Development of THz Band Heterodyne Receiver for Semote-Sensing Spectroscopy of OH Radical and Atomic Oxygen in Middle Atmosphere

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Heterodyne remote-sensing spectroscopy of ions, atoms, and molecules at 1.8-4THz frequency band plays an important role in the study of physical and chemical conditions of the terrestrial and planetary atmospheres. We have developed a THz band superconducting Hot-Electron Bolometer mixer receiver (HEBM) for the remote-sensing of OH, OI, and so on. Our HEBM employed a 6nm thick superconducting Niobium Titanium Nitride (NbTiN) micro-bridge deposited on a 20nm-thick AlN interface layer using a helicon sputtering technique. This HEBM was operated at 4.2K by using the cryostat newly developed with a vibration-free closed-cycle mechanical 4K pulse tube refrigerator. The performance of the HEBM was investigated at the 1.5THz band especially changing the local oscillator power of a multiplier chain solid-state source. The output power of the intermediate frequency (IF) signal at 0.8-1.8GHz band had a maximum peak as a function of the bias current for HEBM. The best receiver noise temperature was obtained widely at around the IF maximum position. It was also found that the IF signal was most stable (more than 10 second) at around the IF maximum position. Considering the RF/LO coupling efficiency and reflection at Si lens the partially corrected receiver noise temperature of our HEBM was better than 1600K although the micro-bridge dimension was not completely optimized.