Session X5: Superconducting Sources of THz-Radiation

THz generation using Fluxon dynamics in high temperature superconductor Josephson junctions

Ever since the Josephson junction was discovered, the possibility of generating high frequency radiation has been considered. Such radiation could have many applications, for example as a local oscillator in an integrated receiver, a spectrometer, an imaging device etc. A brief historical overview of methods and results will be presented. Special emphasis will be given to the BSCCO--type high temperature superconductors that may be considered as a stack of coupled Josephson junctions - potentially having applications as terahertz oscillators. The stack of inductively coupled long Josephson junctions is modeled as a system of coupled sine-Gordon equations. The key point for oscillator applications is to have in-phase coherent motion of fluxons in the different Josephson junctions in the stack. This may be obtained by applying a (large) magnetic field parallel to the layers, or by having the system interacting with a cavity. In both cases the fluxon dynamics is very non-linear, and numerical simulations are necessary. We will also present a few cases where analytical results have provided a new insight in this complicated non-linear system.   

Emission of Coherent THz-Radiation from Superconductors.

Josephson junctions naturally convert dc-voltages into high-frequency electromagnetic radiation, with 1 mV corresponding to 0.483 THz, and many such junctions emitting in phase at the same frequency can produce useful emission power. Stacks of junctions with unsurpassed packing density occur naturally in the layered high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$, in which the superconducting CuO$_{2}$-layers are coupled through the intrinsic Josephson effect. However, achieving synchronization of the high-frequency oscillations of all the junctions in the stack has so far been a major challenge. We demonstrate that coherent continuous-wave THz-radiation of sizable power can be extracted from intrinsic Josephson junctions in the layered high-temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$. In analogy to a laser cavity, the excitation of an electromagnetic cavity resonance inside the sample generates a macroscopic coherent state in which a large number of junctions are synchronized to oscillate in phase. The emission power is found to increase as the square of the number of junctions reaching values of 0.5 $\mu $W at frequencies up to 0.85 THz. The available power is potentially much larger, as there is evidence that 20 $\mu $W of power are pumped into the observed THz cavity resonance. The emission persists up to temperatures of $\sim $50 K. Emission does not require the application of a magnetic field, significantly simplifying the design of superconducting THz-sources. In fact, a single applied D.C. current leads to the efficient excitation of continuous coherent THz-radiation. This work was supported by the U.S. Department of Energy under Contract No. DE-AC02-06CH11357 -- Basic Energy Sciences, the Japanese Ministry of Education, Culture, Sports, Science and Technology, and the Turkish TUBITAK under Project No. 106T053. In collaboration with L. Ozyuzer, A. E. Koshelev, C. Kurter, N. Gopalsami, Q. Li, M. Tachiki, K. Kadowaki, T. Yamamoto, H. Minami, H. Yamaguchi, T. Tachiki, K. E. Gray, W.-K. Kwok.   

Radiation Due to Josephson Oscillations in Layered Superconductors.

The power of direct tunable radiation into free space induced by Josephson oscillations in intrinsic Josephson junctions of highly anisotropic layered superconductors is derived. The super-radiation regime in a current biased crystal is considered when no dc magnetic field is applied. It is assumed that crystal is cut in the form of thin plate parallel to the ac-plane with the thickness of several microns along the b-axis and length of several hundreds microns along the a-axis. At large number of intrinsic junctions oscillations in such BSCCO crystal are synchronized providing high radiation power proportional to squared number of junctions and high efficiency up to 1/3 in the THz frequency range. The radiation correction to the current-voltage characteristic in this regime depends only on crystal shape. When the ac-edge of such a crystal is irradiated by external electromagnetic wave, radiation from both ac-edges of the crystal is enhanced (stimulated radiation) at higher-current part of the Shapiro step. The main part of extra radiation is in the direction opposite to incident wave. This effect of stimulated radiation may be used for amplification of electromagnetic waves. BSCCO crystal with modulated critical current and length several tens microns along the b-axis also provides high THz-radiation power from the ac-edges at discrete Josephson frequencies corresponding to the crystal thickness along the b- axis. Enhancement of radiation power in this case is due to Fiske resonances and super-radiation regime. The powerfuI almost standing electromagnetic wave is excited inside the crystal in the resonance. This wave is homogeneous across the layers meaning that the oscillations are synchronized in all junctions in the stack. In this situation the synchronization of radiation in different junctions is enforced by both super-radiation regime and similar critical current profile in all junctions of the crystal.   

THz emission from a slice of high-\textit{Tc} superconducting single crystal

Copper oxide superconductors possess intrinsically a layered crystalline structure, in which superconducting and non-superconducting layers interleave each other. Therefore the crystal itself consists of a number of superconducting junctions sequentially stacked along the $c$ axis of the crystal, and these junctions are often referred to as intrinsic Josephson junctions (IJJs). In the case of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ (BSCCO), each IJJ measures approximately 1.5 nm thick. Many groups have been exploring the possibilities to develop terahertz (THz) detectors and oscillators based on IJJs, due to the high collective plasma frequencies (up to THz region), the uniformity in junction properties, the easiness to make a large junction array, and the low loss at high frequencies. Some years ago, in IJJs singled out from inside a slice of BSCCO single crystal with a double-sided process, THz response was successfully observed as sharp Shapiro steps at frequencies up to 2.5 THz, and harmonic mixings were carried out with harmonic numbers as large as 90. Recently observed have been THz oscillations in various structures of BSCCO IJJs, which can be excited by dc bias, in-plane magnetic fields, or microwave irradiations at several gigahertz. Needless to say, for practical applications, it is necessary to synchronize the emissions from IJJs, couple the THz oscillations into a finite space, guide them in a controllable way, monitor the frequencies and power levels, and preferably do the jobs using an integrated system. We have been making extensive efforts to explore these ideas, and will report our latest results at the meeting.   

Mechanism of the terahertz wave emission from intrinsic Josephson junctions of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$

We propose a mechanism for the strong emission of terahertz wave recently observed in current-biased mesa-shaped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ single crystals at Argonne National Laboratory. When the mesa width is approximately equal to the $c$-axis penetration depth and an external current is applied along the $c$-axis of the crystal, the Josephson current oscillation can cause the resonance excitation of cavity-mode of the transverse Josephson plasma in the voltage state. When the oscillating electric and magnetic fields of the excited plasma wave have antinodes at the surfaces of mesa, the electromagnetic wave with ac Josephson frequency is strongly emitted in the vacuum outside the mesa as terahertz laser wave. For the coherent continuous-wave terahertz-radiation of sizable power, the nonlinear and nonequilibrium superconducting properties of the current biased intrinsic Josephson junction system play an important role.