HyperTerahertz - High precision terahertz spectroscopy and microscopy

The last 20 years have witnessed a remarkable growth in the field of THz frequency science and engineering, which has matured into a vibrant international research area. The modern THz field arguably began with the development of a pulsed (single-cycle) THz emitter - the semiconductor photoconductive switch - and the subsequent development of THz time-domain spectroscopy (TDS). Since then, considerable success has been achieved in the further development of this and other THz sources, including the uni-travelling carrier (UTC) photodiode and the quantum cascade laser (QCL). However, notwithstanding this, it is only the THz-TDS technology that has been developed sufficiently for commercialization as a complete system, leaving other THz devices, components and techniques still restricted to the academic laboratory. This is unfortunate, since despite the success of THz-TDS, the technique has a number of shortcomings including its high fs-laser dominated cost, low power, and limited frequency and spatial resolution, which could be addressed by QCL and UTC technologies if they were to be engineered into appropriate instruments.

In fact, a cursory comparison with the neighbouring microwave and optical regions of the spectrum reveals that THz frequency science and technology is still in its infancy, and not just in the context of commercial uptake. For example, the THz region significantly lags in the availability of precision spectroscopy instrumentation required to address sharp spectral features inherent to gases, for example, in atmospheric analysis, or in materials with long excited state lifetimes. THz technology also significantly lags in the fields of non-linear spectroscopy and coherent control, where powerful and controlled pulses of electromagnetic radiation interact with matter and manipulate its properties. In the optical and microwave regions, fascinating phenomena including electron-spin resonance and nuclear magnetic resonance were major breakthroughs, revealing a wealth of new science and engineering applications. These techniques, now standard across many disciplines, support much contemporary research and technology activity.

A further example of how THz technology compares unfavourably with other spectral ranges is in the context of THz microscopy and analysis below the diffraction limit, which intrinsically restricts such measurements to ensemble sampling of physical properties averaged over the size, structure, orientation and density of, for example, nanoparticles, nanocrystals or nanodomains. Although near-field imaging approaches have been adapted from the visible/infrared regions enabling THz measurements on the micro/nano-scale, no THz instrument currently provides the required spatial resolution and sensitivity, nor can address the enormous range of length-scales (spanning five orders of magnitude from electron confinement lengths (<10 nm) to the THz wavelength (~300 um)), nor can operate at cryogenic temperatures. In fact, on this point, the THz field is deficient even in the provision of basic technologies such as waveguides and coupling optics required to deliver THz signals with low loss into cryostats or industrial apparatus.

In this programme we will create the first comprehensive instrumentation for precise THz frequency spectroscopy, microscopy, and coherent control. This will be based upon our unique and proprietary capabilities to generate, and manipulate photonically, THz signals of unprecedentedly narrow (Hz) linewidth and with sub-wavelength spatial resolution. The instrumentation will then be exploited to create new challenge-led applications in non-destructive testing and spectroscopic analysis for electronics and atmospheric sensing, inter alia, as well as discovery-led opportunities within physics, quantum technologies, materials science, atmospheric chemistry and astronomy.

This programme will develop new instruments for precise THz frequency spectroscopy, microscopy, and coherent control.

Economic Impact: Recent market studies predict strong growth in THz-based technologies over the next decade: e.g. Prescouter in 2015 predict that the market for current THz imaging and spectroscopy applications will grow from $127 million in 2016 to $570 million in 2021, and importantly, find that new microscopy and spectroscopy instrumentation and methodologies will accelerate this further. We will pursue a number of approaches to develop economic impact, as appropriate for the TRL level of the technology, including collaboration with existing companies to (i) transfer our technology into current commercial products, (ii) develop new commercial products, and (iii) create new companies where this will accelerate product development. Our new spectroscopy and microscopy instrumentation will provide versatile instrumentation for the scientific and R&D markets, with immediate application in non-destructive testing and spectroscopic analysis, inter alia. These include high-throughput microscopy requirements identified by Project Partner TeraView and other end-users in the testing of integrated circuits with international foundries, as well as other high-throughput THz imaging and microscopy applications in the medical, food, and pharmaceutical sectors. Longer-term economic impacts will be delivered through, for example, the applicability of our instrumentation to manipulate coherently quantum dots, and other mesoscopic electronic systems, in a compact and portable footprint. This adds capability to Project Partner Toshiba's development of single-photon and entangled-photon pair sources for applications to quantum key distribution and quantum computing, and the 'THz-to-Telecoms' bridge will lead to on-chip integrated quantum photon converters and hybrid single-photon detectors.

Knowledge Impact: We anticipate very considerable new knowledge generation in the fields of electronic engineering, condensed matter and optical physics, quantum technologies, materials science, atmospheric chemistry, inter alia, with new knowledge not only generated during the course of the Programme, but also created in the future though application of the instrumentation that we will develop. To give one example, our robust frequency-locked QCL spectroscopy systems will find application as local oscillators in satellite instrumentation for Earth-observation and planetary science, and in high-precision gas spectroscopy, and will create knowledge impact when exploited by chemists, atmospheric chemists, astronomers, and electronic engineers. Such instrumentation is necessary for future airborne and satellite-based Earth-observation missions above 2 THz, and with Project Partner DLR, we will integrate our technology onto SOFIA, a joint NASA/DLR project on a modified Boeing 747-SP aircraft to enable e.g. investigations and new knowledge of narrow-frequency outflows in shock regions of the interstellar medium. With RAL, we will address the more stringent requirements of satellite instrumentation enabling e.g. the analysis of OH (3.5 THz) and O (4.7 THz) species in the Earth's mesosphere/lower thermosphere, informing studies and new understanding of climate change, inter alia.

Societal Impact and Impact on People: THz science and technology is of national and global importance, and Society will benefit economically and through provision of new measurement and sensing instrumentation that will support high technology industry, including in the communications, non-destructive testing, and electronics sectors. We will also produce skilled People at doctoral and post-doctoral level having both technology and applications perspective, as well as developing current and future research leaders and role models.