Examining the latest developments in spacetime light vortices

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STOV generation, propagation and conversion. A Generation of STOV via spiral phase modulation using 2D pulse former. Adapted with permission from [29]copyright 2020, NPG. b A spacetime differentiator that breaks the symmetry of the space mirror is capable of generating STOV without Fourier transform. Adapted with permission from [35]copyright 2022, WILEY. c A photonic crystal plate structure possessing a custom nodal line for STOV generation. Adapted with permission from [37]copyright 2021, OPG. d STOV under second harmonic nonlinear process. Adapted with permission from [31]copyright 2021, NPG. And High harmonic generation with transversal OAM. Adapted with permission from [63]copyright 2021, ODA. f Focusing structured light to achieve focal field with transverse SAM and OAM. Adapted with permission from [65], copyright 2022, OPG. Credit: eLight (2023). DOI: 10.1186/s43593-023-00042-6

Vortices of light, regions of zero intensity surrounded by spiral phases, form the skeleton of a wave field and affect the properties of light. Since the discovery of the close connection of optical vortices and the orbital angular momentum (OAM) of light in the seminal paper in 1992, considerable progress has been made in the quest to unravel the beauty of light vortices.

Numerous applications of optical OAM have been discovered in both classical and quantum optics, including optical communication, quantum entanglement and cryptography, optical tweezing, drive torque for a micromachine, rotational Doppler shift, and ‘imaging.

In a new article published in eLightA team of scientists led by Professor Qiwen Zhan of Shanghai University of Science and Technology has examined the latest developments in spacetime light vortices.

Optical vortices exhibit azimuthal phase dependence. The OAM carried by each photon is proportional to the topological charge and quantized. The spin angular momentum (SAM), associated with circular polarization states, is limited, but the OAM per photon is theoretically unlimited. The direction of the angular momentum of light is usually directed along the direction of propagation.

A decade ago, interest in transverse SAM arose and transversely rotating electric fields were discovered in highly focused beams and evanescent waves. The word “photon wheel” describes the orthogonal relationship between the SAM direction and the propagation direction. Similarly, optical vortices carrying transverse OAMs have attracted rapidly growing interest.

The tilted light vortices were predicted using the special theory of relativity. A transversely moving observer sees a spatial optical vortex near the speed of light as an inclined vortex. Space-time optical vortices (STOVs), which occupy a small fraction of the total energy, were first observed in femtosecond filaments in air.

Subsequently, controllable generation of STOVs with transverse OAM was demonstrated using linear optics. Second harmonic generation (SHG) of transverse OAM has been reported and conservation of OAM has been demonstrated. Rigorous calculations of transverse OAM and coupling of transverse OAM and SAM were also performed.

The schemes were designed to generate STOVs using metasurfaces and photonic crystals. Undoubtedly, the experimental realization of STOVs has driven the growing interest in STOVs and stimulated their potential applications in various optical phenomena. It should be noted that this article focuses on the very recent advances related to STOVs. Readers interested in the latest developments in more general spatiotemporal wave packets and structured waves can refer to several review articles cited in this work.

More information:
Chenhao Wan et al, Optical space-time vortices, eLight (2023). DOI: 10.1186/s43593-023-00042-6

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