Enhancement of Surface Second Harmonic Generation in Noncentrosymmetric Crystals

Second harmonic generation (SHG) is an optical technique to probe a variety of surface and interface properties of materials with a submonolayer sensitivity. In media with inversion symmetry, SHG is forbidden within the bulk due to the electric dipole approximation while at the surface, the inversion symmetry breaks by material discontinuity, and the SHG is allowed. In crystals without inversion symmetry, electric-dipole transitions are allowed. Therefore, both the surface and the bulk contribute to the SHG radiation. However, the SHG responses are largely dominated by the intense bulk contribution and the weak surface response is ignored in many studies. We demonstrate that a femtosecond amplifier laser can enhance the surface SHG contributions. In particular, we introduce two incident power regimes where the fundamental light is generated by either an amplifier laser or an oscillator laser. We study the reflected SHG signal from two non-centrosymmetric crystals, GaAs (001) and GaP (001), as a function of azimuthal rotation angle under different polarization geometries of the fundamental and SHG waves. Our results from these two independent laser source investigations reveal that in the oscillator regime, the SHG signal originated from the bulk predominates and the surface response is exceedingly small. Under the same experimental conditions, the amplifier regime manifests strong surface SHG contributions compared to the oscillator regime.