Phase resolved measurements of stimulated emission

TL;DR

This paper presents a novel phase-resolved measurement technique for studying the ultrafast dynamics and gain properties of terahertz quantum cascade lasers, providing new insights into their internal processes.

Contribution

It introduces a direct electric field recording method with sub-100 fs resolution to analyze QCL gain and dynamics, validating quantum models and revealing bandwidth and loss characteristics.

Findings

First measurement of the real bandwidth of THz QCL gain

Demonstration of the current dependence of the gain

Identification of waveguide losses in the laser

Abstract

The development of the semiconductor quantum cascade laser (QCL) [1] has enabled bright coherent sources operating at frequencies between the optical (>100 THz) and electronic (<0.5 THz) ranges opening this frequency region for fundamental science investigations [2-5] as well as for applications [6]. However, detailed information about the internal processes in QCLs and their ultrafast dynamics are needed for their further development towards high output power, longer wavelengths and stable pulsed operation. We introduce a novel approach to study the gain and dynamics of a terahertz (THz) QCL [7] using phase resolved measurements of the stimulated emission. This is enabled by direct recording of the emitted electric field with <100 fs time resolution. For the case of the THz QCL we demonstrate the validity of the quantum mechanical description of the laser. We show for the first time the real bandwidth of the terahertz gain medium, the current dependence of this gain, and the losses associated with the wave propagation in the laser waveguide.