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Superconductor Science and Technology

Terahertz frequency metrology based on high-TcJosephson junctions

J Chen, Hirokazu Horiguchi, H B Wang, Kensuke Nakajima, Takashi Yamashita, Peiheng Wu

Published Nov 21, 2002• 15 citations
Publisher

Abstract

Using YBa 2 Cu 3 O 7 /MgO bicrystal Josephson junctions operating between 6–77 K, we have studied their responses to monochromatic electromagnetic radiation from 50 GHz to 4.25 THz. We have obtained direct detections for radiation at 70 K from 50 GHz to 760 GHz and at 40 K from 300 GHz to 3.1 THz. This indicates that fast detectors can be realized to cover the 10:1 frequency band at one operation temperature, and about 100:1 can be covered by operating only one junction at two different temperatures. Both the highest response frequency and the maximum value of the normalized response are shown to be proportional to the I C R N product of the junction, where I C and R N are the critical current and the normal resistance of the junction, respectively.

DOI
10.1088/0953-2048/15/12/309

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•Jan 1, 1997•0 citations
Using YBa 2 Cu 3 O 7 /MgO bicrystal Josephson junctions operating between 6–77 K, we have studied their responses to monochromatic electromagnetic radiation from 50 GHz to 4.25 THz. We have obtained direct detections for radiation at 70 K from 50 GHz to 760 GHz and at 40 K from 300 GHz to 3.1 THz. This indicates that fast detectors can be realized to cover the 10:1 frequency band at one operation temperature, and about 100:1 can be covered by operating only one junction at two different temperatures. Both the highest response frequency and the maximum value of the normalized response are shown to be proportional to the I C R N product of the junction, where I C and R N are the critical current and the normal resistance of the junction, respectively.
We have studied the responses of high temperature superconducting (HTS) Josephson junctions to monochromatic and polychromatic electromagnetic irradiations from 50 GHz to 4.25 THz. The highest operating frequency and the maximum value of normalized response are shown to be proportional to the I/sub C/R/sub N/ product of the junction, where I/sub C/ and R/sub N/ are the critical current and normal resistance of the junction, respectively. Using a junction with I/sub C/R/sub N/ of 330 /spl mu/V, direct detections of monochromatic irradiations from 76 GHz to 3.1 THz have been obtained, indicating that fast detectors can be realized to cover around 50 : 1 frequency band by operating only one junction at one temperature. Also reported are the superposition of polychromatic signals, as well as some possible applications of the system.
We prepared Josephson junctions made of YBa2Cu3O7 on symmetric 24° LaAlO3 bicrystal substrates. For better coupling of high-frequency radiation, we structured the contacts with logarithmic-periodic antennas made of YBa2Cu3O7. Characterization was done by measuring the voltage–current characteristics to determine the critical current IC, the normal-state resistance RN, the noise parameter Γ, and the Stewart–McCumber parameter βC as predicted by the resistively and capactively shunted junction model (RCSJ). The junctions showed excellent RCSJ-like behavior with ICRN products of up to 2.0 mV at 10 K and 220 μV at 77 K. To demonstrate the possibility to detect high-frequency radiation up to the THz regime, we used the Josephson junction as a detector for radiation from a far-infrared laser and we reconstructed the spectra by Hilbert spectroscopy.
Using high-T/sub c/ grain boundary Josephson junctions (GBJJs) made of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// deposited across silicon bicrystal boundary, we successfully demonstrated direct response at frequency as high as 2.525 THz and the operation temperature up to 70 K. Also, the effects of response on polarization of electromagnetic waves and the harmonic mixing properties are investigated. It is found that there is no polarization dependence of the response for our high-quality GBJJs. This result is favorable to the spectroscopy application. At frequency as high as 2.525 THz, the high-order harmonic mixing at zero and finite bias voltage has been demonstrated.
Detection mechanisms in Josephson junctions with energies Ej comparable to thermal energies kT have been studied. The responses ΔV of YBa2Cu3O7−x bicrystal junctions to monochromatic radiation with frequencies f ranging from 94 GHz to 3.1 THz can be described in terms of classical rectification on a static nonlinear V–I curve at low frequencies and frequency modulation of the ac Josephson current at high frequencies, with an interplay between these mechanisms at intermediate frequencies. An electrical noise-equivalent power of (9 ± 3) × 10−15 W/Hz1/2, a responsivity of (3.4 ± 0.5) × 105 V/W, and a dynamic power range of 5 × 104 have been demonstrated for the square-law classical detection of 94 GHz radiation with the junctions at 50 K. The effect of background radiation on the V–I curves of YBa2Cu3O7−x bicrystal junctions was found to have an optical noise-equivalent temperature of ≤30 mK/Hz1/2. The main contribution to the effect comes from the interplay between the classical and Josephson detection mechanisms. The spectral and power dependencies of the responses ΔV of Josephson junctions have been numerically simulated within the resistively shunted junction model at various values of kT/Ej, and the results are in acceptable agreement with the experimental data.
The high-frequency response of bicrystal YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// Josephson junctions for far infrared laser irradiation is presented. The junctions on silicon and MgO substrates exhibit a remarkable first order Shapiro step for the methanol line at 1.76 THz. dV/dI measurements reveal several steps corresponding to the methanol lines at 1.48, 1.76 and 2.53 THz. The line width of Josephson oscillation at these frequencies is estimated from the width of the Shapiro steps. It is suggested that the excess shot noise broadens the line width.
We report on the mixing properties of high-T, superconductor (HTS) Josephson junctions. Direct radiation and heterodyne mixing experiments in the frequency range 78 GHz-119 GHz have been performed using YBa 2Cu30 7_ x (YBCO) step-edge junctions (SEJ) and bicrystal junctions (BO) on LaA10 3 and MgO substrates. Under external radiation our junctions showed harmonic phase synchronization (Shapiro steps) up to 1.88 THz. Junctions with current voltage characteristics (PVC) dose to predictions of the resistively shunted junction model (RSJ) were mounted into a heterodyne mixing setup. Under the radiation of two monochromatic signals we obtained conversion efficiencies around -14 dB in the 11 GHz intermediate frequency (IF) band. In the fundamental mixing regime we observed response at IF at working temperatures up to 72K. We demonstrated the operation of HTS Josephson junctions in the self-mixing mode. The experimental results are in good agreement with simulations using the RSJ model.