Kazuhiro Nishibayashi

A spin light emitting diode incorporating ability of electrical helicity switching

Fabrication and optical characteristics of a spin light-emitting-diode (spin-LED) having dual spin-injection electrodes with anti-parallel magnetization configuration are reported. Alternating a current between the two electrodes using a computer-driven current source has led us to the observation of helicity switching of circular polarization at the frequency of 1 kHz. Neither external magnetic fields nor optical delay modulators were used. Sending dc-currents to both electrodes with appropriate ratio has resulted in continuous variation of circular polarization between the two opposite helicity, including the null polarization. These results suggest that the tested spin-LED has the feasibility of a monolithic light source whose circular polarization can be switched or continuously tuned all electrically.

Thickness dependence of magneto-optical effects in (Ga,Mn)As epitaxial layers

Rotation angle (θMO) of a linearly polarized light reflected from in-plane, ferromagnetic (Ga,Mn)As layers was measured precisely using a magneto-optical microscope. The θMO value varies non-linearly as a function of (Ga,Mn)As layer thickness d, showing a maximum at d = 50–60 nm. The thickness dependent θMO was analyzed quantitatively with a model based on an interference effect incorporating birefringence and dichroism, and it has been concluded that the contribution of magnetization-vector dependent refractive index, a magnetic birefringence, is responsible for the observed magneto-optical effect. The magnitude of magnetic birefringence appears to be comparable to those of uniaxial birefringence crystals.

Optical Study of Strain-Induced GaAs Quantum Dots

The excitation-intensity-dependent nonlinear luminescence as well as the luminescence of strain-induced GaAs quantum dots (SIQDs) was studied. The luminescence spectrum of SIQDs was composed of three well-resolved transitions and each transition had a different saturation excitation intensity. The increase and saturation of luminescence was more clearly seen in the nonlinear luminescence, where the saturation excitation intensities for each energy level were demonstrated to be proportional to the degeneracies of each level. The nonlinear luminescence spectra were simulated by rate equations, taking account of the relaxation rate, the recombination rate, and the state-filling effect caused by Pauli blocking. As a result of fitting, the relaxation rate was estimated to be about 30 ps.

Pure circular polarization electroluminescence at room temperature with spin-polarized light-emitting diodes

Significance Most of the experiments on the spin manipulation in semiconductors, the principal materials in modern electronic and photonic devices, were carried out at cryogenic temperatures and high magnetic fields because thermal energy tends to randomize spin information in the semiconductor that is nonmagnetic. Here, we report very surprising experimental results of pure circular polarization electroluminescence at room temperature with no external magnetic fields. The results are obtained by electrically injecting moderately high density of spins into semiconductor double heterostructures, the structures that were invented in connection with semiconductor lasers one-half century ago. The results suggest the appearance of some spin-dependent nonlinear processes that lead to recovering and even enhancing the injected, initial spin information in semiconductors. We report the room-temperature electroluminescence (EL) with nearly pure circular polarization (CP) from GaAs-based spin-polarized light-emitting diodes (spin-LEDs). External magnetic fields are not used during device operation. There are two small schemes in the tested spin-LEDs: first, the stripe-laser-like structure that helps intensify the EL light at the cleaved side walls below the spin injector Fe slab, and second, the crystalline AlOx spin-tunnel barrier that ensures electrically stable device operation. The purity of CP is depressively low in the low current density (J) region, whereas it increases steeply and reaches close to the pure CP when J > 100 A/cm2. There, either right- or left-handed CP component is significantly suppressed depending on the direction of magnetization of the spin injector. Spin-dependent reabsorption, spin-induced birefringence, and optical spin-axis conversion are suggested to account for the observed experimental results.

Laser-induced precession of magnetization in ferrimagnetic GdFe thin films with low power excitation

We have investigated thermal effects on the dynamics of laser-induced precession of magnetization in ferrimagnetic GdFe thin films under low-excitation conditions (6-60 μJ/cm2). An increase in quasi-equilibrium temperature by laser heating causes a shift in precession frequency, which is explained analytically by the alteration of the magnetic anisotropy field by 2.2 [Oe] at a pulse fluence of 1 μJ/cm2. We have also demonstrated coherent control of the precession amplitude using a sequence of two laser pulses, each with a fluence of 18 μJ/cm2, and point out the importance of low-power excitation for precise control of the dynamic states.

Investigation of helicity-dependent photocurrent at room temperature from a Fe/x-AlOx/p-GaAs Schottky junction with oblique surface illumination

In view of a study on spin-polarized photodiodes, the helicity-dependent photocurrent in a Fe/x-AlOx/p-GaAs Schottky diode is measured at room temperature by illuminating a circularly polarized light beam (785 nm) either horizontally on the cleaved sidewall or at an oblique angle on the top metal surface. The plane of incidence is fixed to be parallel to the magnetization vector of the in-plane magnetized Fe electrode. The conversion efficiency F, which is a relative value of helicity-dependent photocurrent with respect to the total photocurrent, is determined to be 1.0*10^-3 and 1.2*10^-2 for sidewall illumination and oblique-angle illumination, respectively. Experimental data are compared with the results of a model calculation consisting of drift-diffusion and Julliere spin-dependent tunneling transports, from which two conclusions are obtained: the model accounts fairly well for the experimental data without introducing the annihilation of spin-polarized carriers at the x-AlOx/p-GaAs interface for the oblique-angle illumination, but the model does not fully explain the relatively low F in terms of the surface recombination at the cleaved sidewall in the case of sidewall illumination. Microscopic damage to the tunneling barrier at the cleaved edge would be one possible cause of the reduced F.

Progress in the room temperature operation of GaAs-based lateral-type spin-PD in near-infrared wavelength region

A lateral-type spin-photodiode having a refracting facet on a side edge of the device is proposed and fabricated experimentally. The light impinged on the side of the device is refracted and shed directly on the backside of a spin-detecting Fe contact where spin-polarized carriers are generated in a thin InGaAs active layer and injected in the Fe contact through a crystalline AlOx tunnel barrier. Experiments are carried out at room temperature with photocurrent set up with circular polarization spectrometry, through which light-helicity-dependent photocurrent component, ΔI, is obtained with the spin detection efficiency F ≈ 0.4 %, where F is the ratio between ΔI and total photocurrent. This value is the highest reported so far for lateral-type spin-photodiodes. It is discussed that improving the quality of the p-InGaAs/x-AlOx/Fe interfaces will give rise to higher F values.

Demonstration of polarization modulated signals in a multi-mode GdFe-silica hybrid fiber

We fabricate a hybrid fiber composed of a multi-mode, silica-based fiber and a GdFe thin film adjacent to each other. Magneto-optical (MO) signals modulated by AC magnetic fields on the GdFe layer are detected at the output pupil of the fiber, showing that the polarization state of the propagated light in the mixed-mode condition can be well defined. We find that local modulation of magnetization at different positions of the GdFe layer results in MO signals at the different position of the output, showing the spatial de-multiplexing of polarization modulated signals.

A lateral-type spin-photodiode based on Fe/x-AlOx/p-InGaAs junctions with a refracting-facet side window

A lateral-type spin-photodiode having a refracting facet on a side edge of the device is proposed and demonstrated at room temperature. The light shed horizontally on the side of the device is refracted and introduced directly into a thin InGaAs active layer under the spin-detecting Fe contact in which spin-polarized carriers are generated and injected into the Fe contact through a crystalline AlOx tunnel barrier. Experiments have been carried out with a circular polarization spectrometry set up, through which helicity-dependent photocurrent component, dI, is obtained with the conversion efficiency F ~ 0.4 %, where F is the ratio between dI and total photocurrent Iph. This value is the highest reported so far for pure lateral-type spin-photodiodes. It is discussed through analysis with a model consisting of drift-diffusion and quantum tunneling equations that a factor that limits the F value is unoccupied spin-polarized density-of-states of Fe in energy region into which spin-polarized electrons in a semiconductor are injected.

Enhancement of magneto-optical effect via the evanescent wave and its figure of merit

We investigate the enhancement of the magneto-optical (MO) effect via the evanescent wave in MO structures composed of GdFe thin layers and two dielectrics. The MO Kerr rotation and the figure of merit (FOM) among the Kretschmann, Otto and direct-irradiation configurations are compared. The data obtained by experiment and calculation both show that the standing evanescent wave in the metal layer realized in the former two configurations enhances the MO Kerr rotation and FOM. Furthermore, the Kretschmann configuration appears to be superior to the Otto configuration in terms of the efficiency of MO Kerr enhancement.