Efficient generation of few-cycle pulses beyond 10 µm from an optical parametric amplifier pumped by a 1-µm laser system.

Nonlinear vibrational spectroscopy profits from broadband sources emitting in the molecular fingerprint region. Yet, broadband lasers operating at wavelengths above 7 µm have been lacking, while traditional cascaded parametric frequency down-conversion schemes suffer from exceedingly low conversion efficiencies. Here we present efficient, direct frequency down-conversion of femtosecond 100-kHz, 1.03-µm pulses to the mid-infrared from 7.5 to 13.3 µm in a supercontinuum-seeded, tunable, single-stage optical parametric amplifier based on the wide-bandgap material Cd0.65Hg0.35Ga2S4. The amplifier delivers near transform-limited, few-cycle pulses with an average power > 30 mW at center wavelengths between 8.8 and 10.6 µm, at conversion efficiencies far surpassing that of optical parametric amplification followed by difference-frequency generation or intrapulse difference-frequency generation. The pulse duration at 10.6 µm is 101 fs corresponding to 2.9 optical cycles with a spectral coverage of 760-1160 cm-1. CdxHg1-xGa2S4 is an attractive alternative to LiGaS2 and BaGa4S7 in small-scale, Yb-laser-pumped, few-cycle mid-infrared optical parametric amplifiers and offers a much higher nonlinear figure of merit compared to those materials. Leveraging the inherent spatial variation of composition in CdxHg1-xGa2S4, an approach is proposed to give access to a significant fraction of the molecular fingerprint region using a single crystal at a fixed phase matching angle.

Temperature-dependent phase-matching properties of BaGa2GeS6 in the 0.767-10.5910 µm spectral range.

We report experimental results on the temperature-dependent phase-matching properties of BaGa2GeS6 for second-harmonic generation and sum-frequency generation in the 0.7674-10.5910 µm range. We also derive refined Sellmeier and thermo-optic dispersion formulas that provide a good reproduction of the present experimental results.

Refined Sellmeier equations for BaGa4S7.

This paper reports refined Sellmeier equations for the biaxial nonlinear crystal BaGa4S7 that provide a good reproduction of the phase-matching angles for second-harmonic generation (SHG) and sum-frequency generation (SFG) of the mid-IR outputs of Nd:YAG and Ho:YLF laser-pumped optical parametric oscillators (OPOs) and a frequency-doubled CO2 laser in the 0.7584-8.018 µm range. The theoretical results are found to agree with the recent experimental data for 1-µm-pumped optical parametric processes based on BaGa4S7 beyond 8 µm.

Effective nonlinearity of the new quaternary chalcogenide crystal BaGa2GeSe6.

The unknown magnitude and relative signs of the trigonal crystal ${\rm BaGa}_{2}{\rm GeSe}_{6}$BaGa2GeSe6 nonlinear coefficients have hindered defining an optimum orientation for maximum frequency conversion efficiency. This issue is resolved in the present work, obtaining ${d_{11}}= + {23.6}$d11=+23.6, ${d_{22}}= - {18.5}$d22=-18.5, and ${d_{31}}= + {18.3}\;{\rm pm/V}$d31=+18.3pm/V for frequency doubling of the 10.591 µm radiation.

Thermo-optic dispersion formula for BaGa2GeSe6.

A thermo-optic dispersion formula for ${{\rm BaGa}_2}{{\rm GeSe}_6}$BaGa2GeSe6 nonlinear crystal is reported. The experimentally determined temperature-dependent phase-matching conditions for second-harmonic and sum-frequency generation of Nd:YAG laser-pumped ${{\rm KTiOPO}_4}$KTiOPO4 (KTP) and ${{\rm RbTiOAsO}_4}$RbTiOAsO4 (RTA) optical parametric oscillators as well as a ${{\rm CO}_2}$CO2 laser and its harmonics in the 0.805-10.5910 µm range are precisely reproduced by the formula when combined with the Sellmeier equations previously reported by the present authors [Appl. Opt.57, 7440 (2018)APOPAI0003-693510.1364/AO.57.007440].

Phase-matching properties of BaGa2GeSe6 for three-wave interactions in the 0.778-10.5910 µm spectral range.

We present new experimental results for the phase-matching properties of the recently discovered BaGa2GeSe6 crystal for harmonic generation of a Nd:YAG laser-pumped KTP OPO and a CO2 laser in the 0.778-10.5910 µm spectral range. In addition, we present new Sellmeier equations that provide a good reproduction of the present experimental results as well as the published data points for SHG of a CO2 laser at 10.5910 µm and a 1.85 µm-pumped OPO in the 2.156-3.220 µm and 4.348-13.035 µm spectral ranges.

Thermo-optic dispersion formula for BaGa4Se7.

This paper reports on the thermo-optic dispersion formula for BaGa4Se7 that provides a good reproduction of the temperature-dependent phase-matching conditions for type-1 and type-2 second-harmonic generation of a Nd:YAG laser-pumped AgGaS2 optical parametric oscillator (OPO) and a CO2 laser in the 0.901-10.5910 µm spectral range, as well as the published data points for a Nd:YAG laser-pumped OPO at λi=3.9040 µm when combined with the Sellmeier equations of the present authors [Appl. Opt.56, 2978 (2017)APOPAI0003-693510.1364/AO.56.002978].