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Applied Physics Letters

Coherent emission from large arrays of discrete Josephson junctions

Fangfang Song, F. Müller, Ralf Behr, Alexander M. Klushin

Published Oct 26, 2009• 40 citations
Publisher

Abstract

We observed coherent emission at the frequency of 76 GHz from a large-area array of 7500 niobium junctions embedded in the quasioptical resonator at liquid helium temperature. The maximal detected power of 2 μW was measured by a room temperature superheterodyne receiver. To explain the coherent behavior of the array of Josephson junctions, electromagnetic field simulations were performed. Considering the substrate as a dielectric resonator antenna, the coherent emission was excited if the locations of each subarray coincided with the positions of the maximum electric field intensity in the substrate resonance mode.

DOI
10.1063/1.3253417

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We observed coherent emission at the frequency of 76 GHz from a large-area array of 7500 niobium junctions embedded in the quasioptical resonator at liquid helium temperature. The maximal detected power of 2 μW was measured by a room temperature superheterodyne receiver. To explain the coherent behavior of the array of Josephson junctions, electromagnetic field simulations were performed. Considering the substrate as a dielectric resonator antenna, the coherent emission was excited if the locations of each subarray coincided with the positions of the maximum electric field intensity in the substrate resonance mode.
The experimental results of mm-wave coherent emission from a high temperature superconducting bicrystal Josephson junction array embedded in a Fabry-Perot resonator are reported in this paper.The mm-wave coherent emission from the series junction array results from the interaction of the junctions with the substrate serving as the dielectric resonator as well as the Fabry-Perot resonator.The central frequency of emission of a Josephson junction array containing 166 bicrystal junctions was 75.84 GHz,and the detected emission power was about 10 pW at 77 K.
Josephson junction arrays could be competitive candidates for radiation sources in the frequency range from 0.5 to 1.5 THz. This is the range where compact solid-state continuous wave sources are most lacking. We investigated the millimeter-wave radiation from arrays of high-temperature superconductor bicrystal Josephson junctions embedded in a quasi-optical resonator. The novel approach of utilizing the intrinsic resonance modes of the substrate to improve the impedance coupling between the series array of discrete Josephson junctions and the quasi-optical resonator was investigated by experiments and electromagnetic field simulations. Radiation from the arrays containing up to 536 junctions was detected at about 80 GHz and liquid nitrogen temperature using a low-noise receiver. The detected maximum radiation power of the arrays was about 50 pW and the peak voltage agreed exactly with the Josephson current–voltage relation.
We performed the spectral measurements of a niobium based Josephson junction array in order to estimate the linewidth of Josephson emission. The array is formed by 9996 serially connected Nb/NbSi/Nb Josephson junctions occupying the area of 5×7 mm2on a silicon substrate and divided into 7 distinct subarrays. The array was immersed in a liquid helium dewar from where the emission was brought to open space via an oversize waveguide. Firstly, the measurements on a Fourier-transform spectrometer were carried out in a wide frequency range 139–343 GHz at almost all self-induced steps of current-voltage curve. As in our previous work, the observed linewidth of Josephson emission corresponded to the resolution of the spectrometer. Then, we carried out more precise and sensitive measurements using the 211–275 GHz heterodyne receiver based on a Nb/AlOx/Nb mixer with a spectral resolution better than 0.1 MHz. Dependencies of the linewidth on the step number and on the number of connected subarrays are studied. The peaks corresponding to the 2ndharmonic of the Josephson generation are also observed in the spectra. The linewidth of the main harmonic down to 1.5 MHz was observed in these measurements.
We present experimental results on the emission from phase-locked twodimensional arrays of Josephson junctions. We have coupled the emission from 10 x 10 arrays to a room-temperature mixer throu h a tin-line antenna and WR-12 waveguide. A single voltagetunable peak was detected up to 230 GHz. A stripline resonance in the antenna reduced the array's dynamic resistance and thereby the emission linewidth to as low as 10 kHz. We extract an effective noise temperature of 14 K from the linewidth data. When the array's emission was coupled to an on-chip detector junction through a dc blocking capacitor, we detected voltagetunable emission from 75 GHz up to 300 GHz, and in some circuits emission above 400 GHz. The coherent power spectrum depends primarily on internal resonances.
We report heterodyne measurements of Josephson microwave radiation emitted by a parallel array of small superconductor-insulator-superconductor (SIS) junctions at submillimeter-wave frequencies. The array consists of five Nb/Al–AlOx/Nb junctions nonevenly distributed in a niobium superconducting stripline, and is optimized for rf coupling in the 450–640 GHz range. We observed Fiske-like resonant steps in its I-V curve in the presence of magnetic field. The device was placed in a waveguide mount, and its radiation was quasioptically coupled out of the cryostat, to a SIS-mixer spectrometer in the same frequency range, with a 4–8 GHz band for spectral analysis. We detected a coherent signal in the spectra when the array was biased on the first and third steps, respectively, at the first harmonic frequency of 242 GHz and at the fundamental frequency of 493 GHz, both being the Josephson frequencies associated with their dc voltages. This strongly suggests that this type of parallel arrays optimized for wideband rf coupling, though strongly discretized, hosts dynamic fluxon regimes similar to long Josephson junctions, which could find applications in superconducting digital electronics or integrated heterodyne submillimeter-wave receivers.
This paper describes an array of superconducting planar Josephson junctions (JJs) for terahertz emission. The JJs with bowtie antennae are fabricated with superconducting thin film deposited on the bicrystal dielectric substrate and connected in series by superconducting striplines. The dielectric substrate is regarded as a dielectric resonator to improve the electromagnetic coupling between the JJs for coherent terahertz emission. With the CST electromagnetic software, the S parameters, E-field distribution and farfield pattern of the JJ array are studied by adjusting the width of the bowtie antennae. The results show that standing waves are formed in the dielectric substrate around 420 GHz, which could contribute to the coherent terahertz emission of the superconducting oscillators.