S. Okumi

Surface charge limit in NEA superlattice photocathodes of polarized electron source

The “surface charge limit (SCL)” phenomenon in negative electron affinity (NEA) photocathodes with GaAs–AlGaAs superlattice and InGaAs–AlGaAs strained-layer superlattice structures has been investigated systematically using a 70 keV polarized electron gun and a nanosecond multi-bunch laser. The space-charge-limited beam with multi-bunch structure (1.6 A peak current, 12 ns bunch width and 15 or 25 ns bunch separation) could be produced from the superlattice photocathodes without suffering the SCL phenomenon. From the experimental results, it has been confirmed that the SCL phenomenon is governed by two physical mechanisms at the NEA surface region, the tunneling of conduction electrons against the surface potential barrier (escaping process) and that of valence holes against the surface band bending barrier (recombination process); these effects can be enhanced using the superlattice structure and heavy p-doping at the surface, respectively. We conclude that a superlattice with heavily p-doped surface is the best photocathode for producing the multi-bunch electron beam required for future linear colliders.

Highly polarized electrons from GaAs-GaAsP and InGaAs-AlGaAs strained-layer superlattice photocathodes

GaAs–GaAsP and InGaAs–AlGaAs strained-layer superlattice photocathodes are presented as emission sources for highly polarized electron beams. The GaAs–GaAsP cathode achieved a maximum polarization of 92(±6)% with a quantum efficiency of 0.5%, while the InGaAs–AlGaAs cathode provides a higher quantum efficiency (0.7%) but a lower polarization [77(±5)%]. Criteria for achieving high polarization using superlattice photocathodes are discussed based on experimental spin-resolved quantum efficiency spectra.

Super-High Brightness and High-Spin-Polarization Photocathode

Using a newly developed transmission-type photocathode, an electron beam of super-high brightness [(1.3±0.5)×107 Acm-2sr-1] was achieved. Moreover, the spin-polarization was as high as 90%. We fabricated a transmission-type photocathode based on a GaAs–GaAsP strained superlattice on a GaP substrate in order to enhance the brightness and polarization greatly. In this system, a laser beam is introduced through the transparent GaP substrate. The beam is focused on the superlattice active layer with a short focal length lens. Excited electrons are generated in a small area and extracted from the surface. The shrinkage of the electron generation area improved the brightness. In addition, a GaAs layer was inserted between the GaP substrate and the GaAsP buffer layer to control the strain relaxation process in the GaAsP buffer layer. This design for strain control was key in achieving high polarization (90%) in the transmission-type photocathode.

High brightness and high polarization electron source using transmission photocathode with GaAs-GaAsP superlattice layers

In order to produce a high brightness and high spin polarization electron beam, a pointlike emission mechanism is required for the photocathode of a GaAs polarized electron source. For this purpose, the laser spot size on the photocathode must be minimized, which is realized by changing the direction of the injection laser light from the front side to the back side of the photocathode. Based on this concept, a 20kV gun was constructed with a transmission photocathode including an active layer of a GaAs–GaAsP superlattice layer. This system produces a laser spot diameter as small as 1.3μm for 760–810nm laser wavelength. The brightness of the polarized electron beam was ∼2.0×107Acm−2sr−1, which corresponds to a reduced brightness of ∼1.0×107Am−2sr−1V−1. The peak polarization of 77% was achieved up to now. A charge density lifetime of 1.8×108Ccm−2 was observed for an extracted current of 3μA.

Real Time Magnetic Imaging by Spin-Polarized Low Energy Electron Microscopy with Highly Spin-Polarized and High Brightness Electron Gun

We developed a spin-polarized low energy electron microscopy (SPLEEM) with a highly polarized and high brightness spin electron gun in the present study. Magnetic structures of Co/W(110) were observed with an acquisition time of 0.02 s with a field of view of 6 µm. We carried out a dynamic observation of magnetic structures with the SPLEEM during the growth of Co on W(110).

Strain dependence of spin polarization of photoelectrons from a thin GaAs layer

Abstract We studied the strain dependence of the spin polarization of photoelectrons from a thin GaAs epilayer which was strained by a lattice mismatched GaP x As 1− x buffer substrate. The polarizations were measured for seven samples with different residual strains between 0.34% and 0.80%. Our result shows the clear dependence of the polarization on the residual strain in this strain region. The maximum polarization attained by each sample increases gradually from 67% up to 87% with an increase of the residual strain which corresponds to that of the band splitting from 22 to 52 meV. A brief discussion about this behavior is given.

High-voltage testing of a 500-kV dc photocathode electron gun

A high-voltage dc photocathode electron gun was successfully conditioned up to a voltage of 550 kV and a long-time holding test for 8 h was demonstrated at an acceleration voltage of 500 kV. The dc photocathode electron gun is designed for future light sources based on energy-recovery linac and consists of a Cockcroft-Walton generator, a segmented cylindrical ceramic insulator, guard-ring electrodes, a support-rod electrode, a vacuum chamber, and a pressurized insulating gas tank. The segmented cylindrical ceramic insulator and the guard-ring electrodes were utilized to prevent any damage to the insulator from electrons emitted by the support-rod electrode.

Highly polarized electron source using InGaAs-GaAs strained-layer superlattice

We have studied the polarization of photoemission from an In0.15Ga0.85As–GaAs strained-layer superlattice. The polarization of 82.7±0.3(stat.)±6.1(syst.)% was observed at laser wavelengths from 911 to 916 nm at room temperature. The quantum efficiency at the wavelength of 911 nm was ~0.015% in the vacuum of ~6×10-10 Torr with high cathode voltage of -4 kV.

Spin Relaxation of Electrons in Strained-GaAs-Layer Photocathode of Polarized Electron Source

The luminescence polarization method using a mode-locked Ti:sapphire laser and a streak camera is applied to the measurement of the spin relaxation time and the lifetime of electrons in the strained-GaAs-layer photocathode of a polarized electron source. The spin relaxation time and the electron lifetime are 105 ps and 45 ps at room temperature, respectively. Electron-hole scattering is thought to be the main mechanism of the spin relaxation of our strained-GaAs photocathode.

New-Type Photocathode for Polarized Electron Source with Distributed Bragg Reflector

In order to increase the quantum efficiency of the strained GaAs photocathode for a highly polarized electron source, we designed a new type of photocathode with a distributed Bragg reflector (DBR). A Fabry-Perot cavity is formed by the DBR and the GaAs surface. A large enhancement of quantum efficiency was observed at the laser wavelength which satisfied the condition for the resonant absorption of incident laser light. Based on this experiment, it appears promising to make a photocathode which has the quantum efficiency of more than ~1% together with electron spin polarization higher than 80%.