Matvey Lyatti

Signal and Noise Characteristics of Terahertz Frequency-Selective and Broadband High-

Josephson detectors based on YBa2Cu3O7-x [100]-tilt bicrystal junctions were fabricated and their frequency-selective and broadband modes of operation were studied at terahertz frequencies. The resistances of the [100]-tilt junctions were in the range of 1-200 Ohm and their characteristic voltages were up to 8 mV at 4.2 K. Values of the responsivity up to at radiation frequency of 0.7 THz were demonstrated for the low-resistance frequency-selective detectors at a temperature of 55 K. The responsivity values were found to be in accordance with the predictions of the RSJ model, when only thermal fluctuations are considered. But, the values of noise equivalent power (NEP) and power dynamic range measured in the same experiment at a modulation frequency of 2 kHz were found to be determined by 1/f noise of the junctions and equal to 3ldr10-13 W/Hz1/2 and 5ldr104, correspondingly. It was shown that the values of NEP of 5ldr10-15 W/Hz1/2 and power dynamic range of 106 could be reached in this mode if high-frequency modulation or pulsed radiation were used. Broadband classical detection in high-resistance [100]-tilt junctions was experimentally found to reach the terahertz range. It follows from numerical simulations that broadband detection by the [100]-tilt YBCO junction with the resistance of 300 Ohm might be characterized by NEP-values down to 3ldr10-15 W/Hz1/2 and a bandwidth of 1.5 THz. Josephson detectors based on YBCO [100]-tilt bicrystal junction are promising for various terahertz applications such as real-time spectral analysis of continuous or pulsed radiation sources and terahertz imaging for medical or security screening.

T_{c}

A demonstrator of a terahertz (THz) Hilbert-transform spectrum analyzer with subsecond scanning times and accuracy of the order of 10-3 has been developed. The spectrum analyzer is based on a high-Tc Josephson junction in a compact Stirling cryocooler. Operation of the spectrum analyzer has been critically considered in the THz range and sources of errors have been studied. The voltage and frequency scales of the analyzer have been calibrated with accuracy of 7 ·10-4 by Shapiro steps, induced on the I(V) curve of the junction by intensive monochromatic radiation. Dynamic errors of the analyzer have been minimized with scanning rates up to 4 THz/s. An instrumental function of the spectrum analyzer is of a Lorentz type with a spectral width of 1.5 GHz and free from any harmonic and subharmonic contributions with accuracy of around 10-3. Rapid and detailed characterization of the THz sources, based on frequency multiplication of microwave radiation, has been demonstrated with the developed spectrum analyzer. The developed spectrum analyzer will be effective in new demanding THz applications, where a combination of high speed and accuracy is required.

Josephson Detectors

Fast and reliable identification of liquids is of great importance in, for example, security, biology and the beverage industry. An unambiguous identification of liquids can be made by electromagnetic measurements of their dielectric functions in the frequency range of their main dispersions, but this frequency range, from a few?GHz to a few THz, is not covered by any conventional spectroscopy. We have developed a concept of liquid identification based on our new Hilbert spectroscopy and high- Tc Josephson junctions, which can operate at the intermediate range from microwaves to THz frequencies. A demonstration setup has been developed consisting of a polychromatic radiation source and a compact Hilbert spectrometer integrated in a Stirling cryocooler. Reflection polychromatic spectra of various bottled liquids have been measured at the spectral range of 15?300?GHz with total scanning time down to 0.2?s and identification of liquids has been demonstrated.

THz Hilbert-Transform Spectrum Analyzer Based on High-

We have studied electrical transport and low-frequency noise properties of [100]-tilt bicrystal YBa2Cu3O7-x junctions, which show a lower degree of structural disorder and higher characteristic voltages IcRnmiddot in comparison with conventional [001]-tilt grain-boundary junctions. The oxygen content of the junction barrier was varied by an annealing in atomic oxygen and by aging in molecular oxygen. The modification of the I-V curves and low-frequency noise were monitored. The terahertz losses in the junctions were derived and were shown to decrease with the increase of the oxygen content. It was found that the resistance and critical current fluctuations are completely antiphase correlated and intensities of normalized resistance and critical current low-frequency fluctuations are equal in these junctions. Consequently, quasiparticles and Cooper pairs in the [100]-tilt junctions tunnel directly through the same parts of the barrier, and a band-bending model with the charge fluctuations at the barrier can applied to explain the modification of the [100]-tilt grain-boundary junctions after oxygen loading and aging.

T_{c}

One of future public security techniques will be related with non-invasive, fast and reliable detection of liquids. To distinguish between liquids under concern, we have suggested a concept using our Hilbert spectroscopy, based on high-Tc Josephson junctions. This spectroscopy is the only technique, which covers a frequency range of main dispersions of liquids from a few GHz to a few THz. Several demonstration setups of liquid identifiers, consisting of Hilbert spectrometers integrated on Stirling coolers and polychromatic radiation sources, have been developed. A critical consideration of the main sources of fluctuations in these measurements was carried out and a signal accuracy of around 0.3% has been reached with a total measurement time of a few seconds. Identification of samples of benign and threat liquids was demonstrated.

Josephson Junction in Stirling Cryocooler

Bridging of the terahertz gap in the electromagnetic spectrum between the microwave and infrared ranges requires a variety of new technological developments from basic elements, such as emitters and detectors, to complete systems, such as spectrum analyzers and imagers. As an example of these developments, Hilbert-transform spectral analysis of terahertz radiation sources has been demonstrated. A spectrum analyzer based on a high- Tc square-law Josephson detector has been developed and characterized in the frequency range from 50 to 1800 GHz. Spectra of output terahertz radiation from optically-pumped lasers and frequency multipliers have been studied, and their regimes were optimized for a single-frequency operation. Starting from the optimized multipliers, a polychromatic source has been synthesized and characterized with Hilbert-transform spectrum analyzer.

Liquid identification by Hilbert spectroscopy

Despite impressive progress in the development of superconducting nanowire single-photon detectors (SNSPD), the main obstacle for the widespread use of such detectors is the low operating temperature required for low-temperature superconductors. The very attractive idea of increasing the operating temperature using high-temperature superconductors for SNSPD fabrication is problematic due to the insufficient quality of ultra-thin films from high-temperature superconductors, which is one of the key requirements for the single-photon detection by superconducting nanowires. In this work, we demonstrate the possibility of fabricating ultra-thin YBa2Cu3O7−x films on SrTiO3 substrates with a surface flatness of ±1 unit cell and a high critical current density up to 14 MA cm−2 at T = 78 K. The critical current density of ultra-thin films had very low value in the first three unit cell layers adjacent to the substrate and reached nearly the bulk value at the fifth layer. 97% of the superconducting current is carried by only two upper layers of a 5-unit-cell thick YBa2Cu3O7−x film. Due to such superconducting current distribution over the film thickness and good surface flatness 5-unit-cell thick YBa2Cu3O7−x films could be promising for the fabrication of single-photon detectors.

Electrical Transport and Noise Properties of [100]-Tilt

Fast and reliable identification of liquids is of great importance in developing new security measures at public places. A concept of liquid identification is presented, based on our Hilbert spectroscopy and high-Tc Josephson junctions, that can operate at the frequency range of main dispersions of liquids under concern, i.e. at the intermediate range from microwaves to terahertz frequencies. Several demonstration setups, consisting of synthesized polychromatic radiation sources and compact Hilbert spectrometers integrated in Stirling coolers, have been developed and characterized. Reflection polychromatic spectra of various bottled liquids have been measured at the spectral range of 15 – 400 GHz with total scanning time down to 0.5 second and the possibility of reliable identification of liquids has been demonstrated.

{\hbox{YBa}}_{2}{\hbox{Cu}}_{3}{\hbox{O}}_{7{-}{\rm x}}

Fast and reliable identification of bottled liquids is of great importance for security screening. An unambiguous identification of bottled liquids can be made by electromagnetic measurements of dielectric permittivity functions of the liquid in a frequency range, where bottles are transparent and liquids of concern have specific dispersions. However, this frequency range, from a few GHz to a few THz, cannot be covered by any single conventional spectroscopy. We have developed a concept of liquid identifier, which is based on our new broadband Hilbert spectroscopy and high-Tc Josephson detectors. The first proof-of-principle measurements of reflection spectra from various bottled liquids in the range 40-300 GHz with total scanning time of 1 second have been carried out.

Grain-Boundary Junctions With High

The I-V curves and dynamic resistances Rd (V) of [001]-tilt YBa₂Cu₃O_7-x bicrystal Josephson junctions have been experimentally studied at various levels of oxygen loading. Oxygen content was modified by annealing in ozone atmosphere, in vacuum or in air. Deviations of the <i>IV</i> -curves from those predicted by the RSJC model in the form of additional nonlinearities at the middle and high voltages were found to decrease after enhancement of oxygen content in junctions. Electrodynamic properties of Josephson junctions were analyzed using log-periodic YBa₂Cu₃O_7-x antennas, integrated with the junctions. Fine structures on the <i>IV</i>-curves, related to antenna resonances, were modified with oxygen content due to better antenna-junction coupling and lower electromagnetic losses in YBa₂Cu₃O_7-x film.