Developments on large format array of superconducting tunnel junction detectors are discussed and recent activities in
readout electronics developments and focal plane optics designs are presented. We have been working on submillimeter-wave
SIS photon detectors at 650 GHz using niobium tunnel junctions, which have high sensitivity, large dynamic range
and fast response. Here we discuss on an implementation plan of large format array with cryogenic readout electronics
and compact focal plane optics design. GaAs-JFETs operate at less than 1 K with low noise, low power dissipation and
fast response. We have demonstrated operation of cryogenic integrating amplifiers and digital electronics for SIS photon
detectors with multiplexed readout. Combined with compact focal plane optics, we now have a conceptual design of
large format array of SIS photon detectors in submillimeter-wave. Further development to realize higher sensitivity
superconducting tunnel junction detectors with extremely low leakage current are foreseen.
We have developed the superconductive imaging submillimeter-wave camera with nine detector elements (SISCAM-9)
for Atacama Submillimeter Telescope Experiment (ASTE). SISCAM-9 has nine SIS photon detectors as focal plane
array detector at 680 GHz. To obtain background noise limited sensitivity, we need to operate detectors under the
condition that noise is dominated by shot noise of background photo current. To realize this condition, we fabricated low
noise readout circuits using Si-JFETs. In laboratory, we evaluated performance of nine SIS photon detectors. This is the
first demonstration of 2D array of SIS photon detector for SISCAM-9. Measuring I-V characteristics, detector gap
voltages were 4.9 mV and photo currents were 3 nA. Measuring spectral responses, they had almost same center
frequency of 679 GHz and bandwidths of 77 GHz. They almost matched to 675 GHz atmospheric window from ASTE
site. Detector noise under 300 K radiation were only a few times as large as the shot noise of photo current. Detector
NEP was 1.7x10<sup>-15</sup> W/Hz<sup>1/2</sup>,with the detector quantum efficiencies of 12%. For the first time, SIS photon detectors
worked under shot noise limited condition under 300 K background radiation. We have developed observation system,
SISCAM-9, to realize the first astronomical observations. Detectors were mounted in a cryostat that can be remotely
operated to cool the detectors to 0.46 K. We installed SISCAM-9 in the ASTE telescope and measured system
performance such as photo current and noise characteristics. For the first time, SIS photon detectors operated under the
observing condition. We succeeded in making the first astronomical observation of the moon.
High sensitivity submillimeter-wave focal plane array using SIS photon detector is being developed, which we call SISCAM, the superconductive imaging submillimeter-wave camera. In the course of the detector evaluations, we have measured performance of the SIS photon detectors under various operating conditions. Advantages of the SIS photon detectors are explained by the nature of antenna coupled quantum detectors. Their input coupling can be designed to have band-pass characteristics owing to the distributed junction design. This reduces requirements for infrared blocking filters and enhances optical efficiency. The detector performance is evaluated under background loading and they show background limited performance. Measurement at 4 K shows the SIS photon detector operates under shot noise limit of thermal leakage current and its NEP is 1x10<sup>-14</sup> W/Hz<sup>0.5</sup>, that is better than bolometers at 4.2 K, whereas the same detector has NEP of 10<sup>-16</sup> W/Hz<sup>0.5</sup> at 0.3 K. Dynamic range of SIS photon detectors is estimated to be higher than 10<sup>9</sup>, which surpass the dynamic range achievable with TES bolometers. Nine-element array of SIS photon detector, SISCAM-9, is developed and their performance is evaluated in a submillimeter-wave telescope. With a development of integrated electronics with GaAs-JFET charge integrating readout circuit, the SIS photon detector will be an ideal imaging array in submillimeter-wave region. Due to its large dynamic range and shot noise limited performance under various operating condition, SIS photon detectors can be used for various astronomical instrumentations as well as for other fields of terahertz technologies.
SIS photon detectors are niobium-based superconducting direct detectors for submillimeter-wave that show superior performance when compared with bolometric detectors for ground-based observations. We present the design and development of the SIS photon detectors together with optical and cryogenic components for wide field continuum observation system on Atacama Submillimeter Telescope Experiment (ASTE). Using antenna coupled distributed junctions, SIS photon detectors give wide band response in a 650-GHz atmospheric window as well as high current sensitivity, shot noise limited operation, fast response and high dynamic range. Optical noise equivalent power (NEP) was measured to be 1.6x10<sup>-16</sup> W/Hz<sup>0.5</sup> that is less than the background photon fluctuation limit for ground based submillimeter-wave observations. Fabrication of focal plane array with 9 detector pixels is underway to install in ASTE.
Readout electronics with Si-JFETs operating at about 100 K will be used for this array. Development of readout electronics for larger array is based on GaAs-JFETs operating at 0.3 K. For the purpose of installing 100 element array of SIS photon detectors, we have developed remotely operable low-vibration cryostat, which now cools bolometers for 350, 450, 850-µm observations down to 0.34 K. GM-type 4-K cooler and He3/He4 sorption cooler is used, which can be
remotely recycled to keep detectors at 0.34 K. Since we have large optical window for this cryostat, sapphire cryogenic window is used to block infrared radiation. The sapphire window is ante-reflection coated with SiO<sub>2</sub> by chemical vapor deposition (CVD). The transmittance of the cryogenic window at 650 GHz is more than 95%.
High-sensitivity terahertz direct detectors using superconducting tunnel junctions were fabricated. They were designed for detecting terahertz radiation in the frequency range of 0.4 and 0.65 THz with the fractional bandwidth of above 10 percent. The results of their performance evaluation of five detector elements are presented. We show the results of the frequency response as well as that the absolute efficiency ranged from 10 to 30 percent and that the the
sensitivity was 1.9 x 10<sup>-1</sup>6 W Hz<sup>-0.5</sup> in noise equivalent power.
The Atacama Submillimeter Telescope Experiment (ASTE) is a Japanese development program for the large millimeter and submillimeter array (LMSA/ALMA). The new 10-m submillimeter- wave telescope is a prototype telescope for the LMSA project and to be installed in Chile in 2001. The telescope will be equipped with submillimeter-wave SIS mixer receivers and a submillimeter-wave direct detector camera. The submillimeter- wave camera based on superconducting direct detectors (SIS photon detectors) are under development to realize very wide field observations in submillimeter-wave. Niobium tunnel junctions with low leakage current coupled to antenna structures can be sensitive submillimeter-wave detectors when high quantum efficiency and low leakage current is realized. Inhomogeneous distributed junctions coupled to log-periodic antennas can be used to realize this. Input coupling of more than 50% is calculated using 5-element and 10-element inhomogeneous distributed junctions. With leakage current of less than 100 pA, noise equivalent power can be less than 10<SUP>-16</SUP> W/Hz<SUP>0.5</SUP> under shot noise limited operation. Submillimeter-wave camera of 100 pixels is now under design and will be installed on ASTE 10-m telescope in 2002 or later.