Nonperturbative effects in second harmonic generation

Second harmonic generation (SHG) is a phenomenon where light with twice the frequency of the incident light is generated based on second-order nonlinear optical effects. This research is actively pursued both theoretically and experimentally due to its potential applications in creating blue and UV lasers and improving the performance of optical communication systems. However, the nonperturbative effects on SHG at higher incident light intensities are not well understood. 

In this talk, we investigate the nonperturbative effects on SHG using Floquet formalism. We discovered two types of saturation effects in a general two-band model at higher intensities of incident light. One is the saturation from parabolic to linear dependence of the induced current on the intensity of the incident light, observed when the frequency of the light is larger than the bandgap. The other saturation effect is a stronger saturation, namely, a change from parabolic to constant dependence on the incident light intensity, observed when the frequency is smaller than the bandgap. This nonperturbative behavior has not been found before. Additionally, we numerically analyzed the nonperturbative effects of SHG in light-irradiated transition metal dichalcogenides using a sufficiently large Floquet Hamiltonian and demonstrated that the two types of nonperturbative effects can be observed in large but experimentally feasible amplitudes of light. We also plan to discuss the dependence of SHG on the system's dissipation in this talk.