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

Abstract

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 Cd_0.65Hg_0.35Ga₂S₄. 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. Cd_xHg_1-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 Cd_xHg_1-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.

Figure 1

Schematic layout of the mid-infrared OPA. PR partial reflector, BS beam sampler, SCG supercontinuum generator based on a 6-mm-long YAG crystal, DM1 dichroic mirror, high reflection at 1.03 µm and high transmission at > 1.1 µm, DM2 dichroic mirror, high reflection at 1.0–1.2 µm and high transmission at 6–12 µm, CHGS 2-mm-long, uncoated Cd_0.65Hg_0.35Ga₂S₄ crystal, Ge germanium.

Figure 2

Top: Average power of the idler output beam as a function of center wavelength across the tuning range. Bottom: Representative idler spectra across the tuning range.

Figure 3

Phase matching curves for type-I OPA in Cd_xHg_1−xGa₂S₄ with a Cd content of x = 0.64 (black), 0.65 (red), and 0.66 (blue), at 20 °C and a pump wavelength of 1.028 µm.

Figure 4

(a) Pump-to-idler energy conversion efficiency at 10.6 μm as a function of average pump power at 1.028 μm. The pump power was not corrected for Fresnel losses at the front face of the uncoated OPA crystal and the MIR power was measured at the output after the germanium lens. (b) Average power stability of the idler beam at 10.6 µm as a function of time.

Figure 5

(a) Measured and (b) reconstructed X-FROG traces obtained for chirp-compensated idler pulses. Retrieved temporal (c) and spectral (d) intensity and phase. The retrieved pulse duration is 101 fs. The symbols in (d) represent the measured spectrum.

Figure 6

Idler spectra with broadest bandwidth (solid lines and symbols) and highest power (dashed lines and open symbols) and the corresponding average power delivered by a single-stage OPA based on an 8-mm-long LGS (ref.), an 8.3-mm-long BGS (ref.), and a 2-mm-long Cd_0.65Hg_0.35Ga₂S₄ (this work).