Characterization of photon combination pathways for harmonic generation from indium tin oxide thin films

The harmonic generation in an indium tin oxide (ITO) thin film induced by a ω₀ + 2ω₀ two-color field (ω₀ is the frequency of a fundamental laser field) is investigated based on the numerical solution of the full-wave Maxwell-paradigmatic-Kerr equations. By changing the topological charge number and the amplitude ratio of the ω₀ and 2ω₀ field components, different photon combination pathways in support of each harmonic generation are distinguished, which are manifested as characteristic tempo-spatial field distributions, doughnut-shaped intensity distributions with different diameters, and spiral phase diagrams with different topological charge numbers. The results here provide a new, to the best of our knowledge characterization method to distinguish the photon combination pathways for each order harmonic generation.