SubTHz Arrays of Planar Antennas with SINIS bolometers for BTA

We have investigated the optical and spectral response of a 350 GHz receiver made of annular antenna array with SINIS bolometer elements. Receiver is intended for using at Big Alt-Azimuthal Telescope (BTA). Samples were measured in a dilution cryostat at temperatures in the range 80-400 mK. Voltage responsivity approaches 3*10 V/W. Spectral bandwidth in 230-380 GHz range for single array was measured using Backward Wave Oscillator source.

We have investigated the optical and spectral response of a 350 GHz receiver made of annular antenna array with SINIS bolometer elements. Receiver is intended for using at Big Alt-Azimuthal Telescope (BTA). Samples were measured in a dilution cryostat at temperatures in the range 80-400 mK. Voltage responsivity approaches 3*10 9 V/W. Spectral bandwidth in 230-380 GHz range for single array was measured using Backward Wave Oscillator source.

BTA SAO RAS
The BTA-6 is a 6-metre aperture optical telescope at the Special Astrophysical Observatory located in the Zelenchuksky District on the north side of the Caucasus Mountains at an altitude of 2070 m above sea level. Photo and schematic picture of BTA is presented in Fig.1. The mounting of a cryostat with receiving array is supposed in the Nasmyth focus. The equivalent focus length is 184 meters and diameter of diffraction image is 60 mm. For matching receiving array with incoming radiation we can use horn with large aperture (up to 60 mm) or lens with the same diameter or decreasing of focal length of 24 times by using a large lens. Fig.1. The photo of BTA (left) and schematic picture (right): 1-middle unit; 2 -worm gear; 3 -elevator; 4 -rotation support platform; 5 -spherical support of vertical axis; 6 -spherical pads; 7 -vertical axis; 8 -lower bearing; 9observer's cage; 10 -prime focus unit; 11 -flat mirror; 12platform cover; 13 -oil pads; 14 -main spectrograph; 15observing platform; 16 -support for spectrograph; 17 -pier; 18 -2m camera mirror of main spectrograph; 19 -primary mirror cell; 20 -spur and worm gears; 21 -reinforced concrete.

Matching of incoming signal with receiver array
Receiving array can be matched with incoming signal using arrays of horns, quasi-optical lenses or distributed absorbers. The main advantages and dis-advantages of using feedhorns and filled arrays are represented in [1] and in table.

Feedhorns
Filled array Advantages a) Provides maximum efficiency for detection of a point source with known position; b) the bolometer angular response is restricted to the telescope, giving good stray-light rejection; c) the susceptibility to electromagnetic interference can be controlledthe horn plus integrating cavity act as a Faraday enclosure; d) the number of detectors needed to fill a given array field of view is minimized Disadvantages a) In order to achieve full spatial sampling of the sky, even for a region smaller than the array field of view, jiggling or scanning are needed, which complicates the observing modes; b) the efficiency for mapping is considerably less than the ideal value Advantages a) Provide a higher efficiency for mapping observations; b) they allow full sampling of the instantaneous field of view of the array by use of pixels of 0.5Fλ or smaller, making jiggling unnecessary; c) they yield, for a 0.5Fλ array, a slightly narrower beam profile on the sky for a given telescope size owing to the stronger illumination of the outer parts of the telescope Disadvantages a) the background power per pixel is lower than for the larger feedhorn coupled detectors, typically by a factor of 4 -5, yielding a photon noise NEP that is lower by a factor of 2 or more, and thus more difficult to achieve; b) the detectors are much more vulnerable to stray light because of the very broad pixel angular responseby a factor of πF 2 /4, assuming a pixel beam solid angle of π steradians; c) the vulnerability to electromagnetic interference is also greater owing to the naked array architecture; d) the need for more detectors to fill a given field size In case of using immersion lens close to elliptical lens [2] allows to remove substrate modes and to increase gain coefficient of planar antenna. The spherical wave transforms in plane wave by mounting the source in the second focus.
We suggest two types of arrays for using at BTA: half-wave antenna arrays (with back-to-back horn) and metamaterial arrays (with lens). A single element is an annular antenna with two (or more) SINIS-bolometers (Fig.2). Schematic image of matching such arrays with incoming radiation are presented in Fig.3.
A metamaterial is made of a periodic array of subwavelength metallic resonators that are collective-ly coupled to the free space excitation. In the case of small antennas the matrix can be made more wideband and much smaller that allows placing it in the waist of a single-mode horn or in the focus of immersion lens.

Experimental setup and results
Quasistatic optical response to incoming radiation was measured in dilution cryostat [3] using cold black body (BB) source made of NiCr film on sapphire substrate. Source was equipped with thermometer, connected to current source and mounted on 2.7 K stage. For spectral response measurements we use a 230-380 GHz Backward Wave Oscillator (BWO) illuminating the antennas array via optical window and 3 neutral density filters with transmission below -10 dB placed at radiation shields 100 K, 3 K, 0.3 K temperature stages. Two channels were measured simultaneously by lock-in amplifier, one for signal from bolometer, and another from pyroelectric detector that is monitoring level of the incoming power. Schematic pictures of experimental setup are presented in Fig.4. Results of spectral and optical response measurements are presented in Fig.5,6.