Fig. 1. Tunable metalenses by light source. (a) Schematic illustration of spin-decoupled metalenses.⁶⁹ (b) Simulated and experimental electric field intensity distributions at line y=−1.5mm for multiple polarization states of incident light (LCP, LECP, LP, RECP, RCP from left to right).⁶⁹ (c) Schematic of spin selective metalenses that can focus RCP and LCP incident beams to different focal points.⁷⁰ (d) Schematic view of metalenses using the combination of the PB phase and propagation phase.⁷¹ (e) Schematic of step-zoom metalenses in which the focal length is changed according to the linear polarization of the incident light.⁷² (f) Schematic of metalenses doublet that has different functions depending on the polarization of the incident light.⁷³

Fig. 2. Tunable metalenses by electrical bias. (a) Schematic view of LC integrated metalenses that change functionality achromatic focusing to dispersive focusing when applying voltage bias to LCs.⁷⁸ (b) Side view of TN LCs integrated electrically tunable metalenses. It modulates the polarization state of the incident beam depending on the applied voltage bias.⁷⁹ (c) Schematic of a focus tunable graphene metalens when DC voltage bias is applied to this metalens.⁸⁵ (d) Variation of focal length and focal spot intensity when the design is a packed pattern (top) and shifted bezel pattern (bottom).⁸⁵ (e) Schematic of tunable graphene metalenses in which the chemical potential of graphene is controlled by applying a gate voltage.⁸⁸ (f) Schematic view of electrically tunable metalenses in which the refractive index of BTO antennas is changed by applying voltage bias.⁹⁰

Fig. 3. Tunable metalenses by non-electrical input. (a) Schematic of metalenses that are tuned mechanically by stretching the substrate to tune the focal length.⁹⁶ (b) Measured longitudinal beam profiles according to different stretch ratios (top), intensity distributions of transmitted cross polarized light with different stretch ratios (bottom left), measured and calculated stretch ratio-focal length graph (bottom right).⁹⁶ (c) Schematic illustration of metalenses that are tuned mechanically by rotating the substrate (left) and phase distribution of one of two lenses (right).⁹⁷ (d) Schematic view of a tunable metalenses system that consists of two cubic metasurfaces.⁹⁸ (e) Schematic illustration of MEMS tunable metalenses in which the focal length is tuned by controlling the distance between two lenses.⁹⁹ (f) Schematic illustration of varifocal metalenses in which the focal length is tuned by exploiting the phase change of GSST by furnace annealing.¹⁰⁰ (g) Schematic of varifocal metalenses using phase-change material Sb2S3. The phase of Sb2S3 is changed by controlling the temperature, and the change affects the focal length.¹⁰¹

Fig. 4. Tunable metaholograms by light source. (a) Schematic of tunable metaholograms that generate dual images that can be changed by the polarization state of an incident light beam. (b) Experimentally obtained holograms at incident wavelengths of 24 nm (left) and 475 nm (right).¹²⁹ (c) Design principle of OAM multiplexing metaholograms and metahologram images with a planar wavefront (l=0). (d) Reconstructed images using four different OAM beams with topological charges l=−2, −1, 1, and 2.¹³⁷ (e) Illustration of a video metahologram using OAM. (f) It changes their images depending on the angular momentum of the incident light.⁵⁴ (g) Schematic of the reprogrammable metaholograms with the modulated incident beam by SLM. (h) The recorded images with the characterized incident light (left), with uniform laser light (middle), and the incident laser beam itself (right).¹⁴⁰

Fig. 5. Tunable metaholograms by electrical bias. (a) Schematic of three major states of LC (nematic, smectic, isotropic). (b) Design and demonstration of electrically tunable dielectric metasurfaces that use LCs.¹⁴⁹ (c) Working principle of the electrically controlled digital metasurface device (DMSD). (d) SEM image of the DMSD and experimental results by independent control of seven electrodes.¹⁵⁰ (e) Schematic of the bifunctional vectorial metasurfaces with LC analyzer. (f) SEM image of fabricated metasurfaces, and metaholograms that can be tuned by applying different voltages [scale bars: (left) 50μm, (right) 1μm].¹⁵²

Fig. 6. Tunable metaholograms by non-electrical input. (a) Changing phase geometry (amorphous, semicrystalline, crystalline) that can construct SAM-OAM conversion by tuning the crystallization level of GST. (b) SEM images of fabricated metasurfaces and two different metaholographic images in response to three crystallization levels (top: RCP, bottom: LCP).¹⁵⁸ (c) Schematic of the hydrogenating metasurfaces. (d) Four different holographic images during two hydrogenation and two dehydrogenation processes.¹⁶⁰ (e) Illustration of the surface pressure-responsive designer LC; (f) experimental results before (left, LCP) and after (right, RCP) surface pressure by touch of finger.¹⁶⁴ (g) Schematic of a gas sensor with LC-integrated metahologram. (h) The gas sensor changes optical images when hazardous gas is detected.³⁴

Table 1. Summary of tunable metalenses and metaholograms.