Spatiotemporal optical vortex harmonics in an epsilon-near-zero ITO film

When an intense few-cycle spatiotemporal optical vortex (STOV) laser propagates through an indium tin oxide (ITO) thin film, the vortex harmonic generations are theoretically investigated by numerically solving the full-wave Maxwell-paradigmatic-Kerr equations. It is shown that the fork-shaped dislocation in the spatiotemporal domain becomes center-asymmetric, and the accompanying harmonic radiation is significantly enhanced when the incident laser frequency is within the epsilon-near-zero (ENZ) frequency region of the ITO film. Specially, the accompanying harmonic radiation presents an obvious bright stripe of spatiospectral tilt as the consequence of a spatial chirp, and the corresponding phase spatiotemporal profile of harmonic radiation displays multiple spiral phase periods of 2π. From these distributions of bright stripes and phase spatiotemporal profiles, one can directly determine the topological number of each order harmonic.