Mikhail Tarasov

Carbon nanotube bolometers

A cryogenic bolometer has been fabricated using a bundle of single-walled carbon nanotubes as absorber. A bolometric response was observed when the device was exposed to radiation at 110 GHz. The temperature response was 0.4 mV/K, with an intrinsic electrical responsivity at low frequency up to 109 V/W and noise equivalent power of 3×10−16 W/Hz1/2 at 4.2 K. The response is largest at input power levels of a few femtowatts and decreases inversely proportional to the input power. Low frequency noise shows a 1/f dependence.

Optical Response of a Cold-Electron Bolometer Array Integrated in a 345-GHz Cross-Slot Antenna

Two series/parallel arrays of ten cold-electron bolometers with superconductor-insulator-normal tunnel junctions were integrated in orthogonal ports of a cross-slot antenna. To increase the dynamic range of the receiver, all single bolometers in an array are connected in parallel for the microwave signal by capacitive coupling. To increase the output response, bolometers are connected in series for dc bias. With the measured voltage-to-temperature response of 8.8 μV/mK, absorber volume of 0.08 μm3, and output noise of about 10 nV/Hz1/2, we estimated the dark electrical noise equivalent power (NEP) as NEP = 6 * 10-18 W/Hz1/2. The optical response down to NEP = 2 * 10-17 W/Hz1/2 was measured using a hot/cold load as a radiation source and a sample temperature down to 100 mK. The fluctuation sensitivity to the radiation source temperature is 1.3 * 10-4 K/Hz1/2. A dynamic range over 43 dB was measured using a backward-wave oscillator, a variable polarization grid attenuator, and cold filters/attenuators.

Subharmonic Shapiro steps and noise in high-T c superconductor Josephson junctions

An experimental investigation is made of the subharmonic Shapiro steps observed on the I-V curves of high-Tc superconductor Josephson junctions and on the bias-voltage dependences of the rf noise and detector response when the junctions are subjected to external submillimeter radiation. Structures of this type are ordinarily described by a nonsinusoidal current-phase relation, which is why subharmonic steps appear. Numerical modeling of the processes occurring in a Josephson junction by means of a simple current-phase relation, as in the case of an SNS junction, gives good agreement with experiment. The width of the characteristic Josephson generation line of the junction was estimated on the basis of the noise dependences and the selective detector response. The width can be explained by taking into account the shot noise of the tunneling component of the conductivity. A model of the conductivity of a high-Tc superconductor Josephson junction, consisting of a tunnel junction with microshorts possessing metallic conductivity, is discussed.

Thin multilayer aluminum structures for superconducting devices

Various aluminum-based thin-film structures were manufactured and investigated at temperatures of 50 mK–3 K. Multilayer films of Al and Si, Al and Cr, and Al in the presence of oxygen were deposited by the thermal evaporation technique. As the thickness of pure-Al films decreases from 20 to 3 nm, the temperature of the superconducting transition increases from 1.30 to 2.45 K. An increase in the oxygen pressure to 5 × 10−6 mbar during deposition of Al films results in an increase in the critical temperature to 2.4 K. The presence of a chromium sublayer with a thickness of <0.5 nm may lead to complete suppression of superconductivity, whereas a thicker layer, 1–4 nm, deposited at a higher temperature with preliminary sputtering reduces the critical current of Al/Cr two-layer films to a lower degree. An atomic-force microscope was used to study the surface morphology and granularity and the roughness of manufactured film structures. The smallest linear roughness having a size of 0.29 nm for a 3-nm-thick film shows the advantage of using thinner films for creating a homogeneous tunneling barrier.

Nonthermal optical response of superconductor-insulator-normal metal-insulator-superconductor tunnel structures

Bolometers, which are based on superconductor-insulator-normal metal-insulator-superconductor structures integrated into double slot antennas, are designed and fabricated. The change in the tunneling current in such structures during irradiation at a frequency of 330 GHz is experimentally studied at a temperature of about 0.1 K. It is found that, during irradiation, the energy distribution of electrons has a substantially nonequilibrium form, which differs from the Fermi distribution, and the main contribution to the addition to the tunneling current is made by the electrons that have no time to attain equilibrium within the electron system of the normal metal absorber. The characteristic electron and phonon relaxation times are estimated, and the estimates support the fact that the electron system is strongly nonequilibrium in the radiation field.

Optical response of a cold-electron bolometer array

A multielement bolometric receiver system has been developed to measure the power and polarization of radiation at a calculated frequency of 345 GHz. Arrays of ten series-parallel connected cold-electron bolometers have been pairwise integrated into orthogonal ports of a cross-slot antenna. Arrays are connected in parallel in the high-frequency input signal and in series in the output signal, which is measured at a low frequency, and in a dc bias. Such an array makes it possible to increase the output resistance by two orders of magnitude as compared to an individual bolometer under the same conditions of high-frequency matching and to optimize the matching with the JFET amplifier impedance up to dozens of megohms. Parallel connection ensures matching of the input signal to the cross-slot antenna with an impedance of 30 Ω on a massive silicon dielectric lens. At a temperature of 100 mK, a response to the thermal radiation of a thermal radiation source with an emissivity of 0.3, which covers the input aperture of the antenna and is heated to 3 K, is 25 μV/K. Taking into account real noise, the optical fluctuation dc sensitivity is 5 mK, the estimated sensitivity corresponding to the noise of the amplifier is about 10−4 K/Hz1/2, and the noise-equivalent power is about (1–5) × 10−17 W/Hz1/2.

Carbon nanotube based bolometer

The contacts of single carbon nanotubes and bundles of carbon nanotubes with superconducting and metallic electrodes are investigated in order to create bolometers and electron coolers. Tunneling contacts of the carbon nanotubes with aluminum electrodes are obtained. The current-voltage characteristics of junctions are analyzed for temperatures from room temperature to 300 mK. The resistance of individual nanotubes is primarily determined by defects and is too large for applications. The use of the bundles of carbon nanotubes makes it possible to considerably reduce the resistance of the bolometer, which is determined by a small number of conducting tubes with good tunneling contacts with the electrodes. The energy gap is equal to hundreds and tens of millivolt in the former and latter cases, respectively. Structures containing bundles of carbon nanotubes can be described in a model with a Schottky barrier. The samples with bundles of carbon nanotubes exhibit the bolometric response to external high-frequency radiation at a frequency of 110 GHz with an amplitude up to 100 μV and a temperature voltage response to 0.4 mV/K.

Fabrication and Characteristics of Mesh Band-Pass Filters

Mesh band-pass filters for 300-, 450-, 600-, and 750-GHz central frequencies are designed and manufactured. Copper and aluminum foil and foil-clad Teflon filters were made by chemical and ionic etching methods, and an aluminum film evaporated on a kapton film was formed by the lift-off lithography method. A gold layer was electrolytically applied on copper mesh filters. Characteristics of the filters were measured in the millimeter, submillimeter, and infrared (IR) ranges. Foil filters demonstrate better characteristics at lower frequencies, while filters with evaporated films have better characteristics at upper frequencies. The smallest transmission loss was 0.13 dB. For a combination consisting of four filters, this loss was 0.9 dB. The IR radiation attenuation in a 2.5- to 25.0-μm wavelength region was no less than 11 dB per filter.

Experimental study of a normal-metal hot electron bolometer with capacitive coupling

Normal-metal hot-electron bolometer with capacitive coupling (CCNHEB) is a further development of the concept of a normal-metal hot-electron bolometer with Andreev mirrors (ANHEB). It was proposed to eliminate the frequency and energy restrictions inherent in ANHEB, in which Andreev mirrors act efficiently only with relatively long absorbers and at energies below the superconducting gap. An important advantage of the CCNHEB is its simple topology, in which the same tunnel junctions provide thermal decoupling, noise protection, temperature measurement, and it can be used for electron cooling. The temperature response of the bolometer was measured at temperatures down to 260 mK. The observed response dV/dT=1.7mV/K corresponds to the sensitivity S=0.4×109V/W. The measured noise at the amplifier output with this sample was found to be Vna=4nV/Hz1/2, which corresponds to a noise-equivalent power of 10−17W/Hz1/2. To measure optical response, black-body radiation was used as a source of signal inside the cryostat. The source was a thin NiCr film sputtered on a thin sapphire substrate and suspended by nylon threads. Optical measurements proved to be in good agreement with the dc measurements.

Grain boundary weak link in a-b plane in MgB2 film

The authors have fabricated the Mg B2 bridges 850-6000 nm in width on bicrystal (111) MgO substrates with in-plane grain boundaries of the two types: 13°13° 〈110〉 and 13°13° 〈112〉. Annealing in oxygen of the films on 〈110〉 bicrystal substrates leads to a systematic decrease of critical current, a widening of the transition temperature width, and an improvement of the shape of IV curve that finally looks more like a Josephson junction. They have measured a response of such samples to the microwave radiation at 110 GHz with the voltage amplitude up to 0.5 mV.