In this paper, we study THz generation by optical rectification in GaSe crystal pumped under normal incidence below and around the bandgap. We compared the generated THz signal from 3 crystals with different thicknesses (1 mm, 0.1 mm and 0.02 mm) when pumped with femtosecond laser pulse whose wavelength is tuned from 800 down to 540 nm. With the 1-mm thick crystal, we observe a pronounced THz emission peak when the pumping photon energy is 1.77 eV because of better phase matching conditions. We also notice that the efficiency improvement is more pronounced for higher THz frequencies. The experimental data are nicely fitted by our OR generation model. Since the 2 other crystals are thin, we don’t observe phase matching impact on the signal. When pumping around the bandgap, we detect another, smaller, THz emission peak, due to nonlinearity enhancement this time.
We identified eight nonlinear crystals enabling THz emission from quadratic phase-matched Difference-Frequency-Generation: YCOB, BNA, LBO, CSP, AGS, CdSe, ZnO and GaP. For all these crystals, we performed Time-Domain Spectroscopy in the same conditions to determine their absorption spectra in polarized light as well as their principal refractive indices as a function of wavelength in the 0.5-2.0 THz range. By combining previous data with the Sellmeier equations valid in their visible and infrared transparency ranges, we calculated the coherence length of Difference-Frequency-Generation associated to all possible configurations of polarization and found interesting and complementary phase-matching conditions in the eight studied crystals.
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