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A Tunable 350-780 GHz CW Solid State Oscillator of Intrinsic Josephson Junctions in a high-Tc Superconductor
H. B. Wang#,*, M. Y. Li#,*, J. Yuan#, N. Kinevy, J. Li#, B. Grossz, S. Guenonz, A. Ishii#, T. Hatano#, D. Koellez, R. Kleinerz V. P. Kosheletsy, P. H. Wu* # National Institute for Materials Science, Tsukuba 3050047, Japan * Research Institute of Superconductor Electronics, Nanjing University, Nanjing 210093, China y Kotel'nikov Institute of Radio Engineering and Electronics, Moscow 125009, Russia z Physikalisches Institut and Center for Collective Quantum Phenomena in LISA+, Universitaet Tuebingen, D-72076 Tuebingen, Germany Email: hbwang1000@gmail.com
Abstract--We report on THz emission measurements and low temperature scanning laser imaging of Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks. Coherent emission is observed at large dc input power, where a hot spot and a standing wave, formed in the "cold" part of the stack, coexist. By changing bias current and bath temperature the emission frequency can be varied over a wide range of 350-780 GHz; the variation matches the well-known Josephson-frequency relation. The linewidth of radiation, recently measured with an integrated superconductor-insulatorsuperconductor (SIS) receiver, is as narrow as 6 MHz, much smaller than expected from a purely cavity-induced synchronization. Thus, an additional mechanism seems to play a role. Some scenarios, related to the presence of the hot spot, are discussed.

radiation, can be obtained at high input power where, in addition, a hot spot (i.e., a region heated to above the critical temperature Tc) forms within the mesa structure [3, 4]. The radiation has been further measured with a superconducting integrated receiver. The linewidth at hot-spot regime is at least two orders narrower than that with absence of the hot-spot. To conclude, a novel terahertz oscillator has been successfully fabricated from IJJs of high-Tc superconductors, with a wide tunable range of 350-780 GHz, a narrow linewidth down to 6 MHz at free-running state, and a wide operation temperature up to 60 K. In the near future, a phase-locked oscillator may find itself many applications in bridging the so-called THz gap, e.g., as a tunable local oscillator of a superconducting THz receiver.
Low T emperature Laser Scanning Microscope (LT LSM image) hot-spot area induced by self-heating T Hz standing waves formed Inside sample edge of hot-spot area

I. INTRODUCTION AND BACKGROUND
Power (a.u.)
Frequency (GHz)
700

P

HASE synchronization is one of the prerequisites to use Josephson junction arrays as tunable high frequency sources. While Nb based junctions are limited to frequencies below 1 THz--with applications up to 600 GHz--intrinsic Josephson junctions (IJJs) in Bi2Sr2CaCu2O8 are, at least in principle, able to operate up to several THz [1]. Stacks of many junctions can be made, e.g., by patterning mesa structures on top of single crystals. For many years, investigations focused on small structures consisting of some 10 IJJs, with lateral sizes of a few m. Here, with few exceptions, the IJJs in the stack tended to oscillate out-of-phase or were not synchronized at all. Recently, coherent off-chip THz radiation with an extrapolated output power of some W was observed from stacks of more than 600 IJJs, with lateral dimensions in the 100 m range. Phase synchronization involved a cavity resonance oscillating along the short side of the mesa [2]. This radiation was studied theoretically in a series of recent papers, either based on vortex-type or plasmonic excitations, coupled to cavity modes, or on nonequilibrium effects caused by quasiparticle injection. II. RESULTS While THz emission was currents and moderate dc temperature scanning laser shown that standing wave obtained at relatively low power input [2], using microscopy (LTSLM) we patterns, associated with bias low have THz

(b)

6 0.6662 THz (c) 4 15 K 2 0 0.5 1.0 Frequency (THz)
typical spectrum

(d)
600 500 400 0 10 20 30 40 50 60 Temperature (K)

(a)

emitting T Hz w aves to outside

emission frequency vs. temperature

Fig. 1. (a) Schematic view of a stack of intrinsic Josephson junctions standing on its pedestal of the same Bi2Sr2CaCu2O8 superconductor, (b) standing waves and a hot-spot observed with a low temperature laser scanning microscope, (c) typical spectrum measured with an interferometer with a resolution of 12 GHz, and (d) a wide tunable frequency ranged can be obtained by var ying temperature or bias current. The linewidth measured with an integrated superconducting receiver is not shown here.

REFERENCES
[ 1] [ 2] [ 3] [ 4] R. Kleiner et al., "Intrinsic Josephson effects in Bi2Sr2CaCu2O8 single crystals", Phys. Rev. Lett. 68, 2394, 1992. L. Ozyuzer et al., "Emission of coherent THz radiation from superconductors", Science 318, 1291, 2007. H. B. Wang et al., "Hot Spots and Waves in Bi2Sr2CaCu2O8 Intrinsic Josephson Junction Stacks: A Study by Low Temperature Scanning Laser Microscopy", Phys. Rev. Lett. 102, 017006, 2009. H. B. Wang et al., "Coherent Terahertz Emission of Intrinsic Josephson Junction Stacks in the Hot Spot Regime", Phys. Rev. Lett. 105, 057002, 2010.