Y. Divin

Properties of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films deposited on substrates and bicrystals with vicinal offcut and realization of high I/sub c/R/sub n/ junctions

The surface morphology, microstructure and transport properties of epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ and PrBa/sub 2/Cu/sub 3/O/sub 7/ thin films and heterostructures deposited on slightly vicinal substrates of SrTiO/sub 3/ by high pressure oxygen sputtering were studied. The vicinal angles of the substrates and bicrystals were less then 13/spl deg/. Depending on the tilt angle of the substrate a transition from spiral or island to step-flow growth leading to an improvement of the surface roughness was observed. Atomic force and transmission electron microscopy were used for these investigations. Furthermore, electrical and structural properties of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films on vicinal offcut SrTiO/sub 3/ bicrystals with different grain boundary types were studied. This included junctions with a 2/spl times/12/spl deg/ tilt or twist of the YBa/sub 2/Cu/sub 3/O/sub 7/ c-axis across the grain boundary. In comparison to conventional [001]-tilt grain boundaries bicrystal Josephson junctions [100]-tilt grain boundaries showed high I/sub c/R/sub n/-products of up to 1.2 meV at 77 K and up to 8 meV at 4.2 K. IV-curve instabilities, probably of magnetic origin due to flux flow in the electrodes, often could be observed for junctions biased with high current densities.

High-

Among various discussed ways of explosive detection, the techniques using electromagnetic radiation are considered as having great potential and research activities are recommended in this field. To identify new threats, like liquid explosives, with low rate of false alarms, fast spectral measurements are required in a broad frequency range from microwave to terahertz. We attract attention to a great potential of high-Tc Josephson technology in security applications and present our results in developing high-Tc Josephson junctions for Hilbert spectroscopy and detector arrays.

T_{c}

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.

Josephson Square-Law Detectors and Hilbert Spectroscopy for Security Applications

A prototype of a terahertz Hilbert-transform spectrum analyzer based on a high-T c Josephson junction integrated into a Stirling cooler has been developed. The detector response of YBa 2 Cu 3 O 7-x Josephson grain-boundary junctions to monochromatic radiation with the frequency f in the range from 60 GHz to 5 THz has been studied. Odd-symmetric resonances near the voltages V=hf/2e in the responses ΔI(V) of these junctions to radiation with different frequencies / have been observed in a decade of spectral range for any operating temperature between 30 to 85 K. Decreasing the junction temperature from 85 to 30 K by a Stirling cooler, the spectral analysis could be made in two decades of spectral range. A resolving power δf/f ∼ 10 -3 has been shown in the terahertz spectral analysis with the low-resistive Josephson junctions. As an example of application of this analyzer, an optimization of the tingle-line operation of a far-infrared optically-pumped CH 3 OH laser has been demonstrated.

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

Abstract Hilbert spectroscopy is based on a frequency-selective detection of electromagnetic radiation by a Josephson junction described by the resistively shunted junction (RSJ) model. The YBa 2 Cu 3 O 7− x grain-boundary junctions fabricated on NdGaO 3 bicrystals were found to be close to the RSJ model. General-purpose Hilbert spectrometers were developed and characterized using voltage-biased low-resistance junctions. The spectral bandwidth of Hilbert spectroscopy was shown to be of several frequency decades for any junction temperature between 48 and 85 K. A total bandwidth from 6 GHz to 2.5 THz has been covered using one junction at different temperatures. The spectral resolving power f /δ f of Hilbert spectroscopy was found to be of three orders in the terahertz range. The dynamic range of intensities of electromagnetic radiation in Hilbert spectroscopy was shown to be of five orders.

T_{c}

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.

Josephson Detectors

Abstract A principal possibility of broadband far-infrared (FIR) spectral measurements with Hilbert-transform technique, based on ac Josephson effect, has been experimentally studied. High-pressure 200 W mercury arc lamp was used as a source of FIR broadband radiation. The grain-boundary YBaCuO thin-film Josephson junctions (JJ) with the normal-state resistance R n =1–3 Ω and the IcRn-product up to 1 mV were used in the experiments. Broadband spectra with different FIR mesh-type filters have been measured by this technique. First results on the detection of gas absorption spectra in FIR range with Hilbert spectroscopy (HS) were obtained. Several lines of CO absorption spectrum in the range from 300 up to 1000 GHz have been resolved, when broadband blackbody radiation passed through a gas cell.

Terahertz spectral analysis by ac Josephson effect in high-T/sub c/ bicrystal junctions

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.

Hilbert spectroscopy from gigahertz to terahertz frequencies by high-Tc Josephson junctions

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.

Liquid identification by Hilbert spectroscopy

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.