Towards higher-dimensional structured light

Abstract

Structured light refers to the arbitrarily tailoring of optical fields in all their degrees of freedom (DoFs), from spatial to temporal. Although orbital angular momentum (OAM) is perhaps the most topical example, and celebrating 30 years since its connection to the spatial structure of light, control over other DoFs is slowly gaining traction, promising access to higher-dimensional forms of structured light. Nevertheless, harnessing these new DoFs in quantum and classical states remains challenging, with the toolkit still in its infancy. In this perspective, we discuss methods, challenges, and opportunities for the creation, detection, and control of multiple DoFs for higher-dimensional structured light. We present a roadmap for future development trends, from fundamental research to applications, concentrating on the potential for larger-capacity, higher-security information processing and communication, and beyond.

Fig. 1. Light’s potential DoFs for control.

By tailoring the on-demand structured light distributions of combined DoFs, various non-separable states can be produced so as to pioneer new research fields with theoretically unlimited dimensions to explore for structured light multiplexing

Fig. 2. Structured light towards multiple DoFs and higher dimensions.

a High-order PS representation of two-DoF 2D vector beams. b SU(2) PS representation of complex ray-wave geometric modes. c The Majorana sphere exploits hidden dimensions to extend complex structured Gaussian modes. Adapted from ref. . d Spatial vector beams with spatially dependent polarisation pattern. e Spatio-temporal vortex pulses with transverse OAM at space-time. f Flying electromagnetic toroidal pulses as space-time-polarisation non-separable states

Fig. 3. Illustrations of higher-dimensional structured light multiplexing.

a Multiple wavelengths multiplexing in OAM-based optical communication. b Schematic of the OAM-based six-dimensional encoding and decoding process of nanometric QR codes utilising the combinations of four different OAM states. P polariser, QWP quarter-wave plate, SOP state of polarisation, λ wavelength. c Complex-amplitude OAM-multiplexing metasurface hologram with optically addressable holographic video display at two image planes

Fig. 4. A typical quantum toolbox.

The core quantum toolbox (top panel) typically exploits the generation of entangled photons, shown in red, by spontaneous parametric downconversion (SPDC) of a high-energy photon (shown in blue), often using non-linear crystals. Expressing and controlling the entanglement in the spatial basis comprising patterns of light (bottom panel) allows for high-dimensional entanglement to be exploited in quantum protocols. The more patterns in the superposition, the higher the dimensionality of the quantum state

Fig. 5. Advanced optical technique assisting structured light multiplexing.

a A perfect optical system and an aberrated system, as well as vectorial-AO-enhanced optical system. b Electrically tunable disclination line—visualisation of the movement of different topological defect states under different control voltages. c Metasurface-based techniques for complex beam multiplexing

Fig. 6. Optical Skyrmions.

a PS and Bloch sphere, Skyrmion and Skyrmionic vector beam. b–g are several representative optical skymionic fields (Skymion, anti-Skymion, Bimeron, Bimeronium, Skyrmionium, and high-order Skyrmion); the corresponding Stokes vector fields are also illustrated. The hue colour means the degree of azimuth of polarisation ellipse, and the darkness and brightness mean the ellipticity from left-hand circular polarisation to right-hand circular polarisation