Quantum interference in electron-hole generation in noncentrosymmetric semiconductors

We show that, when fundamental optical beams are present in a noncentrosymm etric medium simultaneously with their sum-frequency beam, quantum interfer ence between single- and two-photon transitions modifies the net absorption , if the sum frequency corresponds to an energy greater than the band gap. At a macroscopic level this effect can be related to the imaginary part of a second-order susceptibility and can be used to coherently control carrier populations and optical absorption. We illustrate this novel effect using phased 1550 and 775 nm, 120 fs pulses incident on GaAs at 295 K.