Abstract
We report the experimental results on the temperature-dependent phase-matching properties of BaGa<sub>2</sub>GeS<sub>6</sub> for second-harmonic generation (SHG) and sum-frequency generation (SFG) of Nd:YAG laser-pumped KTiOPO<sub>4</sub> (KTP) and HgGa<sub>2</sub>S<sub>4</sub> (HGS) optical parametric oscillators (OPOs) and a CO<sub>2</sub> laser in the 0.7674-10.5910 μm range. The experimental values for type-2 90° phase-matching SHG wavelengths in the short and long wavelength branches were measured to be <i>λ</i>1 = 2.0376 and 7.6740 μm, respectively. By using a BaGa<sub>2</sub>GeS<sub>6</sub> crystal cut at <i>θ</i> = 48.1˚ and <i>Φ</i> = 0˚, we measured the type-1 and type-2 phase-matching angles for SHG and SFG of a Nd:YAG laser-pumped KTP OPO at <i>λ</i> = 3.1092 μm and HGS OPO at <i>λ</i> = 7.6740 μm as well as those for SHG and SFG of a waveguide CO<sub>2</sub> laser. In addition, we derived the Sellmeier equations that provide a good reproduction of the experimental results for the phase-matching data. The temperature phase-matching bandwidths (FWHM) were also obtained in eight different phase-matching conditions by using the measured temperature variation of the pump wavelengths, the measured temperature dependence of the phase-matching angles (Δ<i>θ<sub>ext</sub></i>/Δ<i>T</i>) from 20℃ to 120℃ and the angular acceptance (Δ<i>θ<sub>ext</i></sub>‧<i>l</i>) calculated with our Sellmeier equations. Moreover, the temperature phase-matching bandwidth (FWHM) for type-2 90˚ phase-matching SHG was determined to be Δ<i>T‧l</i> = 12.2 ℃‧cm from the measured value of <i>dλ<sub>1</sub>/dT</i> = +0.3 nm/℃ and the calculated spectral bandwidth (Δ<i>λ<sub>1</sub>‧l</i>). From these experimental results and the <i>dn/dT</i> measured at five wavelengths, we derived a thermo-optic dispersion formula valid in the 0.7674 − 10.5910 μm spectral range.
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