A THz Superconducting Imaging Array Developed for the DATE5 Telescope

J Low Temp Phys DOI 10.1007/s10909-015-1355-1 A THz Superconducting Imaging Array Developed for the DATE5 Telescope Sheng-Cai Shi1,2 · Wen Zhang1,2 · Jing Li1,2 · Wei Miao1,2 · Zhen-Hui Lin1,2 · Zheng Lou1,2 · Qi-Jun Yao1,2 Received: 24 September 2015 / Accepted: 22 October 2015 © The Author(s) 2015. This article is published with open access at Springerlink.com Abstract Dome A in Antarctica, located at an altitude of 4093 m and with very low temperature in winter down to −83◦ C, is an exceptionally dry site. Measurements of the atmospheric transmission in the range of 0.75–15 THz by a Far-infrared/THz Fourier transform spectrometer (FTS) strongly suggest that Dome A is a unique site for ground-based THz observations, especially for the 200- and 350-micron windows. A 5-m THz telescope (DATE5) is therefore proposed for Chinese Antarctic Kunlun Observatory. We are currently developing a THz superconducting imaging array (TeSIA) for the DATE5. The TeSIA will be working at the 350-µm window, with a pixel number of 32 × 32 and a sensitivity (NEP) of ∼10−16 W/Hz0.5 . Ti transition-edge sensors with time-domain multiplexing and TiN microwave kinetic inductance detectors with frequency-domain multiplexing are both developed for the TeSIA. In this paper, detailed system designs and some measurement results will be presented. Keywords Terahertz · TES · MKIDs · TeSIA · DATE5 · Dome A 1 Introduction At THz and Far-infrared (FIR) wavelengths, we can observe early distant objects, most cold objects emitting the majority of their light, and obscured objects invisible in the optical due to the presence of dust. In addition, it is a frequency regime where there is an abundance of molecular spectral lines and fine atomic structure spectral lines B Sheng-Cai Shi 1 Purple Mountain Observatory, CAS, Nanjing, China 2 Key Laboratory of Radio Astronomy, CAS, Nanjing, China 123 J Low Temp Phys which are very important tracers for probing the physical and chemical properties and dynamic processes of objects such as stars and planetary systems. As is well known, however, water vapor renders the terrestrial atmosphere opaque at THz and FIR wavelengths. This has driven large ground-based millimeter and submillimeter astronomical observatories to successively higher and drier sites. Dome A, the highest point of the Antarctic ice sheet, is an exceptionally dry site because of high altitude (4093 m) and low temperature (with a record of −83◦ C). Radiometric measurements at 0.66 THz at Dome A have found that typical perceptible water vapor (PWV) in summer is only about 0.14 mm [1,2]. Our recent FTS measurements demonstrate promising results at THz wavelengths [3]. Moreover, Dome A is an excellent year-round observing site in the submillimeter windows below 1 THz. Dome A is therefore a unique site for ground-based THz observations, especially for the 200- and 350-µm windows. China is planning to build an observatory there, namely Chinese Antarctic Kunlun Observatory, with two proposed scientific facilities including a 5-m THz telescope named DATE5 [4]. A science case for the DATE5 telescope is to observe extreme starburst galaxies at different red-shifts to better understand the nature and the evolution of these enigmatic and important objects. To meet this requirement, we are developing a superconducting imaging camera (TeSIA) [5], which has an array size of 32 × 32 pixels and requires a background-limited sensitivity as high as 10−16 W/Hz0.5 at 350 µm (centered at approximately 0.85 THz). Note that the background-limited sensitivity is estimated for a 10% relative bandwidth and 50% atmospheric transmission at 350 µm. Ti transition-edge sensors (TES [6]) with time-domain multiplexing and TiN microwave kinetic inductance detectors (MKIDs [7]) with frequency-domain multiplexing are both adopted for the TeSIA development. A long-wavelength (850 µm or 0.35 THz) prototype with a smaller size of 8 × 8 pixels will be firstly installed on the portable submillimeter telescope (POST [8]) located in western China for demonstration. 2 TES Developments for TeSIA Fig. 1 shows the schematic of the TeSIA developed with TES. The 32 × 32 TES detector array, operated at 300 mK, is indeed composed of four tiles of 16 × 16 Ti TES detectors. Time-domain multiplexing (TDM) is adopted for the readout of the TES detectors. There are eight readout channels in total, with each including 128 SQUIDs at 1 K, an ASIC at 4 K, an SR560 low-noise preamplifier, and a DAQ. SQUIDs used here are developed by STAR Cryoelectronics, with a critical current 2Ic of 26 µA, an input inductance of 650 nH, and current noise lower than 1 pA/Hz0.5 . The ASIC, designed by the Laboratoire AstroParticule et Cosmologie, Université Paris-7, is based on AMS 350-nm BiCMOS SiGe technology. A multiplexing factor of 128 is realized through the use of two levels of multiplexing. Details of the TDM readout are described in [9]. TES detectors adopted here are a single Ti microbridge acting as both absorber and thermometer, which is similar to that proposed by Karasik et al. [10]. As Ti superconducting films of a few tens of nanometers have a critical transition temperature 123 J Low Temp Phys Fig. 1 Schematic of the TeSIA developed with Ti TES. The optics is not shown here (Color figure online) Fig. 2 Scanning-electron-microscopy (SEM) photo of a fabricated Ti TES (left) and schematic of a Ti TES with improved design with four SiN/Nb legs supporting the TES (right) (Color figure online) around 400 mK, they are just suitable for the operating temperature of our detect array. The electric NEP of fabricated Ti TES detectors, as shown in Fig. 2, has reached 4 × 10−17 W/Hz0.5 , which is close to the phonon noise-dominated NEP [11]. The measured effective time constant, however, is about 3 µs, which is still too fast for our TDM readout. Using an electron–phonon decoupled Ti TES, as shown in Fig. 2, can increase the time constant up to ms. As can be seen in Fig. 2 (right), the TES is supported by four SiN/Nb legs, while the signal is coupled from a twin-slot antenna through a CPW transmission line. The twin-slot antennae are coupled to the telescope relay optics by a silicon microlens array. The microlenses are hyper-hemispherical lenses with a diameter of 1.5mm and pixel spacing of ∼1.9Fλ at 350 µm. The telescope relay optics include an ellipsoidal mirror (M5), two planar mirrors (M6 and M7), and a cold silicon lens (L1), which together transform the F/17 beams from the telescope coude focus to the final telecentric beams of F/2.3, maximizing the coupling to the microlenses. A diffraction-limited field of view of greater than 12 arcmin is achieved, satisfying the field requirement by the 32 × 32 TES or MKID array. The schematic of the optics is shown in Fig. 3. 123 J Low Temp Phys Fig. 3 Schematic of the designed optics for the TeSIA on the DATE5 telescope (Color figure online) Fig. 4 Schematic of the TeSIA developed with antenna-coupled T (...truncated)