6533b86ffe1ef96bd12cd0b2

RESEARCH PRODUCT

<title>Spin-polarized electron kinetics under high-intensity picosecond excitation</title>

subject

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ABSTRACT Spin-polarized electron kinetics is studied by time-resolved polarized photoemission with picosecond resolution. The re-sponse time of strained layer photocathodes is found to be in a range of a few picosecond offering an ultrafast response andhigh spin-polarization of emitted electrons. The studies of the sub-picosecond spin dynamics are facilitated in high-intensityexcitation regime when the length of the emission pulse is enlarged due to the dispersion of acceleration time and Coulombrepulsion ofthe electrons in their flight in the vacuum.Keywords: optical orientation, spin kinetics, strained semiconductor layer, time-resolved emission. 1. INTRODUCTION GaAs, layers are known to be promising for design and fabrication of ultrafast room- temperature "spin-electronics" devices 2 for which spin dynamics of the carriers is of a particular interest . The strained layer GaAs-based structures are activelystudied for applications in highly polarized electron sources for accelerators in which uniaxially strained GaAs or GaAsPphotocathodes are used 4-6Forthe polarized r.f.-guns application the pulse-response times in the few pico-second range aredesirable. A scaling of the response time with d2 for the diffusive process provides ultrafast time response ofthin layers and,as a consequence of this, low depolarization because these effects happen on longer time scales. Several factors may modifythe behavior at low thicknesses: first, the diffusive response could be gradually replaced with a ballistic one in the sampleswith the active layer thickness comparable with the electron free path length. Then, multiple electron reflection at the sur-face can reduce the electron capture rate to the band bending region (BBR) which can be expressed in terms oflowering ofthe effective surface recombination velocity. Finally, the effects of the escape processes from the BBR through the surfaceregion could modify the diffusive behavior, e.g. by adding additional time dispersion. These electron-kinetic details are of aparticular importance for the efforts to increase the emitted electron polarization above 90 % so that the polarization lossesat the subsequent stages of the emission processes have to be distinguished and minimized by the appropriate structure de-sign.In this report we present a time resolved study of gradient-doped strained layer GaAs photocathode stmctures at vary-ing excitation intensity. The pulse response experiments enable us to estimate spin relaxation during electron extractionfrom the active layer and in the band bending region, as well as the electron capture rate to the band bending region.