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19 th International Symposium on Space Terahertz Technology P9-6 SIS Mixers for ALMA Band-10: Comparison of Epitaxial and Hybrid Circuits S. V. Shitov 1,2 , M. A. Bukovski 2 , A. V. Uvarov 2 , O. V. Koryukin 2 and Y. Uzawa 1 1 National Astronomical Observatory of Japan, Mitaka, Japan 2 Institute of Radio-Engineering and Electronics, Moscow, Russia To provide a basis for optimum choice of a SIS mixer for ALMA Band-10 (787- 950 GHz), numerical simulations are performed based on properties of both the SIS junction and the possible design of its tuning/coupling circuit. The “traditional” Nb- AlOx-Nb and epitaxial NbN-AlN-NbN junctions are studied within either NbN(NbTiN)/Al or all-epitaxial NbN circuit. The calculations are based on Tucker’s theory, which simplest 3-port approximation, according to M. Feldman, fits the terahertz-range SIS mixers [1]. The extra noise associated with multiple Andreev reflection in NbN junctions [2] is also taken into account. The numerical simulations are finally fitted to the best experimental data available from the literature and to our own experiments. We conclude that, in spite the relatively leaky IV-curve, the performance of an epitaxial NbN junction embedded into a low-loss epitaxial NbN tuning circuit at about 1 THz can be about the same as the performance of a high-quality Nb-AlOx-Nb junction embedded into a (relatively lossy) NbTiN/Al tuning circuit. The abovementioned conclusion is supported by the most recent experiments demonstrating Trx below 300 K (DSB) for the circuits made from the epitaxial NbN. A report on these experimental results is also planned for this conference. [1] M. Feldman, Private communication, 2005. [2] Y. Uzawa and Z. Wang, "Coherent multiple charge transfer in a superconducting NbN tunnel junction," Phys. Rev. Lett. 95, 017002 (2005). 150
19 th International Symposium on Space Terahertz Technology P11-1 A semiconductor quantum dot for spectral sensitive detection of THz radiation R Davis 1 , S Pelling 1 , A. Tzalenchuk 2 and V. Antonov 1 1 Physics Department, Royal Holloway University of London, Egham, Surrey TW20 0EX, UK 2 National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, United Kingdom Spectral sensitive detection of THz radiation can be performed using a quantum dot formed in a two dimensional electron gas of GaAs/AlGaAs heterostructure. Different types of quantum dot sensors have been fabricated and studied. The most sensitive sensor, which is able to detect individual terahertz photons, consists of a quantum dot coupled to a metallic single electron transistor. This sensor however requires state of the art nanofabrication, delicate operation and temperatures below 1K. A more robust but less sensitive sensor is a quantum dot coupled to the point contact. It has reasonable nanofabrication demands and relaxed operation at T~1.5K. We compare two types of detectors, and suggest optimisation of the design aiming at improvement of quantum efficiency. 151
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