Yukihiro Tanaka

Ultrafast Vibrations of Gold Nanorings

We investigate the vibrational modes of gold nanorings on a silica substrate with an ultrafast optical technique. By comparison with numerical simulations, we identify several resonances in the gigahertz range associated with axially symmetric deformations of the nanoring and substrate. We elucidate the corresponding mode shapes and find that the substrate plays an important role in determining the mode damping. This study demonstrates the need for a plasmonic nano-optics approach to understand the optical excitation and detection mechanisms for the vibrations of plasmonic nanostructures.

Real-time imaging of acoustic rectification

We image gigahertz surface acoustic waves normally incident on a microscopic linear array of triangular holes—a generic “acoustic diode” geometry—with a real-time ultrafast optical technique. Spatiotemporal Fourier transforms reveal wave diffraction orders in k-space. Squared amplitude reflection and transmission coefficients for incidence on both sides of the array are evaluated and compared with numerical simulations. We thereby directly demonstrate acoustic rectification with an asymmetric structure.

Lattice thermal conductivity in superlattices: molecular dynamics calculations with a heat reservoir method

We report on a molecular dynamics study of the cross-plane lattice thermal conductivity in GaAs/AlAs superlattices. The layers of the superlattice are modelled by a three-dimensional face centred cubic lattice with cubic anharmonicity, and with atomic scale roughness at the interfaces. We perform the simulation of heat flow for a section of a superlattice with high- and low-temperature thermal reservoirs attached to opposite ends. The calculation reproduces qualitatively the features observed experimentally, i.e., the dramatic reduction of the conductivity relative to the conductivity of the bulk constituent materials, and the variation of the thermal conductivity with the superlattice repeat distance. The results are also in agreement with those obtained previously by Daly et al (2002 Phys. Rev. B 66 024301) who determined the thermal conductivity from the time taken for an initially inhomogeneous temperature distribution to relax.

Damping and decoherence of a nanomechanical resonator due to a few two-level systems

Received 9 July 2009; revised manuscript received 4 September 2009; published 4 November 2009;publisher error corrected 11 November 2009We consider a quantum model of a nanomechanical flexing beam resonator interacting with a bath compris-ing a few damped tunneling two-level systems. In contrast with a resonator interacting bilinearly with an ohmicfree oscillator bath modeling clamping loss, for example , the mechanical resonator damping is amplitudedependent, while the decoherence of quantum superpositions of mechanical position states depends onlyweakly on their spatial separation.DOI: 10.1103/PhysRevB.80.174103 PACS number s : 85.85. j, 03.65.Yz

Rectification of elastic waves in a thin plate

We propose a rectifier of elastic waves in a thin plate, which is made of an elastically isotropic material containing a periodic array of triangular holes as scatterers, and demonstrate numerically that it works both for the symmetric and anti-symmetric Lamb waves as well as shear horizontal waves. The rectification is caused by the geometric effects on wave scattering due to the asymmetric scatterers, while the interplay between the mode conversion and interference effects among the scattered waves owing to the periodic arrangement of scatterers complicates it. The mechanism makes it possible to rectify the typical elastic waves in the system above the threshold frequency corresponding to the wavelength equivalent to the periodicity of scatterers.

Two-dimensional phononic crystals: surface acoustic waves

Abstract We study theoretically the surface localized acoustic waves in two-dimensional phononic crystals consisting of circular cylinders of AlAs embedded in a GaAs matrix. The folding and anisotropy of surface wave branches as well as the existence of pseudosurface wave branches are predicted. We present stop band distributions of both the surface and bulk acoustic waves, which would be relevant to compare with the ultrasound imaging experiment.

Rectifying Acoustic Waves

We propose a system for rectification of acoustic waves. The system consists of an array of triangular holes drilled in an elastically isotropic material. Assuming the propagation of transverse acoustic waves with polarization parallel to the holes, we confirm numerically that the rectification of acoustic waves occurs even in the frequency region with corresponding wavelength comparable to the dimensions of the scatterers. The rectification effects appear above a threshold frequency due to the Bragg diffraction owing to the periodic structures. The threshold frequency depends on the structural parameters, which enables the realization of a tunable rectifier.

Self-Consistent Calculation of the Charging Energy in a Quantum Dot Coupled to Leads.

A self-consistent calculation scheme in the Coulomb-blockade regime is formulated to obtain the charging energy required in adding a single electron to a quantum dot which is electrostatically coupled to leads and a gate electrode. This method is applied to the quantum-dot transistor fabricated recently from a double-barrier heterostructure, showing that the electrostatic coupling to leads significantly influences the charging energy of the dot in such devices when the leads are close to the dot and the electron density is high in the leads.

Phonon images in CaWO4

We study important two aspects related to acoustic phonon group velocity in CaWO4. First, we calculate low-frequency phonon images in CaWO4 with two sets of published elastic constant data. We show that significant difference is found in the images in the X-Y plane due to the variance of the measured C13 and C66. Second, we discuss the origin of the anomalous phonon mode in CaWO4 recently found in the time-of-flight signals of nonequilibrium phonons.

Band structures of acoustic waves in phononic lattices

Abstract The finite-difference time-domain (FDTD) method is applied to the calculation of dispersion relations of acoustic waves in two-dimensional (2D) phononic lattices, i.e., periodic solid–liquid composites for which the conventional plane-wave-expansion method fails. Numerical examples are developed for 2D structures with mercury cylinders forming a square lattice in an aluminum matrix.