Field emission of spin-polarized electrons extracted from photoexcited GaAs tip

A pyramidal-shaped GaAs (tip-GaAs) photocathode for a polarized electron source (PES) was developed to improve beam brightness and negative electron affinity (NEA) lifetime by field emission. The emission mechanism also enables the photocathode to extract electrons from the positive electron affinity (PEA) surface into vacuum, and alleviates the NEA lifetime problem. The measured electrical characteristics of tip-GaAs and its polarization exhibited distinctive field-emission behavior. The polarization of the electron beam extracted from tip-GaAs was 20-38% under irradiation with circularly polarized light of 700-860 nm, and the peak polarization was 37.4 +/- 1.4% at a wavelength of 731 nm. These experimental results indicate that spin-polarized electrons can be extracted from the conduction band into vacuum by a field-emission mechanism. This, in turn, shows that this type of photocathode has the prospect of generating a low-emittance spin-polarized electron beam.