Tomoyuki Sasaki

Electrical Spin Injection into Silicon Using MgO Tunnel Barrier

We demonstrate the evidence of electrical spin injection into silicon (Si) using the Fe/MgO tunnel barrier by the following two methods: 1) non-local (NL) four-terminal magnetoresistance (MR) scheme with their respective corresponding ferromagnetic (FM) electrode and 2) Hanle-type spin precession scheme. These results are compatible and in agreement with each other. The spin injection signals in the non-local scheme were observed up to 150 K, and the spin diffusion length (¿N) was estimated to be 2.8 ¿m at 8 K. The experimental data completely matches every detail of the Hanle measurements equation, and spin lifetime (¿) was estimated to be 9.44 ns at 8 K. We will be able to discuss the physical properties of a pure spin current in silicon by this compelling data.

Three-dimensional vector holograms in anisotropic photoreactive liquid-crystal composites

In this paper, we describe the principle of the three-dimensional vector holograms formed in anisotropic recording medium. The polarization states of the interference light are three-dimensionally modulated due to both the polarization interference and optical anisotropy in the recording medium. The electric field of the polarized light reorients the director and forms the three-dimensional vector hologram in anisotropic photoreactive liquid-crystal composites. The theoretical consideration reveals the formation mechanism and optical characteristics of the resultant three-dimensional vector holograms.

Effects of plasticizer on the transient diffraction properties of polarization holographic gratings made from polarization-sensitive polymeric films

We recorded anisotropic gratings by polarization holographic recording in azobenzene-containing polymeric films doped with a 9-ethylcarbazole (ECZ) plasticizer. Using a polarimeter, we measured in real time the Stokes parameters of the beams diffracted from the gratings. We analyzed these data on the basis of a theoretical model that accounted for the distribution of birefringence caused by molecular reorientation as well as for surface relief deformation caused by mass transportation. Our results indicated that increasing ECZ doping level increased the formation speed of the anisotropic grating, but did not greatly affect the amplitude of the photoinduced birefringence.

Transmission and reflection phase gratings formed in azo-dye-doped chiral nematic liquid crystals

Transmission and reflection gratings were simultaneously formed in azo-dye-doped chiral nematic liquid crystals (N∗LCs) with planar alignment. The formation process is based on a phototuning of the Bragg reflection band of the N∗LC. The helical pitch of the photoreactive N∗LC was spatially controlled with intensity variation of interference light. The resultant periodic structure showed both transmissive and reflective diffractions due to the spatially modulated light intensities. The observed dependence of diffraction efficiencies on the polarization states of the probe beam was well explained by considering a spatial modulation of the helical pitch.

Molecular-Oriented Photoalignment Layer for Liquid Crystals

In this letter, we describe the fabrication of a durable photoalignment layer for liquid crystals (LC) with high azimuthal anchoring and controllability of the tilt angle. A film is irradiated using linearly polarized ultraviolet (LPUV) light followed by annealing which induces the molecular orientation of the photoalignment layer. The nematic LC was aligned parallel to the electric vector of the LPUV light, and the alignment layer showed high thermal stability of the orientational characteristics up to 250 °C.

Liquid crystal phase gratings using photoregulated photocrosslinkable polymer liquid crystals

We have presented liquid crystal phase gratings using photoregulated photocrosslinkable polymer liquid crystals. The phase grating consists of homogeneous and 90°-twisted nematic area and shows unique polarization conversion in the diffraction process. The diffraction properties are well explained by the theory based on Jones calculus.

Effect of spin drift on spin accumulation voltages in highly doped silicon

We investigated the effect of spin drift on spin accumulation in highly doped silicon (Si) by using a non-local three-terminal (NL-3T) and four-terminal (NL-4T) methods, and have clarified that the spin accumulation voltages in a NL-3T device were modulated due to spin drift and that spin lifetime can be accurately extracted by employing a modified spin drift-diffusion equation. The extracted spin lifetime is 4-7 ns, which is slightly shorter than the intrinsic spin lifetime (8 ns) measured in the NL-4T method in the same Si device because of spin drift. It is elucidated that the spin drift effect should be considered for the precise estimation of spin lifetime in Si by NL-3T method.

Local magnetoresistance in Fe/MgO/Si lateral spin valve at room temperature

Room temperature local magnetoresistance in two-terminal scheme is reported. By employing 1.6 nm-thick MgO tunnel barrier, spin injection efficiency is increased, resulting in large non-local magnetoresistance. The magnitude of the non-local magnetoresistance is estimated to be 0.0057 Ω at room temperature. As a result, a clear rectangle signal is observed in local magnetoresistance measurement even at room temperature. We also investigate the origin of local magnetoresistance by measuring the spin accumulation voltage of each contact separately.

Vector holograms using radially polarized light

We have demonstrated the vector holograms using the radially polarized light. The inhomogeneous polarized light well controlled the spatial distribution of the optical anisotropy in the solid state polymeric materials. The theoretical calculation revealed formation mechanism and optical properties of the vector holograms.

Photoinduced reorientation and polarization holography in a new photopolymer with 4-methoxy-N-benzylideneaniline side groups

The photoinduced reorientation and surface relief (SR) formation behaviors of a novel photosensitive polymer, which was transparent in visible region, were investigated using linearly polarized-313-nm light and holographic exposure with a 325-nm He-Cd laser. The polymer was comprised of photosensitive 4-methoxy-N-benzylideneaniline side groups, and exhibited a sufficient photoinduced molecular reorientation with a birefringence of 0.11. Holographic exposure generated a SR structure, which had a periodical molecular reorientation that depended on the polarization of the interference beams. The generated SR height was ∼212 nm, and the inscription of a double holographic exposure yielded a two-dimensional SR structure.