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International Conference on Millimeter and Submillimeter Waves and Applications 1994

Submillimeter radiation from quasioptically coupled Josephson junctions

Michael J. Wengler

Published Jan 1, 1994• 0 citations
Publisher

Abstract

11 nW of 350 GHz radiation is detected from 2-D arrays of 638 niobium Josephson junctions fabricated at Hypres. I.J.W power devices will be used for submillimeter spectroscopy and as local oscillators for SIS mixers.
DOI
10.1117/12.2303104

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The goal of this program was to demonstrate a Quasioptical Josephson Oscillator with Integrated Tuning Elements using standard Conductus niobium technology. This device is based on a novel approach that incorporates integrated tuning and impedance-matching structures at every Josephson junction/ antenna pair. The device has been designed, fabricated and successfully tested. A new, unit cell has been devised, incorporating a sub-array of 1 to 16 Josephson junctions. Microstrip transformers are used locally between each sub- array and its antenna. The oscillator with 110 Josephson junctions and bow-tie antennas was found to radiate close to its maximum available power at 115 GHz. The on-chip SIS radiation detector has detected 2.64 nW of power. The oscillator was tuned across 19 GHz, or 16% of fractional bandwidth, in reasonable agreement with the predicted value of 19%. This is the first demonstration of a distributed array Josephson oscillator, where each junction feeds its own antenna and phase-locks to the radiation of other junctions. It is also the first demonstration of using integrated microstrip tuning and impedance matching elements at every Josephson junction of an oscillator. Finally, this is the first distributed Josephson oscillator reported to date that appears to radiate close to its maximum available power. Submillimeter, Oscillators, Antenna arrays, Superconducting electronics, Quasioptical.
At 190 GHz, 0.36 /spl mu/W has been detected from an 11/spl times/58 array of niobium Josephson junctions. The power is radiated directly into free space above the surface of the two-dimensional array of junctions. Detection is made by a commercial bolometer outside the array cryostat. The junctions in the array are closely spaced in the y-dimension, but spaced by more than one-half wavelength in the x-direction. This asymmetry results in mutual phase-locking between adjacent junctions suitable for the production of y-polarized radiation. The resonance anti asymmetry also results in a reasonable impedance match between the Josephson junctions and the free-space radiation mode.>
Josephson junctions are natural voltage-controlled oscillators capable of generating submillimeter-wave-length radiation, but a single junction usually can produce only 100 nW of power and often has a broad spectral linewidth. The authors are investigating 2-D quasi-optical power combining arrays of 10/sup 3/ and 10/sup 4/ NbN/MgO/NbN and Nb/Al-AlO/sub x//Nb junctions to overcome these limitations. The junctions are DC biased in parallel and are distributed along interdigitated lines. The arrays couple to a resonant mode of a Fabry-Perot cavity to achieve mutual phase-locking. The array configuration has a relatively low impedance, which should allow the capacitance of the junctions to be tuned out at the oscillation frequency.>
Superconducting tunnel diodes (SISs) are investigated for both the generation and detection of submillimeter wavelengths. For detection, heterodyne receivers using SISs are investigated. Josephson currents are found to have a large effect on SIS mixers in the submillimeter, even though their effect is less at lower frequencies. At 492 GHz, fixer conversion gain increased by 4 dB when Josephson currents are not suppressed by a magnetic field. A much higher noise is seen in this gain mode, but further investigation of this recently discovered phenomenon is warranted. For submillimeter generation, a two-dimensional array of SISs is investigated. The ac Josephson effect in the SIS is expected to deliver submillimeter power into a free-space radiation mode traveling perpendicular to the array. No experimental devices have been developed yet, but the authors show computer simulations and microwave scale model measurements of array structures.