An advantage of superconducting detectors is a much lower noise in comparison to their semiconductor counterparts. We have studied the magnitude and spectrum of electric noise in thin superconducting NbN nanostrips carrying a subcritical current. Analysis of the experimental data suggests that the noise appears due to fluctuations in the two-dimensional vortex gas below the Kosterlitz-Thouless phase transition. Implementing this approach to the noise mechanism, we proposed a novel detector. The novelty is the use of the noise, which generally hampers the performance of conventional detectors, as the physical quantity that itself senses radiation. Our detector patterned from a thin NbN superconducting film and integrated in a planar log-periodic antenna. The detector operates at 4.2 K in the current-carrying superconducting state. Optically measured noise-equivalent power amounted at 10-14 W Hz-1 and is likely to improve at lower temperatures.
An advantage of superconducting detectors is a much lower noise in comparison to their semiconductor counterparts. We have studied the magnitude and spectrum of electric noise in thin superconducting NbN nanostrips carrying a subcritical current. Analysis of the experimental data suggests that the noise appears due to fluctuations in the two-dimensional vortex gas below the Kosterlitz-Thouless phase transition. Implementing this approach to the noise mechanism, we proposed a novel detector. The novelty is the use of the noise, which generally hampers the performance of conventional detectors, as the physical quantity that itself senses radiation. Our detector patterned from a thin NbN superconducting film and integrated in a planar log-periodic antenna. The detector operates at 4.2 K in the current-carrying superconducting state. Optically measured noise-equivalent power amounted at 10-14 W Hz-1 and is likely to improve at lower temperatures.
An advantage of superconducting detectors is a much lower noise in comparison to their semiconductor counterparts. We have studied the magnitude and spectrum of electric noise in thin superconducting NbN nanostrips carrying a subcritical current. Analysis of the experimental data suggests that the noise appears due to fluctuations in the two-dimensional vortex gas below the Kosterlitz-Thouless phase transition. Implementing this approach to the noise mechanism, we proposed a novel detector. The novelty is the use of the noise, which generally hampers the performance of conventional detectors, as the physical quantity that itself senses radiation. Our detector patterned from a thin NbN superconducting film and integrated in a planar log-periodic antenna. The detector operates at 4.2 K in the current-carrying superconducting state. Optically measured noise-equivalent power amounted at 10^-13 W / Hz^1/2 and is likely to improve at lower temperatures.
We present a superconducting noise bolometer that is a bolometric detector sensing radiation via radiation-induced changes of its own microwave noise. The detector represents a nanostrip made from a thin NbN superconducting film. It operates in the superconducting state and carries a supercurrent slightly less than the critical current. Radiation couples with the detector via an immersion lens and a planar antenna, which jointly define the useful spectral range from 1to5THz. At an ambient temperature of 4.2K, the optically measured noise-equivalent power amounts at ≈10−13WHz−1∕2 along with a time constant of approximately 150ps.
Advantage of superconducting photon detectors is a much lower noise in comparison to semiconductor counterparts. The purpose of the present work is to advance understanding of the physics of noises in a superconducting nanostructures. We have studied the rate and spectrum of dark counts of an 80-nm wide, 5-nm thick superconducting NbN meander-line and analyzed experimental data considering various noise mechanisms: quantum phase-slips, discrete thermodynamic fluctuations of the superconducting order parameter, and topological fluctuations in the two-dimensional vortex gas. The latter mechanism, i.e. Kosterlitz-Thouless phase transition, is the only one adequately describing the observed noise performance. Besides that, this approach provides clear explanation of the peculiarities of the resistive transition. The other two mechanisms would result in a much stronger current and temperature dependence of the dark count rate than the experiment shows. Basing on our understanding of the noise source, we propose a novel concept of a superconductor submillimeter direct detector. The detector operates in the superconducting state and utilizes current-dependent high-frequency electrical noise that changes under irradiation. The novel aspect is the use of the noise, which generally hampers the performance of conventional detectors, as the physical quantity that itself senses radiation. Predicted noise-equivalent power is 10_-13 W Hz_-1/2 at 4 K and should improve at lower temperatures.
For sensitive wideband spectroscopy at THz frequencies with performance better than for Fourier Transform Spectrometer, one needs a wide-range electrically tunable, narrow linewidth THz source, and a sensitive broadband detector. To obtain a low noise level of sensor we use a superconducting normal metal cold electron bolometer (CEB). A voltage response of the bolometer is 4.108 V/W and an amplifier-limited technical noise equivalent power of the bolometer is 1.25.10-17 W/Hz1/2. The wide band electrically tuned terahertz band cryogenic oscillator is a Josephson junction. Contrary to conventional Josephson junctions based on niobium, and naturally limited in frequency by energy gap of about 700 GHz, the high critical temperature YBaCuO junctions can extend this limit by an order of magnitude. At 4 K such oscillators can radiate at frequencies exceeding 2 THz. Signals received by bolometers integrated with double-dipole and log-periodic antennas and irradiated by the Josephson oscillator show in our experiments the high frequency response corresponding to an oscillation frequency of up to 1.75 THz. Such cryogenic terahertz network analyzer is suitable for low-signal spectral evaluation of cryogenic detectors, quasioptical sub-mm cryogenic filters, planar antennas with planar microwave filters, integrated structures comprising such elements, and arrays of cryogenic receivers.
We present a superconducting noise bolometer for terahertz radiation, which is suitable for large-format arrays. It is based on an antenna-coupled superconducting micro-bridge embedded in a high-quality factor superconducting resonator for a microwave bias and readout with frequency-division multiplexing in the GHz range. The micro-bridge is kept below its critical temperature and biased with a microwave current of slightly lower amplitude than the critical current of the micro-bridge. The response of the detector is the rate of superconducting fluctuations, which depends exponentially on the concentration of quasiparticles in the micro-bridge. Excess quasiparticles are generated by an incident THz signal. Since the quasiparticle lifetime increases exponentially at lower operation temperature, the noise equivalent power rapidly decreases. This approach allows for large arrays of noise bolometers operating above 1 K with sensitivity, limited by 300-K background noise. Moreover, the response of the bolometer always dominates the noise of the readout due to relatively large amplitude of the bias current. We performed a feasibility study on a proof-of-concept device with a 1.0 × 0.5 μm2 micro-bridge from a 9-nm thin Nb film on a sapphire substrate. Having a critical temperature of 5.8 K, it operates at 4.2 K and is biased at the frequency 5.6 GHz. For the quasioptical input at 0.65 THz, we measured the noise equivalent power ≈3 × 10−12 W/Hz1/2, which is close to expectations for this particular device in the noise-response regime.
Superconducting hot electron bolometers (HEBs) are widely used as terahertz (THz) heterodyne detectors (mixers) and becoming the most advantageous candidate for low noise receiver at frequencies above 1.2 THz. However, there is still lack of accurate theoretical modeling which explains the complex mechanism of different aspects of superconducting HEBs. These difficulties do not prevent researchers around the world from making experimental progress. In the last few years, superconducting HEBs are also used as THz direct detectors to develop the array for the applications in biology, medicine and security systems. Recently, a noise equivalent power (NEP) of 0.2 pW/√Hz at 2.5 THz is reported for the superconducting HEBs with thermal biasing. Instead of thermal one, microwave (MW) which was first used to stabilize the HEB mixer is also applied to bias the device as a direct detector. Furthermore, THz can be used to bias the HEBs, if THz sources can be provided easy. Although MW biasing can improve sensitivity of the superconducting HEB direct detectors, little work has been done to compare the responsivity and NEP among the THz direct detectors with thermal, THz and MW biasing. To find a best biasing method, we have studied the similarities and differences of the direct detectors based on the superconducting HEBs with three different biasing methods. The fabricated NbN HEB detectors consist of a complementary logarithmic-spiral antenna made of gold and an NbN film (bridge) connecting across the antenna’s inner terminals. We have fabricated the niobium nitride (NbN) superconducting HEB mixers with the system double sideband (DSB) noise temperature better than 8 times of the quantum limit: hf/kB (where h is the Planck constant, kB is the Boltzmann constant and f is the operating frequency) at the frequencies higher than 1 THz. Here, the mixer chips with the system DSB noise temperature of about 400 K and the intermediate frequency (IF) gain bandwidth (GBW) of larger than 5 GHz at 4.2 K and 0.65 THz have been used. As the direct detectors, the current responsivity of 290 A/W and 105 A/W are obtained for MW and THz biasing, which about one order higher than that of thermal biasing. The NEP of 2.7 pW/√Hz is obtained with MW biasing, which can be expected to be improved in the future.