Session III: Terahertz Emission

III-1: Josephson Vortex Dynamics and Josephson Plasma Excitation

Masashi Tachiki
Graduate School of Frontier Science, The University of Tokyo

The crystal of the high temperature superconductor Ba₂Sr₂CaCu₂O_8+y is composed of stacks of Josephson junction layers. In the crystal an excitation called Josephson plasma appears and its frequency is in the range of terahertz. Since the excitation appears inside the main superconducting energy gap, the damping rate is very weak. Therefore, when the plasma is excited in some way, the excitation spontaneously decays by emitting a terahertz electromagnetic wave. When an external magnetic field is applied along the b-axis parallel to the CuO₂ plane in the crystal, the Josephson vortices are induced in this direction, and when an external current is applied to the c-axis perpendicular to the CuO₂ plane, the vortices are driven in the a-axis direction of the crystal. The motion of the vortices excites the plasma wave. At that time the vortex motion itself is received a strong reaction from the excited Josephson plasma. For describing the behavior, we set up an equation for the gauge invariant phase of the superconducting order parameter, inserting the superconducting and normal currents in the CuO₂ plane, the Josephson and normal currents along the c-axis to the Maxwell equation. The resultant equations are multi-coupled and non-linear equations. For solving the equations, we used a large computer, called Earth Simulator. The obtained solution is as follows. The excited Josephson plasma wave is a transverse standing plasma wave, since the waves are reflected at both the end surfaces of the crystal. The vortices moving in the direction of the a axis have some kinds of coherent configurations along the c-axis, superposed with the vibration of terahertz frequencies along the a axis. The detailed results will be reported at the Workshop.

III-2: Radiation Due to Josephson Oscillations in Layered Superconductors

L. N. Bulaevskii¹⁾ and A. E. Koshelev²⁾
1) Los Alamos National Laboratory, USA
2) Argonne National Laboratory, USA

We derive the power of direct radiation into free space induced by Josephson oscillations in the resistive state in intrinsic Josephson junctions of highly anisotropic layered superconductors in the absence of the magnetic field. We consider the super-radiation regime in a crystal in the form of thin plate parallel to the c-axis. We show that at large enough number of junctions oscillations are synchronized providing high radiation power and efficiency in the THz frequency range. We discuss crystal parameters and bias current optimal for radiation power and crystal cooling for BSCCO crystals.

III-3: Josephson Vortex Motion and THz Emission in Nano-Stacked High-T_c Intrinsic Junctions

M.-H. Bae, J.-H. Choi, and H.-J. Lee
Pohang University of Science and Technology, Pohang, Korea

In the first part of the presentation, we briefly report the experimental confirmation of the collective transverse plasma (CTP) modes excited by the Josephson vortex lattice in stacks of intrinsic Josephson junctions in Bi₂Sr₂CaCu₂O_8+x (Bi-2212) single crystals. In the dynamic vortex state in a sufficiently high magnetic field and for a high bias current, splitting of a single Josephson vortex-flow branch into multiple sub-branches was observed. Detailed examination of the sub-branches for varying H field reveals that sub-branches represent the different modes of the Josephson-vortex lattice along the c axis, with varied configuration from a triangular to a rectangular lattice. We then report the observation of the successful terahertz emission (0.6~1 THz) induced by the Josephson vortex lattice (JVL) motion in a stack of Bi-2212 intrinsic Josephson junctions (IJJs). The emission was continuous and tunable in its frequency and power. For a proper bias to a CTP mode, the emission of the electromagnetic waves by the collective vortex resonance motion in a stack of IJJs (the oscillator stack) was examined using another proximity-arranged stack of IJJs (the detector stack). The THz emission was confirmed by the Shapiro-step response of the detector stack. The emission power estimated from the Shapiro-step response increases rapidly as the bias condition moves toward the in-phase square vortex lattice configuration, which indicates the importance of tuning the stack to the in-phase vortex lattice biasing condition. Our results provide a strong feasibility of developing long-sought solid-state terahertz-wave emission devices.

III-4: Computer Simulation on Terahertz Emission from Intrinsic Josephson Junctions of High-T_c Superconductor

Shi-Zeng Lin and Xiao Hu
National Institute for Materials Science, Tsukuba, Japan

By solving coupled nonlinear Sine-Gordon equations and Maxwell equations numerically [1,2], we have studied the electromagnetic and superconducting properties of the intrinsic Josephson junctions with an external magnetic field applied parallel to the ab-plane and a dc current fed in along the c-axis. We have observed resonance phenomena in the junctions with the frequency in Tera Hertz regime. Using physical boundary conditions, it is revealed that the electromagnetic wave can transmit from the junctions into space. The emitted energy counted by the Poynting vector is about 400W/cm². The frequency as well as the energy of emission can be tuned almost continuously by the current and the magnetic field. The relation among the Tera Hertz radiation and motions of interlayer Josephson vortices will be discussed. Comparison between the experimental observations [3] and our simulation results will also be presented.

[1] M. Tachiki et al. Phys. Rev. B 50, 12831 (2005)
[2] L. N. Bulaevskii and A. Koshelev, J. Supercond. Nov. Magn. 19, 349 (2006)
[3] M. H. Bae et al. Phys. Rev. Lett. 98, 027002 (2007)