H. Aoyagi

Large enhancement of spin polarization observed by photoelectrons from a strained GaAs layer

Abstract We have observed a large enhancement of the spin polarization of photoelectrons emitted from a 0.08 μm thick strained GaAs(001) layer grown on a GaPxAs1−x substrate by the MOCVD method with x=0.17. For this fraction of phosphorus, the lattice-mismatch was estimated to be ∼0.6% and the energy splitting between heavy-hole and light-hole bands at the valence band maximum to be ∼40 meV. The maximum polarization of ∼86% was observed with a quantum efficiency of ∼2 x 10−4, under the conditions that the cathode was at room temperature and the excitation photon wavelength was λ≈860 nm.

The Stanford linear accelerator polarized electron source

The Stanford 3-km linear accelerator at SLAC has operated exclusively since early 1992 using a polarized electron beam for its high-energy physics programs. The polarized electron source now consists of a diode-type gun with a strained-lattice GaAs photocathode DC biased at high voltage and excited with circularly polarized photons generated by a pulsed, Ti:sapphire laser system. The electron polarization at the source is > 80%. To date the source has met all the beam requirements of the SLC and fixed target programs with < 5% downtime.

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.

A High Polarization and High Quantum Efficiency Photocathode Using a GaAs?AlGaAs Superlattice

A charge of 2.3×1011 electrons in 2.5 ns at a laser wavelength of 757 nm with a corresponding quantum efficiency (QE) of 2.0% measured at 752 nm was extracted from a -120 kV biased, 20 mm diameter, GaAs–AlGaAs superlattice photocathode. The maximum electron polarization measured with material from the same wafer, but in a different system, was 71% at 757 nm for a QE of 1.0% measured at 752 nm. The quantity and temporal distribution of the extracted charge is consistent with a space charge limitation, rather than a cathode charge limit. The performance of this type of cathode makes it a possible candidate for future linear colliders.

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%.

Depolarization of photoemission at the surface and interior of an AlGaAsGaAs superlattice

Abstract We have studied the polarization of photoemission from an AlGaAsGaAs superlattice as a function of the total thickness of the superlattice. In three samples with thicknesses of 0.4, 0.1, and 0.05 μm the measured polarizations were 51.0, 69.9, and 73.9% respectively. We obtain 75.5% when we extrapolate these results to the case of a sample so thin that depolarization occurs only at the surface. We tested a fourth sample for which the thickness was 0.1 μm and the Be dopant concentration was a factor of ten below the high concentration of the first three samples. The fourth sample produced a polarization of 74.8%. This result confirms the expectation that reduced doping lowers the spin relaxation in the interior of the superlattice. We conclude that depolarization at the surface dominates interior depolarization when the sample is thin or the dopant concentration is low.

Prospects for generating polarized electron beams for a linear collider using an rf gun

Abstract The next generation of linear colliders — represented by the Japanese Linear Collider (JLC) and the Next Linear Collider (NLC) — will probably utilize polarized electrons generated by a photocathode gun. A photocathode gun with high polarization ( P e ) photocathodes (up to P e ∼ 80% achieved to date) is currently providing polarized electrons for the SLC. The SLC source requires subharmonic bunching at low energy to reduce the bunch length prior to S-band bunching and a damping ring at high energy to reduce the transverse emittance. The use of an rf gun can eliminate the former and possibly simplify the latter. However, rf guns as presently developed have serious problems with vacuum contamination, which would quickly lower the quantum efficiency (QE) of a semiconductor photocathode. In addition, the “charge limit” previously reported for high peak current pulses puts a limit on the laser power usable for photoexciting a low QE cathode near the bandgap threshold. These problems have so far precluded any serious attempt to design an rf gun for polarized electrons. Several technical advances that now improve the prospects for a practical polarized electron rf gun are described. Finally, new ideas for high polarization photocathodes that permit operation in a relatively poor vacuum and techniques being explored to mitigate the low QE “charge limit” are discussed.

Splitting of Degeneracy of Valence Band in Strained GaAs Layers Observed From Polarization of Photoelectrons

Abstract Eight specimens composed of a thin GaAs layer on top of a thick GaAsP layer were grown by metalorganic vapor phase epitaxy on GaAs substrates. The strain in the GaAs layers was estimated by X-ray diffraction, assuming that both the thick GaAsP layers and the substrates are free from strain. The release of the strain in the GaAs layers whose thickness are higher than the critical ones is realized and this results in dividing the layers into many subgrains. Some amount of strain still remains and deforms the subgrains. The rate of the release is described using the layer thickness normalized by the critical thickness. The remaining strains will split the degeneracy of the electron states in the valence band. The dependence of the amount of splitting on the strain was confirmed by the high spin polarization of photoelectrons.

The high peak current polarized electron source of the Stanford Linear Collider

Abstract The Stanford Linear Collider injector requires two 2 ns pulses of 4.5–5.5 × 10 10 electrons, separated by 61 ns at 120 Hz, from its source. Since 1992, these currents have been provided by a polarized electron source based on GaAs photocathodes. A beam polarization of 76 ± 4% has been measured at the end of the 50 GeV linac. At low photocathode quantum efficiencies, and for excitation near threshold, the maximum current delivered by the source is constrained, not by the space charge limit of the gun, but by a “charge limit” of the photocathode. The charge limited current is proportional to the photocathode quantum efficiency, but the proportionality varies for different photocathode types. Experience with high polarization strained GaAs photocathodes on a test beamline and on the SLC is presented.