In this publication, we present a unified photon source and phase modulator. Our integrated device leverages the inherent strong nonlinear properties of intersubband transitions due to their large matrix element. Thus, intersubband transitions can be used as phase-modulators. At the same time intersubband transitions provide gain and amplify light. Therefore phase-modulation and light generation/amplification can be unified in one single device. We demonstrate this by employing a THz Quantum Cascade Laser fabricated in a ring-cavity. 
By applying an RF signal directly to our device, we successfully modulate the phase of the laser, resulting in the generation of sidebands on the original free-running spectrum. This modified laser emission is characterized by phase-locked modes, effectively forming a high-quality frequency comb, unrestrained by traditional cavity modes and enhanced by the spectral gain of the underlying QCL. 

Tunable comb operation in terahertz quantum cascade ring lasers

Michael Jaidl, Dominik Theiner, Florian Pilat, Aaron M. Andrews, Rahul Yadav, Gottfried Strasser, Benedikt Schwarz, Sascha Preu, Juraj Darmo and Karl Unterrainer
Optics Express, Vol. 33, Issue 21, pp. 44042-44050 (2025) , opens an external URL in a new window