Yukihiro Soga

Tunable terahertz band planar Bragg reflectors

A tunable planar narrow-band Bragg reflector based on coupling of the two propagating modes and a cutoff mode is considered. Coupled-wave analysis together with direct numerical simulations demonstrate operation of the proposed scheme up to the terahertz frequency band. Compatibility with the transportation of an intense electron beam encourages the use of a novel Bragg reflector in powerful long-pulse free electron lasers.

Supertransmission channel for an intrinsic localized mode in a one-dimensional nonlinear physical lattice

It is well known that a moving intrinsic localized mode (ILM) in a nonlinear physical lattice looses energy because of the resonance between it and the underlying small amplitude plane wave spectrum. By exploring the Fourier transform (FT) properties of the nonlinear force of a running ILM in a driven and damped 1D nonlinear lattice, as described by a 2D wavenumber and frequency map, we quantify the magnitude of the resonance where the small amplitude normal mode dispersion curve and the FT amplitude components of the ILM intersect. We show that for a traveling ILM characterized by a specific frequency and wavenumber, either inside or outside the plane wave spectrum, and for situations where both onsite and intersite nonlinearity occur, either of the hard or soft type, the strength of this resonance depends on the specific mix of the two nonlinearities. Examples are presented demonstrating that by engineering this mix the resonance can be greatly reduced. The end result is a supertransmission channel for either a driven or undriven ILM in a nonintegrable, nonlinear yet physical lattice.

Intrinsic localized modes in a nonlinear electrical lattice with saturable nonlinearity

This experimental study of driven intrinsic localized modes (ILMs) in an electronic circuit lattice with saturable nonlinearity follows the theoretical work of Hadžievski and coworkers. They proposed that a saturable nonlinearity could introduce transition points where localized excitations in nonintegrable lattices would move freely. In our experiments MOS capacitors provide the saturable nonlinearity in an electric lattice. Because of the soft nonlinearity driver locked, auto-resonance stationary ILMs are observed below the bottom of a linear frequency band of the lattice. With decreasing driver frequency the width of the ILM changes in a stepwise manner as does the softening of the barrier between site-centered and bond-centered ILM locations in agreement with theoretical expectations. However, the steps show hysteresis between up and down frequency scans and such hysteresis inhibits the free motion of ILMs.

Dynamics of impurity attraction and repulsion of an intrinsic localized mode in a driven 1-D cantilever array

Both low frequency and high frequency impurity modes have been produced in a SiN micromechanical cantilever array by illumination with either an infrared or visible laser. When such laser-induced impurities are placed near a driven intrinsic localized mode (ILM), it is either repelled or attracted. By measuring the linear response spectrum for these two cases, it was found that vibrational hopping of the ILM takes place when the natural frequency of the ILM and an intrinsic even symmetry linear local mode are symmetrically located about the driven ILM frequency so that parametric excitation of these two linear modes is enhanced, amplifying the lateral motion of the ILM. Numerical simulations are consistent with these signature findings. It is also demonstrated that the correct sign of the observed interaction can be found with a harmonic lattice-impurity model but the magnitude of the effect is enhanced in a nonlinear lattice.

Design and development of 90 GHz backward-wave oscillator with staggered double-vane slow-wave structure

We designed a 90 GHz band backward-wave oscillator (BWO) with a double-vane slow-wave structure (SWS). A double periodic alignment of metal vanes serves as an SWS wherein electromagnetic waves propagate along a staggered path. We determined the scale parameters of the SWS by dispersion curves and calculated interaction impedance using CST Studio Suite simulation software. Using particle-in-cell (PIC) simulation, we observed the expected radiation with a frequency of 90 GHz, stimulated by a 10 keV electron beam with a 200 mA current. The output power was estimated to be level of Watt. The good agreements between the predictions, based on the interaction impedance and PIC calculation results, offer a key to a timesaving estimation of radiation from a BWO without using PIC simulation.

Experimentally observed evolution between dynamic patterns and intrinsic localized modes in a driven nonlinear electrical cyclic lattice

Locked intrinsic localized modes (ILMs) and large amplitude lattice spatial modes (LSMs) have been experimentally measured for a driven 1-D nonlinear cyclic electric transmission line, where the nonlinear element is a saturable capacitor. Depending on the number of cells and electrical lattice damping a LSM of fixed shape can be tuned across the modal spectrum. Interestingly, by tuning the driver frequency away from this spectrum an LSM can be continuously converted into ILMs and visa versa. The differences in pattern formation between simulations and experimental findings are due to a low concentration of impurities. Through this novel nonlinear excitation and switching channel in cyclic lattices either energy balanced or unbalanced LSMs and ILMs may occur. Because of the general nature of these dynamical results for nonintegrable lattices applications are to be expected. The ultimate stability of driven aero machinery containing nonlinear periodic structures may be one example.

Dynamics of two discrete vortices with different circulations composed of pure electron plasmas

Two-dimensional dynamics of two vortex strings with different circulations were experimentally observed. The strings were composed of pure electron plasmas and were confined in a Malmberg–Penning trap. When one of the two vortices trapped in a conventional potential well had weak circulation, the orbits of the two vortices contradicted the point vortex theory. This disagreement stems from an unavoidable external electric field that exerts a different additional E × B drift velocity on each vortex. We resolved the discrepancy between observation and theory by applying a stepwise potential to the end electrodes, which produced a less non-ideal electric field.

Terahertz band free electron lasers with advanced Bragg reflectors

A new scheme of terahertz band FEL with hybrid Bragg resonator is proposed consisting of an advanced input Bragg mirror and a traditional output Bragg mirror. Advanced Bragg mirror exploiting the coupling between the two counter propagating modes and the cutoff one provides mode selection over the transverse index.