Orbital angular momentum control of third-harmonic generation and vortex dichroism in isotropic media

TL;DR

This paper demonstrates how orbital angular momentum in focused light beams enables third-harmonic generation and vortex dichroism in isotropic media, revealing new chiroptical effects and control mechanisms.

Contribution

It introduces a molecular quantum electrodynamics framework showing nonparaxial beam components enable THG with circular polarization in isotropic fluids, and reveals vortex charge-dependent dichroism.

Findings

Circularly polarized THG is possible in isotropic fluids due to nonparaxial fields.

Electric-magnetic interference causes a chiral contribution to the nonlinear response.

Harmonic intensity depends linearly on vortex charge, reversing with handedness or chirality.

Abstract

Structured light carrying orbital angular momentum enables new regimes of nonlinear light-matter interaction. Here we develop a molecular quantum electrodynamics description of third-harmonic generation (THG) driven by focused Laguerre-Gaussian beams in isotropic molecular media. We show that the nonparaxial longitudinal field components of a tightly focused beam permit THG with circularly polarized excitation in an isotropic fluid, a process forbidden for plane waves and paraxial beams. Within the electric-dipole approximation, the resulting emission is independent of the sign of the vortex charge. Including electric-magnetic dipole interference introduces a chiral contribution to the nonlinear response, giving rise to third-harmonic vortex dichroism (THVD). The emitted intensity then acquires a component linear in the topological charge \(\ell\), reversing sign with either the wavefront handedness or molecular chirality. Numerical modelling reveals corresponding spatial asymmetries in the harmonic field. These results establish both an allowed pathway for circularly polarized THG in isotropic fluids and the first chiroptical analogue of THG in such media, identifying orbital angular momentum as a new control parameter for nonlinear chiral spectroscopy.