Final Report Summary - TOSCA (Terahertz Optoelectronics - from the Science of Cascades to Applications)
The last 20 years have witnessed a remarkable growth in terahertz (THz) frequency science and engineering (300 GHz – 10 THz), which is maturing into a vibrant international research area. A wide range of organic and inorganic crystalline materials and gases exhibit characteristic vibrational/rotational modes in the THz frequency range, which have been exploited using current technology to create new methodologies for process monitoring and non-destructive testing in the pharmaceutical and electronics sectors, inter alia. These THz frequency techniques have also opened the way to a range of new and fundamental scientific investigations including, for example, the determination of time-resolved carrier dynamics in condensed matter systems such as semiconductors, conducting polymers, organic crystals, and superconductors.
However, despite the success and future potential of THz spectroscopy, even a cursory comparison between what is currently possible in this part of the spectrum with that in the neighbouring microwave and optical regions reveals that THz frequency science and technology is still very much in its infancy. The principal reason for this has been the lack of compact, convenient, semiconductor-based THz sources. However, the development of the THz frequency quantum cascade laser (QCL), a solid-state device based on a layered semiconductor superlattice, has opened the way for the development of the field of THz photonics.
The aim of the TOSCA programme was to develop underpinning technologies based on THz frequency QCLs, that would unlock future opportunities for THz science and engineering. Within the programme, we demonstrated the highest-ever reported output power (>1 W) for a THz QCL, together with a broad range of methodologies for controlling precisely the beam profile and output frequency of a THz beam, using techniques such as photonic crystal patterning and injection locking, respectively. We developed a range of solid-state detectors, and integrated THz components on-chip.
We demonstrated a range of QCL imaging and spectroscopy methodologies, with a particular highlight comprising a series of studies in which the emitting QCL cavity itself was used as a coherent self-detector of reflected radiation, exploiting the remarkable phase stability of these lasers. This technique allows absorption-coefficient-sensitive 3D imaging, as well as providing system miniaturization, and is being exploited with collaborators. Such innovative imaging techniques underpin the potential for THz applications across the physical, chemical, and biological sciences, with current on-going studies exploiting the potential of our technology for bio-medical imaging. Developments of THz QCLs on the TOSCA programme also attracted additional investment from the UK and European Space agencies to exploit terahertz QCLs as precise local oscillators for astronomic and atmospheric sensing; one goal is to develop a satellite-borne multi-channel radiometer to determine the abundance of atomic oxygen and the hydroxyl radical in the Earth’s mesosphere and lower thermosphere, a key indicator of global climate change. Underpinning these activities have been the design and demonstration of a range of electronically tunable QCLs (including coupled-cavity Vernier tuning), and multiple frequency QCLs. We also explored a number of other application areas for THz technologies, including studying the high frequency conductivity of mesoscopic semiconductor quantum structures.
Our work has already been published in over 60 high-impact, primary archival journals, including five Nature Group publications; it has also engendered extensive collaborative activity with researchers from Europe, the US, and far East, as well as end-users of THz technology. The success of the TOSCA programme was also a significant contribution to Professors Linfield and Davies being awarded the UK Institute of Physics 2014 Faraday Gold Medal and Prize with the citation: ‘For their outstanding and sustained contributions to the physics and technology of the far-infrared (terahertz) frequency region of the electromagnetic spectrum’, and to Professor Linfield being awarded a 2015 Royal Society Wolfson Research Merit Award.
However, despite the success and future potential of THz spectroscopy, even a cursory comparison between what is currently possible in this part of the spectrum with that in the neighbouring microwave and optical regions reveals that THz frequency science and technology is still very much in its infancy. The principal reason for this has been the lack of compact, convenient, semiconductor-based THz sources. However, the development of the THz frequency quantum cascade laser (QCL), a solid-state device based on a layered semiconductor superlattice, has opened the way for the development of the field of THz photonics.
The aim of the TOSCA programme was to develop underpinning technologies based on THz frequency QCLs, that would unlock future opportunities for THz science and engineering. Within the programme, we demonstrated the highest-ever reported output power (>1 W) for a THz QCL, together with a broad range of methodologies for controlling precisely the beam profile and output frequency of a THz beam, using techniques such as photonic crystal patterning and injection locking, respectively. We developed a range of solid-state detectors, and integrated THz components on-chip.
We demonstrated a range of QCL imaging and spectroscopy methodologies, with a particular highlight comprising a series of studies in which the emitting QCL cavity itself was used as a coherent self-detector of reflected radiation, exploiting the remarkable phase stability of these lasers. This technique allows absorption-coefficient-sensitive 3D imaging, as well as providing system miniaturization, and is being exploited with collaborators. Such innovative imaging techniques underpin the potential for THz applications across the physical, chemical, and biological sciences, with current on-going studies exploiting the potential of our technology for bio-medical imaging. Developments of THz QCLs on the TOSCA programme also attracted additional investment from the UK and European Space agencies to exploit terahertz QCLs as precise local oscillators for astronomic and atmospheric sensing; one goal is to develop a satellite-borne multi-channel radiometer to determine the abundance of atomic oxygen and the hydroxyl radical in the Earth’s mesosphere and lower thermosphere, a key indicator of global climate change. Underpinning these activities have been the design and demonstration of a range of electronically tunable QCLs (including coupled-cavity Vernier tuning), and multiple frequency QCLs. We also explored a number of other application areas for THz technologies, including studying the high frequency conductivity of mesoscopic semiconductor quantum structures.
Our work has already been published in over 60 high-impact, primary archival journals, including five Nature Group publications; it has also engendered extensive collaborative activity with researchers from Europe, the US, and far East, as well as end-users of THz technology. The success of the TOSCA programme was also a significant contribution to Professors Linfield and Davies being awarded the UK Institute of Physics 2014 Faraday Gold Medal and Prize with the citation: ‘For their outstanding and sustained contributions to the physics and technology of the far-infrared (terahertz) frequency region of the electromagnetic spectrum’, and to Professor Linfield being awarded a 2015 Royal Society Wolfson Research Merit Award.