Fig. 1. Metasurfaces for integrated spectrum, polarization and depth sensing

Fig. 2. Tunable micro spectrometer. (a) Schematic of a tunable liquid crystal-subwavelength resonant grating filter^[43]; (b) measured optical transmittance of the tunable filter at different applied voltages to liquid crystal cell^[43]; (c) schematic of phase change in GST induced by laser irradiation on the GST-based metasurface^[42]; (d) transmittance of a GST metasurface as a function of the pump laser fluence^[42]

Fig. 3. Micro spectrometer of metasurfaces array. (a) Schematic diagram of micro spectrometer system of metasurfaces array; (b) schematic diagram of a plasmon filter array, in which the bottom right figure is an SEM image and an optical microscope image illuminated by white light^[49]; (c) experimental transmission spectra for the red, green and blue (RGB) filters^[49]; (d) schematic of a dielectric grating with gradient grating period^[53]; (e) optical images of the fabricated 100-metapixel metasurface^[54]; (f) normalized reflectance spectra for selected 21 metapixels^[54]

Fig. 4. Narrowband filter response detector. (a) Schematic illustration of the graphene detectors with plasmonic nanoparticles^[58]; (b) SEM image of a representative device with the 100-nm diameter nanodisk array^[58]; (c) corresponding spectral response of graphene photodetectors with nanodisk arrays of different sizes^[58]; (d) schematic of a gold grating on an n-type silicon substrate with a 2-nm Ti adhesion layer^[59]; (e) SEM image of gold grating structure^[59]; (f) responsivity peaks redshift with increasing grating interslit distance D(T=200 nm, W=250 nm) ^[59]; (g) schematic diagram of the plasmonic-pyroelectric device^[60]; (h) absorption spectrum obtained by adjusting the size of gold pores^[60]; (i) schematic of the vertical detector structure with a plasmonic metasurfaces deposited on a polycrystalline AlN pyroelectric layer^[61]; (j) reflection spectrum and image of three metasurfaces-pyroelectric detectors and Au reference detector^[61]

Fig. 5. Computational micro spectrometer. (a) Schematic diagram of the reconstruction process of the computational spectrometer; (b) schematic of the plasmonic metasurfaces filter array^[68]; (c) reconstructed transmission spectra of polyethylene^[68]; (d) schematic of the photonic crystals filer array, in which the upper right figure is the optical image of the fabricated 6 × 6 photonic crystal structures^[69]; (e) emission spectrum of a combination of two light-emitting diodes (green and red) ^[69]; (f)SEM images and spectra of three selected metasurfaces units^[70]; (g) photos of fruits captured using an ordinary color camera and our ultraspectral camera (black-and-white), and five single wavelength spectral images are selected to reconstruct^[70]

Fig. 6. Grating metasurface polarimeters. (a) Illustration of the geometric phase GSPM^[75]; (b) reflected power for LCP and RCP incident beams at different wavelengths as a function of reflected angle after passing through GSPM in the experiment^[75]; (c) illustration of the full-Stokes GSPM grating’s measurement principle^[77]; (d) measured diffraction contrasts (denoted by filled circles) for polarization states along the main axes of the Poincaré sphere (indicated by asterisks)^[77]; (e) a unit cell of the 2D grating metasurface for polarization imaging^[81]; (f) polarization images required by the grating metasurface for polarization imaging^[81]

Fig. 7. Waveguide metasurface polarimeters. (a) Sketch of the unit cell of a waveguide GSPM (left) and top view of three combined waveguide GSPMs (right)^[85]; (b) color map of the SPP intensity distribution excited by the incident light with different states of polarization^[85]; (c) illustration of scattering antenna arrays (left), setup for characterizing the antenna array polarimeter (middle), and camera image of the outcoupling gratings, showing polarization-dependent intensities scattered by the four outcoupling gratings^[86]; (d) illustration of polarimeter based on silicon waveguide and the coupling antenna, the measured polariztion states shown in the Poincaré sphere (left), the retrieved polarization states (right)^[87]

Fig. 8. Metalens metasurface polarimeters^[90,93-94]. (a) SEM image of a fabricated DoFP metalens unit cell (upper), in which the polarization basis for each part is shown with the colored arrows and scale bar is 1 μm, and illustration of the DoFP metalens’ working principle (lower); (b) target polarization mask (scale bars:100 μm); (c) fabricated mask imaged using conventional polarimetry(scale bars:100 μm); (d) the same mask imaged using the DoFP metalens(scale bars:100 μm); (e) scheme of the generailzed Hartmann-Shack metalens array; (f) intensity distribution of the focal spot for a radially polarized incident beam, in which the arrows qualitatively indicate the local polarization states and scale bar is 50 μm; (g) image of the focal spot from the metalens array for the radially polarized beam; (h) polarization profile obtained from the metalens array; (i) metasurface-based PMT for simultaneous detection of spin and orbital angular momenta

Fig. 9. Division-of-time metasurface polarimeters^[95]. (a) Schematic of the GIAM-based polarimeter and the SEM image of the fabricated GIAM; (b) reflectivity spectra (R_xx(yy) = |r_xx(yy)|²) for the incident light polarized along the y-axis (circles) at three values of V_g, and along the x-axis (squares) at V_g=0 V, in which vertical dotted line corresponds to wavelength of operation λ₀=6.7 μm; (c) measured intensities (left) of the reflected light at the detector measured for the four incident polarization states (Green, near-RCP; blue, near-LCP; purple, near-x-polarized; red, elliptical polarization), and comparison (right) between the polarization states obtained from the GIAM polarimetry (dashed lines) and those obtained from the rotating analyzer polarimetry (solid lines)

Fig. 10. Photodetector integrated metasurface polarimeters. (a) Schematic diagram of the photodetector integrated polarimeter that contains six differently shaped plasmonic slit structures^[96]; (b) schematic diagram of the four-pixel silicon/graphene hybrid detector integrated polarimeter containing differently orientated plasmonic metasurfaces^[99]; (c) SEM images of the silicon/graphene polarimeter^[99]; (d) measured states of polarization on a Poincaré sphere from the four-pixel polarimeter with elliptically polarized inputs at the wavelength of 1550 nm^[99]

Fig. 11. Other polarimeter metasurfaces. (a) Schematic of ODLM design^[105]; (b) schematic of metasurface design consisting of ODLM and gold nanowire arrays to fully characterize the polarization state of the incident light. The required units are marked by P₀ through P₆. The two additional units labeled P'₅ and P'₆ provide the capability to identify polarization states at different working wavelengths^[105]; (c) Stokes parameters (S₀-S₃) extracted for eigth different random input polarization state, using a polarization analyzer and the metasruface^[105]; (d) schematic of circular-polarization-dependent metahologram for generating RCP and LCP images under LCP and RCP illumination, respectively^[106]; (e) ideal calculated x-polarized image components for illumination of the metahologram by waves with RCP, +45° linear, left-handed elliptical and right-handed ellipical polarization^[106]; (f) holographic images captured by the CCD camera for orientation angles of the HWP’s fast axis of α_HWP=0°, 30° and the QWP’s of α_QWP=-45°, 15°. The inset arrows shcematically indicate the corresponding theoretical polarization state^[106]; (g) Theoretical, experimental and simulated chiral phase difference and Stokes parameters S₁, S₂ and S₃ when rotating the HWP and QWP, respectively^[106]

Fig. 12. Realization of structured light projection by metasurfaces. (a) Geometric phase metasurface for 4×4 spots array projection on the simulation^[108]; (b) binary phase metasurface for polarization-independent 5×5 spots array projection^[109]; (c) scrambling metasurfaces generating random point cloud covering 4π space^[110]; (d) selective diffraction with complex amplitude modulation by geometric phase metasurface^[111]; (e) metasurface for structured light projection with a large field of view^[112]; (f) VCSEL-integrated metasurfaces, realizing wide-ranging dynamic beam steering^[113]

Fig. 13. Realization of active beam steering by metasurfaces. (a) Schematic of metasurface reflecting array and the unit-cell (top)^[114], and experimental demonstration of LiDAR and the acquired depth map (bottom); (b) schematic of actively switchable plasmonic metasurfaces in amorphous (left) and crystalline (right) states^[115]; (c) metasurface active beam steering using MEMS^[116]; (d) schematic of metasurfaces and liquid crystal based SLM^[117]

Fig. 14. Metasurface 3D imaging based on multiview stereo. (a) A chromatic metalens array for full-color light-field imaging^[118](top: schematic of full-color imaging and refocusing; bottom: depth perception of scenes); (b) three-dimensional positioning with a single-shot metalens array, correspondence between the position of the object and the translation of the three images^[120]; (c) correction of aberrations^[120]; (d) recovered vertical distance S_⊥ and horizontal distance S_∥ compared with the experimental setup (red lines) for different object distances^[120]

Fig. 15. Metasurface 3D imaging using DH-PSF. (a) Metasurface integrated with DH-PSF and metalens for three-dimensional imaging^[123](left: rotation of PSF with the change of depth; right: imaging of two off-axis point sources at different depths); (b) 3D imaging by DH-PSF based on dielectric metasurfaces^[124](left: schematic of imaging principle; right: imaging results of a 3D scene and the retrieved depth information); (c) ranging and scene reconstruction using a metasurface^[125]

Fig. 16. Metasurface 3D imaging with improved defocus method. (a) Schematic of single-shot defocus 3D imaging metasurface realized by interleaved off-axis focusing based on spatial multiplexing of metalens^[127]; (b) probability distribution of depth sensing results depending on object depth^[127]; (c) defocus 3D imaging using extreme dispersion in metasurfaces, in which metalens focuses different colors at different depths^[128]; (d) reconstruction results of different scenes^[128]; (e) schematic of 3D (depth and RGB image) reconstruction algorithm using separate U-Net and end-to-end optimization^[128]; (f) analysis of 3D reconstruction results^[128] (left: probability distribution of depth sensing results depending on object depth; right: comparison between reconstruction results of conventional lens and the applied metalens)