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Physical Review Letters

Stimulated Emission and Amplification in Josephson Junction Arrays

Paola Barbara, A. B. Cawthorne, S. V. Shitov, Christopher J. Lobb

Published Mar 1, 1999• 247 citations
Publisher

Abstract

We report measurements of mm-wavelength radiation from two-dimensional arrays of underdamped Josephson junctions. All of our samples emit coherently in a novel synchronized state, which is triggered by a resonance in the array structure. Measurements of the detected power as a function of the number $N$ of active junctions show a threshold, suggesting population inversion. Above threshold, the power scales with ${N}^{2}$ up to an array size bigger than the free-space radiation wavelength. The highest measured conversion efficiency from dc to ac power is about 17%. Our data are consistent with stimulated emission causing coherence and cannot be explained by existing classical coupling mechanisms.

DOI
10.1103/physrevlett.82.1963

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Applied Physics Letters•May 13, 1991•182 citations
We report measurements of mm-wavelength radiation from two-dimensional arrays of underdamped Josephson junctions. All of our samples emit coherently in a novel synchronized state, which is triggered by a resonance in the array structure. Measurements of the detected power as a function of the number NNN of active junctions show a threshold, suggesting population inversion. Above threshold, the power scales with N2{N}^{2}N2 up to an array size bigger than the free-space radiation wavelength. The highest measured conversion efficiency from dc to ac power is about 17%. Our data are consistent with stimulated emission causing coherence and cannot be explained by existing classical coupling mechanisms.
Recent experiments have indicated that Josephson-junction arrays can radiate coherently in a laser-like fashion, as predicted by theoretical work in the 1970s. We present results from measurements of high-efficiency Josephson-junction arrays coupled to resonant cavities. In one of our samples with four columns and 36 rows, the dc to ac (180 GHz) conversion efficiency reaches an asymptotic value of about 32%. Using a simple circuit model we show that we have achieved optimal dc to ac conversion in this sample.
The authors have detected coherent emission from two-dimensional (2D) arrays of superconductor-insulator-superconductor (SIS) Josephson junctions. 2D arrays emit coherent radiation over a frequency range of 60 to 210 GHz, when coupled to detector junctions through DC-blocking capacitors. The detector junctions exhibit Shapiro steps at frequencies corresponding to the voltage across single-array junctions. The maximum power from a 10-by-10 junction-array coupled to a detector junction occurs at 150 GHz and is estimated to be 0.4 mu W, based on simulations of the detector circuit. By varying the number of array junctions, the array geometry, the junction critical current, and the coupling circuit, they have begun determining the essential conditions for observing coherent emission.
We have investigated experimentally a novel type of self-synchronizing two-dimensional Josephson junction array based on special symmetry breaking network architectures. The measurements confirm the theoretical prediction that in such "selector Josephson networks" the Josephson oscillators mutually synchronize into a coherent in-phase state and that such arrays are capable of emitting maximum coherent microwave power in the entire theoretically expected frequency range. The DC biased oscillator array operates coherently even without a reinjection of the generated microwave by an external load or by a resonator cavity. The selector network possesses a much higher tolerance against imperfections, perturbations and load parameter variations than conventional regular two-dimensional or one-dimensional arrays. The experimental sample has been fabricated in an industrial process using Nb-technology, in effect providing relatively large spreads in the array junction parameters. The measured frequency range goes from 85 GHz up to 380 GHz with a maximum microwave power of 0.16 /spl mu/W matched to a coupled load for an array with only 100 active Nb-AlO/sub x/-Nb junctions.
When a parallel external magnetic field is applied to underdamped Josephson-junction arrays, constant-voltage steps appear in their current-voltage characteristics. These steps correspond to different numbers of rows being switched to a new resonant state. If the number of switched rows is larger then a threshold number, the array radiates coherent microwave radiation. When the array is biased on a step, the number of radiating rows stays fixed and we can change the input power, P/sub DC/, by changing the bias current. We measure the output power, P/sub AC/ as a function of P/sub DC/. This dependence is linear at high powers with a slope /spl alpha/, while at low powers P/sub AC/ vanishes nonlinearly with P/sub DC/. For a given array, the slope /spl alpha/ is larger for steps that correspond to a larger number of switched rows. We present a systematic study of the dependence of the slope or on the size of the array and discuss its implications for obtaining optimal DC-to-AC conversion efficiency.
Previous experiments [Barbara et al., Phys. Rev. Lett. 82, 1963 (1999); Vasili\ifmmode \acute{c}\else \'{c}\fi{} et al., Appl. Phys. Lett. 78, 1137 (2001)] showed that two-dimensional, unshunted Josephson-junction arrays emit radiation coherently when a threshold number of junctions is switched to a radiating state. Here, we report low-power radiation being emitted when the number of radiating junctions is smaller than the threshold number. We show that in this regime the output power is incoherent, i.e., the array behavior is qualitatively different from the coherent regime.
Coherent emission has been generated by two-dimensional arrays of SIS Josephson junctions and detected in a junction coupled to the array through a dc-blocking capacitor. The detector junction exhibits Shapiro steps at frequencies corresponding to the voltage across single array junctions and ranging from 60 to 210 GHz. The maximum power coupled to the detector junction occurs at 150 GHz and is estimated to be 0.9 μW, based on simulations of the detector circuit. Possible mechanisms for coherent emission from two-dimensional arrays are discussed.