# Light with a self-torque: extreme-ultraviolet beams with time-varying orbital angular momentum

## TL;DR

This paper introduces and experimentally demonstrates a new class of extreme-ultraviolet light beams with a time-varying orbital angular momentum, called self-torque, arising from high-order harmonic generation driven by time-delayed pulses with different OAM.

## Contribution

The study presents the first optical analog of self-torque in EUV beams, linking angular momentum conservation to the generation of these unique light fields.

## Key findings

- Self-torqued EUV beams naturally arise from high-order harmonic generation.
- Time-dependent OAM induces an azimuthal frequency chirp in the beams.
- Self-torque can be used for advanced imaging and manipulation at nanoscale.

## Abstract

Twisted light fields carrying orbital angular momentum (OAM) provide powerful capabilities for applications in optical communications, microscopy, quantum optics and microparticle rotation. Here we introduce and experimentally validate a new class of light beams, whose unique property is associated with a temporal OAM variation along a pulse: the self-torque of light. Self-torque is a phenomenon that can arise from matter-field interactions in electrodynamics and general relativity, but to date, there has been no optical analog. In particular, the self-torque of light is an inherent property, which is distinguished from the mechanical torque exerted by OAM beams when interacting with physical systems. We demonstrate that self-torqued beams in the extreme-ultraviolet (EUV) naturally arise as a necessary consequence of angular momentum conservation in non-perturbative high-order harmonic generation when driven by time-delayed pulses with different OAM. In addition, the time-dependent OAM naturally induces an azimuthal frequency chirp, which provides a signature for monitoring the self-torque of high-harmonic EUV beams. Such self-torqued EUV beams can serve as unique tools for imaging magnetic and topological excitations, for launching selective excitation of quantum matter, and for manipulating molecules and nanostructures on unprecedented time and length scales.

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Source: https://tomesphere.com/paper/1901.10942