Nowadays, there is a real need in all solid-state sources emitting a high energy and tunable coherent light covering Band II of transparency of the atmosphere (2-5 µm). The best alternative is frequency down-conversion in a nonlinear crystal of a monochromatic wavelength emitted by a commercial laser. The requested performances of the crystal are a high damage threshold and phase-matching conditions associated with large conversion efficiencies. It is the case of the oxyde crystals KTiOPO4 (KTP) and the periodically poled LiNbO3 (PPLN) that are mainly used. The goal of this talk is to give an overview of their full characterization by using the sphere method we developed many years ago. We will also report recent data of new promising oxyde crystals as La3Ga5.5Ta0.5O14 (LGT), NaI3O8 and periodically poled KTiOPO4 (PPKTP). All our data can be used per se. They also lead to the main parameters enabling calculations of the best use of oxyde crystals in optical parametric generators (OPG).
We performed direct measurements of phase-matching conditions of Second-Harmonic Generation (SHG) and Difference-Frequency Generation using the sphere method to determine reliable Sellmeier’s equations valid in the 1-12 microns transparency range of the new BGSe monoclinic nonlinear crystal. We also recorded SHG conversion efficiencies under and out-of phase-matching conditions to determine without the magnitudes and relative signs of the associated non zero quadratic nonlinear coefficients of BGSe. By combining all these data, we were able to calculate pump wavelengths enabling the generation of a widely tunable light in the infrared range from phase-matched Optical Parametric Generation.
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.
Advances in the periodic poling process have led to longer and larger PPLN crystals. Today it is also possible to prepare
PPLN samples with a thickness of about 5 mm which allows the use of pump laser beams with a larger aperture, and so
with a higher energy. Moreover thicker samples give the possibility to consider quasi-phase-matching (QPM) at any
angle with respect to the grating periodicity. We called this scheme angular quasi-phase-matching (AQPM). In order to
illustrate the potentiality of AQPM, we compared its tunability and spectral acceptance with that of BPM in the case of
second harmonic generation (SHG) and difference frequency generation (DFG) in the periodically poled negative
uniaxial 5%MgO:PPLN crystal, with a grating periodicity Λ = 32.2 μm We found that AQPM exhibits complementary
spectral range and acceptances compared with BPM. We experimentally performed the first validation of the theory of
AQPM by cutting 5%MgO:PPLN as a polished sphere with a diameter of 3.9 mm in order to propagate beams in any
direction of the crystal by keeping normal incidence. It allowed us to measure any SHG and DFG AQPM direction, with
the associated efficiencies, the spectral and angular acceptances. They are reported with calculations.
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