Yutaka Takahashi

Adsorption Density Control of N719 on TiO2 Electrodes for Highly Efficient Dye-Sensitized Solar Cells

N719 dye adsorption on anatase TiO 2 surfaces with different adsorption densities was investigated by IR-absorption spectroscopy using a multiple internal reflection technique. When the N719 dye was adsorbed on the TiO 2 surface by immersing it in an N719 solution consisting of acetonitrile and tertiary-butyl alcohol, chemisorbed and physisorbed N719 dyes coexisted on the surface. The ratio of chemisorbed and physisorbed dye densities was dependent on the N719 exposure, and excessive exposure led to the generation of the physisorbed dye. Test fabrications of the dye-sensitized solar cells (DSCs) with various N719 exposures suggested that we need to optimize the dye adsorption density to achieve a higher power conversion efficiency in DSCs.

Polarization-dependent nonlinear gain in semiconductor lasers

We have numerically studied the nonlinear gain coefficients in terms of spectral hole burning for the optical fields in parallel and orthogonal polarizations in semiconductor lasers by solving the equation of motion for the density matrix in perturbation series. The electronic band structures and the transition matrix elements used in the calculations are obtained by diagonalizing Luttingers Hamiltonian. In the present analysis for InGaAsP lasers, the cross-saturation coefficient for the parallel polarizations is twice as large as the self-saturation. Also, the cross-saturation coefficient for the orthogonal polarizations, which affects the polarization switching and polarization bistable operations of the laser, rests between the two. The relative magnitude of self-saturation coefficients and cross-saturation coefficients for orthogonal polarizations satisfies the condition for polarization bistable operations. We also discuss the effect of carrier heating on gain saturation coefficients.

The effect of electric field on the excitonic states in coupled quantum well structures

We have calculated by a variational procedure the excitonic states in symmetric coupled quantum well structures of GaAs/AlGaAs in the presence of an external electric field perpendicular to the interfacial plane using a nonseparable, nonspherical hydrogenic trial wavefunction. The exciton binding energies have been calculated as a function of external electric field for various well and barrier widths. The nonspherical trial wavefunction leads to a better estimate of the exciton binding energy, especially in high electric field, than that calculated with a spherical one. The calculations show that the binding energy is strongly dependent on the external electric fields: the variation (increase or decrease) is related to the spatial distribution of the wavefunctions. The results of the calculation are consistent with our experimental results.

Polarization-dependent gain saturations in quantum-well lasers

Theoretical analyses of polarization-dependent optical gain saturation are given for semiconductor quantum-well (QW) lasers to investigate the conditions of polarization switching and bistable operations. Nonlinear susceptibilities, which give saturation coefficients, are obtained in the perturbative analyses of density matrices, where the relevant electronic states in the QW are calculated by diagonalizing Luttingers Hamiltonian, thus including valence band mixing. The present formulation is applied to InGaAsP QW lasers with edge-emitting and vertical-cavity surface-emitting laser (VCSEL) structures, and the self- and cross-saturation coefficients with parallel and orthogonal optical polarizations are numerically calculated, which are compared with those of bulk lasers. For the edge-emitting case, the saturation coefficients are strongly dependent on the photon energies, and the bistable operation condition is not satisfied in the gain peak, different from a bulk laser which showed only a slight energy dependence. In a VCSEL, the saturation coefficients are also dependent on the photon energies but the bistable operation condition is always satisfied.

Ultrashort-pulse-controlled all-optical modulation by interband and intersubband transitions in doped quantum wells.

The optimum condition for achieving highly efficient ultrafast all-optical pulse modulation by modification of the absorption of a pulsed interband light field by ultrasubband control-light pulses in a doped quantum well is investigated. The modulation efficiency in the femtosecond domain can be maximized by an intersubband control light with a pulse width (200 fs) that is comparable with the phase-relaxation time of the system and an intensity that is close to the intersubband saturation intensity.

Interband nonlinear optical generation in presence of intersubband light in asymmetric quantum wells

The interband sum frequency generation process due to three-wave interaction of interband and intersubband coupling lights has been investigated in a semiconductor quantum well using the perturbational density matrix approach. The origin of the nonlinear process lies in the second-order susceptibility /spl chi//sub c2-h1//sup (2)/ arising due to the optical transition between the second conduction subband and the first heavy hole state. Both the sign and the magnitude of the second-order susceptibility of the well may be controlled by the carrier density level and the frequency of the intersubband field.

Magnetic Properties of Fe(001) Thin Films on GaAs(001) Deposited by RF Magnetron Sputtering

Fe thin films, down to 6 nm thick, were prepared on GaAs(001) substrates by RF magnetron sputtering. The x-ray diffraction (XRD) analyses show that the epitaxial thin films of Fe(001) were grown with cube-on-cube orientation on GaAs(001). Magnetic properties were investigated by vibrating sample magnetometry (VSM) and ferromagnetic resonance (FMR) spectroscopy. The magnetization curves obtained by applying in-plane magnetic fields indicate that easy (hard) direction is along [100] ([110]) and the saturation magnetization is close to the bulk values. The in-plane magnetic anisotropy measured by FMR shows four-fold symmetry, as expected for bcc Fe. We did not observe the in-plane uniaxial magnetic anisotropy reported on the MBE-grown Fe films on GaAs substrates.

Strain-dependence of the gain saturations in InGaAsP/InP quantum-well gain media

Numerical analyses of polarization-dependent optical gain saturations are given for quantum-well (QW) lasers in the presence of strain in the well regions in order to investigate the strain dependence of polarization-bistable operations. Gain saturation coefficients are obtained from nonlinear susceptibilities calculated in the perturbative analyses of density matrices. Band dispersions and dipole matrix elements, which are put into the density matrices, are calculated by diagonalizing Luttingers Hamiltonian, including valence band mixing. The strain induces a change in band dispersions and wavefunctions, leading to strain-dependent saturation coefficients. The self-saturation coefficients and the cross-saturation coefficients (with orthogonal optical polarizations) pertinent to InGaAsP/InP QW vertical-cavity surface-emitting lasers are calculated. We find that the relative magnitudes of self- and cross-saturation coefficients are strongly dependent on the strain; in the presence of compressive strain, the cross-saturation coefficients are larger than the self-saturation in the wide range of the linear gain spectra, especially in the vicinity of the gain peak, indicating that the compressively strained structure is more favorable for the polarization-bistable operations.

Spin diffusion of a two-dimensional electron gas in the random phase approximation

Abstract Spin transport properties of spin-polarized two-dimensional electron gas are studied in the presence of electron–electron interactions. Longitudinal and transverse spin diffusion coefficients are calculated with the quantum transport equation. New results are obtained by evaluating both the drift and the collision terms using the random phase approximation, while the drift term was calculated by Hartree–Fock approximation in our earlier work. The qualitative features remain same, but the present work is valid for larger polarizations. We find that the e–e scattering, which does not contribute to the charge drift mobility, has a significant contribution to the spin diffusion.

Interband difference-frequency generation by means of resonant intersubband transitions in asymmetric quantum wells

We investigate the interband difference-frequency-generation process arising from the three-wave interaction of resonant interband and intersubband coupling lights by theoretical estimation of the linear and the nonlinear susceptibilities of an asymmetric AlGaAs–GaAs quantum well. The three-level quantum-well system is analyzed by the perturbational density-matrix approach. The effect of the variation of carrier density in the quantum well on the interband mixing process is also considered. The nonlinear polarization induced by the intersubband coupling field is significantly enhanced in the case of doped structures. The peak second-order susceptibility estimated at the difference frequency is more than 2 orders of magnitude larger, whereas the coupling-field-induced third-order susceptibility is more than 4 orders of magnitude larger than that of bulk GaAs.