Optical Helicity and Angular Momentum Transfer: From Duality Symmetry to Material Symmetry Breaking

Optical Helicity and Angular Momentum Transfer: From Duality Symmetry to Material Symmetry Breaking

The electromagnetic field possesses a conserved quantity — optical helicity — arising from the duality symmetry of Maxwell's equations. Via Noether's theorem, this symmetry yields a continuity equation that predicts non-dissipative transfer of optical spin angular momentum at interfaces with varying permittivity. This fundamental principle provides a unified framework for understanding how structured light carrying angular momentum couples to matter. I will present the theoretical extension of helicity conservation to dispersive media, resolving the long-standing incompatibility between duality transformations and constitutive relations through a Lagrangian formalism that includes explicit material degrees of freedom and enables the identification of chiral polaritons, where helicity conservation governs enantioselective coupling in cavity quantum electrodynamics.

Drawing on my collaboration with Chemistry at the University of Glasgow, I demonstrate how this transfer of angular momentum, mandated by the continuity equation, is manifest in material-specific response channels. Depending on which degrees of freedom can absorb the transferred helicity, light with angular momentum can exert electronic torques in nanofilms, breaking effective symmetry without nuclear displacement and producing strain-equivalent effects of gigapascal pressures; induce mechanical deformation of atomically-thin 2D materials via radiation pressure gradients from Laguerre-Gaussian beams; or modulate the optical activity of chiral metasurfaces through field-gradient coupling to dark multipolar modes.