GHz control of THz QCL band structure and gain by standing acoustic strain

TL;DR

This paper demonstrates a novel method for dynamically controlling the bandstructure and gain of terahertz quantum cascade lasers using GHz-frequency standing acoustic waves, enabling new possibilities for frequency combs and waveform control.

Contribution

It introduces the first use of GHz acoustic strain to modulate the bandstructure of THz QCLs, offering a new approach for device control beyond bias modulation.

Findings

Demonstrated tunable standing BAWs at 5-12 GHz in QCLs

Observed PL energy modulation up to a few meV due to BAW strain

Developed a model predicting BAW effects on QCL bandstructure

Abstract

Active frequency comb generation and waveform control are central challenges in the terahertz (THz) domain. In THz quantum cascade lasers (QCLs), these functions have typically been achieved through active bias modulation, which alters the operating point of the device and imposes severe limitations on its flexibility. To address these challenges, we propose an approach based on the direct modulation of the QCL bandstructure using GHz-frequency standing bulk acoustic waves (BAWs), promising direct and localized control of the optical gain and chromatic dispersion. To this end, we fabricated a bulk acoustic transducer on top of a THz QCL in order to excite GHz standing BAWs within its active region. We demonstrate that radio-frequency driving of the transducer leads to the tunable generation of standing BAWs in 5-12 GHz frequency range with wavelengths commensurate to the QCL period length. The effect of the BAW on the QCL bandstructure is revealed by measuring photoluminescence (PL) of the active region, where the BAW strain leads to a considerable modulation of the PL energy up to a few meV around its non-modulated value. We also develop a model and perform bandstructure simulations to predict the effect of the BAW on the QCL subband structure and gain. These results mark the first demonstration of dynamic bandstructure modulation in a THz QCL using GHz acoustic strain, introducing a fundamentally new paradigm that establishes a powerful synergy between QCLs and BAWs towards coherent control and frequency comb engineering in the THz domain.