Fig. 1. Schematic diagram providing an overview of this review. LCs can be categorized into nematic, smectic, columnar, and cholesteric LC based on the alignment of the molecules. Recently, metasurfaces have been integrated with LCs to achieve tunability and this integrated optical platform can be applied to virtual reality (VR), augmented reality (AR), encryption, and sensors.

Fig. 2. Tunable metaholograms with LC. (a) Dynamic metaholographic displays that respond to various external stimuli. The left image shows a voltage-responsive display, the middle image shows a heat-responsive display, and the right image shows a touch-responsive display. (b) An electrically-controlled DMSD for light projection. The left two images show the DMSD device and its working principle. The right two images show an SEM image of the DMSD and experimental results of the DMSD with independent control of seven electrodes each. (c) Schematic of the security platform using an electrically tunable vectorial holographic device. (d) Chiral metasurfaces integrated with LC enable precise control of hologram intensity, offering the ability to switch between fully "on" and "off" states. The top images show illustrations of the adjustable intensity of holograms utilizing chiral metasurfaces. Upon illumination of LCs with 45° linearly-polarized light, an output polarization state of RCP is observed at a voltage of 1.39 V, and a transition to LCP state occurs at a voltage of 1.18 V. The bottom images show the measured results when the applied voltages were (i) 1.39, (ii) 1.30, (iii) 1.24, and (iv) 1.18 V. (e) Schematic of the broadband metahologram operating in both the UV and visible regions. The image on the right shows the experimental results of the designed broadband metahologram. Figure reproduced with permission from: (a) ref.¹⁰⁴, © 2020 John Wiley and Sons; (b) ref.⁸¹, (c) ref.¹⁰⁸, (d) ref.¹⁰⁹, under a Creative Commons Attribution 4.0 International License; (e) ref.¹¹⁰, © 2023 Royal Society of Chemistry.

Fig. 3. Tunable metalenses combined with LCs. (a) Schematic of the varifocal metalens integrated with TN LCs designed by sweeping lengths and widths of the meta-atom. (b) Schematic of an LC-cell combined bifocal metalens. The right images were captured by using the bifocal metalens in each focal plane of f₁ and f₂ with LCP and RCP incident light, respectively. (c) Schematic of the electrically tunable platform using metasurface integrated with LCs. The right images show the demonstration of the electrically tunable metalens using the platform. Focusing and imaging were demonstrated at each focal plane at the target wavelength of 450, 532, and 635 nm. (d) A varifocal metalens encapsulated in an electrically biased LC cell. The focal length continuously varies by changing bias voltage between its “off” and “on” values. (e) Schematic of a tunable metalens switching from bright-field mode to edge-enhanced imaging mode depending on incident polarization beam. The middle images show captured images of the resolution target obtained through each mode for RGB wavelengths. The right images show captured images of biological samples by bright-field optical microscope (top), bright-field mode in metalens (middle), and edge-enhanced imaging mode in metalens (bottom). Figure reproduced with permission from: (a) ref.⁸², © 2020 Optica Publishing Group; (b) ref.³⁰, under a Creative Commons Attribution 4.0 International License; (c) ref.¹²⁵, (d) ref.¹²⁷, © 2021 American Chemical Society; (e) ref.¹²⁸, © 2023 American Chemical Society.

Fig. 4. Metasurface-based dynamic beam steering devices combined with LCs. (a) The left images show the concept of a metasurface-based beam-switching device. It transmits light straight through in nematic state and deflects it at a fixed angle at isotropic state. The right shows 2D images and measured intensity of three main diffraction orders (0, +1, +2) for different temperatures. The total power of transmitted light varying with temperature is also measured. (b) The left image shows a three-level-addressing scheme of beam steering device with tunable deflection angle. The right image shows the concept of large aperture device with integration of active-matrix electrodes. (c) Schematic of LC-tunable metasurface using inverse design. A designed grating deflects incident light to the target angle in the opposite direction depending on the voltage “on” and “off” states. (d) The left image shows the schematic of a programmable metasurface performing THz beam steering. Shifting the applied coding sequence changes the beam deflection angle. The right image shows the measured and calculated distribution of the reflected beam for the applied five coding sequences at 672 GHz. (e) The left image shows the schematic of the digital coding metasurface using a MIM resonator and CASR pattern, performing various THz beam manipulation functions including dual beam steering. The middle images show the coding patterns and simulated 3D, and 2D scattering patterns for dual beam steering at 0.408 THz. The rightmost figure shows a comparison between the measured and simulated scattering patterns of the transmitted beam for five different coding sequences. Figure reproduced with permission from: (a) ref.¹³³, © 2018 American Chemical Society; (b) ref.¹³⁴, © 2019 The American Association for the Advancement of Science; (c) ref.¹³⁵, © 2020 American Chemical Society; (d) ref.¹³⁶, © 2020 AIP Publishing; (e) ref.¹³⁷, © 2021 John Wiley and Sons.

Fig. 5. Color generation metasurfaces combined with LCs. (a) The left images show a schematic of aluminum grating metasurface integrated with LC. The right images show experiment results of Al grating metasurface with incrementing the applied voltage from 0 V to 10 V. (b) Schematic of wide tuning range color filter and optical photographs of proposed structure for the initial voltage of 0 V and the saturated voltage of 5 V. (c) The left image shows a schematic of proposed color filter achieving a color coverage exceeding 70% of the sRGB color gamut. The right image shows the CIE chromaticity diagram of the transmitted colors for the proposed structure. (d) The left image shows a schematic of the LC plasmonic device producing full RGB color. The right image shows the CIE chromaticity diagram of the LC plasmonic device and that of the comparative standard. (e) The top image shows a schematic of tunable all-dielectric LC system. The bottom image shows experimental results with rotated subpixels and the CIE chromaticity diagram of the all-dielectric LC device. Under an LP axis of 90°, the intended primary color is generated, while at an LP axis of 0°, successful mixing of two colors is achieved. (f) The left image shows a schematic of the switchable transparent displays which can be turned on and off. The right image shows experimental images of the device at the applied voltage of 0 and 20 V. Figure reproduced with permission from: (a) ref.¹⁴³, © 2017 Optica Publishing Group; (b) ref.¹⁴⁴, © 2017 American Chemical Society; (c) ref.¹⁴⁵, © 2020 American Chemical Society; (d) ref.¹⁴⁶, (e) ref.¹⁴⁷, under a Creative Commons Attribution 4.0 International License; (f) ref.¹⁴⁹, © 2019 American Chemical Society.

Fig. 6. Spectral tuning in the NIR and THz region using LC-combined metasurface. (a) The left image depicts a schematic of the nonlinear metasurface capable of electrically tuning nonlocal second-harmonic generation by combining LC, along with a SEM image. The right image illustrates the results of electrical switching for second-harmonic generation and the second-harmonic signal. (b) This image shows a simplified representation of computational spectropolarimetry using a tunable LC metasurface. When light with an unknown polarization and spectrum strikes the LC metasurface, the polarization and spectrum of the incident light can be computationally reconstructed based on the measured intensity of the reflected light. (c) The left image shows a schematic of the proposed system for electrically tunable anisotropy and chirality. The right image shows FDTD simulations of electric field distribution at γ=0°, 45°, and 90°, where γ represents the angle between the long axis of the LC molecule orientation and the x−y plane. (d) The left image displays a schematic of the demonstrated active Fano resonance cloaking system. The right image presents numerical simulations, measured transmission spectra, and corresponding phase spectra for incident waves in both x-polarization (red curve) and y-polarization (blue curve). (e) The left image shows a schematic of the proposed tunable LC-loaded metasurfaces. The right image shows experimental results based on the ratio of width to length. Figure reproduced with permission from: (a) ref.¹⁵⁰, (b) ref.¹⁵¹, under a Creative Commons Attribution 4.0 International License; (c) ref.¹⁵², © 2022 Optica Publishing Group; (d) ref.¹⁵³, © 2019 AIP Publishing; (e) ref.¹⁵⁴, © 2021 John Wiley and Sons.

Fig. 7. Various applications using LC-based metasurfaces. (a) Schematic of a gas-sensitive holographic sensor. The middle image shows proposed metasurfaces successfully combined with a curved surface. The right image demonstrates broadband properties with experimental results. (b) The left image illustrates a schematic of the proposed polarization-based encryption platform and possible cases based on polarization states. The right image shows the demonstrated encryption platform using this concept. (c) Schematic of dynamic AR system using layer-folded metasurface integrated with LC. The right image shows dynamically switchable holographic images superimposed on real-world scenes, by controlling the polarization beam with tunable LC platform. Figure reproduced with permission from: (a) ref.¹⁶³, under a Creative Commons Attribution 4.0 International License; (b) ref.¹⁶⁴, © 2021 American Chemical Society; (c) ref.¹⁶⁵, © 2022 John Wiley and Sons.