First Light with an 800 GHz Phonon-cooled HEB Mixer Receiver

Phonon-cooled superconductive hot-electron bolometric (HEB) mixers are incorporated in a waveguide receiver designed to operate near 800 Gliz. The mixer elements are thin-film niobium nitride microbridges with dimensions of 4 nm thickness, 0.2 to 0.3 p.m in length and 2 jun in width. At 780 GHz the best receiver noise temperature is 840 K (DSB). The mixer IF bandwidth is 2.0 GHz, the absorbed LO power is —0.1 1.1W. A fixed-tuned version of the receiver was installed at the Submillimeter Telescope Observatory on Mt. Graham, Arizona, to conduct astronomical observations. These observations represent the first time that a receiver incorporating any superconducting HEB mixer has been used to detect a spectral line of celestial origin. Introduction Superconducting HEB mixers look to become the technology of choice for heterodyne detection above 1 THz. This technology has thus far fulfilled, in the laboratory at least, all the basic requirements for efficient astronomical observing at subraillimeter wavelengths: low-noise performance, low local oscillator power requirement and large intermediate frequency bandwidth. However, performance in the laboratory is often a poor substitute for performance in the field. Even with the detection of molecular lines in the laboratory with this type of receiver, many people, in particular potential users, remain cautiously skeptical about whether or not this new technology will be useful in practice. In order to address this final concern directly, we have aimed our efforts in the past six months to build a complete receiver system employing a superconducting HEB mixer to take to a submillimeter telescope facility and test its performance definitively. Our receiver employs a phonon-cooled HEB mixer [1] fabricated from niobium nitride [2]. The mixer elements are thin-film microbridges with typical thickness of 4 nm fabricated on crystalline quartz substrates. The critical temperature, T„ is —9 K. with a transition width of —1 K. The sheet resistance ranges from 1000 to 2000 Over the course of our experiments [3], we have learned that all of our mixers fabricated using conventional optical photolithography require more local-oscillator power present address: Caltech 320-47, Pasadena, CA 91125 than can be conveniently provided by frequency multiplied solid-state sources, especially at the highest frequencies. For example, we were able to pump an 800 GHz mixer at only two frequency points. Furthermore, with optical lithography it is difficult to make a mixer that has simultaneously a low impedance and low LO power requirement. Since our goal was to build a receiver for a telescope, we absolutely needed to be able to pump the mixer continuously across the operating band of the receiver. Also, we desired to lower the mixer impedance from about 400 or so, which was typical of optically fabricated mixers, to about 100 S2. For a phonon-cooled mixer the optimal LO power simply scales with the volume of the microbridge. With the thickness fixed, the area of the mixer can be flexibly adjusted in order to give a wide range in impedance and in the level of localoscillator power. We have therefore fabricated NbN microbridges defined by electron beam lithography that have in-plane areas —10 times smaller than those manufactured for our previous studies. Such mixers have LO power requirement reduced by 10 dB compared to that of larger mixers. The mixers also have favorably lower impedance. In this paper we state the performance of our HEB receiver, and show results of its operation on a telescope. Receiver Performance A current-voltage curve of a mixer is shown in Figure 1. This mixer is 2 gm wide and 0.3 puri long, with a normal room-temperature resistance of 90 Q. The general shape of the IV curve is similar to the larger mixers. One major difference, however, is that the voltage scale is compressed in the voltage-total IF power curve, which is also plotted in the figure. This difference is a clue that the new mixer will require less local-oscillator power and dc power to reach the optimal operating point. Incidentally, this difference also has interesting consequences for the mixer saturation level, which is discussed below. The mixer is incorporated in a waveguide block with a mechanically driven backshort. The block was designed to accommodate an SIS mixer, the details of which can be found in [4]. The corrugated feed illuminates a cold off-axis paraboloid and an optical flat before exiting the cryostat. Several layers of porous Teflon provides near-infrared filtering, and a 0.5 mm Teflon window seals the cryostat. The local-oscillator is a multiplied solid-state source, and a Martin-Puplett diplexer is used to combine the local-oscillator and signal beams. Receiver noise temperature The sensitivity of the receiver was measured using the standard Y-factor technique of alternatively placing a room temperature load and a cold load at the temperature of liquid nitrogen at the input of the receiver. No corrections were made. We were able to make a continuous measurement of the receiver noise temperature across the operating band of the local-oscillator source. The noise performance across the 800 GHz band is plotted in Figure 2, and across the band, the noise temperature is always less than 2 K GHz -1 . The best noise temperature at 780 GHz is 850 K, where we estimate that the conversion loss is 14 dB and that Lax= 750 K. The receiver will actually work all the way down to the cutoff frequency of the waveguide, which is near 600 GHz.