Below-bandgap excitation of bulk semiconductors by light carrying linear momentum

FABIAN CORREA QUINCHIA; G. F. QUINTEIRO

Conferencia; XVII Giambiagi Winter School; 2015

We theoretically investigate the response of bulk semiconductors to excitation by plane waves light below the energy bandgap. To do its we modify the generalized assumption, known as ?vertical transitions?(VT), that neglects the photon?s momentum in direct absorption/emission processes. Exceptions are found in early and recent works, reporting theoretical and experimental results strongly dependending on the conservation of the linear momentum of the photon that results in new selection rules not accounted for by VT. The dynamics of electrons in a semiconductor having a valence and a conduction bands is described by Heisenberg equations of motion for populations in each band plus the quantum coherence between them; these equations form a coupled system. Under the condition of low field excitation, the equations can be treated perturbatively and can be decoupled. The first order term in the light field is the quantum interband coherence whereas, populations are second order in thelight field. Then, the linear response of the unexcited system (zero conduction band population) can be obtained. We show that the excited states can be thought of as a superposition of slightly perturbed exciton states undergoing center-of-mass motion. The susceptibility shows the expected features, i.e. no spatial dependence but ?spatial dispersion?(dependence). The COM-motion correction introduces a tiny shift in the absorption line. Used in conjunction with Maxwell?sequations, the susceptibility yields information about the effect that electrons has on the EM field, eg. attenuation of the beam.