A. L. Ivanov

Optics of excitonic molecules in semiconductors and semiconductor microstructures

Abstract A bipolariton concept of an excitonic molecule in direct-band-gap semiconductors is reviewed. The bipolariton model interprets a molecule as two quasi-bound polaritons provided that both the exciton-exciton Coulombic interaction and the exciton-photon coupling (polariton effect) are treated simultaneously and beyond a low-order perturbation theory. The molecule-mediated optics is reformulated within the bipolariton concept for bulk semiconductors (e.g., CuCl and CdS) and semiconductor microstructures (e.g., GaAs quantum wells and wires). We discuss such phenomena as two-photon absorption and four-wave-mixing due to excitonic molecules, molecule-mediated optical Stark effect and other coherent processes which effectively involve excitonic molecules, and polariton solitons coupled through the molecule state.

Transient optical emission from excitonic molecules in GaAs quantum wells: Coherent quantum evolution in momentum space

Resonantly-excited photoluminescence (PL) of excitonic molecules with femtosecond resolution reveals a surprising initial transient characterized by a finite rise and an extremely fast non-exponential decay. We show that this initial transient stems from coherent quantum evolution of relative momentum of two excitons in the molecule, in sharp contrast to radiative dynamics of excitons. The initial transient also reflects a type of quantum beats that differ from the usual quantum beats in transient optical measurements.

Mesoscopic Semiconductor Switching Element with Giant Electro-Optical Nonlinearities due to Intrinsic Photoconductivity

We propose and analyze a new efficient mesoscopic room-temperature electro-optical device consisting of long-period GaAs/AlGaAsMQW under a vertical voltage bias through a resistor in series. Both the MQW and the substrate are designed to be undoped. Intrinsic photoconductivity, initiated by the photo-active GaAs layers, gives rise to an effective feedback through Kirchhoffs law between the light beam intensity and the voltage drop over MQW. The giant electro-optical nonlinearities of this element stems from (i) Franz-Keldysh effect, (ii) photocarriers accumulation in the GaAs layers due to the charge separation induced by the vertical quasi-static electric field, (iii) dependence of a mean free-path of the ballistic carriers on this electric field. The device operates at light intensities less than 100 mW/cm2 with a switching time of about 100 ns.

Lamb Shift for an Excitonic molecule in GaAs Quantum Well

We study the influence of the radiative decay of an excitonic molecule (xx) in quasi-2D quantum well (QW) on the xx effective binding energy (xx Lamb shift). For the xx wave function, the new Schrodinger equation which includes the polariton effects self-consistently is analyzed. In quasi-2D GaAs QW the radiative renormalizations give rise to a Mexican hat structure in the xx dispersion at small momenta together with a considerable increase of both the xx effective binding energy exx and the inverse xx radiative lifetime Γxx. Our theory explains a quantitative contradiction between the value of exx obtained in the recent experiments and the conventional theoretical one.

Bipolariton in quantum wells

SummaryWe develop the bipolariton concept for a quasi-2D excitonic molecule (m) in quantum wells (QWLs). The bipolariton wave equation, which includes both the exciton-exciton (x-x) attraction and thex-photon (γ) coupling, is analysed. In the 2D bipolariton model, a quasi-2Dm exists in QWLs mainly as two quasi-boundsurface polaritons and, consequently, decays mainly into two outgoing surface polaritons. In quasi-2D GaAs QWLs the polariton renormalizations give rise to a «Mexican hat» structure in them dispersion at small momenta together with a considerable increase of both them effective binding energy ɛm and the inversem radiative lifetimeГm. A van Hove singularity in the joint density of polariton states is responsible for this effect.

Analysis of an electro-optical switching device due to intrinsic photoconductivity

We analyse an electro-optical device consisting of a stack of undoped alternating layers of narrow- and wide-gap materials (e.g. GaAs/AlGaAs) together with a series resistor under constant voltage bias. The device depends crucially on the photoconductivity of the narrow-gap layers with a width of the order of a carrier mean free path. The Franz-Keldysh effect, the charge separation of photoexcited carriers in a static field, and the ballistic part of the current are responsible for the resulting very large electro-optical nonlinearity. In a Fabry-Perot cavity a room temperature electro-optical bistability is obtained at light intensities smaller than 10 mW cm-2.