Unveiling spectral purity and tunability of terahertz quantum cascade laser sources based on intra-cavity difference frequency generation

TL;DR

This paper characterizes the spectral purity and tunability of room-temperature terahertz quantum cascade lasers based on intra-cavity difference frequency generation, demonstrating their suitability for high-precision applications.

Contribution

First measurement of linewidth, tuning, and absolute frequency of THz DFG-QCLs using a terahertz frequency comb synthesizer, revealing their potential for metrological and spectroscopic uses.

Findings

Emission linewidth of 400 kHz at 1 ms integration

Precise measurement of frequency tuning characteristics

Absolute frequency uncertainty of 4 x 10^-10

Abstract

Terahertz sources based on intra-cavity difference-frequency generation in mid-infrared quantum cascade lasers (THz DFG-QCLs) have recently emerged as the first monolithic electrically-pumped semiconductor sources capable of operating at room-temperature (RT) across the 1-6 THz range. Despite tremendous progress in power output, that now exceeds 1mW in pulsed and 10 {\mu}W in continuous-wave regime at room-temperature, knowledge of the major figure of merits of these devices for high precision spectroscopy, such as spectral purity and absolute frequency tunability, is still lacking. Here, by exploiting a metrological grade system comprising a terahertz frequency comb synthesizer, we measure, for the first time, the free-running emission linewidth (LW), the tuning characteristics, and the absolute frequency of individual emission lines of these sources with an uncertainty of 4 x 10-10. The unveiled emission LW (400 kHz at 1ms integration time) indicates that DFG-QCLs are well suited to operate as local oscillators and to be used for a variety of metrological, spectroscopic, communication, and imaging applications requiring narrow-linewidth THz sources.