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Journal of Applied Physics

600 GHz resonant mode in a parallel array of Josephson tunnel junctions connected by superconducting microstrip lines

V. K. Kaplunenko, Britt H. Larsen, J. Mygind, N. F. Pedersen

Published Sep 1, 1994• 12 citations
Publisher

Abstract

The high frequency properties of the one-dimensional transmission line consisting of a parallel array of resistively shunted Josephson tunnel junctions have been studied in the limit of relatively low damping where this nonlinear system exhibits new and interesting phenomena. Here we report on experimental and numerical investigations of a resonant step observed at a voltage corresponding to 600 GHz in the dc current-voltage characteristic of a parallel array of 20 identical small NbAl2O3Nb Josephson junctions interconnected by short sections of superconducting microstrip line. The junctions are mutually phase locked due to collective interaction with the line sections excited close to the half wavelength resonance. The phase locking range can be adjusted by means of an external dc magnetic field and the step size varies periodically with the magnetic field. The largest step corresponds to a superconducting phase difference of π between neighboring junctions.

DOI
10.1063/1.357500

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IEEE Transactions on Applied Superconductivity•Jun 1, 1995•2 citations
The high frequency properties of the one-dimensional transmission line consisting of a parallel array of resistively shunted Josephson tunnel junctions have been studied in the limit of relatively low damping where this nonlinear system exhibits new and interesting phenomena. Here we report on experimental and numerical investigations of a resonant step observed at a voltage corresponding to 600 GHz in the dc current-voltage characteristic of a parallel array of 20 identical small NbAl2O3Nb Josephson junctions interconnected by short sections of superconducting microstrip line. The junctions are mutually phase locked due to collective interaction with the line sections excited close to the half wavelength resonance. The phase locking range can be adjusted by means of an external dc magnetic field and the step size varies periodically with the magnetic field. The largest step corresponds to a superconducting phase difference of π between neighboring junctions.
The high frequency properties of the one-dimensional transmission line consisting of a parallel array of resistively shunted Josephson tunnel junctions have been studied in the limit of relatively low damping where this nonlinear system exhibits new and interesting phenomena. Here we report on experimental investigations of a resonant step observed at a voltage corresponding to 600 GHz in the dc current-voltage characteristic of a parallel array of 20 identical small NbAlO/sub x/Nb Josephson junctions interconnected by short sections of superconducting microstrip line. The junctions are mutually phase locked due to collective interaction with the line sections excited close to the half wavelength resonance. The phase locking range can be adjusted by means of an external dc magnetic field and the step size varies periodically with the magnetic field.>
We investigated parallel arrays of superconducting Nb/AlOx/Nb tunnel junctions nonevenly distributed in a superconducting Nb/SiO/Nb microstrip transmission line. Such devices are discretized Josephson transmission lines (DJTLs) in which, from theory, magnetic flux quanta (“fluxons”) can travel as solitonic waves when a dc current bias and a dc magnetic field are applied. We observed a reproducible series of resonant branches in each device’s I−V curve, at Josephson submillimeter-wave frequencies (from 240 to 720 GHz) matching the resonances predicted using a transmission line analysis, where the loading of the N=5 junctions is fully taken into account. The nonperiodic distribution was optimized to provide rf matching over a large bandwidth (450–650 GHz typically), implying that the plasma resonance of junctions is inductively tuned out over a similar band by the array. A confirmation of this comes from the observation, at frequencies higher than the untuned junctions plasma frequency, of several Josephson phenomena reported in this article: Fiske-like resonances, phase-locking of the n=3 resonance to an external 600 GHz microwave source, rf-induced zero crossing, and resonances at fractional harmonics of the rf signal. These experimental results are all compatible with a fluxon-based resonances interpretation, as in the extensively studied long Josephson junctions yet at higher frequencies. As reported elsewhere, we could detect ∼500 GHz microwave radiation emitted by our devices in the n=1 and n=3 modes. In light of these unique properties, we propose nonuniform DJTLs as a promising type of Josephson device for submillimeter-wave oscillators and fast fluxon-based electronics.
High-Tc Josephson junction arrays with coupling by superconducting short- circuited (5 µm wide and 0.1 µm thick) coupled microstrip lines, provided dc bias in parallel, were fabricated on bicrystal sapphire substrates for experimental investigation of the phase- locking phenomenon. Josephson self radiation, emitted by the array was measured by an external 90 GHz receiver for different applied dc magnetic fields. The numerical simulation of array shows that strong interaction between Josephson junctions and the standing waves within structure require small enough damping α≤ 10 -3 per coupling section for reduction of Josephson oscillation linewidth.
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.
The authors report on measurements performed on integrated superconducting devices based on arrays of long Josephson tunnel junctions operating in the resonant fluxon oscillation regime (i.e. biased on the zero field steps). The electromagnetic coupling among the junctions causes a mutual phase-locking of the fluxon oscillations with a corresponding increase of the emitted power and a decrease of the signal linewidth. This phase-locked state can be controlled by means of an external DC bias current and magnetic field. The effect of the generated microwave signal has been observed on a small Josephson tunnel junction coupled to the array via a microstrip transmission line. The feasibility of the reported devices as local oscillators in an integrated microwave Josephson receiver is discussed.>
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.>