IR-stimulated second harmonic generation in Sb₂Te₂ Se-BaF₂-PbCl₂ glasses

Abstract

Theoretically predicted and experimentally observed infraredinduced second-harmonic generation of glasses in the mid-infrared spectral region can be described by fifth-order nonlinear optical susceptibility. The effect is observed in the mid-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. As subjects for investigation, chalcohalide Sb₂Te₂Se-BaF₂-PbCl₂ glasses were chosen. They are transparent, over a spectral range of 1.1–10.9 μm. The second-harmonic generation (SHG) output signal within the 1.5–4.8 μm spectral range has significant spectral dependence. Correlation of the SHG spectral maxima positions with spectral positions of anharmonic phonon frequencies confirms that the fifth-order steady-state process occurs due to cascading processes and IR-induced charge density non-centrosymmetry. A maximum value of the SHG is achieved at a pump-probe delay time of 18–36 ps, which is typical for anharmonic electron-phonon interactions. As temperature rises, the values of the photoinduced SHG signal susceptibility increases up to 3 × 10⁻³⁹ m⁴/V⁴. The SHG signal reaches a saturation point for the IR-pump power densities of about 0.8 GW/cm², which corresponds to the output SHG intensities of about 6 × 10⁻⁴ with respect to fundamental one. The values of the diagonal fifth-order tensor component χ^(2ω) _xxxxx are at least one order of magnitude larger than the off-diagonal tensor components.