We show theoretically that interband transitions in a bulk semiconductor via coherent one-and twophoton absorption leads to the formation of an electrical current whose direction is controlled by the relative phase of the beams. The phenomenon can occur in centrosymmetric and noncentrosymmetric materials; easily measurable currents are predicted for GaAs under realistic experimental conditions.

We demonstrate room temperature coherent generation and control of a directional photocurrent in bulk GaAs via simultaneous one-and two-photon interband absorption processes using phase-related 1 ps or 175 fs pulses at 0.775 and 1.55 mm. Electrical currents generated in low-temperature-grown (LT) and normal bulk GaAs are collected via gold electrodes. Current densities as high as 3 nA͞mm 2 in LT-GaAs are measured for injected carrier densities as low as 10 14 cm 23 and for peak irradiances of 18 MW cm 22 (1.550 mm) and 3 kW cm 22 (0.775 mm).

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We give a detailed procedure for the calculation of the second-harmonic-generation susceptibility tensor in single and multiple asymmetric quantum wells (AQW's). The effect of nonhomogeneity is explicitly included. Exciton states and continuum states are considered. The contributions due to the asymmetry of the confining potential and to the inversion asymmetry of the bulk material are separately assessed. Numerical calculations are carried out for two typical AQW's of GaAs/Gai Al As. The results indicate larger values than in the bulk, in agreement with preliminary experimental data.