Executive Summary

The HyperTerahertz programme finished in December 2022.  The vision for HyperTerahertz was to open up new opportunities for the terahertz (THz) frequency range through the development of new instrumentation for precise THz spectroscopy, microscopy, and coherent control, and to prove and apply the technology in proof-of-concept studies. The five-year programme was defined by a series of formal objectives and exceptional progress has been made against each of them.

We demonstrated the first continuous-wave (cw) injection locking of a quantum cascade laser (QCL), with the QCL emission locked to a fibre-based near-infrared telecommunications frequency comb, itself referenced to a GPS-locked microwave frequency synthesiser, providing traceability of the QCL frequency to primary standards. This not only stabilizes the QCL frequency, and enables independent control of the frequency and phase for the first time, but also reduces the QCL linewidth to <1 Hz, and allows the phase-locked cw QCL emission to be detected coherently. This brings the frequency precision and accuracy available at microwave frequencies to the THz region of the spectrum for the first time, and as all components are semiconductor-based, compact integration is possible benefitting scientific application. We also demonstrated a bench-top QCL-based instrument capable of producing synchronized intense, narrowband, transform-limited pulses, with pulse lengths down to 650 ps. Sequences of twin pulses were also demonstrated where the pulse widths, delay and amplitude were independently controlled electronically. This narrowband pulse spectrometer will find application in the study of narrowband excitation and relaxation processes at THz frequencies and in coherent communications. Considerable work has furthermore been completed on packaging fibre-coupled uni-travelling-carrier photodiodes (UTC-PDs) with integrated antennas for future integrated QCL locking and high-precision spectroscopy.

We have pursued both room temperature and cryogenic near-field scanning THz microscopy, exploiting scattering- and aperture-based approaches. Our THz s-SNOM exploiting self-mixing detection, for example, was successfully integrated into a commercial platform and <30 nm resolution microscopy demonstrated. To enable the a-SNOM to image coherently and operate with cw QCL sources, we integrated semiconductor nanowire and nano-scale FET THz detectors into the probe aperture. These instruments have been applied to the investigation of samples and materials including: study of phonon-polariton-resonant crystals; imaging of localized plasmonic resonance modes in antenna structures and metamaterials; and, the study of TI materials, the latter requiring considerable aligned effort in the growth and device fabrication of these contemporary materials systems. The cryogenic microscopy programmes faced significant technical challenges caused by vibrational interference arising from the cryo-cooler compressor that affected the microscope resolution. A spring-coupled floating stage arrangement was developed with the cryo-cooler manufacturer, dramatically reducing the tip vibration, and opening the way for full cryogenic THz microscopy in future programmes.

We developed a toolkit of components including waveguides, reflectors, windows, and feedthroughs to couple THz radiation into difficult to access environments, focussing on 4.2 K and 1.5 K cryostats, and dilution refrigerators. QCL transmission within a dilution refrigerator was demonstrated, resulting in base and electron temperatures of, respectively, 109 mK and 430 mK, the latter in an InGaAs-AlGaAs two-dimensional electron system (2DES) during cyclotron absorption measurements. We also successfully designed, developed, and demonstrated antenna-coupled direct THz detectors exhibiting high responsivities, leading to the discovery of a new physical phenomenon—the in- plane photoelectric effect. And finally, excellent progress was made developing efficient large-area photoconductive array structures for generation and detection of high-power THz pulses, and in the use of these in proving work on 2D non-linear spectroscopy of the atom-in-solid material Ge:As.

The programme has demonstrated significant consortium cohesion and collaboration, with many inter-site visits for joint experiments, sample and technology transfer, and discussion. The covid-19 pandemic disruption inevitably had a negative impact on the breadth and coverage of the programme’s later activities as originally envisaged, but we successfully prioritized and focussed research to maximize impact and publications. We actively pursued public understanding of science and community engagement; for example, we presented at the Royal Society Summer Science Exhibition, developed a series of YouTube videos, worked with the Royce Institute to bring together the THz microscopy and 2D materials communities, and we lead the TeraNet UK Network. Programme sustainability includes both aligned and follow-on funding ranging from new programme grants (EP/V001914/1, EP/W028921/1) and responsive mode grants (EP/T034246/1, EP/V004743/1, EP/W03252X/1), to UKRI Future Leader Fellowships (Valavanis, Balakier, Ponnampalam). The development of our diverse and talented PGR and PDRA cohort and their progress from this programme into subsequent academic and industrial positions has been a particular highlight.