Terahertz heterodyne spectral imager of planets and comets

Science goals and objectives. The objective of this proposal is the development of a sensitive, frequency-tunable heterodyne sensor for submillimeter/THz molecular line spectroscopy suitable for planetary missions. Such a device could be used for limb sounding the atmospheres of Jupiter, Saturn, Uranus, Neptune, Titan and Triton, observing water vapor plumes on Europa and volcanic plumes on Io, as well as performing spectroscopy of the tenuous atmospheres of icy moons (including Enceladus) and comets. The heterodyne capability allows very high spectral resolution spectroscopy to be carried out and thus individual spectral line shapes and Doppler velocities to be measured. The proposed Tunable Antenna-Coupled Intersubband Terahertz (TACIT) mixer will provide access to the 1-5 THz frequency range, which contains numerous strong spectral lines originating from the rotational transitions of many molecules, including H2O, CO, CO2, CH4, HCl, H2O2, CS, SO and HCN and their isotopologues. Future planetary missions that will benefit from this technology include the Uranus and Enceladus Flagship Orbiters and all New Frontier candidates identified in the current Vision and Voyages Decadal Survey other than the Venus in-situ mission. The TACIT mixer is expected to have 4 times the sensitivity than that of the state-of-the-art heterodyne sensor on MIRO/Rosetta. The mixer will be able to operate at 60-70 K (passive cooling in space) and will require just a few microwatts of local oscillator (LO) power. The outcome of this work will enable a multi-pixel heterodyne imager (16 pixels or more) which will allow the spatial mapping to be done 16 times faster than with a single pixel. The proposed R&D task will be a 3-year effort, which will build on the work performed under the previous 2014 PICASSO funding, during which we successfully demonstrated the operation of a single-pixel TACIT mixer at 2.5 THz. It will optimize the design of TACIT devices further, develop the technique for embedding such devices into THz waveguide which are required for achieving a multipixel imager, and conclude with a demonstration of a lab receiver consisting of a mixer, first-stage low-noise amplifier, and required optics. Methodology. The approach to the mixer is to explore a photoconductive mechanism due to the intersubband transition in a 40nm GaAs/AlGaAs quantum well (QW) heterostructure. The mixer device is fabricated from a pure material (electron mobility > 1E6 cm^2/V-s), in order to achieve the high sensitivity of the device resistance to temperature at 50-70 K. Using gate electrodes, the subband separation can be electrically tuned to any frequency in the 1-5 THz range. In order to be imbedded into a waveguide, the mixer QW structure is suspended on a thin Si membrane. The mixer bandwidth is set by the relaxation rate of the two-dimensional electron gas, which can be as large as 10 GHz. TACIT mixer devices fabricated on Si membrane will enable the waveguide-based mixer which will be the starting point for the follow-on 16-pixel imager development effort. Relevance to the call. The proposal is relevant to the 2018 PICASSO call, which supports the development of spacecraft-based instrument systems that show promise for use in future planetary missions. We will develop a new component (multipixel heterodyne sensor) that significantly improves submillimeter spectroscopic measurement capabilities for planetary science missions. The entry TRL for this technology is 3 and the exit TRL will be 4. Under this proposal, we will closely interact with an SBIR-funded effort to develop a novel quantum cascade laser THz LO source, which will be required to develop an imaging heterodyne array instrument we expect to pursue under the MatISSE program.