Metasurfaces are two-dimensional artificial engineered optical elements composed of subwavelength meta-atoms that have highly flexible design freedom. Therefore, metasurfaces have exhibited unprecedented electromagnetic modulations and performance beyond nature materials, opening up new possibilities for compact and versatile wavefront engineering. As a result, many novel, efficient, and versatile devices are emerging, including polarized elements, beam generators, and surface wave modulators, leading to the refinement of intelligent devices and compact integrated systems. Furthermore, cutting-edge technologies, including novel displays, virtual reality, augmented reality, optical anti-counterfeiting, and information encryption and storage, are expected to be empowered by this fantastic optical platform.

With the advent of the metaverse, more vivid display technologies are required. Computer-generated holography provides efficient solutions for whole wavefront reconstruction and is envisioned as one of the most promising display technologies. By providing depth cues required by human eyes, holography can bring an immersive and realistic visual experience. Unlike conventional optical holography, which has limited functions, metasurface holography features subwavelength modulation units, versatile multiplexed capacities, and high spatial bandwidth products. The combination of metasurface and holography enables the development of compact and lightweight holographic displays, cameras, and sensors with a wide range of practical applications in science and industry.

We expound on the advance of metasurfaces and introduce our research group's progress in multi-dimensional light field manipulation using metasurfaces and holographic displays.

The use of metasurfaces to achieve multi-dimensional modulations of optical field amplitude, polarization state, phase, and frequency is currently a hot direction in metasurface research. Our research group has conducted in-depth research on polarization modulation, novel beam generation, optical detection, optical field imaging, and surface wave modulation based on the metasurface, so as to continuously achieve efficient and multifunctional multi-dimensional optical field modulation. In the field of polarization modulation, we have achieved the conversion from linear to circular polarization, and actively tunable metasurfaces present further possibilities for polarization modulation. Novel beam generation methods based on metasurfaces have been proposed, enabling random dot beams, diffraction-free beams, novel vortex beams with multiple dimensions and modes, and generalized vortex beams. We have improved the performance of optical sensing systems by using metasurface devices as the core devices of optical fiber-end sensors, and the ultra-thin nature of metasurface devices provides a compact optical platform for integrated physical and chemical inspection systems. We have also applied the optical field modulation capability of the metasurface to novel imaging, including phase imaging measurements, single-pixel imaging, and three-dimensional (3D) imaging. For on-chip metasurfaces, we have achieved mode conversion and wavefront modulation with a slit waveguide structure to enable on-chip equal-intensity beam splitting devices. An on-chip tunable mode converter based on liquid crystals is designed, which can convert fundamental modes to multiple higher-order modes.

Moreover, based on the abundant properties of metasurfaces, a great deal of work in the holographic display is described in detail, which can realize the reconstruction of multiplexed holographic images combined with polarization, orbital angular momentum, multi-wavelength, and nonlinear effect. In terms of polarization multiplexing, different polarization channels can load various holograms. Our research group has studied birefringent metasurfaces for the complete control of polarization channels. Twelve polarization channels and seven different image combinations are demonstrated. Also, we demonstrate full-Stokes polarization multiplexing, polarization and holography recording in real-space and k space, non-reciprocal asymmetric polarization encryption, and simultaneous control of phase and polarization of different diffraction orders with metasurfaces. We also verify a polarization-encrypted metasurface based on orbital angular momentum (OAM) multiplexing, and the OAM selective holographic information can only be reconstructed with the exact topological charge and a specific polarization state. The cascaded metasurfaces integrate multiple functional metasurfaces according to specific combination methods. Our research group demonstrates holographic multiplexing of a variety of different cascaded metasurfaces, including the combination, relative rotation, and relative movement of two different metasurfaces, which improves the information density and realizes multi-dimensional dynamic modulation. In terms of wavelength multiplexing, we demonstrate a metasurface integrating color printing and holography simultaneously. Furthermore, we prove dual wavelength and polarization multiplexed photon sieve holograms, correlated triple amplitude and phase holographic encryption, and color holographic display based on multi-wavelength code-division multiplexing and polarization multiplexing with metasurface. In terms of dynamic holographic display, we propose dynamic metasurfaces based on liquid crystal materials, phase change materials, and magnetic materials. By changing the external conditions such as electromagnetic field and temperature, the display image can be switched. According to nonlinear effects, we show spin and wavelength multiplexing, bicolor holography, and four-wave mixing holographic multiplexing based on nonlinear metasurfaces. Besides, we demonstrate nonlinear third-harmonic signals controlled by dielectric matesurfaces.

Metasurface can flexibly manipulate the parameters of electromagnetic waves, including amplitude, phase, polarization, and OAM. By optimization-designed metaatoms, metasurfaces can manipulate light fields in multiple dimensions, which enhance the information density and make metasurfaces more versatile. Hence metasurface holography is promising to expand hologram information density, increase the number of display channels, and improve imaging performance. Metasurfaces have led to breakthroughs in many fields and are of milestone significance in the miniaturization, integration, and lightweight of optical devices, and related research has brought unlimited possibilities for the development of micro-nano optics, but it still faces some challenges. The new principle of the interaction between light and material needs to be further explored, and designing and fabricating large-diameter multi-dimensional metasurface devices are still facing plenty of issues. The research on pixel-level actively adjustable metasurfaces needs to be further developed. In summary, exploring new metasurface modulation physical mechanisms and more advanced design and processing methods can unlock more application scenarios and fully leverage the advantages of metasurface devices.