# Nonlinear silicon waveguides generating broadband, spectrally engineered frequency combs spanning 2.0-8.5 um

## TL;DR

This paper demonstrates fully air clad suspended-silicon waveguides capable of generating broadband mid-infrared frequency combs spanning 2.0-8.5 um, enabling advanced spectroscopy and chemical analysis with high resolution and stability.

## Contribution

The work introduces novel suspended-silicon waveguides with engineered dispersion for broadband mid-IR frequency comb generation, including efficient coupling and high coherence.

## Key findings

- Spectra span 2.1 octaves in the mid-infrared range.
- Achieved high signal-to-noise ratio with 112,200 comb lines.
- Enabled broadband dual-comb spectroscopy with high resolution.

## Abstract

Nanophotonic waveguides with sub-wavelength mode confinement and engineered dispersion profiles are an excellent platform for application-tailored nonlinear optical interactions at low pulse energies. Here, we present fully air clad suspended-silicon waveguides for infrared frequency comb generation with optical bandwidth limited only by the silicon transparency. The achieved spectra are lithographically tailored to span 2.1 octaves in the mid-infrared (2.0-8.5 um or 1170--5000 cm-1) when pumped at 3.10 um with 100 pJ pulses. Novel fork-shaped couplers provide efficient input coupling with only 1.5 dB loss. The coherence, brightness, and the stability of the generated light are highlighted in a dual frequency comb setup in which individual comb-lines are resolved with 30 dB extinction ratio and 100 MHz spacing in the wavelength range of 4.8-8.5 um (2100-1170 cm-1). These sources are used for broadband gas- and liquid-phase dual-comb spectroscopy with 100 MHz comb-line resolution. We achieve a peak spectral signal-to-noise ratio of 10 Hz^0.5 across a simultaneous bandwidth containing 112,200 comb-lines. These results provide a pathway to further integration with the developing high repetition rate frequency comb lasers for compact sensors with applications in chip-based chemical analysis and spectroscopy.