Beecroft Building, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU
Zixin Zhang, University of Oxford
Abstract
The global commissioning of multi-Petawatt laser systems provides unprecedented access to interactions of ultra-high electromagnetic fields with themselves. At extreme electromagnetic field strengths, quantum electrodynamics (QED) predicts that the vacuum behaves as a nonlinear optical medium, enabling phenomena such as vacuum birefringence and stimulated photon–photon scattering (SPPS). Understanding these effects in realistic laser geometries, however, often lies beyond the reach of existing analytical models.
In my DPhil, I collaborated on the development of a large-scale, fully 3D computational model for generic quantum vacuum effects. The numerical framework was first benchmarked against analytical predictions for vacuum birefringence and then applied to SPPS driven by Gaussian laser pulses. These simulations provide access to the full spatial and temporal evolution of the generated signal, allowing realistic interaction times, interaction volumes, beam profiles, and propagation effects to be quantified beyond plane-wave approximations.
Building on this framework, I investigated how the angular momentum carried by structured laser fields is transferred through SPPS. Orbital and spin angular momentum were found to follow fundamentally different mechanisms. Orbital angular momentum is transferred vectorially to the generated signal, enabling the production of higher-order and spatio-temporal optical vortices. Spin angular momentum, by contrast, does not generally obey an analogous conservation rule: while it is conserved for circularly polarised fields in collinear interactions, the signal polarisation in non-collinear geometries is governed by the helicity-selection rules of leading-order QED. These results reveal distinct mechanisms governing orbital and spin angular momentum in QED and, more generally, in nonlinear optical interactions, and establish the phase and polarisation structure as new observables for probing the quantum vacuum.