With the continuous development of computer technology and holography, the wavefront recording of holography can be realized through computer-generated holography (CGH), avoiding complex optical path and material consumption, while the developed computational holography simplifies the recording process. This technique can create holograms of virtual objects that in reality do not exist. At present, the main devices loaded with CGH are digital micromirror devices (DMDs) and liquid crystal spatial light modulators (LC-SLMs). These optical elements have large cell sizes and thicknesses owing to the limited refractive index of natural materials. The pixel size of the device is usually a few micrometers, creating shortcomings such as the small field of view, low resolution, and high-order diffraction. Therefore, it is necessary to find an optical device with a sub-wavelength size in thickness and pixel size to improve the quality of holographic imaging.

In recent years, with the continuous progress of micro? and nanomanufacturing technology, metamaterials and metasurface devices with sub-wavelength structures have created new possibilities for optical device design. Metamaterials are man-made structures with properties not found in natural materials. The study of metamaterials began with the exploration of a negative refractive index that included the realization of various optical functions, particularly the arbitrary control of light waves at the nanoscale. A metasurface, as a two-dimensional version of a metamaterial, is an artificial layered structure composed of a single-layer sub-wavelength meta-atom. A planar structure can be designed to modulate optical parameters such as the amplitude, phase, and polarization of light. Meta-holography combines metasurface and holography and is an important frontier application of nanotechnology. Owing to the small-period characteristics of a sub-wavelength structure, a metasurface has high resolution, overcomes high-order diffraction, and has a strong ability to regulate a light field. A metasurface has a higher degree of integration than that of bulky traditional optical components. Meta-holography has been applied in display, encryption, imaging, communication, and other fields. It is necessary to summarize the existing research of metasurface holography and clarify the possible development direction in the future.

Progress The application fields of metasurface holography are summarized in Fig.1. First, we introduce the milestone research achievements of metasurface holography in the field of display. These include the realization of the control of the complex amplitude of a light field to improve the quality of a reconstructed image, the three-dimensional image reconstruction in the visible light range, and the color holographic image reconstruction. In the field of dynamic holographic display, the dynamic high frame-rate holographic display using space multiplexing and orbital angular momentum (OAM) multiplexing is described, including the utilization of different metasurface spaces and the modulation of incident beams with different topological nuclear charges. We then introduce metasurface holography combined with augmented reality (AR) technology to superimpose virtual information into the real world to improve imaging performance, field of view, and system compactness, to demonstrate the potential capabilities of metasurface in AR display, and to provide a reference route for the design of lightweight wearable AR displays. Second, the application of metasurface holography in information encryption is introduced. By means of optical field parametric multiplexing or the active metasurface combination of different information to interfere with decryption combined with single-pixel imaging, the security of information is greatly improved. Then, the application of metasurface holography in imaging is introduced, and a new method is provided for the design of metalenses by using a meta-hologram to design a multi-focus achromatic metalens. Finally, the application of metasurface holography in miniaturized spectrometers is introduced, where a metasurface demultiplexer focuses incident light with different wavelengths, polarizations, and topological nuclear charge numbers to different positions.

Conclusions and Prospects Owing to its exceptional advantages in light-field regulation, metasurface holography has attracted the attention of researchers in many application scenarios outside the fields of display, encryption, and imaging. We believe that with the further development of micro? and nanomanufacturing technology and related theories, metasurface holography will play an important role in more fields.

Originally invented by Dennis Gabor in 1948, holography records and reconstructs the complete wavefront information of an object. In optical holography, an object beam interferes when it meets a reference beam, and the three-dimensional information of the object is recorded on a plate and a hologram is obtained. A hologram records the amplitude and phase information of light waves, and the object with three-dimensional information can be reconstructed by illuminating the hologram with coherent light. While traditional photography only stores the intensity of light and loses the phase information, a hologram contains the complete information of an object beam. Because of its powerful ability to reconstruct an arbitrary light field, holography is used in many optical fields, including three-dimensional display, encryption, data storage, and component detection.