| The total absorption rate of optical beams with commensurate frequencies in a semi-conductor is dynamically controlled by the phase of the incident light fields. Phase control of free carrier injection arises from quantum interference between the linear and nonlinear absorption pathways that access the same initial and final states in the continuum bands, as detailed by a Fermi Golden Rule analysis. Carrier density and optical transmission control was demonstrated in bulk GaAs for two different interference schemes: one- vs. two-photon absorption (with beams of wavelengths 0.775 and 1.55 μm) and one- vs. three-photon absorption (with beams of wavelengths 0.675 and 2.03 μm). All experiments were done at room temperature using subpicosecond optical pulses. The schemes are representative of two classes of interference control categorized according to their material symmetry dependence. Interference between A- and B-photon absorption, where A+B is an odd number, can be observed only in a crystal without a centre of inversion. If A+B is even, interference can be observed in both centro- and noncentrosymmetric materials. The establishment of the quantum interference process within the nonlinear susceptibility formalism provides a new interpretation of the origins of certain tensor contributions to the optical response. The phase-dependent contribution to carrier injection arising from interference in the A+B scheme is related to the imaginary part of the nonlinear electric susceptibility, χA+B −1. For the first time, χ2 and other odd-rank tensors are linked to energy absorption. Experimental results in conjunction with numerical modeling of the nonlinear mixing process provide a novel method of extracting the magnitude of the imaginary relative to real components of χ 2 and χ3 for GaAs at 1.55 μm and 2.03 μm respectively. |
