Seisuke Nakashima

Large Faraday effect in a short wavelength range for disordered zinc ferrite thin films

We have prepared a zinc ferrite (ZnF2O4) thin film 85nm thick deposited on a silica glass substrate by using a radio frequency sputtering method. Faraday effect measurements have been carried out not only for as-deposited but also for annealed thin films. The thin film annealed at 300°C as well as the as-deposited thin film exhibit a large Faraday rotation angle at a wavelength of around 390nm. In particular, the thin film annealed at 300°C manifests the largest Faraday effect among the present thin films; the rotation angle of 1.65°∕μm is attained at a wavelength of 386nm. The thin films 1.08μm thick exhibit a large magnetization at room temperature, and the dependence of the magnetization on the external magnetic field is suggestive of a ferrimagnetic behavior.

Plasmonically enhanced Faraday effect in metal and ferrite nanoparticles composite precipitated inside glass

Using femtosecond laser irradiation and subsequent annealing, nanocomposite structures composed of spinel-type ferrimagnetic nanoparticles (NPs) and plasmonic metallic NPs have been formed space-selectively within glass doped with both α-Fe(2)O(3) and Al. The Faraday rotation spectra exhibit a distinct negative peak at around 400 nm, suggesting that the ferrimagnetic Faraday response is enhanced by the localized surface plasmon resonance (LSPR) due to metallic Al NPs. At the interfaces in the nanocomposites, the ferrimagnetism of magnetite NPs is directly coupled with the plasmon in the Al NPs. The control of the resonance wavelength of the magneto-optical peaks, namely, the size of plasmonic NPs has been demonstrated by changing the irradiation or annealing conditions.

Template method for nano-order positioning and dense packing of quantum dots for optoelectronic device application

In this paper, we present recent progress of our researches on positioning of quantum dot in the field of optoelectronics. The first research is aimed at quantum information device application. As a technology for production of quantum devices, we developed the method for positioning of a single colloidal quantum dot. Oxide lines on a Si substrate drawn by a scanning probe microscope were used as a negative etching mask with controlling their cross section in order to create a nanohole for trapping of a single quantum dot. The other research is aimed at solar cell application. It has been predicted that quantum-dot superlattice solar cell will achieve photoelectric conversion efficiency of more than 70%. After the sedimentation of the colloidal quantum dots into the pyramidal holes processed by anisotropic wet etching on a Si substrate, we observed characteristic photoluminescence from the quantum-dot sheet.

Position control of PbS quantum dot using nanohole on silicon substrate processed by scanning probe lithography

We report a method of processing of nanometer-size holes using an oxide mask drawn by scanning probe microscopy (SPM), and show that a nanohole can be used for the position control of a single colloidal quantum dot. An apertureless mask process was developed for the formation of nanometer-wide holes. The process conditions used to obtain a large slope angle at the edge of the oxide mask and high Si/oxide selectivity during dry etching were investigated to make a nanohole sufficiently deep to trap quantum dots. SPM observation suggested that a 6 nm PbS quantum dot was trapped by the smallest nanohole with a width of 10 × 18 nm2 and a depth of 5 nm.

Formation of nanohole for positioning of colloidal quantum dot

We propose a method of positioning a colloidal quantum dot (QD) using a nanohole on a Si substrate processed by oxidation with scanning probe microscope (SPM). We investigated two methods for the formation of the nanohole. Using oxide as a positive mask, we found that the hole of QD diameter and depth was not possible to fabricate since the oxidation does not progress deeply under the existing oxide. Using oxide as a negative mask, we found that the fabrication of a QD-size hole is achievable by dry etching. SPM observations suggest that a single sheet of PbS QDs can be trapped using a nanohole array.

Temperature-Controlled Symmetry of Linear Polarization of Photoluminescence from InGaAs-Buried InAs/GaAs Quantum Dots

We have succeeded in adjusting the symmetry of the linear polarization of exciton emission from self-assembled InAs/GaAs quantum dots by controlling the combination of temperature and composition of the InGaAs burying layer. The anisotropic shape of the Stranski–Krastanow-type quantum dot is a drawback to the generation of a polarization-entangled photon pair. We found that the polarization symmetry of the intensity and wavelength of photon emission depends on the sample temperature and the composition of the burying layer. The ground-state emission peaks in two linear polarization directions were tuned to overlap by lowering the temperature and using a high indium composition of the burying layer. Our results will aid in the development of an entangled-photon generator using the emission of exciton molecule in self-assembled quantum dots.

Amorphous/epitaxial superlattice for thermoelectric application

An amorphous/epitaxial superlattice system is proposed for application to thermoelectric devices, and the superlattice based on a PbGeTeS system was prepared by the alternate deposition of PbS and GeTe using a hot wall epitaxy technique. The structure was analyzed by high-resolution transmission electron microscopy (HRTEM) and X-ray analysis, and it was found that the superlattice consists of an epitaxial PbTe-based layer and a GeS-based amorphous layer by the reconstruction of the constituents. A reduction in thermal conductivity due to the amorphous/epitaxial system was confirmed by a 2ω method. Electrical and thermoelectric properties were measured for the samples.

Interband absorption in PbTe/PbSnTe-based type-II superlattices

Short-period PbTe/Pb1−xSnxTe/CaTe (x = 0.36, 0.48) superlattices were prepared on a KCl (100) substrate and their interband optical absorption was measured. The superlattice indicated strong absorption of more than 2500 cm−1, corresponding to the electron transition from the first valence subband in the PbSnTe layer to the conduction subband in the PbTe layer, which can give enough optical gain for laser operation under inversion population. The absorption coefficient increased with lowering of temperature in the wavelength region from 7 to more than 20 μm. The enhancement of the absorption coefficient was explained by the enhancement of the overlap of wave functions and the two-dimensional density of states in the type-II superlattice retaining a large value even with reduced band gap. The conduction-band offset of the PbTe/Pb0.64Sn0.36Te type-II heterojunction was estimated to be 120 meV.

PbSrS/PbS mid-infrared short-cavity edge-emitting laser on Si substrate

A PbSrS/PbS short-cavity edge-emitting laser consisting of a SrS/PbS short period superlattice was prepared on the Si (111) substrate. The laser with a cavity length of 25 μm and a cavity width of 3 μm operated up to 313 K around 3 μm in wavelength under pulsed laser excitation. Maximum peak output power was 40 mW at 260 K which indicated a high quantum efficiency.

Magneto-optical properties of waveguide structures containing magnetic nanoparticles using femtosecond laser

This paper demonstrates the advantages of a femtosecond-laser-processing method as a novel fabrication technique of waveguides containing such nanocomposites inside glass materials. By focusing the femtosecond laser inside the transparent glass, the nanocomposites are space-selectively precipitated as results of atomic diffusion and crystallization processes. At the same time, the refractive index of the irradiated area becomes higher, leading to the formation of optical waveguides. Due to the advantages, this method can be potentially applied to produce magneto-optical micro-devices, such as three-dimensional high density magnetic recording devices or micro-sized optical isolators integrated with waveguides. Recently, optical interfaces between electronic circuits are actively studied. In such advanced high-speed devices, micro lasers and the appropriate micro isolators are required.