Cascaded half-harmonic generation of femtosecond frequency combs in mid-IR

TL;DR

This paper demonstrates a highly efficient cascaded half-harmonic generation technique that converts near-IR frequency combs into broadband, phase-locked mid-IR combs spanning two optical octaves, enabling new applications in molecular spectroscopy.

Contribution

It introduces a novel cascaded half-harmonic generation method achieving two-octave down-conversion with high efficiency and intrinsic phase locking, extending frequency comb coverage into the mid-IR.

Findings

Achieved ~64% efficiency in first stage, ~18% in second stage.

Generated broadband, phase-locked combs at ~2 μm and ~4 μm.

Produced pulses of ~50 fs and ~110 fs at respective wavelengths.

Abstract

For the growing demand of frequency combs in mid-infrared (mid-IR), known as the "molecular fingerprint" region of the spectrum [1], down conversion of near-IR frequency combs through half- harmonic generation offers numerous benefits including high conversion efficiency and intrinsic phase and frequency locking to the near-IR pump [2]. Hence cascaded half-harmonic generation promises a simple path towards extending the wavelength coverage of stable frequency combs. Here, we report a two-octave down-conversion of a frequency comb around 1 {\mu}m through cascaded half-harmonic generation with ~64% efficiency in the first stage, and ~18% in the second stage. We obtain broadband intrinsically-frequency-locked frequency combs with ~50-fs pulses at ~2 {\mu}m and ~110-fs pulses at ~4 {\mu}m. These results indicate the effectiveness of half-harmonic generation as a universal tool for efficient phase- and frequency-locked down-conversion, which can be beneficial for numerous applications requiring long-wavelength coherent sources.