Acoustic control of quantum cascade heterostructures: the THz "S-LASER"

Our vision is to develop active terahertz (THz) frequency devices in which the stimulated emission of coherent THz acoustic phonons is achieved simultaneously together with the stimulated emission of coherent THz photons in a quantum cascade laser (QCL); specifically, we will create a new epitaxially integrated device, which we term the THz "S-LASER" (Sound & Light Amplification by Stimulated Emission of Radiation) which will overcome electrical limits on the speed of modulation of current THz sources, by allowing self-oscillating sound-generating regions of the device to control at 100s of GHz regions in which THz lasing occurs.

Furthermore, we will achieve precise frequency modulation in the same device using lateral surface acoustic waves (SAWs) to control the Fabry-Perot modes with unprecedented frequency range under full electrical control. THz QCLs offer an ideal platform upon which to develop such devices, owing to their gain recovery time being suitably fast, their layer structure being ideal for phonon perturbation, and since they will permit monolithic integration with active phononic devices (SASERs) in the GaAs/AlGaAs material system.

THz QCLs are a well-established laser technology pioneered by the Leeds team, offering multi-Watt power levels, and spanning the spectral region from ~1 to 5 THz. Communications links formed by modulated THz QCL laser sources have been proposed as ideal candidates for military and satellite communication systems, as well as for other short-range high-throughput secure applications including in data centres. Until now, however, the maximum modulation rate that one can achieve has been limited to a few GHz by the RLC (resistive/inductive/capacitive) time constants associated with the electrical circuits in which THz QCLs are embedded.

Here, we will exploit acoustic perturbation of the QCL bandstructure to modulate the electronic states and hence control the light output on picosecond timescales, yielding unprecedented modulation bandwidths of 100s of GHz. We will then use these developments to demonstrate an S-LASER, combining concepts from THz QLCs with self-oscillating SASERs.

Building on a proven experimental and theoretical collaboration between Leeds and Nottingham, and using our combined world-leading expertise in THz devices and ultrafast acoustics, we will investigate experimentally the interaction of acoustic waves with THz QCL heterostructures. By exploiting the spatial overlap of confined THz photons and acoustic waves, our work will open up exploration of the physical regime of strong opto-mechanical coupling at THz frequencies. New regimes of opto-acoustic interaction have been investigated recently in the 10's of GHz frequency range, but here our chosen system will increase the frequency of operation by three orders of magnitude, enabling new physics, technology, and applications to be realised.