Ultrabroadband Gain-Switched and Superluminescent Terahertz Semiconductor Lasers

TL;DR

This paper demonstrates ultrabroadband terahertz emission from quantum cascade lasers using microwave modulation, achieving gain-switched and superluminescent regimes for spectroscopy and imaging applications.

Contribution

It introduces a novel method of modulating THz QCLs to produce ultrabroadband emission covering 1.9-4.1 THz, with comprehensive modeling of the laser dynamics.

Findings

Gain-switched spectrum spans 1.9-4.1 THz with smooth envelope.

Increasing modulation broadens modes to a superluminescent regime.

Devices enable gapless absorption spectroscopy and broadband THz sources.

Abstract

Terahertz quantum cascade lasers (THz QCLs) are chip-scale semiconductor lasers operating in the frequency range between 1-6 THz, useful as compact sources for spectroscopy, communications, and non-destructive imaging and testing. Here, we apply low-frequency microwave modulation on a planarized THz QCL to generate ultrabroadband emission in the THz range. For very low modulation frequencies below 1 GHz, a gain-switched octave-spanning spectrum with a smooth spectral envelope is generated between 1.9 - 4.1 THz. Increasing the modulation frequency broadens the lasing modes until a low-coherence, continuous emission spectrum is achieved in the superluminescent regime, covering the spectral region between around 3 - 4 THz, without any discrete lasing modes or spectral gaps. We complement the experimental results with extensive analytical models and numerical simulations that capture the intracavity laser dynamics and fully explain the different operation regimes. These devices could prove useful for absorption spectroscopy without any spectral gaps, and as ultrabroadband sources of THz radiation.