The Cold-Electron Bolometer (CEB) is a sensitive millimetre-wave detector which is easy to integrate with superconducting
planar circuits. CEB detectors have other important features such as high saturation power and very fast response. We
have fabricated and tested CEB detectors integrated across the slot of a unilateral finline on a silicon substrate. Bolometers
were fabricated using two fabrication methods: e-beam direct-write trilayer technology and an advanced shadow mask
evaporation technique. The CEB performance was tested in a He3 sorption cryostat at a bath temperature of 280mK. DC
I-V curves and temperature responses were measured in a current bias mode, and preliminary measurements of the optical
response were made using an IMPATT diode operating at 110GHz. These tests were conducted by coupling power directly
into the finline chip, without the use of waveguide or feedhorns. For the devices fabricated in standard direct-write technology,
the bolometer dark electrical noise equivalent power is estimated to be about 5×10-16W/√Hz, while the dark
NEP value for the shadow mask evaporation technique devices is estimated to be as low as 3×10-17W/√Hz.
For sensitive wideband spectroscopy at TeraHertz frequencies one needs a wide-range electrically tunable THz source and a sensitive detector. In this paper a superconducting normal metal cold electron bolometer (CEB) was used as a broadband sensor. Bolometers were integrated with broadband log-periodic antenna designed for 0.2-2 THz frequency range and double-dipole antennas designed for 300 and 600 GHz central frequency. A Josephson junction was used as a wide band electrically tuned terahertz cryogenic oscillator. Bicrystal YBaCuO Josephson junctions demonstrated a characteristic voltage IcRn of over 4 mV that corresponds to characteristic frequency about 2 THz. The bolometer chip is attached to a Si substrate lens at 260 mK and the oscillator chip is attached to the sapphire substrate lens at 1.8 K, with lenses facing each other at the distance of few centimeters. High signal was measured in the whole frequency range up to 1.7 THz by simple changing the bias voltage of Josephson junction from zero to 3.5 mV. A voltage response of the bolometer up to 4*108 V/W corresponds to an amplifier-limited technical noise equivalent power of the bolometer NEP=1.25*10-17 W/Hz1/2. Combining a Terahertz band Josephson junction, a high-sensitive hot electron bolometer, and a sample under test in between, makes it possible to develop a cryogenic compact Terahertz-band transmission spectrometer with a resolution below 1 GHz corresponding to the linewidth of Josephson oscillations. For frequencies below 600 GHz a conventional Nb shunted SIS junction can be used as Josephson oscillator.
A capacitively coupled hot-electron nanobolometer (CC-HEB) is the simplest and most effective antenna-coupled bolometer. The bolometer consists of a small absorber connected to the superconducting antenna by tunnel junctions. The tunnel junctions used for high-frequency coupling also give perfect thermal isolation of hot electrons in the small volume of the absorber. The same tunnel junctions are used for temperature measurements and electron cooling. This bolometer does not suffer from the frequency limitations in the submillimeter range due to the high potential barrier of the tunnel junctions as does the microbolometer with Andreev mirrors (A-HEB), which is limited by the superconducting gap. Theoretical analyses show that the two-junction configuration more than doubles the sensitivity of the bolometer in current-biased mode compared to the single-junction configuration used for A-HEB.
Another important advantage of CC-HEB is its simple two-layer technology for sample fabrication. Samples were fabricated with an absorber made of a bilayer of Cr and Al to match the impedance of the antenna. Electrodes were made of Al and tunnel junctions were formed over the Al oxide layer. The coupling capacitances of the tunnel junctions, C ≈ 20 fF, in combination with the inductance of the 10 μm absorber create a bandpass filter with a central frequency around 300 GHz. Bolometers are integrated with log-periodic and double-dipole planar antennas made of Au. The temperature response of bolometer structures was measured at temperatures down to 256 mK. In our experiment we observed dV/dT=1.3 mV/K, corresponding to responsivity S=0.2.109 V/W. For amplifier noise Vna=3nV/Hz1/2 at 1 kHz the estimated total noise equivalent power is NEP=1.5.10-17 W/Hz1/2. The intrinsic bolometer self noise Vnbol=0.5 nV/Hz1/2 corresponds to NEP=3.10-18 W/Hz1/2. For microwave evaluation of bolometer sensitivity we used a black body radiation source comprising a thin NiCr stimulator placed on the cold plate of cryostat in front of a CC-HEB attached to an extended hemisphere sapphire lens. This measurements were consistent with estimates based on the dc responsivity of the bolometer.
We present the results of experimental development of an ultrasensitive normal metal hot-electron microbolometer with Andreev mirrors and electronic cooling by superconductor- insulator-normal metal (SIN) tunnel junctions. A value NEP equals 5 (DOT) 10-18 W/Hz1/2 for the temperature fluctuations component of noise and the thermal time constant (tau) equals 0.2 microseconds at 300 mK have been estimated for one of the realized devices with thermal conductance G approximately equals 6 (DOT) 10-12 W/K. At 100 mK, the thermal conductance was decreased to G approximately equals 7 (DOT) 10-14 W/K, that gives NEP equals 2 (DOT) 10-19 W/Hz1/2 for the temperature noise component and a thermal time constant (tau) equals 5 microseconds. Such microbolometer is intended as a detector of millimeter and submillimeter wave radiation for space applications.
The IV curves and the direct detector response of HTS bicrystal grain-boundary Josephson junctions have been measured under submm-wave irradiation at frequencies 350-750 GHz. The Josephson oscillation spectrum has been measured using the Hilbert transformation of detection response dependence. The Josephson oscillation linewidth and corresponding value of effective noise is compared to the estimates from the dc measurements of noise rounding of IV curve. To separate sources of noise the direct measurements of junction's output noise has been performed at 1.5 GHz and low frequencies 0.5-20 kHz.
An integrated quasioptical receiver circuit comprising planar complementary log-periodic antenna, SIS mixer with microstrip matching transformers and parallel array of unshunted SIS junctions operating as a local oscillator has been designed, fabricated and tested. The array of parallel SIS junctions united in a microstrip transmission line can be described as a Josephson transmission line (JTL) with single flux quanta synchronously moving along the array of quantum interferometers. The JTL with 20 junctions each 4 micrometers in diameter has critical current 4.5 mA, normal resistance 0.3 (Omega) and a self-detection step at about 1.1 mV. The step width is over 1.5 mA in current scale and 50 (mu) V in voltage scale that corresponds to about 1 (mu) W oscillation power and 25 GHz tuning range with 550 GHz central frequency. A current applied along the top electrode of the JTL allows to vary the output power of such oscillator. The advantages of such oscillator in comparison with Flux-FLow Oscillator are higher impedance of microstrip line about 10 (Omega) that makes easier matching to SIS mixer, lower losses at frequencies about the energy gap for Nb, narrow linewidth and the possibility to tune output power. THe SIS junction IV curve shows current steps at about 1.2 mV that corresponds to the matching circuit central frequency about 600 GHz that is close to JTL central frequency.
A quasioptical Josephson detector with resistively shunted superconducting tunnel junctions of superconductor-insulator-superconductor (SIS) type and different integrated matching circuits for mm waveband region has been designed, fabricated and experimentally studied. A special quasioptical cryogenic probe has been designed and fabricated for measurements of microwave response of such integrated receiving structures. For low-frequency matching a cold transformer has been used at the output port. Many efforts have been made for reducing the external noise influence on Josephson junction IV curve. Quasioptical beamguide has been optimized and adjusted using Bi bolometers evaporated instead of Josephson junctions in the center of planar antenna. Beampatterns of several types of planar antennas, including self-complementary and non-complementary logarithmic spiral and log-periodic antennas, have been measured using the same technique.
Josephson detector in the selective response mode is a promising device for wideband sensitive quick spectrum analysis. It allows to detect spectral lines with resolution of the order 1 GHz. The specific feature of such mode is that it is sensitive only to the narrow spectral lines and the wideband radiation does not make contribution into the output signal. Further processing by Hilbert transform method allows to plot spectrum of ncoming signal. Th1/2 ultimate temperature of selective Josephson detector, according to should be TN=2Tr T=15K at helium temperatures, spectral resolution for 50 junctions array of the order of 10 MHz and the sensitivity of 1 10-15W. The bandwidth of the device without duplicated response at subharmonics and harmonics equals to octave.