"We present an ab initio study of the GaAs(11) surface. In particular, the spin injection current, the electrical injection current and the degree of spin polarization are calculated for one-photon excitations. First, we present a formal derivation of the formulas used to obtain such responses; we explicitly obtain the expressions for both the total and the layered contributions. In the second chapter, we give an introduction of the method used to calculate the wave function for a crystalline surface along with all its approximations. In the third chapter, we make a convergence study to ensure that our calculations will be correct within our approximations. Finally, in chapter 4, we present our results and its possible applications."