Josephson emission with frequency span 1\u201311 THz from small Bi2Sr2CaCu2O8+\u03b4 mesa structures

E. A. Borodianskyi,V. M. Krasnov

doi:10.1038/s41467-017-01888-4

Abstract

Mesa structures made of Bi2Sr2CaCu2O8+δ high-temperature superconductor represent stacks of atomic scale intrinsic Josephson junctions. They can be used for generation of high-frequency electromagnetic waves. Here we analyze Josephson emission from small-but-high mesas (with a small area, but containing many stacked junctions). We have found strong evidence for tunable terahertz emission with a good efficacy in a record high-frequency span 1–11 THz, approaching the theoretical upper limit for this superconductor. Emission maxima correspond to in-phase cavity modes in the mesas, indicating coherent superradiant nature of the emission. We conclude that terahertz emission requires a threshold number of junctions N ~ 100. The threshold behavior is not present in the classical description of stacked Josephson junctions and suggests importance of laser-like cascade amplification of the photon number in the cavity.

Highlights

Mesa structures made of Bi2Sr2CaCu2O8+δ high-temperature superconductor represent stacks of atomic scale intrinsic Josephson junctions
It is much more difficult to built compact, continuouswave, narrow line-width THz lasers, which would be tunable in a broad frequency range
A limited tunability of such lasers can be partially obviated by multi-mode frequency comb operation, allowing a factor-two frequency span[7], or by post-processing[8]

Summary

Introduction

Mesa structures made of Bi2Sr2CaCu2O8+δ high-temperature superconductor represent stacks of atomic scale intrinsic Josephson junctions. It is much more difficult to built compact, continuouswave, narrow line-width THz lasers, which would be tunable in a broad frequency range Significant progress in this direction is achieved by semiconducting quantum cascade lasers[5,6]. It has long been anticipated that small Bi-2212 mesas may have many benefits as THz oscillators: (i) Edge effects[21,22] and capacitive coupling[23] persuade in-phase synchronization of junctions, needed for superradiance; (ii) Self-heating is reduced proportional to the mesa size[24], which allows operation at high voltages and frequencies; (iii) The frequency and the quality factor Q of the primary geometrical resonance (cavity mode) increase inversely proportional to the mesa size. This is puzzling because theoretical analysis univocally predicted significant emission from small mesas[23,26,27,31,32]

Methods

Results

Conclusion