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.

Project partners
Our research programme has been designed to have impact in a number of areas, and has been planned as such in conjunction with project partners who are leaders in their sectors. Each partner has the capability to help us towards commercialising our research: NPL for terahertz waveguides standards and metrological applications; the Rutherford Appleton Laboratory for their expertise in high resolution THz spectroscopy and space applications, and Teledyne E2V (T-E2V) as a major UK-based international company capable of commercial device fabrication. Our project partners will contribute resources, expertise, mentoring, and use of facilities to the programme as specified in their Letters of Support and as detailed in our Pathways to Impact.

Exploitation of applications
There are two main pathways to exploit commercial opportunities: (i) collaboration with companies to translate relevant expertise/technology into economic performance; and (ii) patent licensing/sale(s). Both the School of Electronic & Electrical Engineering at Leeds and the School of Physics & Astronomy at Nottingham have well established track records of protecting and licensing IP from their research. IP will be identified and protected through Leeds University's Commercialisation Team based in the Research & Innovation Service. This office will lead on the filing and licensing of new IP generated, and will act as the interface to the University's venture capital partner, IP Group plc. Together, these provide a comprehensive service at all stages in the commercialisation process from the identification of new ideas, through to realization and protection of IP, to licenses or via spin-out companies. The UoL Research and Innovation Services office also works with companies by administering KTPs and PhD placements. The Nottingham team will hold regular meetings with their School's Business Development Executive (BDE), Dr. Peter Milligan, who is the main contact to the University's Business Engagement and Innovation Services. The BDE supports interaction with industrial partners and government agencies, leads on the identification of new intellectual property, and works with academic and research staff to identify opportunities for setting up spin-out companies.

Our impact work will be achieved, in part, through support from colleagues in the Leeds Research & Innovation Service and the University's Communications Team. We will establish a website, initially to promote our work on acoustic perturbation, but as the programme progresses, to highlight areas of the research with particular potential industrial impact, e.g. in gas sensing and Earth Observation relating to the climate change agenda. The website will provide a high-level description of the work and its main results to encourage new collaborations, and give examples of the latest projects and outputs from the consortium. We will identify and liaise with companies to promote this programme. The University of Leeds business Hub, Nexus, will provide a platform for us to proactively make new contacts with companies interested in fast THz modulation / communications applications. The investigators will work with the Commercialisation Team to coordinate patent applications and subsequent licensing arrangements based on the technologies developed. We will attend major international conferences as a means of disseminating the science and engineering developments, but we will also use these trips as an opportunity to engage with industrial end-users who attend such conferences and associated trade shows. PI JEC will co-chair European Microwave Week in 2021, which will include the largest trade in microwave devices and systems in Europe, providing one such opportunity.