Determination of Edge Purity in Bilayer Graphene Using micro-Raman Spectroscopy

Polarization resolved micro-Raman spectroscopy was carried out at the edges of bilayer graphene. We find strong dependence of the intensity of the G band on the incident laser polarization, with its intensity dependence being 90 degrees out of phase for the armchair and zigzag case, in accordance with theoretical predictions. For the case of mixed-state edges we demonstrate that the polarization contrast reflects the fractional composition of armchair and zigzag edges, providing a monitor of edge purity, which is an important parameter for the development of efficient nanoelectronic devices.Comment: 3 pages, 3 figures, to appear in Applied Physics Letter

Optical Anisotropy of Electronic Excitations in Elliptical Quantum Dots

The authors report that anisotropic confining potentials in laterally-coupled semiconductor quantum dots (QDs) have large impacts in optical transitions and energies of inter-shell collective electronic excitations. The observed anisotropies are revealed by inelastic light scattering as a function of the in-plane direction of light polarization and can be finely controlled by modifying the geometrical shape of the QDs. These experiments show that the tuning of the QD confinement potential offers a powerful method to manipulate electronic states and far-infrared inter-shell optical transitions in quantum dots.Comment: 8 pages, 4 figure

A molecular state of correlated electrons in a quantum dot

Correlation among particles in finite quantum systems leads to complex behaviour and novel states of matter. One remarkable example is predicted to occur in a semiconductor quantum dot (QD) where at vanishing density the Coulomb correlation among electrons rigidly fixes their relative position as that of the nuclei in a molecule. In this limit, the neutral few-body excitations are roto-vibrations, which have either rigid-rotor or relative-motion character. In the weak-correlation regime, on the contrary, the Coriolis force mixes rotational and vibrational motions. Here we report evidence of roto-vibrational modes of an electron molecular state at densities for which electron localization is not yet fully achieved. We probe these collective modes by inelastic light scattering in QDs containing four electrons. Spectra of low-lying excitations associated to changes of the relative-motion wave function -the analogues of the vibration modes of a conventional molecule- do not depend on the rotational state represented by the total angular momentum. Theoretical simulations via the configuration-interaction (CI) method are in agreement with the observed roto-vibrational modes and indicate that such molecular excitations develop at the onset of short-range correlation.Comment: PDF file only; 24 pages, 7 figures, 2 table. Supplementary Information include

Optical control of energy-level structure of few electrons in AlGaAs/GaAs quantum dots

Optical control of the lateral quantum confinement and number of electrons confined in nanofabricated GaAs/AlGaAs quantum dots is achieved by illumination with a weak laser beam that is absorbed in the AlGaAs barrier. Precise tuning of energy-level structure and electron population is demonstrated by monitoring the low-lying transitions of the electrons from the lowest quantum-dot energy shells by resonant inelastic light scattering. These findings open the way to the manipulation of single electrons in these quantum dots without the need of external metallic gates.Comment: To appear in NanoLetter

Effets de fortes photo-excitations sur les puits et boîtes quantiques à base de nitrures d'éléments III (phase wurtzite)

MONTPELLIER-BU Sciences (341722106) / SudocSudocFranceF

Nonlinear behavior of photoabsorption in hexagonal nitride quantum wells due to free carrier screening of the internal fields.

International audienceWe investigate the effects of large electron-hole pair densities on the energy spectra of quantum wells based on hexagonal group-III nitrides, such as GaN/AlxGa1−xN or InyGa1−yN/GaN systems. More specifically, we solve self-consistently the Schrödinger and Poisson equations in order to calculate the changes in emission and absorption spectra, induced by the screening of the large internal electric fields that are present in these systems. In particular, we find that pair densities of a few times 1012cm−2 induce not only a blueshift of the fundamental transition but also a significant enhancement of the absorption coefficient, in the region corresponding to transitions between excited states. We estimate the typical optical power densities necessary to induce such effects

Donor-acceptor-like behavior of electron-hole pair recombinations in low-dimensional (Ga,In)N/GaN systems.

International audienceWe propose a model for the radiative recombination of electron-hole pairs in (Ga,In)N/GaN quantum objects, including huge internal electric fields and strong carrier localization. This model explains why the time decay of the photoluminescence keeps a constant nonexponential shape, while its time scale can be varied over several orders of magnitude. Instead of localized excitons, we consider an electron and a hole independently localized at sharp potential fluctuations, along two parallel sheets, forming a two-dimensional pseudo-donor-acceptor pair