Orbital angular momentum (OAM), which is manifested by the helical wavefront of light, has emerged as a new degree of freedom of light that can significantly enhance both the optical and quantum information capacities. Holography is an intriguing technique that can be used to reconstruct the three-dimensional (3D) light fields of an object. Because the helical-mode index of OAM beams is theoretically unlimited, numerous OAM-dependent information channels can be multiplexed in a single hologram, indicating that it has tremendous potential for achieving ultrahigh capacities in holographic devices and systems. Traditional OAM holograms rely on spatial light modulators (SLM) to modulate the wavefront. However, their low resolution and bulky size limit their practical applications. Metasurfaces, which are artificially engineered materials and are composed of optical thin films and subwavelength structure arrays, can be used to flexibly and accurately modulate optical fields, making them ideal candidates for high-performance optical applications. By designing the metasurface structure, it is possible to adjust the phase as well as independently and simultaneously adjust other physical dimensions such as the amplitude, polarization, angle, and frequency. A combination of these physical dimensions and OAM can be used to create independent information channels for high-capacity holographic multichannel multiplexing. The development of new degrees of freedom and improvement in the image quality open up new opportunities for future research in OAM holography, which can lead to the creation of safer and more integrated optical components.
The recent progress in OAM multiplexed holography is reviewed based on optical metasurfaces, from the basic concepts to the practical implementation. The OAM is divided into two categories according to its multiplexing form and other optical degrees of freedom (Fig.1): extrinsic degree of freedom multiplexed OAM holography, and intrinsic degree of freedom multiplexed OAM holography. Extrinsic degrees of freedom multiplexed OAM holography mainly includes polarization multiplexed OAM holography (Fig.3), frequency-multiplexed OAM holography (Fig.4), space-time multiplexed OAM holography (Fig.5), and chiro-optical field multiplexed OAM holography (Fig.6). Intrinsic degree of freedom multiplexed OAM holography includes phase jump gradient factor-multiplexed OAM holography, angle-multiplexed OAM holography (Fig.7), ellipticity-multiplexed OAM holography, and radial-angle-multiplexed OAM holography (Fig.8).
OAM holography can achieve an infinite number of orthogonal and independent channels, which significantly increases the information capacity for various applications. This can be achieved using a compact metasurface to miniaturize the device and improve the imaging quality by eliminating unnecessary diffraction. Furthermore, metasurfaces provide an excellent platform for combining OAM holography with other optical degrees of freedom, resulting in an enriched information capacity for potential applications in optical communications and information encryption. However, some pressing issues still require immediate resolution. First, as the topological charge of the OAM increases, the image resolution and information capacity of the OAM holography deteriorates because of the increased discrete sampling interval. Second, the current dynamic OAM holography primarily employs switchable metasurfaces; however, to achieve real-time and continuously tunable OAM holographic displays, adjustable materials should be utilized to design metasurface structures. Finally, further exploration of new optical degrees of freedom is imperative for achieving greater information capacity.
