shown in Figure 3(d). Inspired by the fact that jumping spiders decode depth information from a series of simultaneously obtained defocus images, Guo et al. utilized a metalens incorporating phase profiles of 2 off-axis lenses to split the incident light and the depth map was calculated by point spread function (PSF) analysis of 2 images with different defocus on the same sensing plan [102], as shown in Figure 4(a). Their methods require a low budget of computation; thus, a real-time operation is possible for compact systems.
The ultrathin, large NA metalenses are also a potential approach to address the challenges in miniaturized optical systems for augmented/virtual reality (AR/VR) [

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Lee et al. finished a prototype of AR glass system achieving a large field-of-view (90°) by incorporating a metalens with a relatively large diameter of 20 mm fabricated by nanoimprinting [28], as shown in Figure 4(b). For circularly polarized light, the metalens based on geometric phase design acts as transparent glass for copolarization transmission and a converging or diverging lens for cross-polarization transmission. This feature enabled the fusion of actual scenes and virtual objects, and the optimization was performed for copolarization mode to circumvent the distortion [28].
Capabilities of polarization-sensitive detection enable metalenses to acquire information unavailable from intensity or spectral analysis, which facilitates the implementation of polarization imaging using one optical component [104][105][106]. Recently, in order to circumvent the image blurring in underwater detection, Zhao et al. developed a metalens-based polarization imaging instrument assisted by the differences of polarization states between the light reflected from target objects and from unwanted background particles, and a proper estimation of extinction coefficients of the propagation medium begot accurate depth information (Figure 4(c)) [106].
Metalens arrays are applicable to implement highperformance IR focal plane arrays (IR-FPAs) with suppressed spatial cross talk and increased sensitivity operating at the room temperature [107,108]. Zhang et al. designed an architecture of the back-illuminated IR-FPA with a solidimmersion metalens array fabricated through directly etching the GaSb substrate, the focal spots of which are located on pixels of the detector array embedded in the substrate [107], as shown in Figure 4(d). Implementation of the IR-FPA requires a large meta-atom array and monolithic integration with the detector array, and this will be enabled by the constant developing progress of the batch fabrication methods [29,109]. The perfect absorbers enabled by infrared metasurfaces provide a new architecture of spectral selective infrared detectors [110]. Multifunctional metasurfaces consisting of structured metals and pyroelectric materials exploit the same structure for both optical enhancement and electrical readout. The metasurface-based detector provides a potential solution to implement compact, multicolor, and highly responsive infrared detectors for hyperspectral imaging.
In addition to the aforementioned research, metasurfaces have shown significant potential in light sources. Applications such as structural light projection [111], enhanced LED light extraction [112], radiation sources of X-ray [113], vacuum ultraviolet light [114], and IR light [115,116] Figure 5(a). The quality factor is 4~5 times higher with the terahertz (THz) polarization approach. The polarization ellipse of the output THz waves from the tumor cells (293T, B16, and HepG2) shows significant difference after aspirin treatment [117]. The polarization sensing technology amplifies the tumor cells' spectrum difference before and after the aspirin treating. The result demonstrates that the well-designed and portable THz metasurface provides a new route to the detection of antiproliferation in the tumor cell research area as well as other medical fields, potentially widely adopted in future clinical practices. Mainstream antimicrobial susceptibility testing (AST) technologies of the current clinics have made a great improvement to the modern medical science. Combined with the metasurface, a new technology namely phase-shift reflectometric interference spectroscopic measurement (PRISM) was developed for AST [118], as shown in Figure 5(b). While the conventional AST technologies take around 20 hours to finish the entire process, the PRISM takes less than 5 h. The effect of antibiotic treatment can be identified by measuring the temporal responses of PRISM, determining the minimum inhibitory concentration (MIC). PRISM can find the most effective recipe of antibiotics for a special patient in a short time among a wide range of antibiotics. With the development of artificial intelligence, metasurface technologies may detect the mutation of the bacteria gene and the status of the porins on the bacteria membrane, thus providing more accurate information for doctors to find the suitable therapies.
Mid-IR spectrum, on the other hand, offers a nondestructive and label-free approach for biological molecule (proteins, lipids, and DNA) detection. However, due to the mismatch between mid-IR wavelengths and dimensions of molecules, the sensitivity of mid-IR is not enough to detect the nanometer-scale samples (biological membranes). In order to improve the sensitivity and tell the unique

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fingerprint of the nanometer-scale biological molecules (protein, lipids, and DNA), a new method based on the mid-IR metasurface is proposed. When the resonance spectrum of the metasurface is overlapped with the absorption fingerprints of the molecules, either the frequency or the strength of the resonance will be changed. This concept is called surface-enhanced IR absorption (SEIRA) [21], as shown in Figure 5(c). Distinct biological molecule absorption fingerprint is originating from the amid I and amid II vibrational bands located near 1660 cm -1 and 1550 cm -1 , respectively. With the special structural design, the mid-IR metasurface has the extremely high Q (Q > 200) absorption spectrum peaks between 1350 cm -1 and 1750 cm -1 with a gap 60 cm -1 , which is much narrower than the spectral feature size of the individual amide I and II absorption band. Therefore, the SEIRA metasurface can easily distinct the different biological molecules and quantify the molecules concentrations with artificial intelligence (AI), facilitating the improvement in biosensing. The ubiquitous SEIRA is foreseeable by decreasing the cost in material and fabrication processes of the nanostructures [119]. Aluminum plasmonic disks were fabricated using colloidal mask processes to form infrared metamaterial perfect absorbers. The absorbers were functionalized using phosphonic acid and enhance the absorbance response of bovine serum albumin by at least 8 times with the surface plasmonic enhancement effect. These works pave a way to achieve highly sensitive biomolecule detection using SEIRA.
In addition to bacteria and biomolecule detection, metasurface can provide an efficient tool for virus screening and detection. For example, in order to realize fast diagnosis of COVID-19, IR metasurface is developed as an alternative approach to achieve high-efficiency patient screening. COVID-19 is induced by a new coronavirus consisting of single-stranded positive-sense RNA genome and four structural proteins (spike surface glycoprotein (S), small envelope protein (E), matrix protein (M), and nucleocapsid protein (N)) [120]. Each of them has different resonance frequencies. The normal and mutation viruses are capable to be distinguished with these 5 absorption peaks by artificial intelligence IR metasurface. This metasurface biosensor with an ultrahigh sensitivity (1.66%/nm) has a wide detection range within the diversity detection environment (gas/liquid), as shown in Figure 5(d). It provides a pathway towards the ultrarapid, label-free, multifunctional, and unique IR fingerprint detection of the COVID-19. The ability of the mutation virus detection can make great efforts to address the current global pandemic.
Besides standalone metasurface membranes for biological spectrum detection, metasurfaces can be integrated into lab-on-a-chip devices to achieve advanced functions. For example, classical microfluidic devices based on the binding assays which require fluorescent or enzymatic tags have done the great contributions to biological detection [121]. Metasurface integrated into the microfluidic devices called "optofluidic" is the label-free assays which eliminate the need for time-consuming labeling process and can monitor binding kinetics in real time. In order to realize the early diagnosis of cancer patients, a novel lateral flow-through biosensor consisting of a metasurface with a twodimensional (2D) periodic array of silicon nanoposts (SNPs) is reported to detect cancer biomarker, as shown in Figure 5(e). With the incident angle 1°, the absorption peaks of the metasurface will get the highest Q value [121]. By the spectral analysis, the antigen and antibody binding process can be observed as the spectral shifts. Overall, the optofluidic devices offer a new insight of the biomolecule detection technology and give a new direction of the metasurfaces integrated devices. Combined with the deep learning technologies, this system may have the great potential in diagnosis of a wide range of diseases.

Metasurfaces in Chemical
Sensing. Gaseous target detection and quantitative remote sensing attract massive efforts in metasurface-assisted spectral analysis technologies. In order to achieve highly sensitive gas detection, metasurfaces integrated with metal organic frameworks (MOFs) are introduced. It provides both excellent absorption selectivity and high gas affinity [122]. With the design of the metasurface, the absorption peaks are adjusted to the wavelength range between 4.25 μm and 7.66 μm to match the vibration modes of the CO 2 and CH 4 , as shown in Figure 6(a) [122]. After modifying the surface with MOFs, the absorption peak value increases from 0.022 to 0.221. The MOF-SEIRA platform achieves simultaneous on-chip sensing of CO 2 and CH 4 with the fast response time (<60 s), high accuracy (CO2: 1.1%, CH4: 0.4%), small footprint (100 × 100 μm 2 ), and excellent linearity in a wide dynamic range (0 − 2:5 × 10 4 ppm).
To meet the demands of rapid, low cost, and portable deployment, a metasurface based on the multiwalled carbon nanotubes is developed to detect chemical residues, such as pesticides. The limited performance of the traditional metasurface with metallic meta-atoms is mainly due to inherent losses in metals. Alternatively, carbon nanotubes with the outstanding electrical and optical properties offer the new opportunities for applications in THz science and technology, as shown in Figure 6(b). Different concentrations of pesticides (2,4-dichlorophenoxyacetic and chlorpyrifos solutions) can be detected by this new platform with lowest detection mass of 10 ng and the sensitivities of 1:38 × 10 −2 / ppm and 2:0 × 10 −3 /ppm, respectively [123]. Good linear relationship between transmission amplitude and pesticide concentration, acceptable reliability, and stability can be achieved in this multiwalled carbon nanotube metasurfacebased chemical sensing platform.
In addition to pesticides, metasurface may also be employed in quantitative sensing of specific drugs for healthcare applications. In order to monitor the harmful effects of the drug abuse, a rapid, noninvasive, accurate detecting method is highly desired. To meet these demands, the metasurface is a candidate due to the improved signal to noise ratio in the spectroscopy. A new metasurface based on the hybrid Au/Ag hybrid nanoparticles is proposed to work as a surface-enhanced Raman spectrometry (SERS) substrate, as shown in Figure 6(c). Combining with the Raman spectroscopy, the obvious Raman peaks of cocaine at 1001 cm -1 , 1027 cm -1 , 1275 cm -1 , and 1598 cm -1 can be 8 Advanced Devices & Instrumentation observed with the concentration as low as 10 μg/mL. Considering the metasurface area 20 mm 2 , the average of the detection limit is 5 ng/mm which is outperforming previous results [124]. Therefore, metasurface is a potential solution to develop highly sensitive, low cost sensors for therapeutic drug monitoring. Metasurfaces can be integrated in the optoelectronic detectors to form a compact system for chemical detection. In order to detect the target gas, the IR detector has to pair with a band pass filter making the sensor bulky and expensive. A new gas sensing platform is proposed by integrating the pixeled metasurface absorber into the detector to solve this shortcoming, as shown in Figure 6(d). By modifying the geometry of metallic plasmonic resonators, the central wavelength of each pixeled cell can be independently fitted the characteristic absorption bands of different target gases. With the metasurface design, the platform can sense different gases, including H 2 S, CH 4 , CO 2 , CO, NO, CH 2 O, NO 2 , and SO 2 with the detection limits of 489, 63,2,11,17,27,54, and 104 ppm, respectively [125]. The concentrations of gases in mixtures can be detected by multiple narrowband detectors. In the future, with the development of the MEMS technologies, integrated multiplexed gas sensors with miniaturized dimension can be achieved.

Metasurfaces for Multifunctional Optoelectromechanical
Systems. Metasurfaces, composed of subwavelength metaatoms, have demonstrated unprecedented capabilities for manipulating EM waves from the microwave to optical regimes [126][127][128]. However, most of early generation meta-surfaces were static, and their EM responses were immutable due to the fixed configurations. Tunable and reconfigurable metasurfaces are able to achieve dynamic manipulation of EM waves towards multifunctional optoelectromechanical systems. Since the EM responses are tuned in the subwavelength scale, the modulation efficiency of the device is high, fulfilling the requirements of intelligent and integrated devices and instruments, such as advanced wireless communications [129,130], LiDAR [19], and dynamic holography [131,132]. In the microwave range, active electronic devices (varactors, diodes, and semiconductor switches) have been integrated with the meta-atoms to control the EM waves dynamically [133,134]. Combined with the digital controllers, the programmable metasurface systems are realized [135]. When the working wavelength of reconfigurable metasurfaces moves from the microwave to the THz, IR, or even the visible regime, additional tuning mechanisms based on various materials, including liquid crystals [136,137], 2D materials [138][139][140], phase change materials (PCMs) [141,142], and epsilon-near-zero (ENZ) thin films [143,144], have been explored to dynamically control EM responses.
The demonstration of an on-chip electrical switching metasurface platform based on the PCM, such as Ge 2 Sb 2-Se 4 Te (GSST), was developed to enable binary switching and beam steering at the 1550 nm wavelength, as shown in Figure 7(a) [141]. In this design, a large-scale GSST Huygens' metasurface was fabricated on an optimized metallic heater (reflector), and the device was wire bonded and mounted onto a PCB. The phase (amorphous/crystalline) of the PCM was changed by electrical pulsing to tune the 9 Advanced Devices & Instrumentation amplitude and phase responses. This electrically reconfigurable metasurface is capable of deflecting beam with an angle of 32°. In addition, ENZ thin films, such as doped semiconductors and transparent conducting oxide (TCO) materials, have also been employed in metasurfaces to realize advanced optical systems. As shown in Figure 7(b), an array of Au plasmonic nanoresonators (top layer) and an Al mirror (bottom layer) were separated by an indium tin oxide (ITO) layer (middle layer) [143]. The charge depletion layers were formed at the upper and lower interfaces between the ITO layer and the insulting oxide layers by applying two appropriate gate voltages to shift the phase responses in a wide range. The integrated spatial light modulator consisted of an active metasurface, including 550 individually addressable nanoresonators, and the driving electronics have been applied in constructing LiDAR in the NIR regime.
High-resolution spatial light modulators are a class of optical devices that create arbitrary light patterns. Metasurfaces enable efficient spatial light modulation at long-wave infrared, terahertz, and microwave regimes. Spatial light modulators based on tunable metasurfaces with semiconductors [145] and liquid crystals [146] have given rise to single pixel and computational imaging systems. More comprehensive overview of metasurface-based spatial light modulator may be found in the latest review [147].
Besides, MEMS-based tunable/reconfigurable metasurfaces are capable of manipulating the near-field interactions between meta-atoms significantly by the mechanical defor-mation to achieve tunable response [148,149]. Massive efforts have been made to develop MEMS-based reconfigurable metasurfaces due to their large tunability and highpower handling capabilities [150].

Modulation of Amplitude and Polarization.
The homogenously reconfigured metasurfaces can efficiently modulate the amplitude [151][152][153] and polarization [33,154,155] of EM waves. For example, arrays of split ring resonators were fabricated on bimaterial cantilevers to form the first integration scheme of a mechanically reconfigurable metasurface [151]. In this design, the bimaterial cantilevers underwent mechanical deformation by a thermal stimulus, and the EM responses were tunable. Many mechanically reconfigurable metasurfaces were then developed from the terahertz to optical regime. To increase the diversity of functions, a MEMS-based reconfigurable metasurface with multiple-input-output (MIO) states was demonstrated, as shown in Figure 8(a) [129,130]. In this design, logic operations (XOR, XNOR, and NAND) were exhibited with two independently controlled electrical inputs and an optical output at terahertz frequencies. The device served as an important tool for the cryptographically secured terahertz wireless communication. Recently, a novel reconfigurable metasurface platform with combined tuning mechanisms has been demonstrated to be capable of realizing the efficiently multidomain control of terahertz waves [156]. As shown in Figure 8(b), a microcantilever array was fabricated 10 Advanced Devices & Instrumentation on an ion-irradiated silicon substrate to achieve the advanced spatiotemporal modulator. In this design, the MEMS tuning and femtosecond laser pulses provided large spectral tunability and ultrafast amplitude modulation, respectively. In addition to tuning amplitude, the polarization of EM waves is also desirable to be modulated dynamically. To obtain ultrathin tunable optical components, a birefringent reconfigurable metasurface was proposed to replace traditional polarization modulators based on birefringent materials. In this design, an Au nanograting was fabricated on the out-of-plane electrostatic MEMS actuator [157]. The retardation was modulated from 21.5°to 46.8°at a wavelength of 633 nm with an actuation voltage of 0-200 V.
The anisotropic behavior for different polarization angles is introduced by the asymmetrically structured unit cells [158,159]. MEMS actuators not only break the symmetry of symmetrical meta-atoms [154] but also generate different EM responses in asymmetrical meta-atoms [36,160]. With a microcantilever array design, the anisotropic reconfigurable metasurface was able to change the polarization of transmitted EM waves from circular to linear at 0.81 THz by a voltage of 40 V, as shown in Figure 8(c) [33]. In the polarization tunable metasurface, the single-layer microcantilevers were fabricated on a silicon substrate, which was coated with an insulating silicon nitride thin film using surface micromachining. This CMOS-compatible reconfigurable terahertz metasurface is able to be applied in material characterization and enhanced imaging. Furthermore, chiral metasurfaces are important for numerous applications such as optical circular polarizers, chiral light imaging [161], and quantum computing [162]. As depicted in Figure 8(d), an array of asymmetric bended split ring resonators (SRRs) exhibited giant chiroptical responses at 5.2 μm due to the symmetric breaking along z-axis [155]. The bending angle of the 3D SRR was tuned by the tensile stress resulted from focused ion beam (FIB). The measured CD was -0.29 for the forward incidence and 0.71 for the backward incidence at a bending angle of 60°.
Different from the standard MEMS processes, nanoorigami/kirigami provides an efficient microfabrication/nanofabrication for transforming planar sheets into 3D structures [163,164]. To date, origami/kirigami-based metasurfaces have been applied in modulating amplitude [165,166] and chirality responses [167,168]. In an electromechanically reconfigurable optical nanokirigami shown in Figure 8(e), the modulation contrast achieved 88% and 494% at 953 nm and 1734 nm by controlling the applied   11 Advanced Devices & Instrumentation voltage, respectively [169]. The dynamic modulation frequency was measured up to 200 kHz, which exhibited a high modulation speed in electromechanical optical reconfigurations. Recently, eight electromechanical nanokirigami structures with different azimuth angles were proposed to cover the 2π phase difference under a single-voltage control at visible wavelengths [170].

Dynamic Wavefront Manipulation.
To manipulate wavefront dynamically, the early scheme of MEMS-based reconfigurable metasurface with the local control strategy was presented at terahertz frequencies [36]. As shown in Figure 9(a), the suspension angle of each bimorph cantilever was precisely controlled by the applied voltage. In this device, beam steering and holographic display was realized through 1D and 2D encoding of unit cells, respectively. However, the global control approach was commonly used due to the feature size of the meta-atoms operated in the IR and visible range [171][172][173][174]. As shown in Figure 9(b), the dynamic wavefront shaping was achieved by silicon antenna arrays, which were fabricated by the standard silicon-on-insulator (SOI) technology [171]. In this scheme, the width of the high-index Si nanobeams (100 nm thick) increased from 80 nm to 160 nm. The gap between the Mie resonator-based metasurfaces and the Si substrate was changed with the different actuation voltage (<4 V), and the deflection angle was changed from 2°to 12°at a wavelength of 600 nm. Gap surface plasmon-(GSP-) based gradient metasurfaces are possible solutions for controlling light at the nanoscale [175,176]. As depicted in Figure 9(c), an optical metasurface (OMS) has been combined with a thin-film piezoelectric MEMS mirror to form the GSP-based MEMS-OMS platform [172]. The phase and amplitude of the reflected light were well modulated by varying the small air gap between the OMS and the MEMS mirror. By adjusting the applied voltage within the range of 3.75 V, dynamic polarization-independent beam steering and reflective 2D focusing have been experimentally demonstrated. The beam steering efficiency reached about 50% at an operating wavelength of 800 nm, and the rise/fall times of the MEMS-based device were less than 0.4 ms.
Tunable metalenses are an important component for intelligent optical systems [173,177]. In the early concept of dynamic metalens based on MEMS technology, a 10 μm thick metasurface-based flat lens was attached onto the processed MEMS actuator [177]. However, the monolithic integration of MEMS and metasurfaces was not realized in this design, and it required delicate operation to combine the

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Advanced Devices & Instrumentation two components. Subsequently, a focal-tunable metalens with two layers of all-dielectric metasurfaces generated more than 60 diopters changes at 915 nm wavelength when one metasurface moves 1 μm, as shown in Figure 9(d) [173]. In this design, the stationary and moving metasurfaces were patterned on a glass substrate and a silicon nitride membrane, respectively. The focal point was modified by controlling the distance between the two Si metasurfaces using electrostatic forces. In addition, the optical intensity is able to be modulated by Alvarez lenses, which consist of paired optical elements with complementary cubic surface profiles [178,179]. According to the Alvarez principle, the focal length is varied when there is a relative lateral displacement between the two identical elements. Currently, an ultrathin Alvarez lens based on metasurfaces was presented to replace the conventional bulky optical element that needs the complicated fabrication [180,181]. As illustrated in Figure 9(e), a silicon nitride metasurface-based Alvarez lens yielded a total in-plane displacement of 6.3 μm with an actuation voltage of 20 V, and a tuning range of 68 μm in focus was produced at 1550 nm wavelength [174]. The MEMS-based tunable metasurfaces are an alternative approach to conventional optical MEMS devices, such as digital mirror devices (DMDs), in some applications. DMDs are well known for the capability of modulating the amplitude of light using micro mirror arrays. They are widely employed in displays, laser 3D printing, adaptive optics, and optical imaging, among others [182]. The dimension of each MEMS mirror in DMDs is usually larger than 10μm × 10μm, and the band of the dynamic tuning is 1 kHz, enabling fast modulation and wide-angle optical beam manipulation. The MEMS-based tunable metasurfaces may not only modulate the amplitude but also provide additional functions, such as modulating the phase, wavefront, and polarization. Moreover, the tunable meta-atoms may modulate light at the subwavelength scale, giving rise to larger tuning range of the optical response with microscale physical displacement [171]. However, metasurfaces exhibit high dispersion and limited spectral bandwidth, while DMDs can efficiently modulate light over a wide spectrum. Therefore, the MEMS-based tunable metasurfaces may be an alternative approach and provide additional tunability to DMD for narrow band applications.

Conclusion
Metasurfaces provide a platform to manipulate EM waves in classical optics, and the interests in exploiting novel functions based on flat, metasurface optics in the quantum optics are increasing dramatically. The quantum states of photons include, but not limited to, the polarization, momentum, and orbital angular momentum, which might be controlled by metasurfaces as discussed above. For instance, metalens array fabricated on the nonlinear crystal, such as barium borate (BBO), is capable of generating the spontaneous parametric down-conversion photon source to demonstrate the multiphoton quantum entanglement for on-chip, integrated quantum devices. The metasurfaces, or meta-atoms, can be modulated in space and time, giving rise to dynamically tun-able quantum correlation for nonreciprocal quantum routers and isolators. In addition to quantum phenomena, photonic topological insulators, parity-time symmetry, and exceptional points are enabled by coupled metasurfaces, which hold the promise to control the propagation of photons and EM waves in desired ways and bring novel functions. It is difficult to cover every aspect of metasurfaces in a review article. The development of metasurface theory and physics may be found in other review articles, such as [5,8,9,60], and the myriad applications of metasurfaces may be found elsewhere [47,65,74]. As complementary to the published review articles, our review provides insights of metasurface applications in advanced devices and instrument.
In summary, metasurfaces can manipulate the propagating wavefront and near-field confinement by engineering meta-atoms. The extraordinary optical response may improve the performance of microsystems for imaging and sensing applications. In turn, the integration of MEMS with metasurfaces enables dynamically tunable optical responses, paving the way towards intelligent microsystems with capability of arbitrary control over the EM waves.

Conflicts of Interest
The authors declare no competing interests.