Supra-terahertz technology for atmospheric observations of the mesosphere and lower thermosphere

Advances in satellite remote-sensing measurements of the constituents of the Earth's mesosphere and lower thermosphere (MLT) have increased our knowledge of atmospheric composition over the last decade. Nonetheless, global measurements of key atmospheric species have not been made directly by previous satellite missions and these species, particularly atomic oxygen and the hydroxyl radical (OH), are targets for a low Earth orbit mission operating in the multi-terahertz (THz) spectral range (3 - 5 THz). A LOw Cost Upper-Atmosphere sounder (LOCUS) has therefore been proposed to ESA, which would be able to detect a broad range of important species (O, O3, OH, NO, CO, H2O and HO2) between altitudes of 50 and 400 km.

Heterodyne radiometry provides a spectral resolution that is well suited to characterising emission signatures originating from the MLT. The technique has been demonstrated and proven at sub-terahertz frequencies through a number of space flight missions over the past two decades. However, operation above 3 THz (supra-terahertz) has never been attempted from a space environment, and measurements of a number of important atmospheric species that have potential impact on climate change and related space weather effects have therefore not been made. Even systems operated from an airborne platform are rare and require large instruments that are completely unsuitable for space flight. There is therefore a need to develop compact, high-sensitivity, supra-terahertz heterodyne systems capable of undertaking global atmospheric measurements from space. To achieve this goal, technical development of the heterodyne mixer detector and its local oscillator (LO) is required.
The preferred heterodyne mixing device for Earth observation is the Schottky barrier diode. Although a well-known semiconductor device, it has not been demonstrated in a planar form beyond ~3 THz and challenges related to fabrication and circuit embedding need to be solved to allow this technical evolution. Additionally, the provision of LO power and its coupling to the mixer diode, whilst already presenting a technical barrier at sub-terahertz frequencies, is a particularly difficult problem to resolve in the supra-terahertz range. Fortunately, the advent of the quantum cascade laser (QCL) semiconductor device provides the prospect of a miniaturized, low power, supra-terahertz LO source with sufficient output power to 'pump' the mixer diode as a part of the frequency down-conversion process. Additionally, electromagnetic simulation software now permits the analysis and optimisation of QCL and Schottky diode devices and their respective electrical embedding circuits, with new and advanced micro-fabrication techniques allowing corresponding manufacture. However, technical development is required before a supra-terahertz MLT remote sounding instrument can be realised. For instance, QCL and Schottky device performance optimisation, physical integration into a common (waveguide) package, and frequency stabilisation are necessary.

We therefore propose a proof-of-concept development programme with an objective of demonstrating key component technologies (QCL and Schottky diode) to a minimum technical readiness level of TRL 3. Within this programme we will significantly advance core heterodyne technologies through a stepwise development approach, and with a goal of integrating and testing a QCL and Schottky diode in a common waveguide mount. Consideration will also be given to the scientific application and future technical development towards TRL 4 and beyond.
ESA has accepted the LOCUS concept as one requiring further evaluation as a prelude to a future in-orbit demonstration. Technical advancement of a terahertz frequency spectrometer through this NERC Proof of Concept Programme would provide a step-change in the progress towards this important scientific objective, as well as positioning the UK ideally for future in-orbit programmes with ESA.

Our proof-of-concept research addresses core technical objectives for advancing terahertz (THz) technology in support of climate change studies. This has a direct relationship to society through increased understanding of our environment and how human activity may be detrimentally affecting it. Our work will increase the technical feasibility of a new mission concept, LOCUS, and will therefore benefit research scientists undertaking atmospheric studies, in addition to technical scientists and engineers pursuing THz research. The proposed activity will be completed with a 12 month period, a timescale consistent with the LOCUS mission, and will raise the technical readiness level and substantially increase the prospect of the mission launch in demonstration form during the next 5 years. Moreover, it will provide PI and CoI leadership opportunities for UK scientists and engineers, and be of direct benefit to UK industry via, for example, satellite payload provision to ESA. Thus, the impact is not only proof of a breakthrough concept relating to THz device development, but also novel and innovative science of direct pubic and industrial benefit.

The work will be of interest to planetary scientists and astronomers. For instance, THz spectroscopy allows sounding of the key chemical species in the dense atmospheres of the Giant Planets and potentially the atmospheres of Mars and Venus. The development of a compact, low power/low mass heterodyne THz receiver would allow high-resolution in situ measurements of planetary atmospheres from orbital spacecraft. This would return important information about the complex physical and chemistry of the planets leading to a better understanding of how the solar system formed and evolved. Additionally, the next generation of far-infrared astronomical space based facilities will require very much higher spatial resolution than presently possible with single dish space telescopes. Launching large deployable apertures is one solution to this problem, but is a hugely expensive and risky operation. QCL local oscillators operating in the supra-THz range would allow the formation of an interferometric system offering high-spatial resolution.

Our development work will enhance UK strengths in THz technology and lay the foundations for a new generation supra-THz detection systems that will generate opportunities for UK industry and achieve economic gain. Previous experience has demonstrated a strong industrial interest in, and potential commercial benefit to be gained from, applying THz sensors to weather monitoring and forecasting, communications, security surveillance, biological sensing, medical and plasma diagnostics, in addition to a growing identified interest in advancing far-infrared laboratory based spectroscopy for industrial process control, materials examination, and local pollution monitoring. Each of these topics has excellent near-term growth potential and, through commercial and industrial exploitation, represents opportunities for delivering a considerable financial return to the UK and improvements to society. For instance, Leeds work on THz time-domain spectroscopy of crystalline molecules has allowed detailed experimental and modelling studies of materials of security relevance including explosives and drugs-of-abuse. This has been collaborative with UK Government security agencies including HM Government Communications Centre, the Police Scientific Development Branch, the Home Office Centre for Applied Science and Technology, and the Ministry of Defence. Also, RAL's diode development work has attracted considerable attention from industrial organisations within the UK and overseas and has led to a spinout company (Teratech Ltd). The proof-of-concept will increase national prestige in the THz field and will be exploited in a broad range of potential applications additional to Earth observation. A forum for our work is the International Space Terahertz and Technology conference.