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IEEE Transactions on Applied Superconductivity

Mixing and detection in YBaCuO step-edge Josephson junction arrays up to 670 GHz

Janusz Konopka, I. Wolff, S. Beuven, M. Siegel

Published Jun 1, 1995• 6 citations
Publisher

Abstract

Submillimeter wave detectors and mixers based on YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) step-edge Josephson junctions incorporated into planar antennas were designed and tested up to 670 GHz at temperatures from 12 to 65 K. A quasi-optic coupling system with a LaAlO/sub 3/ lens was used. In some samples the noise intrinsic in the detection process does not follow the Johnson noise law but has a minimum, which for a given sample may occur in the temperature range between 35 to 50 K. Harmonic mixing recorded at 670 GHz proves that the system equilibrium is established in a time shorter than 500 fs.>

DOI
10.1109/77.403085

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Submillimeter wave detectors and mixers based on YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) step-edge Josephson junctions incorporated into planar antennas were designed and tested up to 670 GHz at temperatures from 12 to 65 K. A quasi-optic coupling system with a LaAlO/sub 3/ lens was used. In some samples the noise intrinsic in the detection process does not follow the Johnson noise law but has a minimum, which for a given sample may occur in the temperature range between 35 to 50 K. Harmonic mixing recorded at 670 GHz proves that the system equilibrium is established in a time shorter than 500 fs.>
We have fabricated YBCO step-edge Josephson junctions and measured the responses to millimeter and submillimeter irradiation. In the substrate milling process for the purpose of fabricating the step, the step angle was controlled using a Nb metal mask. At a step angle of 20 degrees, we obtained Josephson junctions, the characteristics of which are consistent with the RSJ model. We estimated the capacitance to be 4.4 fF using the RSJ model, but such a value for an 8 /spl mu/m wide and 300 nm thick junction seems too small. When the junctions were irradiated by 300 GHz radiation, Shapiro steps up to the 4th were observed. The junctions were used as mixers in the 100 GHz band, and as harmonic mixers in the 100 GHz LO and 700 GHz RF bands. Beat signals were observed in both cases. We also estimated the quasi-optical conversion losses of the step-edge junctions as mixers.>
We have investigated YBCO step-edge Josephson junctions as millimeter and submillimeter wave mixers. These mixers were integrated with an Au thin-film log-periodic antenna designed for broadband coupling of the radiation. We estimated the noise temperature of the mixer using conventional hot/cold blackbody irradiation techniques in the 100- and 300-GHz bands. We found that the Au thin-film antenna yielded about 5 dB higher coupling efficiency than that of YBCO thin film antenna, and the noise temperature decreased from 2,800 K to 1,800 K. The best receiver noise temperature, 1,200 K, was achieved in the 300-GHz band through improvement in impedance matching.
A Josephson quasioptical detector comprising YBaCuO junction on bicrystal MgO substrate, integrated log-periodic Au antenna, MgO extended hyperhemisphere lens attached to the substrate and 1.4 GHz cold amplifier with cold circulator has been designed, fabricated and experimentally studied. We fabricated several integrated Josephson junctions 1-2 /spl mu/m wide which at 4.2 K demonstrate RSJ IV curves with critical currents 100-300 /spl mu/A and normal resistance of 10-20 /spl Omega/. The noise temperature (T/sub N/) at 4.2 K and 330 GHz measured by a hot/cold load method in fundamental mixing mode with external BWO local oscillator brings T/sub N/=1100 K for the junction shunted by low-inductance resistive shunt and T/sub N/=1400 K without shunting. At bath temperatures of 20-40 K the noise temperature was in the range 1500-3000 K, at 50 K the noise temperature was below 2000 K, and at 60-70 K increased over 3000 K in the frequency range 310-586 GHz. The noise temperatures measured in the self-pumping mode vary between 580-2300 K with dc bias in the range 0.1-0.5 mV. The linewidth of Josephson oscillations has been extracted from a selective detector response and IF noise voltage dependence. The measured excess noise can be attributed to shot noise and multiple Andreev reflection in pinholes with Ss'S structure. We observed small subharmonic steps in IV curve and noise dependencies, which are typical for SNS junctions.
A high-temperature superconducting (HTS) terahertz (THz) frequency down-converter or mixer based on a thin-film ring-slot antenna coupled YBa 2 Cu 3 O 7– x (YBCO)/MgO step-edge Josephson junction is reported. The frequency down-conversion was achieved using higher order harmonics of an applied lower frequency (19–40 GHz) local oscillator signal in the Josephson junction mixing with a THz signal of over 600 GHz, producing a 1–3 GHz intermediate frequency signal. Up to 31st order of harmonic mixing was obtained and the mixer operated stably at temperatures up to 77 K. The design details of the antenna, HTS Josephson junction mixer, the matching and isolation circuits, and the DC and RF performance evaluation are described in this paper.
We have fabricated YBCO step-edge Josephson junctions and measured the responses to the millimeter and submillimeter irradiation. In the substrate milling process for the purpose of fabricat- ing the step, the step angle was controlled using a Nb metal mask. At a step angle of 20 degrees, we obtained Josephson junctions, the characteristics of which are consistent with the RSJ model. We estimated the ca- pacitance to be 4.4 fF using the RSJ model, but such a value for an 8 pm wide and 300 nm thick junction seems too small. When the junctions were irradiated by 300 GHz, Shapiro steps up to the 4th were ob- served. The junctions were used as mixers in the 100 GHz band, and as harmonic mixers in the 100 GHz LO and 700 GHz RF bands. Beat signals were observed in both cases. We also estimated the quasi-optical con- version losses of the stepedge junctions as mixers.
Detectors made from superconducting cuprate YBa/sub 2/Cu/sub 3/O/sub x/ bicrystal Josephson junctions (BJJs) on sapphire and NdGaO/sub 3/ substrates have been fabricated and characterized in the frequency band 200-900 GHz. Junctions on sapphire substrates had a normal state resistance R/sub N//spl ap/15-60 /spl Omega/, and I/sub C/R/sub N/ product up to 2.5 mV at T=4.2 K. Junctions on NdGaO/sub 3/ substrates had lower R/sub N/=1-5 /spl Omega/ and I/sub C/R/sub N/=0.4-0.9 mV at T=77 K. Three types of detecting devices have been investigated in both the broadband and the frequency-selective detection modes. One type was patterned with log-periodic antenna, and two others with Pt-metal double-slot antenna designed for a central frequency f=300 GHz and f=400 GHz, respectively. Measurements at f=320 GHz of the reception bandwidth /spl Delta/f for a device with double-slot antenna gave a quality factor Q=f//spl Delta/f/spl ap/10. A low-noise cooled 1-2 GHz bandwidth amplifier enables a better sensitivity in the self-pumping frequency mixing mode, avoiding the 1/f noise. The dependence of the spectral density of noise on voltage was compared to the data for the Josephson emission linewidth obtained by the selective detector response method. Also discussed are measurements at 500 GHz of the NEP values carried out at different experimental conditions.