Photon Self-Induced Spin to Orbital Conversion in TGG crystal at high laser power

TL;DR

This paper experimentally demonstrates a third-order nonlinear optical process called Self-Induced Spin-to-Orbital Conversion (SISTOC) in TGG crystals at high laser power, revealing a new depolarization mechanism involving photon entanglement.

Contribution

It provides the first experimental evidence of SISTOC in TGG crystals and elucidates its role in laser beam depolarization at high powers.

Findings

SISTOC causes depolarization in intense laser beams.

Maximal entanglement between photon spin and orbital angular momentum occurs during SISTOC.

Depolarization can be mitigated by understanding SISTOC's mechanism.

Abstract

In this paper, we present experimental evidence of a newly discovered third-order nonlinear optical process Self-Induced Spin-to-Orbital Conversion (SISTOC) of the photon angular momentum. This effect is the physical mechanism at the origin of the depolarization of very intense laser beams propagating in isotropic materials. The SISTOC process, like self-focusing, is triggered by laser heating leading to a radial temperature gradient in the medium. In this work we tested the occurrence of SISTOC in a terbium gallium garnet (TGG) rod for an impinging laser power of about 100~W. To study the SISTOC process we used different techniques: polarization analysis, interferometry and tomography of the photon orbital angular momentum. Our results confirm, in particular, that the apparent depolarization of the beam is due to the occurrence of maximal entanglement between the spin and orbital angular momentum of the photons undergoing the SISTOC process. This explanation of the true nature of the depolarization mechanism could be of some help in finding novel methods to reduce or to compensate for this usually unwanted depolarization effect in all cases where very high laser power and good beam quality are required.