Two-photon- photoluminescence excitation spectroscopy of single quantum-dots

We present experimental and theoretical study of single semiconductor quantum dots excited by two non-degenerate, resonantly tuned variably polarized lasers. The first laser is tuned to excitonic resonances. Depending on its polarization it photogenerates a coherent single exciton state. The second laser is tuned to biexciton resonances. By scanning the energy of the second laser for various polarizations of the two lasers, while monitoring the emission from the biexciton and exciton spectral lines, we map the biexciton photoluminescence excitation spectra. The resonances rich spectra of the second photon absorption are analyzed and fully understood in terms of a many carrier theoretical model which takes into account the direct and exchange Coulomb interactions between the quantum confined carriers.Comment: Accepted for publication in PR

Shell structure and electron-electron interaction in self-assembled InAs quantum dots

Using far-infrared spectroscopy, we investigate the excitations of self-organized InAs quantum dots as a function of the electron number per dot, 1<n<6, which is monitored in situ by capacitance spectroscopy. Whereas the well-known two-mode spectrum is observed when the lowest s - states are filled, we find a rich excitation spectrum for n=3, which reflects the importance of electron-electron interaction in the present, strongly non-parabolic confining potential. From capacitance spectroscopy we find that the electronic shell structure in our dots gives rise to a distinct pattern in the charging energies which strongly deviates from the monotonic behavior of the Coulomb blockade found in mesoscopic or metallic structures.Comment: 4 pages, 3 PostScript figure

Slowly Rotating Homogeneous Stars and the Heun Equation

The scheme developed by Hartle for describing slowly rotating bodies in 1967 was applied to the simple model of constant density by Chandrasekhar and Miller in 1974. The pivotal equation one has to solve turns out to be one of Heun's equations. After a brief discussion of this equation and the chances of finding a closed form solution, a quickly converging series solution of it is presented. A comparison with numerical solutions of the full Einstein equations allows one to truncate the series at an order appropriate to the slow rotation approximation. The truncated solution is then used to provide explicit expressions for the metric.Comment: 16 pages, uses document class iopart, v2: minor correction

Stimulated and spontaneous optical generation of electron spin coherence in charged GaAs quantum dots

We report on the coherent optical excitation of electron spin polarization in the ground state of charged GaAs quantum dots via an intermediate charged exciton (trion) state. Coherent optical fields are used for the creation and detection of the Raman spin coherence between the spin ground states of the charged quantum dot. The measured spin decoherence time, which is likely limited by the nature of the spin ensemble, approaches 10 ns at zero field. We also show that the Raman spin coherence in the quantum beats is caused not only by the usual stimulated Raman interaction but also by simultaneous spontaneous radiative decay of either excited trion state to a coherent combination of the two spin states.Comment: 4 pages, 3 figures. Minor modification

Externally mode-matched cavity quantum electrodynamics with charge-tunable quantum dots

We present coherent reflection spectroscopy on a charge and DC Stark tunable quantum dot embedded in a high-quality and externally mode-matched microcavity. The addition of an exciton to a single-electron charged quantum dot forms a trion that interacts with the microcavity just below strong coupling regime of cavity quantum electrodynamics. Such an integrated, monolithic system is a crucial step towards the implementation of scalable hybrid quantum information schemes that are based on an efficient interaction between a single photon and a confined electron spin.Comment: 10 pages, 4 figure

Dislocation Free Island Formation in Heteroepitaxial Growth: An Equilibrium Study

We investigate the equilibrium properties of strained heteroepitaxial systems, incorporating the formation and the growth of a wetting film, dislocation free island formation, and ripening. The derived phase diagram provides a detailed characterization of the possible growth modes in terms of the island density, equilibrium island size, and wetting layer thickness. Comparing our predictions with experimental results we discuss the growth conditions that can lead to stable islands as well as ripening.Comment: 4 pages, LaTeX, 3 ps figure

Imaging and spectroscopy of single InAs self-assembled quantum dots using ballistic electron emission microscopy

Single InAs self-assembled quantum dots buried spatially beneath a Au/GaAs interface are probed for the first time using the imaging and spectroscopic modes of ballistic electron emission microscopy (BEEM). BEEM images show enhanced current through each dot. Spectra taken with the tip positioned on a dot show shifted current thresholds when compared with the off dot spectra, which are essentially the same as those of Au on bulk GaAs. Shifts in the γ and L conduction band thresholds are attributed to strain in the GaAs cap layer. Fine structure below the γ threshold is consistent with resonant tunneling through zero-dimensional states within the quantum dots