Tomoki Ishikawa

Measurement of pressures in 10−10Pa range from the damping speed of field emission current

The damping behavior of field emission (FE) currents was monitored with W(111) tips under pressures down to 5×10−10Pa. Semilogarithmic damping curves of FE currents were linear for 0.3L hydrogen exposure in the thoroughly degassed extreme high vacuum (XHV) FE system. The slope of semilogarithmic damping curves was linearly proportional to the operation pressure, suggesting a method of measuring pressure in an XHV range. Electron stimulated desorption was not observed.

Coherent and intense multibeam generation by the apex of sharp nano-objects: Electron half-circular prism

Mutually coherent multiple electron beams (multibeam) were generated from diffraction at the apex of sharp nano-objects, especially carbon nanotubes, in a field emission projection microscope. Simulation using a simple scalar diffraction model showed that the apex of sharp nano-objects played the role of an electron half-circular prism, deflecting an electron beam through a constant angle toward the prism axis. The multibeam diffraction pattern gave a high visibility of ∼0.8 and high intensity.Mutually coherent multiple electron beams (multibeam) were generated from diffraction at the apex of sharp nano-objects, especially carbon nanotubes, in a field emission projection microscope. Simulation using a simple scalar diffraction model showed that the apex of sharp nano-objects played the role of an electron half-circular prism, deflecting an electron beam through a constant angle toward the prism axis. The multibeam diffraction pattern gave a high visibility of ∼0.8 and high intensity.

Analysis of quantum efficiency improvement in spin-polarized photocathode

GaAs/GaAsP strain-compensated superlattices (SLs) were developed for spin-polarized photocathode applications. High crystal quality was maintained with SL thicknesses up to 720 nm (90-pairs); however, the quantum efficiency (QE) did not increase linearly with the SL thickness but became saturated starting from an SL thickness of 192 nm (24-pairs). Time-resolved photoluminescence measurements revealed that the carrier lifetime in the GaAs/GaAsP strain-compensated SL was as short as 20.5 ps at room temperature, which causes the elimination of photoexcited electrons before emission. A simulation based on a diffusion model was implemented to quantitatively evaluate the effect of the carrier lifetime on the QE. The simulation results were in good agreement with the experimental results and demonstrate that a carrier lifetime of over 120 ps is required for a two-fold improvement of the QE.

Prolonged spin relaxation time in Zn-doped GaAs/GaAsP strain-compensated superlattice

A GaAs/GaAsP strain-compensated superlattice (SL) is a highly promising spin-polarized electron source. To realize higher quantum efficiency, it is necessary to consider spin relaxation mechanisms. We have investigated the electron spin relaxation time in a Zn-doped GaAs/GaAsP strain-compensated SL by time-resolved spin-dependent pump and probe reflection measurements. The long spin relaxation time of 104 ps was observed at room temperature (RT), which is about three times longer than that of conventional undoped GaAs multiple quantum wells. Even when the excitation power increases from 30 to 110 mW, the change in the spin relaxation time at RT was small. This relationship implies that the intensity of the electron beam can be increased without affecting the spin relaxation time. These results indicate that a Zn-doped GaAs/GaAsP strain-compensated SL has the great advantage for use as a spin-polarized electron source.

One hundred picosecond spin relaxation in GaAs/GaAsP strain-compensated superlattice as highly spin-polarized electron source

Summary form only given. A GaAs/GaAsP strain-compensated superlattice (SL) is a highly promising spin-polarized electron source. In this paper, we report the spin relaxation for a GaAs/GaAsP strain-compensated SL obtained by time-resolved spin-dependent pump and probe reflectance measurements. The spin relaxation time of the GaAs/GaAsP strain-compensated SL was measured to be 104 ps at room temperature. This longer spin relaxation time than that of conventional GaAs multiple quantum wells shows that the present SL is suitable for use as a highly spin-polarized electron source.

Picoseconds carrier spin relaxation in In 0.8 Ga 0.2 As/Al 0.5 Ga 0.5 As/AlAs 0.56 Sb 0.44 coupled double quantum wells

InGaAs/AlAsSb coupled double quantum wells (CDQWs) have been attracting much attention for their use in all-optical switches at 1.55 μm wavelength because of their large conduction band offset of 1.7 eV. In this study, we have investigated the spin relaxation in In0.8Ga0.2As/Al0.5Ga0.5As/AlAs0.56Sb0.44 CDQWs using time-resolved spin-dependent pump and probe reflectance measurements to observe the time evolution of the spin polarization. By the double-exponential fitting of the time evolution of spin polarization, we obtained the spin relaxation times of 4.2 ps and 50.2 ps at room temperature. The observed spin relaxation time of 4.2 ps at room temperature indicates high potential for applications to high-speed optical devices.

Analysis of InAlGaAs/InAlAs/InP multi-mode interference photonic switch with Mach-Zehnder interferometer for higher switching performances

We analyzed the switching performances of an InAlGaAs/InAlAs/InP multi-mode interference photonic switch with Mach-Zehnder interferometer. In the result, it is shown that the device can achieve high performances concerning lower crosstalk, lower power consumption and less polarization dependence.