Abstract
A theory of the shift current induced by direct two-photon and indirect one-photon absorption is developed for noncentrosymmetric crystals. A formula is derived for the microscopic shifts of Bloch electrons induced by two-quantum processes. It is shown that the ratio of the two-photon photocurrent at the photon frequency ω to the photocurrent induced by direct one-photon transitions at the photon frequency 2ω, as compared to the corresponding absorption-rate ratio, contains a large factor {ie152-1}ω/(2{ie152-2}ω − Eg), where Eg is the bandgap; i.e., these photocurrent can be comparable in order of magnitude. For crystals of Td symmetry, the photocurrents induced by one- and two-photon absorption are compared in terms of polarization dependence.
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