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Spectroscopy for Terahertz Frequency Bands

#3308


Terahertz Josephson Spectroscopy

Tech Area / Field

  • INS-DET/Detection Devices/Instrumentation
  • PHY-RAW/Radiofrequency Waves/Physics
  • PHY-SSP/Solid State Physics/Physics

Status
8 Project completed

Registration date
27.06.2005

Completion date
14.05.2010

Senior Project Manager
Lapidus O V

Leading Institute
Russian Academy of Sciences / Institute of Radioengineering and Electronics, Russia, Moscow

Collaborators

  • Technical University of Denmark, Denmark, Copenhagen\nForschungszentrum Jülich GmbH / Institut für Festkörperforschung, Germany, Jülich\nInstitute of Electrical Engineering, Slovakia, Bratislava

Project summary

Exploring the subterahertz and terahertz frequency bands is one of the most significant problems of radio engineering. It is related to practical tasks of modern nanoelectronics, wireless communications, computer science, ecology and medicine, as well as fundamental scientific problems of biology, chemistry, condensed matter physics and astrophysics (see, for example, [1]). It becomes increasingly prevalent to use a phenomenon of superconductivity for development of electronic devices of subterahertz and terahertz frequency bands. Today, high temperature superconductors (HTSC) are used for more and more electronic components in measurement instrumentation.

It is known that the ac Josephson effect, which is observed in the weak linked superconductors under certain conditions, can be used for analysis of spectra of weak electromagnetic radiation that affects Josephson junction. Recent years a number of research works were performed, that denote a possibility to develop new spectroscopic devices. These devices use quantum effects in superconducting nanostructures and Hilbert transform for the reconstruction of the spectrum of the external signal from the modification of the current-voltage curve of the Josephson junction, caused by effect of the signal [2]. The devices of this kind demonstrate a number of advantages over another spectroscopic devices for the subterahertz and terahertz frequency bands such as high operating speed and simplified measurement procedure, compactness and mobility, high resolution in real time scale and so on. Laboratory prototypes of Hilbert-spectrometer based on thin film HTSC Josephson junctions in a liquid-nitrogen cryostat with the operating temperatures of 50-90 K or in a Stirling cryocooler with operating temperatures of 35-80 K were developed [3]. It was achieved noise equivalent power (NEP) about 8∙10-15 W/Hz0.5 at T=77K and relative spectral resolution δf/f~10-3 up to 4THz. However, as this area of study expands, new tasks and new capabilities are discovered in this way.

First, one of the promising and important capabilities of laboratory prototypes of Hilbert-transform spectrometer, already developed by us [3], is an investigation of continuous as well as discrete line spectra of subterahertz and terahertz frequency electromagnetic radiation sources. Recently, a rapid progress has taken place in a development of pulse sources of terahertz radiation, such as GaAs heterostructure quantum cascade lasers, and of sources with continuous spectrum, which are based on relativistic electron beams. Fourier-transform spectrometers with semiconductor bolometers, operated at liquid helium temperatures are usually employed for spectral characterization of these sources. Weak points of Fourier-transform spectrometers are low speed and large dimensions. Application of compact and fast Hilbert-transform spectrometers for the spectral analysis of these radiation sources gives a possibility to simplify greatly and speed up an optimization of their operation and to reduce a time of their practical realization. Hilbert-transform spectroscopy, based on HTSC junctions makes it possible to measure transmission and absorption spectra of substances with the scanning time of the order of millisecond [4].

Second, there is no data on high-frequency limit of Josephson oscillations in high temperature superconductors, determining the high-frequency limit of Josephson spectroscopy. So, it is necessary to provide a detailed investigation of physical conditions and phenomena, which define this limit in practical HTSC weak-link junctions. By now, Josephson junctions were realized, which let to cover the frequency range from 6 GHz to 4 THz [5]. These junctions were fabricated from the c-oriented YBa2Cu3O7-x films on the bicrystal NdGaO3 substrates. Thereby, optimal misorientation angles of the bicrystal substrate and conditions of bicrystal weld forming were determined [6]. Moreover, according to the values of a characteristic voltage Vc of the available Josephson junctions (Vc=IcRn, Ic is a critical current, and Rn is a resistance of the Josephson junction in a normal state), high frequency limit of the ac Josephson effect should exceed 10 THz. However, the advance towards higher frequencies is blocked up by the strong dumping of Josephson oscillations in a near-field zone of the weak link, which is supposed to be caused by an effective interaction of Josephson oscillations with optical phonons in thin-film YBa2Cu3O7-x electrodes [5]. The Vc –values are defined by the superconductor energy gap and in conventional isotropic “low-temperature” superconductors they correspond to energies that are substantially smaller than optical phonon excitation energy. An opposite condition may be fulfilled in anisotropic HTSC Josephson weak links. An excitation of optical phonons with the c-axis polarization appears to be at the energies smaller then the energy gap in the ab plane. Preliminary experiments allow to suppose that the investigation of physical origins of the Josephson effect high-frequency limit in HTSC will make it possible to increase the frequency range to tens of terahertz.

Third, the ac Josephson effect allows to investigate absorption spectra of substances, as well as frequency dependence of impedances of micro- and nanostructures, which are placed in the vicinity of the weak link [7]. It is possible to reconstruct a frequency dependence of the impedance of the close environment of Josephson junction, by measuring a modification of the current-voltage characteristic of the weak link, caused by the interaction of it with substance or system under consideration. By means of HTSC Josephson junctions, these characteristics can be measured within the whole terahertz frequency range. Small dimensions of the junction area allow to perform near-field impedance spectral analysis at terahertz frequencies. Consequently, the technique, which is proposed in [7], extends a functionality of Josephson junctions in spectroscopic research. Also, use of HTSC materials allows to realize new methods of Josephson spectroscopy in subterahertz and terahertz frequency ranges. It is supposed to use thin-film YBa2Cu3O7-x Josephson junctions on bicrystal NdGaO3 substrates, which are already developed for Hilbert-transform spectroscopy by IRE RAS staff, for this spectroscopic research, because the basic requirements for junctions are the same in both cases. Indeed their properties should correspond to the predictions of resistively shunted junction model. Consequently, Josephson impedance spectroscopy of collective excitations in solid state, micro- and nanostructures in subterahertz and terahertz frequency bands can be implemented.

According to aforesaid, the main goals of the project proposed are the following:

  1. clearing up of physical conditions and phenomena, which define a high-frequency limit of Josephson generation in HTSC thin-film weak-link structures and possible ways of its enhancement;
  2. study of discrete line and continuous spectra of external electromagnetic radiation sources of subterahertz and terahertz frequency bands as well as transmission and absorption spectra of substances using laboratory prototypes of Hilbert-transform spectrometer;
  3. investigation of absorption spectra of substances, micro- and nanostructures in subterahertz and terahertz frequency bands using Josephson generation of HTSC thin-film junctions placed in the vicinity of the sample under consideration.

To achieve these goals, it is required to solve the following scientific and engineering tasks:
  1. improvement of technology methods of fabrication of thin-film Josephson junctions of YBa2Cu3O7-x on bicrystal substrates of NdGaO3, MgO and others with various misorientation angles and adjustable mutual tilts of the c-axes.
  2. Development of testing methods of tuning frequency range of Josephson oscillations in thin-film YBa2Cu3O7-x junctions up to tens terahertz;
  3. development of methods of optimal wide-band coupling of electromagnetic radiation sources with thin-film YBa2Cu3O7-x Josephson junction up to far infrared band for “far-field zone”, if, for example, terahertz laser is used as a radiation source, as well as for “near-field zone”, if a sample of substance, which absorption spectra are investigated, is placed directly to Josephson junction;
  4. development of software and numerical methods of data processing, which are used for reconstruction of radiation or absorption spectral density for “far-field zone” and “near-field zone”;
  5. study of conditions of one-dimensional arrays of thin-film Josephson junctions application for antenna structures design with improved ultimate parameters (improved sensitivity, expanded dynamic reserve and so on);
  6. development and fabrication of new types of laboratory prototypes of high-speed spectral devices of subterahertz and terahertz frequency ranges based on HTSC thin-film Josephson junctions.

Thus, the present project is aimed on solving of important problems of physics of transient processes in weak-linked high-Tc superconductors up to frequencies of the order of 10 THz and for problems of application of terahertz Josephson spectroscopy for investigation of electromagnetic radiation sources and absorption spectra of substances. So, the project contains as problems of basic condensed matter physics and weak superconductivity, as applied problems of development of electromagnetic radiation sources and research of absorption spectra in subterahertz and terahertz frequency ranges. Theoretical and experimental methods of cryogenic radio physics and electronics, micro- and nanoelectronic technology, and methods of data processing and computer science are used for solving these problems.

The project is presented by a group of research scientists of the Institute of Radioengineering and Electronics of the Russian Academy of Sciences with broad experience (for 20-30 years) of research and development of weak-link superconductor structures, high sensitive receivers and spectral devices of millimeter, submillimeter and far infrared wave length bands; computer systems of data acquisition and processing. Measurement workbenches and computers are available. The group includes high skilled technologists and it is supplied with modern equipment for fabrication high quality thin-film Josephson junctions of high-Tc superconductors with required characteristics. IRE RAS researchers, which will run the project, have close international contacts; their works on weak superconductivity, application of Josephson effect in millimeter wave and far infrared devices, research and application of HTSC Josephson junctions are well-known in the world.

This project corresponds completely to the goals of ISTC. Its realization makes possible for IRE RAS research group members, which have knowledge and skills in military research and development, to exclude involuntary activity on contracts, related to development of new military technology (which provides IRE RAS with additional financing) from their work plans.

LITERATURE

  1. E.R. Mueller. The Industrial Physicist, v.9, Issue 4, pp. 27-29, 2003
  2. Y.Y.Divin, O.Y.Polyanskii, A.Y.Shul’man. Sov.Teckn.Phys.Lett., v. 6, No.17, pp.1056-1061, 1980
  3. Y.Y.Divin, O.Y.Volkov, V.V.Pavlovskii et al. Adv. Solid State Phys, v.41, p.301, 2001
  4. V. Shirotov et al., IEEE Trans. Appl. Supercond., v.13, n.2, pp.172-175, 2003
  5. Y.Y.Divin, O.Y.Volkov, M.V.Liatti, V.N.Gubankov. IEEE Trans. Appl. Supercond. v.13, No.6, p.2872, 2003
  6. Y.Y.Divin, I.M.Kotal’anskii, V.N.Gubankov. J. Comm. Tech.and Electr., v.48, pp.1137-1147, 2003
  7. A.F.Volkov. Radiotehnika i elektronika, v.17, No12, p.2581, 1972 (in Russian)


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