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Théodore Remark presents

 Evaluation of non-linear crystals for the generation of THz waves by frequency difference under phase matching conditions

Friday, September 8th 2023 at 14:00

Room F418 – Building F – Institut Néel

The defence will be in French.


Abstract: Energetic emission of a Terahertz (THz) electromagnetic wave, corresponding to wavelengths typically ranging between 15 µm and 3000 µm, is attractive for many applications in the fields of imaging and spectroscopy. Our strategy is to generate THz waves from Difference Frequency Generation (DFG) or Optical Rectification (OR) that are nonlinear processes, by using birefringent crystals pumped in the visible or the infrared range. The aim is to achieve phase-matching conditions to maximize the conversion efficiency. We studied eight crystals identified in previous works: BNA, LBO, YCOB, AGS, CdSe, CSP, ZnO and GaP that were obtained thanks to international collaborations. We also considered two new crystals: CTAS, whose optical properties were characterized for the first time in its ultraviolet-infrared and THz transparency domains; and GeO2, whose linear and nonlinear optical properties were missing in the ultraviolet-infrared transparency domain. The approach was at first theoretical, with calculations of phase-matching conditions. This was achieved by exploiting the index surface of crystals, which depends on their optical class, and equations describing the variation of the principal refractive indices as a function of wavelength. In the ultraviolet-infrared transparency range, these equations are derived from the interpolation of directly measured phase-matching angles or from literature. In THz range, the time-resolved spectroscopy (TDS) technique was used. Then, the approach was experimental, with a setup implemented to generate THz in the ten selected nonlinear crystals. A commercial femtosecond source continuously tunable between 0.48 microns and 2.6 microns was used for the two incident wavelengths of DFG or OR. Optical components were used to transport and focus the THz beam in a bolometer to quantify the energy generated. The need for a suitable detection able to record the spectrum of the THz wave was discussed: we targeted the Fourier transform of the correlation between a probe pulse emitted by the femtosecond source and the THz pulse of picosecond duration, the latter’s electric field inducing an electrooptical effect in a ZnTe crystal.