Valery Pavlovskiy

THz Hilbert-Transform Spectrum Analyzer Based on High-

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.

T_{c}

Application of Josephson admittance spectroscopy for the spectral analysis of a broad-band log-periodic antenna was demonstrated at the frequency range from 50 to 700 GHz. A real part of the antenna admittance was reconstructed as a function of the frequency, when the antenna was connected to a Josephson junction. Small-scale features were observed on this dependence. They demonstrated a periodicity in the logarithmic frequency scale and were related to the antenna resonances.

Josephson Junction in Stirling Cryocooler

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 admittance spectroscopy of log-periodic antenna at the submillimeter wavelength range

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.

Josephson Spectroscopy for Identification of Liquids

High-Tc Josephson technology looks promising for THz applications. One of the remaining problems, important both for detection and emission of THz radiation, is an optimum coupling of high-Tc junctions with environment. Recently, we have demonstrated a possibility to characterize electromagnetic systems integrated with high-Tc junctions using dc I-V curves of these junctions. Here, we report in details on modification of the dc I-V curves of YBa2Cu3O7-x bicrystal junctions, related to interaction of Josephson oscillations with terahertz resonance antennas. Thin-film log-periodic antennas with various resonance structures were fabricated on the same substrate as Josephson junctions and were excited by Josephson oscillations. Frequency-dependent admittances of the antennas were derived from fine log-periodic structures on the dc I-V curves of the junctions and compared with the results of numerical simulations. Data obtained can be used for optimization of coupling between the junctions and antennas.

Terahertz Applications of Hilbert-Transform Spectral Analysis

Development of a quasioptical set-up based on a high-T c Josephson detector and intended for detection of liquids inside dielectric containers is presented. First results of reflectance measurements for water/ethanol mixtures are shown in the frequency range from 50 to 500 GHz. A dynamic range in reflectance measurements of 200 has been reached for a scanning time of a few seconds. A possibility of recovery of the residual alcohol content in “alcohol-free” beverages has been demonstrated with this set-up.

Terahertz Characterization of External Resonant Systems by 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.

T_{c}

Application of Josephson admittance spectroscopy for the spectral analysis of a broad-band log-periodic superconducting antenna was demonstrated at the frequency range from 50 to 700 GHz. The [001]-tilt YBa2Cu3O7−x bicrystal Josephson junctions, integrated with sinuous log-periodic YBa2Cu3O7−x antennas, were fabricated on NdGaO3 bicrystal substrates. A real part of the antenna admittance ReY(f) as a function of the frequency f was reconstructed from the modification of the dc current-voltage characteristic of the junction, induced by the antenna. Resonance features were observed in the recovered ReY(f)-spectra with a periodicity in the logarithmic frequency scale, corresponding to log-periodic geometry of the antenna. The ReY(f)-spectra, recovered by Josephson spectroscopy, were compared with the ReY(f)-spectra, obtained by CAD simulation, and both spectra were shown to be similar in their main features. A value of 23 was obtained for an effective permittivity of the NdGaO3 bicrystal substrates by fitting simulated data to those obtained from Josephson spectroscopy.

Josephson Junctions

To distinguish between liquids in security screening, we have applied Hilbert spectroscopy and high-Tc Josephson detectors. Compact set-ups have been developed and reflection spectra of various liquids in bottles have been measured in the spectral range of 15–300 GHz with scanning times down to 0.2 s.

Screening of Liquids with Quasioptical High-Tc Josephson Detectors

Josephson admittance spectroscopy was applied for spectral analysis of broadband log-periodic sinuous antenna with modified log-periodic geometry, which provides pronounced resonances. A correspondence of experimentally observed spectral features to the results of numerical simulations was established.