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Register a new userPolarization sensitive spectroscopy of charged Quantum Dots
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We present an experimental and theoretical study of the polarized photoluminescence spectrum of single semiconductor quantum dots in various charge states. We compare our high resolution polarization sensitive spectral measurements with a new many-carrier theoretical model, which was developed for this purpose. The model considers both the isotropic and anisotropic exchange interactions between all participating electron-hole pairs. With this addition, we calculate both the energies and polarizations of all optical transitions between collective, quantum dot confined charge carrier states. We succeed in identifying most of the measured spectral lines. In particular, the lines resulting from singly-, doubly- and triply- negatively charged excitons and biexcitons. We demonstrate that lines emanating from evenly charged states are linearly polarized. Their polarization direction does not necessarily coincide with the traditional crystallographic direction. It depends on the shells of the single carriers, which participate in the recombination process.
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We characterize the positively charged exciton (X1+) in single InGaAs quantum dots using resonant laser spectroscopy. Three samples with different dopant species (Be or C as acceptors, Si as a donor) are compared. The p-doped samples exhibit larger inhomogeneous broadening (x3) and smaller absorption contrast (x10) than the n-doped sample. For X1+ in the Be-doped sample, a dot dependent non-linear Fano effect is observed, demonstrating coupling to degenerate continuum states. However, for the C-doped sample the X1+ lineshape and saturation broadening follows isolated atomic transition behaviour. This C-doped device structure is useful for single hole spin initialization, manipulation, and measurement.
We measured, for the first time, two photon radiative cascades due to sequential recombination of quantum dot confined electron hole pairs in the presence of an additional spectator charge carrier. We identified direct, all optical cascades involving spin blockaded intermediate states, and indirect cascades, in which non radiative relaxation precedes the second recombination. Our measurements provide also spin dephasing rates of confined carriers.
We measured the polarization memory of excitonic and biexcitonic optical transitions from single quantum dots at either positive, negative or neutral charge states. Positive, negative and no circular or linear polarization memory was observed for various spectral lines, under the same quasi-resonant excitation below the wetting layer band-gap. We developed a model which explains both qualitatively and quantitatively the experimentally measured polarization spectrum for all these optical transitions. We consider quite generally the loss of spin orientation of the photogenerated electron-hole pair during their relaxation towards the many-carrier ground states. Our analysis unambiguously demonstrates that while electrons maintain their initial spin polarization to a large degree, holes completely dephase.
We study the absorption and emission polarization of single semiconductor quantum dots in semiconductor nanowires. We show that the polarization of light absorbed or emitted by a nanowire quantum dot strongly depends on the orientation of the nanowire with respect to the directions along which light is incident or emitted. Light is preferentially linearly polarized when directed perpendicular to the nanowire elongation. In contrast, the degree of linear polarization is low for light directed along the nanowire. This result is vital for photonic applications based on intrinsic properties of quantum dots, such as generation of entangled photons. As an example, we demonstrate optical access to the spin states of a single nanowire quantum dot.
The electronic states of lateral many electron quantum dots in high magnetic fields are analyzed in terms of energy and spin. In a regime with two Landau levels in the dot, several Coulomb blockade peaks are measured. A zig-zag pattern is found as it is known from the Fock-Darwin spectrum. However, only data from Landau level 0 show the typical spin-induced bimodality, whereas features from Landau level 1 cannot be explained with the Fock-Darwin picture. Instead, by including the interaction effects within spin-density-functional theory a good agreement between experiment and theory is obtained. The absence of bimodality on Landau level 1 is found to be due to strong spin polarization.
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