Homogeneous quantum cascade lasers operating as Terahertz frequency combs over their entire operational regime

TL;DR

This paper demonstrates a homogeneous quantum cascade laser operating as a stable, broadband terahertz frequency comb with high power, narrow linewidth, and tunability, suitable for precise metrological applications.

Contribution

It introduces a homogeneous THz QCL that functions as a stable, broadband frequency comb across its entire operational regime without external dispersion compensation.

Findings

Operates as a broadband THz frequency comb with 36 modes

Achieves a minimum beatnote linewidth of 500 Hz

Can be injection-locked over a 1.2 MHz range

Abstract

We report a homogeneous quantum cascade laser (QCL) emitting at Terahertz (THz) frequencies, with a total spectral emission of about 0.6 THz centered around 3.3 THz, a current density dynamic range of Jdr=1.53, and a continuous wave output power of 7 mW. The analysis of the intermode beatnote unveils that the devised laser operates as optical frequency comb (FC) synthesizer over the whole laser operational regime, with up to 36 optically active laser modes delivering ~ 200 uW of optical power per comb tooth, a power level unreached so far in any THz QCL FC. A stable and narrow single beatnote, reaching a minimum linewidth of 500 Hz, is observed over a current density range of 240 A/cm2, and even across the negative differential resistance region. We further prove that the QCL frequency comb can be injection locked with moderate RF power at the intermode beatnote frequency, covering a locking range of 1.2 MHz. The demonstration of stable FC operation, in a QCL, over the full current density dynamic range, and without any external dispersion compensation mechanism, makes our proposed homogenous THz QCL an ideal tool for metrological application requiring mode-hop electrical tunability and a tight control of the frequency and phase jitter.