Discrete Vernier tuning in terahertz quantum cascade lasers using coupled cavities

Abstract

Terahertz-frequency quantum cascade lasers (THz QCLs) are compact solid-state sources of coherent radiation in the 1–5 THz region of the electromagnetic spectrum . The emission spectra of THz QCLs typically exhibit multiple longitudinal modes characteristic of Fabry–Pérot (FP) cavities. However, widely-tunable (single-mode) THz QCLs would be ideally suited to many THz-sensing applications, such as trace gas detection, atmospheric observations , and security screening . Here we demonstrate discrete Vernier tuning using a simple two-section coupled-cavity geometry. A monolithic THz QCL ridge cavity was etched using focused ion beam milling to create two coupled FP cavities separated by an air gap. In this scheme, one of the two sections (the ‘lasing section’) is electrically driven above the lasing threshold, while the other is driven below threshold and acts as a ‘tuning section’. The lengths of the two sections and the air gap were designed such that the longitudinal FP modes of the respective sections coincide at a selected (‘resonant’) frequency. The dominant lasing mode of the coupled cavity occurs at this frequency owing to the reduction in threshold . A small perturbation to the frequency of the modes in either section of the device will detune the resonance, causing the dominant mode of the coupled-cavity to ‘hop’ to a different frequency, in a manner analogous to the Vernier effect. The longitudinal modes of the tuning section are controlled by perturbing its refractive index through current-induced heating