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We present experimental evidence for the final state stimulation of the exciton-exciton scattering rate into the exciton-polariton state in a semiconductor microcavity. We have studied the exciton density, polariton density, and time delay dependence of the stimulated emission, and have found the behavior to be in agreement with a prediction from a rate equation model of the dynamics.

We investigate a GaAs/(Ga,Al)As Fabry-Perot microcavity, into which (In,Ga)As quantum wells have been inserted. The cavity is wedge shaped, i.e., the detuning between the bare-exciton resonance and the bare optical cavity mode depends on the spatial position on the sample. Linear transmission spectra reveal a well-resolved Rabi splitting of 8 meV at resonance, an inhomogeneously broadened exciton transition of 4-5-meV width, and an 0.7-meV-wide Fabry-Perot mode. The time-resolved transmission exhibits deep beatings and a subpicosecond exponential decay: a behavior similar to that foreseen in the strong-coupling regime and in the absence of electronic disorder. Conversely, the four-wave mixing response appears weakly influenced by the cavity and not much different from what is expected for bare excitons. A photon echo, dephasing times as long as 50 ps, and only weak Rabi oscillations are observed. The experimentally observed features can be explained by a model based on the numerical solutions of the Maxwell-Bloch equations. This model confirms the dramatic influence structural disorder in the quantum wells has on the coherent nonlinear exciton-photon dynamics.

A theoretical investigation of the exciton formation process from free carriers in a single GaAs/Al1-xGaxAs quantum well is presented. The mechanism for the formation processes is provided by the interaction of the electrons and holes with phonons. The contributions from both the acoustic and optical phonons are considered. The relative importance of exciton creation from a thermalized electron-hole gas (bimolecular formation) versus a direct creation of excitons from the crystal ground state through the electromagnetic field (geminate formation) is discussed. The formation process is analyzed for different densities and temperature of the free carriers, and for different intensities of the exciting optical pump. The results compare reasonably well with recent experiments.

We solve the problem of the scattering of an electromagnetic plane wave from a planar array of equispaced cylindrical scatterers with strong resonances. We apply this theory to calculate the reflectivity of an array of quantum wires using the electromagnetic properties of the single wire as the only input. We find electromagnetic interference between the wires, whose effects are additional shifts and broadenings of the isolated wire resonances. A detailed analysis of some commonly used experimental configurations is also presented, with evidence of interference effects. Resonance shifts and broadenings are also interpreted as properties of the mixed modes of the quantized electromagnetic fields and the full grating excitations, i.e., grating polaritons. Stationary modes of the grating polaritons are also found, which have the peculiar property of being guidelike modes sustained by electronic excitations confined along one propagation direction.

We study the convergence and the stability of fictitious dynamical methods for electrons. first, we show that a particular damped second-order dynamics has a much faster rate of convergence to the ground state than first-order steepest-descent algorithms while retaining their numerical cost per time step. Our damped dynamics has efficiency comparable to that of conjugate gradient methods in typical electronic minimization problems. Then, we analyze the factors that limit the size of the integration time step in approaches based on plane-wave expansions. The maximum allowed time step is dictated by the highest frequency components of the fictitious electronic dynamics. These can result either from the larger wave vector components of the kinetic energy or from the small wave vector components of the Coulomb potential giving rise to the so called charge sloshing problem. We show how to eliminate large wave vector instabilities by adopting a preconditioning scheme in the context of Car-Parrinello ab initio molecular-dynamics simulations of the ionic motion. We also show how to solve the charge sloshing problem when this is present. We substantiate our theoretical analysis with numerical tests on a number of different silicon and carbon systems having both insulating and metallic character.

We prove that reflectivity measurements as a function of angle of incidence allow the measurement of the exciton-polariton modes in quantum wells. The resonant polaritons at ${\mathit{k}}_{\mathrm{?}}$${\mathit{k}}_{0}$=\ensuremath{\omega}/v manifest themselves as peaks in the reflectivity while surface polaritons at ${\mathit{k}}_{\mathrm{?}}$g${\mathit{k}}_{0}$ manifest themselves as dips in the attenuated total reflection. A comparison of the reflectivities with polarization perpendicular and parallel to the plane of incidence allows us to detect the different polariton modes and to measure the dispersion relation. We also give expressions to compute reflectivity and transmission for the case of multiple quantum wells, including the effects of multiple reflections. Examples are given for standard reflectivity of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells and for attenuated total reflection of CuCl/${\mathrm{CaF}}_{2}$ quantum wells.

Based on density-functional perturbation theory we have computed from first principles the photoelastic tensor of few crystalline phases of silica at normal conditions and high pressure (quartz, $\alpha$-cristobalite, $\beta$-cristobalite) and of models of amorphous silica (containig up to 162 atoms), obtained by quenching from the melt in combined classical and Car-Parrinello molecular dynamics simulations. The computational framework has also been checked on the photoelastic tensor of crystalline silicon and MgO as prototypes of covalent and ionic systems. The agreement with available experimental data is good. A phenomenological model suitable to describe the photoelastic properties of different silica polymorphs is devised by fitting on the ab-initio data., Comment: ten figures