Shaping terahertz harmonic frequency combs with frequency dependent external reflectors

TL;DR

This paper introduces a novel method for engineering terahertz harmonic frequency combs using frequency-dependent external reflectors, enabling passive generation of ultrashort pulses and improved coherence for spectroscopy applications.

Contribution

The study presents a new approach that controls both frequency offset and comb spacing of terahertz HFCs via external reflectors, enhancing pulse shaping and coherence without active modulation.

Findings

Enables passive generation of picosecond pulses

Improves coherence of quantum cascade laser emission

Allows triggering of HFCs from free-running dynamics

Abstract

We present a method for engineering harmonic frequency combs (HFCs) in the terahertz spectral region. This approach involves interfacing a quantum cascade laser (QCL) with an external reflector featuring frequency-dependent reflectivity. A notable advantage of this method over existing ones is its dual functionality in shaping HFCs, allowing for control over both the frequency offset and comb spacing based on the external reflectivity profile. Moreover, the resulting HFCs manifest as sequences of short pulses in the time domain. Consequently, our method enables the generation of ultrashort picosecond pulses passively, providing a distinct alternative to conventional pulse generation systems reliant on active bias current modulation, which struggle with modulation frequencies significantly higher than the first beatnote. This offers intriguing prospects for utilizing HFCs in pump and probe spectroscopy, a field already recognized in the literature as one of the most compelling applications of these states. Furthermore, we demonstrate that these HFCs can be triggered from an initial condition of free-running unlocked dynamics, eliminating the need for assuming free-running comb emission. Thus, the utilization of an external, frequency-dependent reflector is capable of enhancing the coherence of the QCL emission.