Direct Observation of Terahertz Frequency Comb Generation in Difference-Frequency Quantum Cascade Lasers

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, being the only electrically pumped device able to operate in the 0.6-6 THz range without the need of bulky and expensive liquid helium cooling. Here we present comb operation obtained by intra-cavity mixing of a distributed feedback laser at lambda = 6.5 mu m and a Fabry-Perot device at around lambda = 6.9 mu m. The resulting ultra-broadband THz emission extends from 1.8 to 3.3 THz, with a total output power of 8 mu W at 78 K. The THz emission has been characterized by multi-heterodyne detection with a primary frequency standard 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 MHz accuracy. In the future, this setup can be used for Fourier transform based evaluation of the phase relation among the emitted THz modes, paving the way to room-temperature, compact, and field-deployable metrological grade THz frequency combs.

Automated summary

This article presents the ultra-broadband THz emission obtained by intra-cavity mixing of different wavelengths, as well as the characterization and measurement methods of this emission. The new device can operate at low temperatures without the need for expensive liquid helium cooling, offering high stability and frequency accuracy.