Multi-mode Dynamics of Terahertz Quantum Cascade Lasers: spontaneous and actively induced generation of dense and harmonic coherent regimes

TL;DR

This paper investigates the complex dynamics of terahertz quantum cascade lasers, including spontaneous and RF-induced regimes, demonstrating new methods for generating high-contrast pulse trains and harmonic mode-locking for advanced applications.

Contribution

It provides a comprehensive analysis of dense and harmonic regimes in THz-QCLs, introducing systematic procedures for pulse generation and harmonic mode-locking based on experimental and theoretical insights.

Findings

Reproduces key features of optical frequency combs in THz-QCLs

Demonstrates RF injection can lock laser dynamics and generate short, high-contrast pulses

Shows harmonic mode-locking produces pulse sequences suitable for communication and imaging

Abstract

We present an extended study concerning the dynamics of dense and harmonic coherent regimes in quantum cascade lasers (QCLs) in a Fabry--Perot (FP) configuration emitting in the terahertz (THz) spectral region. Firstly, we study the device in free running operation, reproducing the main features of the self-generated of optical frequency combs (OFCs) and harmonic frequency combs (HFCs) in this spectral range, commenting on the points of difference from the mid-infrared region, and finding excellent agreement with the most recent experimental evidences. Then, we analyze the THz-QCL dynamics under radiofrequency (RF) injection, with a focus on the effect of the modulation of the current on the degree of locking of the system, and we perform a systematic investigation aimed to provide a procedure for the generation of train of pulses with short duration and high contrast. Furthermore, we extend our study to the generation of sequences of pulses with repetition frequency which is a multiple of the free-spectral range (FSR) of the laser cavity, reproducing harmonic mode-locking (HML) of the laser, a method which has been recently demontrated in experiments with THz-QCLs, and the results of which are particulary promising for a large stream of applications, ranging from optical communication to imaging.