Polarization-induced phase shift of ultrafast photocurrents

Abstract

Shift and injection currents are known to occur for linearly and circularly polarized optical excitations of semiconductors, respectively. Here, we show with room-temperature experiments that for excitation of discrete transitions the frequency dynamics of the coherent polarization changes this phase rule significantly. For above-bandgap optical excitation of semiconductors two main components of all-optically induced photocurrents have been recognized and are known as shift and injection currents (1). These currents can be linked to a second-order optical nonlinear process associated with difference- frequency mixing. Microscopically, shift currents and injection currents result from a polar distribution of carriers in real and momentum space, respectively. They are also known as linear and circular photogalvanic currents, since they occur for optical excitation with linearly and circularly polarized light, respectively (1-3). Here, we show that the frequency dynamics of the coherent polarization resulting from excitation of discrete transitions is capable of significantly altering this phase rule such that shift and injection currents occur for both, linear and circular polarizations. The measurements are performed on exciton transitions in a (110)-oriented, 5-nm wide GaAs quantum well (QW) sample, with the x, y, and z directions being parallel to the (001), (1 0), and (110) crystallographic directions, respectively.