Sheng-Cai Shi

Development of Superconducting Spectroscopic Array Receiver: A Multibeam 2SB SIS Receiver for Millimeter-Wave Radio Astronomy

We have developed a 3×3 multibeam sideband separation superconducting receiver-Superconducting Spectroscopic Array Receiver (SSAR)-for the 85-115 GHz frequency band. The receiver employs 2SB superconductor-insulator-superconductor (SIS) mixers with a typical single sideband (SSB) noise temperature of 60 K and image rejection ratio above 10 dB over the frequency band. Digital techniques are extensively applied in this receiver system for excellent operational stability and efficiency. They include fast Fourier spectrometers, digital LO and digital bias supplies. In our knowledge this is the first 2SB multibeam millimeter wavelength receiver in the world. This receiver has been successfully put into observation and its considerable enhancement of mapping speed has been demonstrated.

An SIS mixer using two junctions connected in parallel

We have fabricated a superconductor-insulator-superconductor (SIS) mixer using a couple of junctions connected in parallel through a stripline inductor. The junctions have significantly larger area, i.e. larger capacitance, and smaller normal resistance than conventional ones. In order to obtain a good impedance matching between the source and the junctions, an impedance transformer made of a superconducting stripline was integrated with the junctions. The capacitance of the junctions was tuned out by the inductor to obtain a broadband operation without mechanical tuning elements. It was shown that the double sideband noise temperature of the receiver employing this type of mixer device was less than 40 K over the bandwidth of 90-115 GHz. The lowest receiver noise temperature of /spl sim/20 K, which is only 4 times as large as the quantum limited photon noise of hv/k/sub B/, was obtained around 105 GHz.<<ETX>>

Large-scale mapping observations of the CI 3P1-3P0 line toward heiles cloud2 in the Taurus Dark Cloud

A distribution of the neutral carbon atom (C i) in Heiles cloud 2 (HCL2) has been investigated with the Mount Fuji submillimeter-wave telescope. A region of 1.2 deg 2 covering a whole region of HCL2 has been mapped with the 3 P1‐ 3 P0 fine-structure line (492 GHz) of C i. The global extent of the C i emission is similar to that of 13 CO, extending from southeast to northwest. However, the C i intensity is found to be rather weak in dense cores traced by the line of C 18 O. On the other hand, strong C i emission is observed in a south part of J= 1‐0 HCL2 in which the C 18 O intensity is fairly weak. The C i/CO abundance ratio is greater than 0.8 for the C i peak, whereas it is 0.1 for the dense cores such as the cyanopolyyne peak. The C i‐rich cloud found in the south part may be in the early evolutionary stage of dense core formation where C i has not yet been converted completely into CO. This result implies that formation of dense cores is taking place from north to south in HCL2. Subject headings: ISM: atoms — ISM: clouds — ISM: evolution — ISM: individual (Heiles’s cloud 2)

Atomic carbon and CO isotope emission in the vicinity of DR 15

We present observations of the P-3(1)-P-3(o) fine-structure transition of atomic carbon [C I], the J = 3-2 transition of CO, and the J = 1-0 transitions of (CO)-C-13 and (CO)-O-18 toward DR 15, an H II region associated with two mid-infrared dark clouds (IRDCs). The (CO)-C-13 and (CO)-O-18 J = 1-0 emissions closely follow the dark patches seen in optical wavelength, showing two self-gravitating molecular cores with masses of 2000 and 900 M-circle dot, respectively, at the positions of the cataloged IRDCs. Our data show a rough spatial correlation between [C I] and (CO)-C-13 J = 1-0. Bright [C I] emission occurs in the relatively cold gas behind the molecular cores but does not occur in either highly excited gas traced by CO J = 3-2 emission or in the H II region/molecular cloud interface. These results are inconsistent with those predicted by standard photodissociation region models, suggesting an origin for interstellar atomic carbon unrelated to photodissociation processes.

Noise temperature and beam pattern of an NbN hot electron bolometer mixer at 5.25 THz

We report the measured sensitivities of a superconducting NbN hot electron bolometer (HEB) heterodyne receiver at 5.25 THz. Terahertz (THz) radiation is quasioptically coupled to a HEB mixer with a lens and a spiral antenna. Using a measurement setup with black body calibration sources and a beam splitter in vacuo, and an antireflection coated Si lens, we obtained a double sideband (DSB) receiver noise temperature (TrecDSB) of 1150 K, which is nine times h?/2k, where h is the Planck constant, ? the frequency, and k the Boltzmann constant. In addition, the measured far field beam patterns of the integrated lens antenna show nearly collimated beams from 2.5 to 5.3 THz that allow reliable measurement of TrecDSB using the vacuum setup. Our experimental results in combination with an antenna-to-bolometer coupling simulation suggest that the HEB mixer can work well at least up to 6 THz, making it suitable for next generation of high-resolution spectroscopic space telescopes and, in particular, for the detection of the neutral atomic oxygen line at 4.7 THz.

Conceptual design studies of the 5 m terahertz antenna for Dome A, Antarctica

As the highest, coldest and driest place in Antarctica, Dome A provides exceptionally good observing conditions for ground-based observations over terahertz wavebands. The 5m Dome A Terahertz Explorer (DATE5) has been proposed to explore new terahertz windows, primarily over wavelengths between 350 and 200 mu m. DATE5 will be an open-air, fully-steerable telescope that can function by unmanned operation with remote control. The telescope will be able to endure the harsh polar environment, including high altitude, very low temperature and very low air pressure. The unique specifications, including high accuracies for surface shape and pointing and fully automatic year-around remote operation, along with a stringent limit on the periods of on-site assembly, testing and maintenance, bring a number of challenges to the design, construction, assembly and operation of this telescope. This paper introduces general concepts related to the design of the DATE5 antenna. Beginning from an overview of the environmental and operational limitations, the design specifications and requirements of the DATE5 antenna are listed. From these, major aspects on the conceptual design studies, including the antenna optics, the backup structure, the panels, the subreflector, the mounting and the antenna base structure, are explained. Some critical issues of performance are justified through analyses that use computational fluid dynamics, thermal analysis and de-icing studies, and the proposed approaches for test operation and on-site assembly. Based on these studies, we conclude that the specifications of the DATE5 antenna can generally be met by using enhanced technological approaches.

Low-noise 0.5THz all-NbN superconductor-insulator-superconductor mixer for submillimeter wave astronomy

We report the detection of spectral line emission from CO (J=4–3) at 0.46THz using a low-noise 0.5THz superconductor-insulator-superconductor (SIS) mixer incorporating NbN∕AlN∕NbN tunnel junctions (with an energy gap of 5.6meV) and a NbN∕MgO∕NbN tuning circuit, which is installed on a 30cm submillimeter telescope. Uncorrected receiver noise temperatures as low as 149K are measured on the telescope from 0.45–0.50THz for an unoptimized device. Moreover, the 0.5THz all-NbN SIS mixer exhibits high stability in a considerably large dc bias range and low noise at significantly high temperatures compared with a 0.5THz Nb SIS mixer.

Experimental results of SIS mixers with distributed junction arrays

The heterodyne mixing performance of three respective distributed junction arrays, i.e., a number of superconductor-insulator-superconductor (SIS) junctions distributed along a thin-film transmission line involving two, five, and ten junctions are measured and compared to their Fourier transform spectroscopy (FTS) detection responses. It has been found that distributed junction arrays have a rather large bandwidth in comparison to conventional SIS junction devices, while still keeping a quantum-limited noise performance. Detailed experimental results are presented.

Development of a 500-GHz band SIS mixer

This paper presents detailed experimental and analytical results for a 500-GHz tuneless waveguide SIS mixer, which is incorporated with parallel-connected twin junctions. The 500-GHz SIS mixer exhibits as low an overall receiver noise temperature as 176 K at 482 GHz, despite relatively poor quality of the SIS junction. The effect of the junction subgap leakage is therefore investigated. The experimental results are finally compared to those simulated by the quantum theory of mixing.

A 100-GHz fixed-tuned waveguide SIS mixer exhibiting broad bandwidth and very low noise temperature

This paper presents design and performance data for a 100-GHz waveguide superconductor-insulator-superconductor (SIS) mixer, which is incorporated with a twin-junction device (i.e., two identical junctions connected in parallel through a tuning inductance) and a fixed-tuned broad-band waveguide mixer mount. The 100-GHz mixer is actually developed as a prototype of the 500-GHz one. The constructed 100-GHz SIS receiver results in an overall receiver noise temperature (DSB) of less than 40 K over the frequency range from 80-120 GHz, with a minimum value of just 20 K that is four times as large as the quantum limit (i.e., 4/spl planck//spl omega//k). Such a performance is comparable or even superior to those of the best mechanically tunable waveguide SIS receivers at the same frequency band. In addition, some interesting mixing behaviors related to this two-junction device are described in detail.