Manifestation of electron-phonon interactions in IR-induced second harmonic generation in a sulphide glass-ceramic with β-GeS2 microcrystallites

The non-linear optical response of sulphide glass-containing β-GeS2 microcrystallites, has been studied at a fundamental wavelength of λ=5.5 μm under additional irradiation by IR-pulses of λ=3.7 μm. A substantial increase of the non-linear susceptibility with IR-power and size of the microcrystallites has been found. Molecular dynamics calculations linked to quantum chemical calculations show that this effect could be explained by substantial contributions of the microcrystallite interfaces and due to the phonon sub-system. The role of anharmonic electron-phonon effects is experimentally confirmed by the observation of an optimal delay of 8 ps between the pump laser pulses at 3.7 μm and the probing laser pulses at 5.5 μm, which generate the observed second harmonic signal.