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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Two dimensional Josephson junction array oscillators

Bin Liu

Published Dec 7, 2017• 0 citations
Publisher

Abstract

We have designed and fabricated two dimensional Josephson junction arrays as sub- THz oscillators. These arrays are integrated with various coupling structures and fabricated on one side of substrates with the other side of substrates metalized to form resonant cavities behind the arrays. The radiation is coupled out from broadside. The I-V curve measurement has been done, but none of the I-V curves has shown a strong indication of phase locking of the array at this time.

DOI
10.1117/12.2298125

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We have designed and fabricated two dimensional Josephson junction arrays as sub- THz oscillators. These arrays are integrated with various coupling structures and fabricated on one side of substrates with the other side of substrates metalized to form resonant cavities behind the arrays. The radiation is coupled out from broadside. The I-V curve measurement has been done, but none of the I-V curves has shown a strong indication of phase locking of the array at this time.
We have investigated two-dimensional arrays of overdamped Josephson junctions that have been fabricated in Nb/Al/sub 2/O/sub 3//Nb-technology, together with microwave coupling structures. Different arrays have shown to emit coherent microwave radiation by using microwave-coupled on-chip detectors. Low-temperature scanning electron microscopy is used to study the coupling of the microwave radiation to an on-chip load. From the appearance of Shapiro steps in the current vs. voltage curves of detector junctions for different degrees of disorder in the array-junctions, margins of the spread in the critical currents of the Josephson junctions for coherent oscillation can be drawn.
Arrays of phase locked Josephson junctions are attractive for use as sub-mm wavelength voltage controlled oscillators for heterodyne mixers and free space radiators. We have performed phase locking experiments on 40 junction, parallel biased arrays of Josephson junctions coupled through transmission lines of varying delays. The longest junction to junction delay implemented was 5.4 ps for a design frequency of 90 GHz, while the shortest was 1.45 ps, for a design frequency of 345 GHz. The resonant frequency f/sub 0/ of the arrays was found to be almost independent of the transmission line delay with f/sub 0/, scaling with the plasma frequency for arrays fabricated at critical current densities of 10/sup 3/ and 4.5/spl times/10/sup 3/A/cm/sup 2/. The data indicate that at the resonant frequency, strong phase locking is present among the junctions for junction separations significantly less than one half wavelength even in arrays many wavelengths long.>
Interaction between two Josephson junctions and a microstrip resonator is investigated using a resonator shunted junction (RLCSJ) model. A sharp current step resulting from coupling with the resonator is observed in simulated I-V characteristics. There is strong phase-locking and decreasing of dynamic resistance at the current step. We design and fabricate two types of Josephson array oscillators using these effects to obtain stable phase locking and narrow linewidth for the oscillation. The fabricated oscillators are formed with nine shunted Nb/AlOx/Nb tunnel junctions and Nb microstrip resonators. The power of the oscillator delivered to the load resistor was estimated to be about 3.6 /spl mu/W at 330 GHz. A composite linewidth of the oscillators is also measured at about 19 MHz at 4.2 K using two oscillators with a log-periodic antenna and a SIS detector.
Work with large two-dimensional arrays of Josephson junctions for submillimeter power generation is discussed. The basic design of the quasioptical Josephson oscillator (QJO) is presented. The reasons for each design decision are discussed. Superconducting devices have not yet been fabricated, but scale models and computer simulations have been done. A novel method for characterizing array RF coupling structures is described, and initial results with this model are presented. Microwave scale models of the radiation structure were built, and a series of measurements was made with a network analyzer. These measurements were summed in a computer to find properties of the structure when all elements in it are operating in phase. The goal of these measurements was to develop structures for oscillators which are tunable over a very broad band. Computer programs simulating a Josephson junction in any frequency-dependent coupling structure have been developed. An iterative harmonic balance technique finds the time-domain current and voltage waveforms across the junction for an arbitrary frequency-dependent RF load impedance. DC bias, and therefore oscillator frequency, can be held fixed in this technique, in contrast to time-domain techniques where the oscillation frequency is found after the waveform is found. With these programs design goals for maximum oscillator power and design limits to achieve tolerable harmonic distortion were found.
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.>