Using a newly developed transmission-type photocathode, an electron beam of super-high brightness [ð1:3 AE 0:5Þ Â 10 7 AÁcm À2 Ásr À1 ] 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. #

Articles you may be interested in30-kV spin-polarized transmission electron microscope with GaAs-GaAsP strained superlattice photocathode Development of spin polarized electron photocathodes: GaAs-GaAsP superlattice and GaAs-AlGaAs superlattice with DBR AIP Conf.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 20 kV 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 1.3 m for 760-810 nm laser wavelength. The brightness of the polarized electron beam was ϳ2.0ϫ 10 7 A cm −2 sr −1 , which corresponds to a reduced brightness of ϳ1.0ϫ 10 7 A m −2 sr −1 V −1 . The peak polarization of 77% was achieved up to now. A charge density lifetime of 1.8 ϫ 10 8 C cm −2 was observed for an extracted current of 3 A.

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 m. We carried out a dynamic observation of magnetic structures with the SPLEEM during the growth of Co on W(110). #

Extremely low emittance electron beams are required for next generation accelerators. GaAs semiconductor photocathodes with negative electron affinity ͑NEA͒ surfaces have an intrinsic advantage for generating such low emittance beams and the thermal emittance as low as 0.1 mm mrad is expected in ideal case. The thermal emittance of photoelectrons was measured for two different NEA photocathodes: a bulk-GaAs photocathode and a GaAs-GaAsP superlattice strained photocathode. The normalized root-mean-sqare emittances for the beam radius of 1.0 mm were as low as 0.20− 0.29± 0.02 and 0.15± 0.02 mm mrad, respectively. A comparison of these results shows that the superlattice photocathode minimizes the thermal emittance for photon excitation energies higher than the band gap energy.

We have successfully developed a transmission-type GaAs/GaAsP strained superlattice (SL) photocathode, and a high spin-polarization (SP) (90%) with a super-high brightness (~10 7 A⋅cm −2 ⋅sr −1 ) of electron beam was achieved [1]. In this study, we report the design and fabrication of an optimized transmission-type photocathode with strain-compensated SL for higher quantum efficiency (QE).In the GaAs/GaAsP strained SL, a compressive strain was introduced in the GaAs well layers to obtain a large band-splitting between heavy-hole and light-hole mini-bands. The increasing SL pair-number causes strain relaxation with resultant SP degradation. A smaller SL layer thickness is one reason behind the limited value of the QE. To overcome this problem by increasing the SL layer thickness without degradation, the use of strain-compensated SL was proposed [2]. In this structure, a strain is introduced in the SL barrier layers to the opposite direction to compensate the strain in the SL well layers. Figure 1 shows the GaAs/GaAsP strain-compensated SL structure. The maximum pair of the prepared SL is 90.X-ray diffraction revealed that the strain relaxation by thickness increase was effectively controlled. Figure 2 shows the change of maximal spin-polarization with the SL pair number. The superlattice photocathodes up to 36-pair maintain high SP of about 90%. Then, the SP obviously decreased. During the transport, the spin-polarized electrons should flip by scattering with holes. The scattering effect becomes stronger in the thicker SL photocathodes. The thickness effect on the QE and transport time will be investigated.

No abstract

GaAs/GaAsP strain-compensated superlattices (SLs) with thickness up to 90-pair were fabricated. Transmission electron microscopy revealed the SLs are of high crystal quality and the introduced strain in SLs layers are fixed in the whole SL layers. With increasing SL pair number, the strain-compensated SLs show a less depolarization than the conventional strained SLs. In spite of the high crystal quality, the strain-compensated SLs also remain slightly depolarized with increasing SL pairs and the decrease in spin-polarization contributes to the spin relaxation time. 24-pair of GaAs/GaAsP strain-compensated SL demonstrates a maximum spin-polarization of 92% with a high quantum efficiency of 1.6%.

We verify that each wave packet of spontaneous radiation from two undulators placed in series has a double-pulsed temporal profile with pulse spacing which can be controlled at the attosecond level. Using a Mach–Zehnder interferometer operating at ultraviolet wavelengths, we obtain the autocorrelation trace for the spontaneous radiation from the tandem undulator. The results clearly show that the wave packet has a double-pulsed structure, consisting of a pair of 10-cycle oscillations with a variable separation. We also report the characterization of the time delay between the double-pulsed components in different wavelength regimes. The excellent agreement between the independent measurements confirms that a tandem undulator can be used to produce double-pulsed wave packets at arbitrary wavelength.