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Study of Novel Low Noise Superconducting Mixers for Terahertz Radio Astronomy

Final Report Summary - TERAMIX (Study of Novel Low Noise Superconducting Mixers for Terahertz Radio Astronomy)

Atoms and molecules emit electromagnetic waves whose frequency and intensity bare information about their nature and physical properties (temperature, concentration, velocity). Though such emission (and absorption as well) occurs at various wavelength/frequencies, observations in the terahertz (THz) part of the electromagnetic spectrum (0.1-10THz) offer great advantages over e.g. infrared or visible: better penetration through interstellar media and often higher intensity. Furthermore, THz heterodyne spectrometers allow for remote sensing of planetary atmospheres, comets, etc. Sensitive THz wave detection with a high spectral resolution is achieved in THz mixers, which are essentially detectors for THz waves.
Teramix project studied interaction of terahertz waves (photons) with thin films of superconducting magnesium diboride (MgB2) and fast processes of electron energy relaxation in an attempt to create very broadband and sensitive THz mixers. Integrated with on-chip THz micro antennas, and shaped as nano scale bridges, such mixer is essentially an electronic bolometer, where absorption of THz photons is sensed via variation of its electrical resistance. Considering the operation principles, it is called Hot-Electron Bolometer (HEB). However, in order to achieve the goal bandwidth of 10GHz, electrons in MgB2 nanobridges have to be able to cool very quickly, within a few picoseconds. No other material could provide that. However, our preliminary studies showed that this might be possible if ultrathin films of MgB2 are utilized, yet with best superconducting quality. The later one requires the superconducting transition temperature to be above 20-25K.
During the course of the project, we developed a method for such high quality MgB2 deposition yet as thin as 5nm (10 atomic layers) and with a critical temperature 30-32K. Initially, we have proved that in such films electron temperature can be modulated as fast as 10GHz, which is a factor 3-4 faster than in any other superconductor. We fabricated MgB2 HEB mixers as small as 300nm squares. Studies of mixer sensitivity was conducted at frequencies from 700GHz to 2.6THz using THz gas lasers. Our best mixers had a noise bandwidth of 13GHz and an input noise temperature of about 1000K. Furthermore, due to unprecedented high critical temperature, such HEB mixers do not require expensive liquid helium cooling (4K) anymore, but can operate as high as at 20K, where much more compact and cheaper coolers exist.
Apart of astronomical applications, we see potential for our devices in many other fields: spectroscopy, security imaging, atmospheric studies, etc. Investigation of commercial and societal potential of MgB2 HEB mixers will be conducted during the ERC-Proof of Concept project, which starts in November 2017.