Session Y5: Optical Properties and Excitations in Semiconductor Quantum Dots

Elastic vs Inelastic Light Scattering from a Quantum Dot

We spectrally resolve the light scattered by a single InAs semiconductor quantum dot and analyze in detail the contribution from elastic and inelastic scattering processes. The measurements are well described by the theoretical expression given by Mollow. High resolution measurements reveal that the elastically scattered light is highly phase coherent with the laser. Thus a quantum dot elastically scattering a pulsed laser may serve as a triggered single photon source. In this regime, spectral diffusion and other broadening mechanisms are not a bottleneck for obtaining transform-limited photons.   

Polarized luminescence characterization of charged quantum dot molecules

Polarization sensitive spectroscopy of self assembled quantum dots (QDs) has been shown to yield important information about spins associated with the charge carriers in various excitonic states. As pairs of quantum dots are brought together, and the formation of molecular states via tunneling becomes relevant, the interactions that determine the polarization state can be modified. In this talk we will present polarization dependent photoluminescence and photoluminescence excitation studies on vertically stacked InAs QDs grown by molecular beam epitaxy. We will discuss the characterization of the Stokes parameters of singly and doubly charged exciton states in these coupled QDs and compare with the results reported for single QDs. This study will help in identifying appropriate charge states for potential spin manipulation and entanglement measurements.   

Exciton fine structure splitting in self-assembled semiconductor quantum dots: Intrinsic and extrinsic effects

We investigate the excitonic fine structure splitting (FSS) in InGaAs/GaAs and GaAs/AlGaAs quantum dots (QDs) of different shapes and sizes using an atomistic pseudopotential approach [1,2]. We consider intrinsic effects originating from the atomistic symmetry of the structure. We highlight the effects of the growth direction and the repercussions it has on the point group symmetry and the FSS. We give predictions for the cases where the semiconductor alloy has a certain degree of atomic order, and for the case, where the QDs are influenced by charged point defects. These effects are contrasted to the extrinsic effect of applied stress.\\ \noindent [1] R. Singh and G. Bester, {\it Lower bound for the excitonic fine structure splitting in self-assembled quantum dots.} Phys. Rev. Lett. {\bf 104}, 196803 (2010).\\ \noindent [2] R. Singh and G. Bester, {\it Nanowire Quantum Dots as an Ideal Source of Entangled Photon Pairs.} Phys. Rev. Lett. {\bf 103}, 063601 (2009).   

Coulomb Enhanced Nonlinear Optical Properties of Strongly Confined Excitons in InAs/GaAs Quantum Dots

Nonlinear optical response in parametric crystals has become widely used in entanglement production; however, low generation rate is an undesirable feature of this technique. Self-assembled quantum dots arrays (QDAs) may be a promising alternative with a larger nonlinear coefficients respect to higher dimensionality systems. Previously, some works have dealt with nonlinear susceptibilities of InAs/GaAs quantum dots, studying intraband transitions of either electrons or holes. The second order susceptibilities found there, are substantially bigger than those in bulk samples, although interband excitations and Coulomb effects were not considered. In this work we study the effects of strong confinement and Coulomb interactions on exciton states in fully 3D axially symmetric QDs. By using partial CI approach, we obtain eigenenergies and envelope eigenfunctions. Second order optical susceptibilities and their dependence on quantum dot size and aspect ratio are calculated. As a main result, we observe a Coulomb related enhancement in the second order optical susceptibilities of exciton transitions as compared to those in bulk, 2D, and even 0D purely intraband systems. This increased nonlinear response suggests interband excited QDAs as efficient entanglement sources.   

Diamagnetic Exciton Properties in Quantum Dot Molecules

The magnetic properties of nanostructures like quantum dots and rings are the subject of intense research. In particular, magnetic control of coupled quantum dots (artificial molecules) has become subject of interest. The diamagnetic shift of confined excitons complexes has been used as a measured of the wave function spatial extent in semiconductor nanostructures. In weak magnetic field, the diamagnetic shift is expected to exhibit quadratic dependence. However, for exciton complexes the diamagnetic behavior is expected to exhibit more complicated features related to electron-hole asymmetry effects on Coulomb interactions. In this work we study the magnetic response of neutral and charged excitons in InAs/GaAs asymmetric artificial molecules By using a first order perturbation approach, and within the effective mass approximation, we calculate magnetic field dependent electronic structures of confined excitons and trions in vertically coupled quantum dots. These predicted regions, which show coexistence of crossing and anticrossing exciton states, because of allowing control of charge localization and polarization of emitted photons. .   

Observed Shifts in Unoccupied States for Cu Doped CdSe Quantum Dots Observed via Synchrotron Techniques

Recent work has been targeted on examining the optical properties of guest ions in quantum dot (QD) lattice; however, very few studies have attempted to understand the effect the dopant has on the host electronic structure. In this talk, we will present data that suggests copper doping of CdSe QDs leads to trapped states below the conduction band (CB) minimum of the host CdSe particle. We propose that one possible reason for this could be hybridization between copper and cadmium, lowering the energy for the cadmium 5s states below the CB minimum of bulk CdSe. X-ray absorption near edge structure spectroscopy measurements at the Cd $M_3$-edge for bulk, undoped, and doped QDs are compared and an unexpected lowering in the CB minimum is observed. We also present a first order theoretical model, for describing our results considering the effects caused by confinement, doping, and hybridization. Numerical approximations for atomic interactions suggest the hybridization parameter can lead to a lowering of the CB minimum by as much as 1.5 eV, as observed experimentally. Future work will include more in depth modelling of hybridization starting from tight binding calculations, developing a predictive model, applicable to more than existing data.   

Elementary electronic excitations in quantum wires made up of vertically stacked quantum dots

We investigate the elementary electronic excitations in quantum wires made up of vertically stacked (self-assembled) InAs/GaAs quantum dots. The length scales (of a few nm) involved in the experimental setups prompt us to consider an infinitely periodic system of two-dimensionally confined (InAs) quantum dot layers separated by GaAs spacers. The resultant quantum wire is characterized by the Bloch functions and the Hermite functions. We make use of the Bohm-Pines' RPA in order to derive a general nonlocal, dynamic dielectric function. The theoretical framework is then specified to work within a two-subband model that enables us to scrutinize the single-particle as well as collective responses of the system. We also size up the importance of studying the inverse dielectric function in relation with the quantum transport phenomena. It is remarkable to notice how the variation in the barrier- and well-widths can allow us to tailor the excitation spectrum in the desired energy range. Given the foreseen applications in the single-electron devices and in the quantum computation, it is quite tempting to explore the electronic, optical, and transport phenomena in such systems.\footnote{M.S. Kushwaha, J. Chem. Phys. {\bf 135}, 124704 (2011).}   

Anomalous Suppression of Valley Splittings in Lead Salt Nanocrystals

Atomistic $sp^3d^5s^*$ tight-binding theory of PbSe and PbS nanocrystals is developed. It is demonstrated, that the valley splittings of confined electrons and holes strongly and peculiarly depend on the geometry of a nanocrystal. When the nanocrystal lacks a microscopic center of inversion and has $T_d$ symmetry, the splittings are strongly suppressed as compared to the more symmetric nanocrystals with $O_h$ symmetry, having an inversion center. This effect is quite unusual because typically a higher symmetry of a physical system implies a higher degeneracy of its energy levels, while in our case the suppression of the splittings occurs in NCs having lower symmetry. Nevertheless, we were able to explain this puzzling behavior using mathematical apparatus of the group theory.   

First-principles spectroscopic characterization of PbSe nanoparticles passivated with Fe complexes

Given that defining characteristics of nanoparticles -- morphology, catalytic reactivity, optical and electronic properties -- are often dictated by their surfaces, it is informative to investigate how surface chemistry and structure change as different ligands are introduced to the surface. Starting with oleate-passivated PbSe nanoparticles, we remove the oleate ligands and replace them with an organometallic complex: cyclopentadienyl iron dicarbonyl. Measured and calculated x-ray photoemission core-level shifts indicate a charge transfer between surface Pb atoms and Fe atoms. We investigate the nature of this charge transfer in more detail through analysis of x-ray absorption spectra (XAS) at the Fe L-edge. Fe XAS are calculated from first-principles using a GW-based Bethe-Salpeter approach. The spectra reveal that the extent to which pi-backbonding is possible between the Fe and associated carbonyls varies with the charge density on the Fe atom.   

Preservation of the optical properties of small Si quantum dots in the face of oxidation

Rapid oxidation occurs when small, unpassivated Si quantum dots are subjected to ambient conditions, and it may at first seem that complete oxidation of 1-3 nm QDs is inevitable, since their initial oxidation rate is expected to be greater than that for bulk Si for a more open surface structure, and the oxide layers on bulk Si tend to be thicker than the dot radii. We use computations based on density functional theory show that sufficiently small, appropriately terminated dots might actually be able to resist oxidation better than their larger brethren. This is because, on well-passivated bulk surfaces, oxygen attacks silicon through vulnerable sites and defects, and defects are much less likely to be present as dot size decreases. Although the more open surface structure of these small dots does leave certain keys sites more vulnerable to oxidation than their bulk counterparts, the oxygen atoms absorbed there are essentially immobile due to large hopping barriers. Furthermore, computations employing the many-body Green function perturbation theory show that the oxidation of these QDs has a relatively small impact on optical character. Therefore, the best defense against oxidation is to eliminate defects; a strategy that becomes increasingly reasonable as dot size decreases.   

Confinement effects on the vibrational properties of colloidal quantum dots

We present a first-principles study of the confinement effects on the vibrational properties of colloidal III-V and II-VI nanoclusters with thousand atoms and radii up to 16.2~\AA. We describe the connection between the vibrational properties including surface-optical and -acoustic modes, coherent acoustic modes and the structural changes induced by the surface. We highlight the qualitative difference between III-Vs and II-VIs. We can clearly ascribe most of the observations to the large relaxation of the clusters dominated by an inward relaxation of the surface penetrating deep inside the cluster in case of the III-Vs and a large distribution of bond length at the surface of II-VIs. These strong confinement effects tend to disappear for clusters with more than 1000 atoms, where a small red shift of the Raman peaks remains, due to a softening in response to undercoordination. The coherent acoustic phonons are identified and found to be in good agreement with results from the Lamb model and experiment. We explain why the simple model by Lamb gives an accurate description in case of the breathing modes while the vibrational properties of small NCs are poorly described by continuum models in general.