Correction of broadband terahertz electro-optic sampling with GaSe crystals

Gallium selenide (GaSe) is an efficient nonlinear crystal for electro-optic (EO) sampling in the multi-terahertz (THz) frequency range. However, the lattice resonance at several THz frequencies hampers broadband EO sampling, resulting in distorted pulse waveforms. In this work, we experimentally evaluated the frequency-dependent response function in EO sampling, considering the effects of phonons, phase mismatch, and gate pulse waveforms. The phonon effect is described using an effective Faust-Henry coefficient, which was determined to be -0.19 for a thin GaSe on a substrate and -0.23 for a free-standing one. The corrected field amplitude of multi-THz pulses aligns with additional measurements of average power and a beam diameter. The successful compensation of the frequency characteristics in GaSe will contribute to a more accurate evaluation of multi-THz transients.