Bolometer mixers based on ultrathin NbN films find their applications in far infrared heterodyne detection. They consist of a microbridge, which is brought to its transition temperature by radiation and current. It was recently proposed that the resistivity increase should be understood in terms of vortex-antivortex unbinding as described in the Berezinskii-Kosterlitz-Thouless theory, although finite size effects, material properties, granularity and pinning will complicate a description of real devices. We have measured, close to the transition temperature, the current-voltage characteristics of a variety of NbN film samples, sputtered on a silicon substrate. Four terminal measurements of patterned films with different sizes are analyzed. We report the effect of the geometrical size on the logarithmic dependence of the voltage on the current. Results will be related to the properties of the NbN bolometric mixers.

We have conducted an investigation of the optimal embedding impedance for a waveguide superconducting hot-electron bolometric (HEB) mixer. Three mixer chip designs for 800 GHz, offering nominal embedding resistances of 70 /spl Omega/, 35 /spl Omega/, and 15 /spl Omega/, have been developed. We used both High Frequency Structure Simulator (HFSS) software and scale model impedance measurements in the design process. We subsequently fabricated HEB mixers to these designs using 3-4 nm thick NbN thin film. Receiver noise temperature measurements and Fourier Transform Spectrometer (FTS) scans were performed to determine the optimal combination of embedding impedance and normal-state resistance for a 50 Ohm IF load impedance. A receiver noise temperature of 440 K was measured at a local oscillator frequency 850 GHz for a mixer with normal state resistance of 62 /spl Omega/ incorporated into a circuit offering a nominal embedding impedance of 70 /spl Omega/. We conclude from our data that, for low noise operation, the normal state resistance of the HEB mixer element should be close to the embedding impedance of the mixer mount.

The Receiver Lab Telescope (RLT) is a ground-based terahertz telescope; it is currently the only instrument producing astronomical data between 1 and 2 THz. The capabilities of the RLT have been expanding since observations began in late 2002. Initial observations were limited to the 850 GHz and 1.03 THz windows due to the availability of solid state local oscillators. In the last year we have begun observations with new local oscillators for the 1.3 and 1.5 THz atmospheric windows. These oscillators provide access to the 11-10 and 13-12 lines of CO at 1.267 and 1.497 THz, as well as the [N II] line at 1.461 THz. We report on our first measurements of these high CO transitions, which represent the highest-frequency detections ever made from the ground. We also present initial observations of [N II] and discuss the implications of this non-detection for the standard estimates of the strength of this line.

We present recent measurements of receiver noise temperature and intermediate frequency (IF) bandwidth in the frequency range 0.8-1.3 THz for waveguide NbTiN HEB mixers of various dimensions. These devices are fabricated from an NbTiN film deposited on crystalline quartz substrates with AlN buffer layer. The lengths of the mixer elements vary from 0.3 to 0.5 /spl mu/m and their widths vary from 3 to 10 /spl mu/m. Critical temperatures are typically at 8.5 K, and the measured normal state resistance of the devices is about 1000 ohms per square. All the device DC parameters demonstrate a high degree of uniformity. A double side band noise temperature at 0.8 THz as low as 550 K has been measured at an IF frequency of 1.8 GHz, with a conversion loss of around 14 dB. At an IF of 3 GHz, the noise temperature increases to 750 K. We have also made extensive measurements of the IF bandwidth as a function of bias voltages and currents. At the optimal low-noise operation point, a 3-dB IF bandwidth of 1.2 GHz is obtained for a wide variety of device dimensions and bath temperature.

We have developed a 1.5 THz superconducting NbN Hot-Electron Bolometer mixer. It is operated by an all-solid-state Local Oscillator comprising of a cascade of 4 planar doublers following an MMIC based W-band power amplifier. The threshold available pump power is estimated to be 1 /spl mu/W.

NbN hot- electron bolometer mixers have reached the level of 10hv/k in terms of the input noise temperature with the noise bandwidth of 4-6 GHz from subMM band up to 2.5 THz. In this paper we discuss the major characteristics of this kind of receiver, i.e. the gain and the noise bandwidth, the noise temperature in a wide RF band, bias regimes and optimisation of RF coupling to the quasioptical mixer. We present the status of the development of the mixer for Band 6 Low for Herschel Telescope.

A low noise heterodyne receiver is being developed for the terahertz range using a phonon-cooled hot-electron bolometric mixer based on 3.5 nm thick superconducting NbN film. In the 1–2 GHz intermediate frequency band the double-sideband receiver noise temperature was 450 K at 0.6 THz, 700 K at 1.6 THz and 1100 K at 2.5 THz. In the 3–8 GHz IF band the lowest receiver noise temperature was 700 K at 0.6 THz, 1500 K at 1.6 THz and 3000 K at 2.5 THz while it increased by a factor of 3 towards 8 GHz.

We present a broadband and low noise heterodyne receiver for 1.4-1.7 THz designed for the Hershel Space Observatory. A phononcooled NbN HEB mixer was integrated with a normal metal doubleslot antenna and an elliptical silicon lens. DSB receiver noise temperature Tr was measured from 1 GHz through 8GHz intermediate frequency band with 50 MHz instantaneous bandwidth. At 4.2 K bath temperature and at 1.6 THz LO frequency Tr is 800 K with the receiver noise bandwidth of 5 GHz. While at 2 K bath temperature Tr was as low as 700 K. At 0.6 THz and 1.1 THz a spiral antenna integrated NbN HEB mixer showed the receiver noise temperature 500 K and 800 K, though no antireflection coating was used in this case. Tr of 1100 K was achieved at 2.5 THz while the receiver noise bandwidth was 4 GHz.

The direct responses of NbN phonon-cooled hot electron bolometer (HEB) mixers,integrated with different planar antennas, are measured, using Fourier TransformSpectrometer (F1S). One spiral antenna and several double slot antennas, designed for0.6, 1.4, 1.6, 1.8 and 2.5 THz central frequencies, are investigated. The Optimization ofthe measurement set-up is discussed in terms of the beam splitter and the F11S-to-HEBcoupling. The result shows that the spiral antenna is circular polarized and has abandwidth of about 2 THz. The frequency bands of double slot antennas show some shiftfrom the design values and their relative bandwidth increases by increasing the designfrequency. The antenna responses do not depend on the HEB bias point and temperature,as long as the device is in the resistive state. I. Introduction Hot electron bolometer (HEB) mixers currently have better performance as heterodynereceivers at THz frequencies, where SIS and Schottky mixers have worse sensitivity. Ithas been shown that NbN HEB mixers have noise temperature about

The local oscillator power required for NbN hot-electron bolometric mixers (PLO) was investigated with respect to mixer size, critical temperature and ambient temperature. PLO can be decreased by a factor of 10 as the mixer size decreases from 4×0.4 μm to 0.6×0.13 μm. For the smallest volume mixer the optimal local oscillator power was found to be 15 nW. We found that for such mixer no signal compression was observed up to an input signal of 2 nW which corresponds to an equivalent input load of 20,000 K. For a constant mixer volume, reduction of Tc can decrease optimal local oscillator power at least by a factor of 2 without a deterioration of the receiver noise temperature. Bath temperature was found to have minor effect on the receiver characteristics

The performance of NbN based HEB mixers has been investigated at THz frequencies in a receiver with quasi—optical coupling. The best performance is achieved with devices made from NbN films deposited on crystalline MgO substrates, with film thickness d = 3.5 nm and a critical temperature T, -= 9 — 10K. Double—side band receiver noise temperatures are 530 K at 0.6 THz, 650K at 1.6 THz and 1100 K at 2.5 THz. The intermediate frequency (IF) bandwidth is 4.5 GHz under operating conditions yielding the lowest noise temperature. Operating the bolometer at high bias voltages can increase the IF bandwidth up to 9 GHz at the expense of a drop in sensitivity by a factor of two.

Near-field vector beam pattern of the 1.5 THz superconducting Hot Electron Bolometer (HEB) receiver currently in operation in Northern Chile has been performed in our laboratory. Using an open waveguide probe, we have mapped both the amplitude and phase of the beam emerging from our 1.5 THz HEB receiver package, across a number of planes along the line of propagation of the radio-beam. With an integration time of about 100 ms per point, a signal-to-noise ratio of about 25 dB was achieved for a beam waist of 3.5 mm. These measurements have proved to be invaluable in achieving good alignment between the cryostat housing the HEB mixer and the remainder of the receiver and telescope optics. The accuracy of our beam measurement is estimated to be ±0.2 mm in position and ±5 arc minutes in angular displacement.