arXiv subjects

Kayn A Forbes

Publications and source records attributed to Kayn A Forbes.

4 recordsLinked to original sources

arXiv2026

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

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.

physics.optics↗

arXiv2025

Vortex Light at the Nanoscale: Twists, Spins, and Surprises -- A Review

For over three decades, the study of optical vortex beams carrying orbital angular momentum (OAM) has been at the forefront of optics, driven by fundamental questions about optical momentum as well as diverse applications in quantum information, communications, and optical manipulation. Most work has focused on paraxial beams, whose transverse fields are accurately described by conventional wave optics and the Stokes formalism. By contrast, when light is confined to the nanoscale and tightly focused beyond the paraxial regime, vortex beams exhibit complex electromagnetic structures that transcend these conventional models. In this deeply non-paraxial regime, the resulting fields display rich and often counterintuitive behaviour, opening new perspectives on light-matter interactions. This review unifies the emerging physics of nanoscale optical vortices by developing a coherent theoretical framework and offering a critical synthesis of recent advances, guiding readers toward a deeper understanding and stimulating future work in this rapidly evolving field.

physics.optics↗

arXiv2024

Nonlinear vortex dichroism in chiral molecules

The recent discovery that linearly polarized light with a helical wavefront can exhibit vortex dichroism (also referred to as helical dichroism) has opened up new horizons in chiroptical spectroscopy with structured chiral light. Recent experiments have now pushed optical activity with vortex beams into the regime of nonlinear optics. Here we present the theory of two-photon absorption (TPA) of focused optical vortices by chiral molecules: nonlinear vortex dichroism (NVD). We discover that highly distinct features arise in the case of TPA with focused vortex beams, including the ability to probe chiral molecular structure not accessible to current methods and that the differential rate of TPA is significantly influenced by the orientation of the state of linear polarization. This work provides strong evidence that combining nonlinear optical activity with structured light provides new and improved routes to studying molecular chirality.

physics.optics↗

arXiv2021

Enantioselective optical gradient forces using 3D structured vortex light

Here we highlight enantioselective optical gradient forces present in 3D structured optical vortex tweezing systems. One chiral force originates from the circular polarization of the light, while remarkably the other is independent of the input polarization, even occurring for unpolarized light, and is not present in 2D structured light nor propagating plane waves. This latter chiral sorting mechanism allows for the enantioselective trapping of chiral particles into distinct rings in the transverse plane through conservative radial forces.

physics.optics↗