Multiple-octave spanning mid-IR supercontinuum generation in bulk quadratic nonlinear crystals

TL;DR

This paper demonstrates the generation of multiple-octave spanning supercontinua in the mid-IR range using a lithium indium sulfide crystal, leveraging cascaded quadratic nonlinearities to produce broad, coherent supercontinuum spectra.

Contribution

It introduces a novel experimental approach for mid-IR supercontinuum generation using phase-mismatched second-harmonic generation in bulk quadratic nonlinear crystals.

Findings

Achieved supercontinuum spanning 1.6-7.0 μm in LiInS₂ crystal.

Demonstrated self-defocusing soliton formation via cascaded nonlinearities.

Showed potential for using other mid-IR crystals for supercontinuum generation.

Abstract

Bright and broadband coherent mid-IR radiation is important for exciting and probing molecular vibrations. Using cascaded nonlinearities in conventional quadratic nonlinear crystal like lithium niobate, self-defocusing near-IR solitons have been demonstrated that led to very broadband supercontinuum generation in the visible, near-IR and short-wavelength mid-IR. Here we conduct an experiment where a mid-IR crystal pumped in the mid-IR gives multiple-octave spanning supercontinua. The crystal is cut for noncritical interaction, so the three-wave mixing of a single mid-IR femtosecond pump source leads to highly phase-mismatched second-harmonic generation. This self-acting cascaded process leads to the formation of a self-defocusing soliton at the mid-IR pump wavelength and after the self-compression point multiple octave-spanning supercontinua are observed (covering 1.6-$7.0~\mu$m). The results were recorded in a commercially available crystal LiInS$_2$ pumped in the 3-$4~\mu$m range, but other mid-IR crystals can readily be used as well.