In a microcavity, light-matter coupling is quantified by the vacuum-Rabi frequency ${\Omega }_R$. When ${\Omega }_R$ is larger than radiative and nonradiative loss rates, the system eigenstates (polaritons) are linear superposition of photonic and electronic excitations, a condition actively investigated in diverse physical implementations. Recently, a quantum electrodynamic regime (ultrastrong coupling) was predicted when ${\Omega }_R$ becomes comparable to the transition frequency. Here we report signatures of this regime in a quantum-well intersubband microcavity. Measuring the cavity-polariton dispersion in a room-temperature linear optical experiment, we directly observe the antiresonant light-matter coupling and the photon-energy renormalization of the vacuum field.

The optical response of the intersubband excitation of multiple two-dimensional electron gases within a semiconductor microcavity has been studied through angle-dependent reflectance measurements. Using a resonator based on total internal reflection, a clear splitting of about 14 meV of the coupled intersubband cavity modes is observed from 10 K to room temperature, with resulting polaritonlike dispersion. The experimental findings are in good agreement with theoretical calculations performed in a transfer-matrix formalism.

We present an analytic model that explains the self-ordering of quantum nanostructures grown on nonplanar surfaces. Self-limiting growth in these structures results from the interplay among growth-rate anisotropy, curvature-induced capillarity, and, for alloys, entropy of mixing effects. Experimental results on self-limiting organometallic chemical vapor deposition on corrugated surfaces are in quantitative agreement with the model. The implications of the self-limiting growth characteristics on the self-ordering of quantum wells, wires, and dots are discussed.

We have developed an analytic model that describes in detail the establishment of self-ordered profiles during semiconductor epitaxy on corrugated surfaces. Lateral, self-ordered epitaxy derives from surface gradients in the chemical potential due to the nonplanarity of the profile (capillarity). The growth rate variation on the different facets composing the profile determines whether the profile sharpens up at the bottom of the grooves or at the apex of the corrugations. For alloy growth, additional entropy of mixing effects affect the profile shape and composition. The predictions of the model were applied to explain the self-limiting surface profiles obtained by organometallic chemical vapor deposition on $\left[011¯\right]$-oriented grooves. Finally, this model is used to design a variety of low-dimensional quantum-confined nanostructures.

The structure of GaAs/AlGaAs quantum wires (QWRs) and vertical quantum wells (VQWs) grown by low-pressure organometallic chemical vapor deposition was investigated by conventional and high resolution transmission electron microscopy, and by low-temperature cathodoluminescence. The lower wire boundaries show a much smaller radius of curvature, as compared with atmospheric pressure growth of similar structures, and the upper boundaries show distinct faceting. More abrupt interfaces are obtained due to the kinetically limited growth, with measured interface grading as small as one or two monolayers. The VQW structures formed in the AlGaAs barrier exhibit several branches related to the faceting of the QWR boundaries. These characteristics of the low-pressure nonplanar growth should allow the fabrication of quasi-one-dimensional QWRs with size and shape controlled on the monolayer level.