Overview of optical spatiotemporal vortexes

Recently, the nanophotonics team led by Professor Qiwen Zhan of the Future Optics International Laboratory of University of Shanghai for Science and Technology was invited to write a review of optical space-time vortexes (STOV), and the article was published in the new journal eLight of the Excellence Program with the title “Optical spatiotemporal vortices”.

Optical space-time vortices have attracted more and more attention in recent years because they carry the angular momentum of photon transverse orbits and energy circulation in space-time. The realization of experimentally controllable generation of space-time vortices has triggered a series of studies in this field. This article covers the latest developments in optical space-time vortexes, from theoretical physics, experimental generative protocols, characterization methods, to applications and future prospects. This new degree of freedom in the angular momentum of the transverse orbit of photons conferred by space-time vortices paves the way for the discovery of new physical mechanisms in optics and photon applications.

Figure 1: STOV generation method

The tight link between optical vortex and orbital angular momentum (OAM) was discovered in a seminal paper by Allen in 1992. Researchers have found numerous applications of optical OAM in classical and quantum optics, including optical communications, quantum entanglement and cryptography, optical tweezers, micromechanical drive torque, rotational Doppler effect, and imaging. The OAM carried by each photon in the optical vortex is proportional to the topological charge and is measured by Quantization of units. The spin angular momentum (SAM) associated with the circularly polarized state is limited to each photon[-?,?]。 However, OAM per photon is theoretically unlimited. The angular momentum direction of light usually follows the direction of propagation. Ten years ago, interest in transverse SAM began to emerge, and transverse rotating electric fields were discovered in strongly focused beams and hidden lost waves. The term “photon wheel” was coined to describe the orthogonal relationship between the direction of SAM and the direction of propagation. Similarly, optical vortices carrying lateral OAM have attracted increasing attention. This article is a review of the latest research advances on optical space-time vortices. Section 1 introduces mathematical theory and physical explanations. Section 2 discusses the generation, propagation, and conversion of STOVs. Section 3 summarizes current characterization methods and their limitations. Section 4 describes the different types of STOVs. Section 5 discusses the application of STOVs.

Figure 2: Different types of STOVs

STOV represents a new type of optical vortex with topological and conservation properties similar to space vortexes, as well as new properties such as carrying lateral OAM. Different kinds of STOVs exhibit peculiar photonic properties in various optical phenomena and are suitable for optical manipulation, space-time differentiators, sub-light and superluminal pulse propagation, and free-space optical communications. In order to fully understand the spatiotemporal phase singularity of STOV and explore its applications, many research works need to be carried out, including: propagation of STOV through waveguides such as optical fibers; Reflecting, refracting and scattering STOVs in non-homogeneous, anisotropic or nonlinear materials; Mode excitation and light manipulation in nanostructures using STOV; STOV-assisted dichroism is used to detect molecular chirality; STOV’s optical parametric chirp pulse amplification; Characterization and application of ultra-fast and ultra-strong STOV pulses. It can be expected that the new physical mechanisms and applications related to STOV will be more manifested in the near future. (Source: China Optics WeChat public account)

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