Mitsuharu Uemoto

Spatial polarization variation in terahertz electromagnetic wave focused by off-axis parabolic mirror

We have investigated the spatial distribution of the polarization state of a terahertz electromagnetic wave focused by an off-axis parabolic mirror (OPM) in the focal plane. We employed polarization-resolved terahertz time-domain spectroscopy and found that a steep spatial variation in the polarization state appears slightly distant from the focus when a linearly polarized terahertz wave is focused. The spatial variation includes an abrupt change in the polarization state (states change between circular and linear polarizations) within a wavelength. The observed phenomena are confirmed by numerical calculations and are shown to be intrinsic to the reflection from the OPM.

Simulation method for resonant light scattering of exciton confined to arbitrary geometry.

We develop an electromagnetic (EM) simulation method based on a finite-element method (FEM) for an exciton confined to a semiconductor nanostructure. The EM field inside the semiconductor excites two transverse exciton polariton and a single longitudinal exciton at a given frequency. Established EM simulation methods cannot be applied directly to semiconductor nanostructures because of this multimode excitation; however, the present method overcomes this difficulty by introducing an additional boundary condition. To avoid spurious solutions and enhance the precision, we propose a hybrid edge-nodal element formulation in which edge and nodal elements are employed to represent the transverse and longitudinal polarizations, respectively. We apply the developed method to the EM-field scattering and distributions of exciton polarizations of spherical and hexagonal-disk quantum dots.

Performance Optimization and Evaluation of Scalable Optoelectronics Application on Large Scale KNL Cluster

“ARTED” is an advanced scientific code for electron dynamics simulation which has been ported to various large-scale parallel systems including the “K” Computer, the ex-fastest supercomputer in the world, and many other MPP and cluster systems.

Ab-initio simulation for propagation of ultrashort laser pulse in solids (Conference Presentation)

Nowadays ab-initio calculations are recognized as an essential and indispensable tool in materials science. Although density functional theory has been widely used, it is a theory for electronic ground states. To describe electronic excitations and dynamics, time-dependent density functional theory (TDDFT) has been developed. Solving the time-dependent Kohn-Sham equation, the basic equation of the TDDFT, in real time, it has been possible to explore ultrafast electron dynamics induced by ultrashort laser pulses with typical resolutions of 0.02 nm in space and 1 as in time. We are developing a novel ab-initio simulation method to describe a propagation of ultrashort laser pulses in a bulk medium based on the TDDFT. A key innovation in our simulation method is the multiscale combination of simulations in two different scales, electromagnetic field analysis for the propagation of pulsed light and the TDDFT calculation for the electron dynamics in atomic scale induced by the pulsed light. Our method allows us to describe interactions between an ultrashort laser pulse and bulk materials without any empirical parameters, in particular the energy transfer from the pulsed light to electrons in the medium. The energy transfer is significant in practical usages of the pulsed light, for example, to understand the initial stage of non-thermal laser processing. Our method provides a useful platform of numerical experiments that faithfully describe optical experiments such as pump-probe measurements. We believe that the simulation method will contribute much to progresses in wide fields of optical sciences. We apply the method to interactions between an intense and ultrashort pulsed light and nanoscale semiconducting materials: silicon nanofilms and silicon 3D nanostructures. Under the irradiation of the intense pulsed light, our calculations indicate that the optical properties of the silicon changes from insulator to metal, owing to the multi-photon carrier excitations. For a propagation of a pulsed light in silicon nanofilms, we solve a coupled problem of 1D-Maxwell equations for the electromagnetic fields of the pulsed light and 3D electron dynamics described by the time-dependent Kohn-Sham equation. Penetrating the silicon nanofilms, the waveform of the pulsed light is found to be modulated during the propagation in the film: suppression in the high intensity amplitude, distortion in the tail part, and so on. A collaboration with an experimental research group is ongoing on this subject. As 3D silicon nanostructures, we consider two cases: a nanospheres of about 500 nm diameter in which a focusing of pulsed light takes place, and a bowtie-shaped nanogap composed of square nanoblocks of about 400 nm side in which a near field enhancement is expected. For the strong intensity beam, the spatial distribution of the energy transfer is modulated by the carrier excitation induced by the focused light, and it decreases the lifetime of the light confinement.

Computational materials design for efficient red luminescence: InGaN codoped with Eu and the donor–acceptor pair of Mg and O

We propose that InGaN is superior to GaN as a host material for GaN-based red-light-emitting diodes (LEDs). In our previous paper, we proposed that codoping of Eu and a Mg and O pair generates an efficiently luminescent center in GaN. This is caused by the quantum confinement of the quantum dot constructions generated by the codoping method. The present report illustrates that InGaN allows the expansion of such electronic structures throughout the crystal owing to spontaneous phase decomposition. This can be used for self-organized fabrication and self-regenerated products.

One-dimensional subwavelength position determination exploiting off-axis parabolic mirror

We demonstrate a subwavelength position determination method for the terahertz region. Previously, we reported that an off-axis parabolic mirror generates a peculiar transient rotational distribution around the focus on the subwavelength scale. In the method proposed herein, the position is determined by measuring the electric field scattered by a sample placed at this rotational distribution. We perform a realistic numerical calculation and show that this method is feasible for a sample on the wavelength scale and can distinguish a displacement of the order of 0.01 wavelengths. This method can be easily implemented for micro and nanoscale measurement and processing in the terahertz region.

Conditions for stronger field enhancement of semiconductor bowtie nanoantennas

A semiconductor bowtie nanoantenna acts as a high-quality cavity because a strongly enhanced field with a narrow spectral width appears at a nanogap region owing to exciton resonance. We theoretically investigate suitable antenna structures to obtain a strong field enhancement, and the following conditions are found: (i) the antenna structure is long in the direction of light polarization, and (ii) the tip structure near the nanogap is blunt. Condition (ii) is opposite to that for a metallic bowtie nanoantenna because the exciton wave function is distributed to avoid a narrow area near the sharp tip. These conditions are expected to be guidelines for designing efficient semiconductor nanoantennas for various applications.