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We have observed a large enhancement of the spin polarization of electrons extracted from an AlGaAs-GaAs superlattice illuminated by circularly polarized light. A polarization of 71.2 ±1.1 (stat) ±6.1(syst)% was obtained with a photon wavelength of 802 nm at room temperature. We have also confirmed the removal of the degeneracy between a heavy-hole band and a light-hole band at the T point from the laser-wavelength dependence of the polarization.PACS numbers: 73.60. Br, 29.25.Bx, 29.75,+x, 79.60.Eq Introduction.-Recently, the desire for developing a highly polarized electron source has been increasing in high-energy physics [1], especially for applications to linear colliders such as the JLC. A GaAs photocathode with a negative-electron-affinity (NEA) surface illuminated by circularly polarized light is the standard method to produce an intense beam of polarized electrons. The intrinsic upper limit of polarization for a GaAs photocathode is 50% because of the degeneracy between the heavy-and the light-hole bands at the T point. An AlGaAs-GaAs superlattice has been studied as a possible electron source with polarization greater than 50%, due to the removal of the degeneracy by the periodic potential wells in the superlattice. However, none of the experiments performed in the 1980s [2] achieved a polarization of extracted electrons greater than 50%. The reasons for the low polarization were assumed to be the depolarization of the electron spin before emission, the lack of a large separation between the heavy-and the light-hole bands, or a combination of both. Recently, we observed an enhancement of the spin polarization slightly higher than 50% by using a 0.4-jum-thick superlattice consisting of alternate layers of Al*Gai-v As (31.1 A thick and x=0.35) and GaAs (19.8 A thick) [3]. SLAO Wisconsin-UC Berkeley-CEBAF group [4] also reported an enhancement which was slightly higher than 50% by using an AlAs-GaAs monolayer superlattice. However, the achieved polarizations in both experiments were still not high enough to justify the use of superlattices. In our previous sample, three parameters of the superlattice (the thickness of the GaAs and AlGaAs layers and the fraction of aluminum in AlGaAs) were chosen to give a large enough energy splitting between the heavy-and the light-hole bands compared to the thermal noise and to get a large transition rate between neighboring layers [5]. From the data of this sample we concluded that the energy splitting was large enough to excite the heavy-hole band only. However, depolarization inside and/or on the surface of the superlattice yielded only a small enhancement of the polarization of the extracted electrons. In or-

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%.