Quantum-limited frequency fluctuations in a terahertz laser

Abstract

Quantum cascade lasers(1,2) can be considered the primary achievement of electronic band structure engineering, showing how artificial materials can be created through quantum design to have tailor-made properties that are otherwise non-existent in nature. Indeed, quantum cascade lasers can be used as powerful testing grounds of the fundamental physical parameters determined by their quantum nature, including the intrinsic linewidth of laser emission(3), which in such lasers is significantly affected by the optical and thermal photon number generated in the laser cavity. Here, we report experimental evidence of linewidth values approaching the quantum limit(4,5) in far-infrared quantum cascade lasers. Despite the broadening induced by thermal photons, the measured linewidth results narrower than that found in any other semiconductor laser to date. By performing noise measurements with unprecedented sensitivity levels, we highlight the key role of gain medium engineering(6) and demonstrate that properly designed semiconductor-heterostructure lasers can unveil the mechanisms underlying the laser-intrinsic phase noise, revealing the link between device properties and the quantum-limited linewidth

Phase locking of a 3.4 THz third-order distributed feedback quantum cascade laser using a room-temperature superlattice harmonic mixer

“…It is interesting to note that the measured frequency locked linewidth is at least one order of magnitude larger than the extracted intrinsic linewidth derived from the noise measurement in a FP QCL ($100 Hz in Ref. 20). The larger linewidth in our case can be attributed primarily to the loop bandwidth, which is unable to suppress the frequency noise beyond 100 kHz, but also more fundamental differences in the QCL itself (i.e., use of resonant-phonon over singleplasmon bound-to-continuum design and DFB over FP frequency selection mechanism).…”

Subcycle measurement of intensity correlations in the terahertz frequency range

“…The fundamental linewidth of such a system was measured recently [29,30], demonstrating very narrow (100's Hz) linewidth that are Schawlow-Townes limited, as expected [31]. Nevertheless, the possible role of an additional broadening caused by amplified thermal emission remains experimentally and theoretically open [32].…”

Frequency Locking and Monitoring Based on Bi-directional Terahertz Radiation of a 3rd-Order Distributed Feedback Quantum Cascade Laser

“…The latter has roughly −0.6 GHz/V determined from a separate FTS experiment. We are therefore able to estimate a free running QCL linewidth of around 800 kHz, which is much larger than the intrinsic linewidth [23] because of time-dependent jitters. Strictly speaking, this is not the laser linewidth, but rather the range of laser emission frequency averaged in a measured time interval [11].…”