轨道角动量超表面全息复用研究进展

Recent Advances in Orbital Angular Momentum Multiplexed Metasurface Holography

光子的轨道角动量(OAM)由于理论上的无限模式数而被广泛用于提高数据传输的信道容量。由光学薄膜和亚波长结构阵列组成的超表面可以操纵光学维度,实现高性能的光子集成,而利用超表面实现OAM复用有利于光学器件向小型化、多功能化方向发展。通过设计超表面的幅度与相位分布可以实现OAM全息,将OAM与偏振、波长、角度等其他光学维度结合可以获得多自由度大容量OAM复用全息。本综述将从这些方向展开,论述近年来利用超表面实现OAM复用全息的研究进展。提高OAM复用全息的自由度并改善成像质量,实现安全性、集成度更高的光学元器件,是未来OAM全息的发展方向。

Significance 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 highperformance 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.Progress 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 factormultiplexed OAM holography,angle-multiplexed OAM holography(Fig.7),ellipticity-multiplexed OAM holography,and radialangle-multiplexed OAM holography(Fig.8).Conclusions and Prospects 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.