Terahertz frequency combs exploiting an on-chip solution processed graphene-quantum cascade laser coupled-cavity architecture

TL;DR

This paper demonstrates a miniaturized terahertz frequency comb using a graphene-quantum cascade laser with an integrated on-chip saturable absorber, achieving high power, many modes, and stable operation for advanced spectroscopy and metrology.

Contribution

It introduces a novel on-chip graphene saturable absorber integrated with a THz QCL to enable stable, high-power, broadband frequency combs with over 90 modes, surpassing previous dispersion limitations.

Findings

Achieved a 8 mW power frequency comb with over 90 modes.

Demonstrated stable injection-locking of the frequency comb.

Operated over more than 55% of the laser’s range.

Abstract

The ability to engineer quantum-cascade-lasers (QCLs) with ultrabroad gain spectra and with a full compensation of the group velocity dispersion, at Terahertz (THz) frequencies, is a fundamental need for devising monolithic and miniaturized optical frequency-comb-synthesizers (FCS) in the far-infrared. In a THz QCL four-wave mixing, driven by the intrinsic third-order susceptibility of the intersubband gain medium, self-lock the optical modes in phase, allowing stable comb operation, albeit over a restricted dynamic range (~ 20% of the laser operational range). Here, we engineer miniaturized THz FCSs comprising a heterogeneous THz QCL integrated with a tightly-coupled on-chip solution-processed graphene saturable-absorber reflector that preserves phase-coherence between lasing modes even when four-wave mixing no longer provides dispersion compensation. This enables a high-power (8 mW) FCS with over 90 optical modes to be demonstrated, over more than 55% of the laser operational range. Furthermore, stable injection-locking is showed, paving the way to impact a number of key applications, including high-precision tuneable broadband-spectroscopy and quantum-metrology.