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.

Second-order nonlinear optical coefficients of the monoclinic crystal BaGa4Se7.

We characterize the magnitude and relative signs of the second-order nonlinear optical coefficients of the biaxial crystal BaGa4Se7 (BGSe) combining measurements of phase-matched and non-phase-matched second-harmonic generation at different wavelengths using slabs and a sphere.

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.

Intracavity difference-frequency mixing of optical parametric oscillator signal and idler pulses in BaGa4Se7.

An overall quantum conversion efficiency of 7.8% is achieved by intracavity mixing the signal and idler of a 1.064 µm pumped Rb:PPKTP optical parametric oscillator in BaGa4Se7. In this way, a pulse energy of ∼0.71 mJ is generated at ∼7 µm for a repetition rate of 100 Hz. Tuning of the mid-IR radiation is demonstrated by heating of the Rb:PPKTP crystal.

Widely tunable in the mid-IR BaGa4Se7 optical parametric oscillator pumped at 1064 nm.

A BaGa4Se7 nanosecond optical parametric oscillator (OPO) shows extremely wide idler tunability in the mid-IR (2.7-17 µm) under 1.064 µm pumping. The ∼10 ns pulses at ∼7.2 µm have an energy of 3.7 mJ at 10 Hz. The pump-to-idler conversion efficiency for this wavelength reaches 5.9% with a slope of 6.5% corresponding to a quantum conversion efficiency or pump depletion of 40%.

Phase-matching directions and refined Sellmeier equations of the monoclinic acentric crystal BaGa4Se7.

We directly measured the phase-matching directions of second-harmonic and difference frequency generation in the principal planes of the biaxial BaGa4Se7 (BGSe) crystal using the sphere method. The simultaneous fit of the data allowed us to refine the Sellmeier equations of the three principal refractive indices of BGSe, and to calculate the tuning curves for infrared optical parametric generation.

Angle noncritical phase-matched second-harmonic generation in the monoclinic crystal BaGa4Se7.

We performed a complete study of angle noncritical phase-matched second-harmonic generation in BaGa4Se7 in the mid-infrared region. We measured the dielectric frame orientation and showed that it does not rotate as a function of wavelength. We determined the phase-matching wavelengths and the associated spectral and angular acceptances along the x and y axes of the dielectric frame. We also estimated the magnitude of the nonlinear coefficients involved.

Narrow bandwidth, picosecond, 1064 nm pumped optical parametric generator for the mid-IR based on HgGa2S4.

We report on optical parametric generation in a mercury thiogallate (HgGa2S4) crystal pumped by 16 ps, 1064 nm pulses at 250 kHz. A broad tuning range extending from 1.19 to 1.47 µm (signal) and from 3.85 to 10 µm (idler) is achieved. Narrow bandwidth, quasi-Fourier limited operation with high beam quality for the signal/idler pulses and >14% pump depletion is obtained by continuous wave seed injection at 1.29 µm.

Mid-infrared rotated image singly resonant twisted rectangle optical parametric oscillator based on HgGa(2)S(4) pumped at 1064 nm.

We compare linear, planar ring, and rotated image singly resonant twisted rectangle (RISTRA) type nanosecond optical parametric oscillator cavities using HgGa2S4 nonlinear crystal pumped by 8 ns pulses at 1064 nm from a low beam quality pump source. The input-output characteristics and the output idler beam quality at 6300 nm are compared for two values of the pump beam diameter presenting different cavity Fresnel numbers and magnitudes of the spatial walk-off effect due to birefringence. The RISTRA cavity ensures in all cases a circular output beam profile but is advantageous in terms of beam quality with respect to the planar ring only at a large pump beam diameter.

Midinfrared optical parametric oscillator based on the wide-bandgap BaGa4S7 nonlinear crystal.

The orthorhombic biaxial crystal BaGa(4)S(7) has been employed in a 1064 nm pumped optical parametric oscillator generating <6 ns long idler pulses with energies as high as 0.5 mJ at 6.217 µm and average power of ~50 mW at 100 Hz. Notwithstanding the relatively low nonlinearity, ~3 times above threshold operation has been achieved at pump intensities more than 5 times below the crystal surface damage limit.

Picosecond mid-infrared optical parametric amplifier based on the wide-bandgap GaS(0.4)Se(0.6) pumped by a Nd:YAG laser system at 1064 nm.

Operation of a GaS(0.4)Se(0.6) optical parametric amplifier is demonstrated with a 5-11 µm idler tuning range, maximum energies of ∼10 µJ for sub-30-ps pulse durations, and performance ∼3 times better than with pure GaSe.

Continuous-wave and Q-switched Tm-doped KY(WO4)2 planar waveguide laser at 1.84 µm.

High-quality monoclinic planar waveguide crystals of Tm-doped KY(WO4)2 codoped with Gd3+ and Lu3+ were grown by liquid-phase epitaxy. For the first time, planar waveguide lasing was demonstrated in a monolithic cavity in the 2 µm spectral range. The laser was operated in the Q-switched mode using a Cr2+:ZnSe crystal as saturable absorber and in the continuous-wave regimes. The Q-switched planar waveguide laser delivered pulse energies up to 120 nJ at a repetition rate of 7 kHz.

Broadly tunable compact continuous-wave Cr(2+):ZnS laser.

We report the development of a continuous-wave, room-temperature Cr(2+) ZnS laser that is compact and tunable over 700 nm. The laser is pumped by a diode-pumped Er-fiber laser and generates 0.7 W of linearly polarized radiation at 2.35microm , at up to 40% slope efficiency. Cr(2+) ZnS directly diode-pumped at 1.6microm yields polarized radiation that is tunable over 400 nm at up to 25 mW of output power. A comparison of Cr(2+) ZnS with Cr ZnSe (70 mW, 350 nm) in a similar setup is given. As opposed to Cr ZnSe, the Cr ZnS laser is intrinsically polarized. Finally, we observe sensitization of the output radiation by a few milliwatts of the visible (470-500-nm) and near-infrared (740-770-nm) radiation.