Mid-infrared light-induced second-harmonic generation in specific glasses

Abstract

We have theoretically predicted and experimentally confirmed a possibility of optically induced second-harmonic generation (SHG) for glasses in the middle-infrared (IR) spectral region described by fifth-order nonlinear optical susceptibility. This effect may be observed in the middle-IR region when the value of the electronic energy gap is comparable to the energies of actual phonons participating in the anharmonic (non-centrosymmetric) electron–phonon interactions (EPIs). As a subject for the investigations we have chosen Sb2Te3–BaF2–SrBr2 glasses which are transparent in the range of 0.8–10.1 µm. The SHG output within the 1.5–4.8 µm spectral range demonstrates a crucial spectral dependence. Correlation of the SHG spectral dependences with spectral positions of the anharmonic phonon mode density confirms that a fifth-order steady-state process occurs from both the cascading processes and IR-induced charge density non-centrosymmetry. A maximal value of the photoinduced SHG is achieved at a pump–probe delaying time of about 20 ps which is typical for the anharmonic EPI.