Fundamental and harmonic submillimeter-wave emission from parallel Josephson junction arrays

Abstract

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.

Fluxon modes and phase-locking at 600 GHz in superconducting tunnel junction nonuniform arrays

“…57 It turns out that the SIS receiver bandwidth ͑450-650 GHz͒ included the Josephson frequencies of the first harmonic of the n = 1 mode ͑242 GHzϫ 2͒ and the fundamental of the n = 3 mode ͑493 GHzϫ 1͒, but no harmonic of the n = 2 step ͑406 GHz͒. As reported elsewhere, submillimeter-wave radiation could be detected when the arrays were biased on certain of their resonant steps, using a SIS receiver and a spectrum analyzer.…”

“…Impedance transformers will do the job at the cost of truncating the naturally huge ͑hundreds of GHz͒ tuning bandwidth of FFOs. 57 This result was expected since these two frequencies fall within the coupling range measured by FTS, while no harmonic of the n = 2 mode would lie in that range. 1͑a͔͒.…”

Inhomogeneous parallel arrays of Josephson junctions

“…These systems present many advantages, they can be made fairly simply, they are robust because the oxide layer is protected from the atmosphere, they can be easily integrated into a microwave transmission line and their description in terms of electric circuits is simpler than the series arrays. These devices generate microwave radiation as shown by Boussaha et al [5]. Note also that cuprate high Tc superconductors can be seen as a natural superposition of these parallel arrays with intrinsic Josephson junctions.…”

“…Their positions are given in Table 3 in lm. The values of the other physical parameters, the critical current density J c , the capacities per unit area and the magnetic thickness d 0 are Table 3 Positions of the junctions for the five junction parallel array studied in [5]. This leads to the following reduced parameters k J ¼ 5:55 Â 10 À6 m;…”

“…Finally to show the effectiveness of our modeling approach we compute the resonances in the I-V curve of a five junction parallel array made by Boussaha et al [5] and shown to generate microwave radiation. The junctions of equal area 1 lm  1 lm are placed in a microstrip of length l = 126 lm and width 5 lm.…”