Kouhei Takahashi

Terahertz Radiation by an Ultrafast Spontaneous Polarization Modulation of Multiferroic BiFeO 3 Thin Films

Terahertz (THz) radiation has been observed from multiferroic BiFeO3 thin films via ultrafast modulation of spontaneous polarization upon carrier excitation with illumination of femtosecond laser pulses. The radiated THz pulses from BiFeO3 thin films were clarified to directly reflect the spontaneous polarization state, giving rise to a memory effect in a unique style and enabling THz radiation even at zero-bias electric field. On the basis of our findings, we demonstrate potential approaches to ferroelectric nonvolatile random access memory with nondestructive readability and ferroelectric domain imaging microscopy using THz radiation as a sensitive probe.

Studies of ultra-intense laser plasma interactions for fast ignition

Laser plasma interactions in a relativistic parameter regime have been intensively investigated for studying the possibility of fast ignition in inertial confinement fusion (ICF). Using ultra-intense laser systems and particle-in-cell (PIC) simulation codes, relativistic laser light self-focusing, super hot electrons, ions, and neutron production, are studied. The experiments are performed with ultra-intense laser with 50 J energy, 0.5–1 ps pulse at 1053 nm laser wavelength at a laser intensity of 1019 W/cm2. Most of the laser shots are studied under preformed plasma conditions with a 100 μm plasma scale length condition. In the study of laser pulse behavior in the preformed plasmas, a special mode has been observed which penetrated the preformed plasma all the way very close to the original planar target surface. On these shots, super hot electrons have been observed with its energy peak exceeding 1 MeV. The energy transport of the hot electrons has been studied with making use of Kα emissions from a see...

Fast ignitor research at the Institute of Laser Engineering, Osaka University

The physics element relevant to the fast ignitor in inertial confinement fusion has been extensively studied. Laser-hole boring with enormous photon pressures into overcritical densities was experimentally proved by density measurements with XUV laser probing. Ultra-intense laser interactions at a relativistic parameter regime were studied with a 50-TW glass laser system and a 100-TW glass laser system synchronized with a long pulse laser system. In the study of relativistic laser beam propagation in a 100-μm scale-length plasma, a special propagation mode (super-penetration mode) was observed, where the beam propagated into overdense regions close to the solid target surface. At the super-penetration mode, 20% of the laser energy converted to energetic electrons toward the target inside, while the coupling efficiency was 40% without the long scale-length plasmas. The high-density energetic electron transport and heating of solid material was also studied, indicating beamlike propagation of the energetic electrons in the solid target and effective heating of solid density ions with the electrons. Based on these basic experimental results, the heating of imploded plasma by short-pulse-laser light with three different ways of injecting the heating pulse has been studied.

Tailoring effective thermoelectric tensors and high-density power generation in a tubular Bi0.5Sb1.5Te3/Ni composite with cylindrical anisotropy

Transverse thermoelectric responses in heterogeneous composites made of periodically laminated Bi0.5Sb1.5Te3/Ni in a tubular shape were investigated. Numerical calculations quantitatively clarify the relationship between geometrical parameters and effective thermoelectric tensors. In the present tubular heterogeneous composites, the temperature gradient across the radial direction yields a transverse voltage along the axial direction due to the unnatural cylindrical anisotropy. The tubular configuration allows for direct and efficient heat transfer from fluid heat sources. A high-density power generation of 417 W m−2 was achieved under the small temperature difference of 83 K.

Bifunctional thermoelectric tube made of tilted multilayer material as an alternative to standard heat exchangers

Enormously large amount of heat produced by human activities is now mostly wasted into the environment without use. To realize a sustainable society, it is important to develop practical solutions for waste heat recovery. Here, we demonstrate that a tubular thermoelectric device made of tilted multilayer of Bi0.5Sb1.5Te3/Ni provides a promising solution. The Bi0.5Sb1.5Te3/Ni tube allows tightly sealed fluid flow inside itself, and operates in analogy with the standard shell and tube heat exchanger. We show that it achieves perfect balance between efficient heat exchange and high-power generation with a heat transfer coefficient of 4.0 kW/m2K and a volume power density of 10 kW/m3 using low-grade heat sources below 100°C. The Bi0.5Sb1.5Te3/Ni tube thus serves as a power generator and a heat exchanger within a single unit, which is advantageous for developing new cogeneration systems in factories, vessels, and automobiles where cooling of excess heat is routinely carried out.

Influence of Mn Doping on Ferroelectric-Antiferromagnet BiFeO3 Thin Films Grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 Substrates

We have fabricated phase-pure BiFe1-xMnxO3 (x=0, 0.05, and 0.2) thin films on (LaAlO3)0.3(Sr2AlTaO6)0.7 substrates by pulsed laser deposition technique and examined the Mn substitution effect on the electric and magnetic properties. The undoped BiFeO3 films exhibited simultaneous ferroelectricity and antiferromagnetism at room temperature, which was consistent with the bulk property. The increase in Mn content resulted in an enhancement in electric conductivity, leading to an increase in spontaneous magnetization at room temperature along with a deterioration in ferroelectricity. Among the three compositions, BiFe1-xMnxO3 (x=0.05) thin film showed the best multiferroic property exhibiting simultaneous ferroelectricity and weak ferromagnetism.

Multifrequency high-speed phase-modulation atomic force microscopy in liquids.

We have developed a new technique, called multifrequency high-speed phase-modulation atomic force microscopy (PM-AFM) in constant-amplitude (CA) mode based on the simultaneous excitation of the first two flexural modes of a cantilever. By performing a theoretical investigation, we have found that this technique enables the simultaneous imaging of the surface topography, energy dissipation and elasticity (nonlinear mapping) of materials. We experimentally demonstrated high-speed imaging at a scan speed of 5 frames/s for a polystyrene (PS) and polyisobutylene (PIB) polymer-blend thin-film surface in water.

Breaking the trade-off between thermal and electrical conductivities in the thermoelectric material of an artificially tilted multilayer

Breaking the trade-off between thermoelectric (TE) parameters has long been demanded in order to highly enhance its performance. Here, we report the ‘trade-off-free’ interdependence between thermal conductivity (κ) and resistivity (ρ) in a TE/metal tilted multilayer and significant enhancement of TE power generation based on the off-diagonal thermoelectric (ODTE) effect, which generates transverse electrical current in response to vertical thermal current. ρ and κ can be simultaneously decreased by setting charge flow along more-electrically conductive layer and thermal flow across less-thermally conductive perpendicular direction by decreasing the tilting angle. Moreover, introducing porosity in the metal layer enables to decrease in κ without changing ρ, because the macroscopic ρ and κ of the tilted multilayer is respectively governed by the properties of the TE material and the metal with large dissimilarity. The obtained results reveal new strategies for developing trade-off-free TE materials, which will stimulate practical use of TE conversion for waste-heat recovery.

Implications of phase-segregation on structure, terahertz emission and magnetization of Bi(Fe1-xMnx)O3 (0⩽x⩽0.5) thin films

Structural, magnetic and terahertz emission properties of Bi(Fe1-xMnx)O3 (0≤x≤0.5) thin films of various thicknesses were studied. A transition from coherently strained structure to relaxed structure at a film thickness (t) of ~80–90 nm occurs only for x<0.2. It is shown that terahertz-emission efficiency is not deteriorated with increasing Mn-doping (x). The magnetic moment of thin films (t≤85 nm) exhibits only a weak enhancement with increasing x —a feature suggesting that Mn-doping is ineffective in inducing ferromagnetism in BiFeO3. The thicker films (t≥150 nm), on the contrary, possess larger magnetic moment which evidently arises from the segregated magnetic MnFe2O4 phase.

Studies of intense laser-plasma interactions for the fast ignitor concept at ILE, Osaka University

We experimentally studied laser-hole boring with 100 ps/1 TW laser light and ultra-intense laser-plasma interactions using a 0.5 ps/100 TW laser system. Investigation into laser-hole boring was made by measurements of backscattered light spectra, x-ray image and electron density profiles in under- and overdense regions. For the first time x-ray laser light was used to directly observe the channel in the overdense plasma. We further examined short pulse laser interactions as well as the generation of high-energy particles with 100 TW laser light. Neutrons by D-D fusion reaction in the interaction plasma were measured to diagnose ion acceleration. High-energy electrons (MeV) were measured by a x-ray method.