Direct observation of terahertz frequency comb generationin difference-frequency quantum cascade lasers

TL;DR

This paper demonstrates the generation of a terahertz frequency comb using intra-cavity difference-frequency mixing in quantum cascade lasers, achieving broad spectral coverage and precise mode spacing, advancing THz spectroscopy and metrology.

Contribution

It presents the first direct observation of a THz frequency comb from intra-cavity difference-frequency generation in quantum cascade lasers, with detailed characterization and potential for portable applications.

Findings

Broadband THz emission from 1.8 to 3.3 THz achieved

Confirmed equally spaced modes with 1 MHz accuracy

Output power of 8 μW at 78K

Abstract

Terahertz quantum cascade laser sources based on intra-cavity difference frequency generation from mid-IR devices are an important asset for applications in rotational molecular spectroscopy and sensing, beingthe only electrically pumped device able to operate in the 0.6-6 THz range without the need of bulky andexpensive liquid helium cooling. Here we present comb operation obtained by intra-cavity mixing of adistributed feedback laser at {\lambda} = 6.5 {\mu}m and a Fabry-P\'erot device at around {\lambda} = 6.9 {\mu}m. The resultingultra-broadband THz emission extends from 1.8 to 3.3 THz, with a total output power of 8 {\mu}W at 78K.The THz emission has been characterized by multi-heterodyne detection with a primary frequencystandard referenced THz comb, obtained by optical rectification of near infrared pulses. The down-converted beatnotes, simultaneously acquired, confirm an equally spaced THz emission down to 1 MHzaccuracy. In the next future this setup can be used for Fourier transform based evaluation of the phaserelation among the emitted THz modes, paving the way to room-temperature, compact and field-deployable metrological grade THz frequency combs.