Broadband Direct Frequency Down-Conversion to Wavelengths Beyond $5\ \mu \mathrm{m}$ Using Wide-Gap Non-Oxide Crystals

Resonant light-matter interaction in the mid-infrared (MIR,
$3-30\ \mu \mathrm{m}$
) provides valuable structural and functional information on complex chemical, physical, and biological systems. High-brightness light sources with broad spectral coverage in the molecular fingerprint region (
$7-20\ \mu \mathrm{m}$
) that can simultaneously access multiple vibrational modes at high temporal resolution can benefit a wide range of related spectroscopic applications, from strong-field physics to nonlinear and time-resolved vibrational spectroscopy [1], [2]. However, currently only quasi-continuous-wave gas lasers and narrowband quantum cascade lasers with limited tunability are available in the MIR and solid state lasers are restricted to wavelengths below
$5\ \mu \mathrm{m}$
. Recent progress in two key technical fields have been tremendously beneficial for advancing broadband parametric MIR sources in the fingerprint region. On the one hand, Ti:sapphire laser technology is currently being gradually replaced by power-scalable, diode-pumped ultrafast Yb lasers. In addition to an increase in average power, more robust supercontinuum seed pulse generation and optical synchronization are enabled in optical parametric devices via this change of laser technology, thanks to the longer pulse duration and ruggedness of Yb laser amplifiers compared to Ti:sapphire laser systems. On the other hand, progress in the manufacture of novel wide-bandgap non-oxide crystals offer the possibility of direct frequency conversion of the output of near-infrared pump lasers to the mid-infrared beyond
$5\ \mu \mathrm{m}$
in a parametric optical amplifier (OPA) without employing lossy cascaded frequency conversion schemes. A growing number of non-oxide crystals are becoming available that are both transparent and phase-matchable beyond the infrared cutoff of oxide crystals and can also by pumped by Yb laser systems at
$1\mu \mathrm{m}$
without detrimental two-photon absorption. This presents an enormous potential for power scaling and boosting the efficiency of few-cycle mid-infrared sources at wavelengths beyond
$5\ \mu \mathrm{m}$
. The additional benefit of obtaining carrier-envelope phase-stabilized MIR pulses by such supercontinuum-seeded OPAs can be exploited in applications requiring repeatable and controllable waveforms for electric field-resolved detection.