Probing quantum nanostructures with near-field optical microscopy and (vice versa)

A theory is presented to show how near-field optical microscopy can be used to probe quantum nanostructures. Calculations are done for a quantum dot. Results for different tip/dot configurations and sizes show that near-field excitation can enhance light-hole transitions, excite selection-rule breaking transitions with rates comparable to allowed transitions, and map electron-hole pair wave functions. Conversely, dot response can be used to characterize tip near-fields.Comment: 8 pages of ReVTex, 5 ps figures, submitted to Appl. Phys. Let

Controlling the layer localization of gapless states in bilayer graphene with a gate voltage

Experiments in gated bilayer graphene with stacking domain walls present topological gapless states protected by no-valley mixing. Here we research these states under gate voltages using atomistic models, which allow us to elucidate their origin. We find that the gate potential controls the layer localization of the two states, which switches non-trivially between layers depending on the applied gate voltage magnitude. We also show how these bilayer gapless states arise from bands of single-layer graphene by analyzing the formation of carbon bonds between layers. Based on this analysis we provide a model Hamiltonian with analytical solutions, which explains the layer localization as a function of the ratio between the applied potential and interlayer hopping. Our results open a route for the manipulation of gapless states in electronic devices, analogous to the proposed writing and reading memories in topological insulators

Semiconductor-metal nanoparticle molecules: hybrid excitons and non-linear Fano effect

Modern nanotechnology opens the possibility of combining nanocrystals of various materials with very different characteristics in one superstructure. The resultant superstructure may provide new physical properties not encountered in homogeneous systems. Here we study theoretically the optical properties of hybrid molecules composed of semiconductor and metal nanoparticles. Excitons and plasmons in such a hybrid molecule become strongly coupled and demonstrate novel properties. At low incident light intensity, the exciton peak in the absorption spectrum is broadened and shifted due to incoherent and coherent interactions between metal and semiconductor nanoparticles. At high light intensity, the absorption spectrum demonstrates a surprising, strongly asymmetric shape. This shape originates from the coherent inter-nanoparticle Coulomb interaction and can be viewed as a non-linear Fano effect which is quite different from the usual linear Fano resonance.Comment: 5 pages, 5 figures, submitted to Phys. Rev. Let

Incorporation of random alloy GaBix_{x}As1−x_{1-x} barriers in InAs quantum dot molecules: alloy strain and orbital effects towards enhanced tunneling

Self-assembled InAs quantum dots (QDs), which have long hole-spin coherence times and are amenable to optical control schemes, have long been explored as building blocks for qubit architectures. One such design consists of vertically stacking two QDs to create a quantum dot molecule (QDM). The two dots can be resonantly tuned to form "molecule-like" coupled hole states from the hybridization of hole states otherwise localized in each respective dot. Furthermore, spin-mixing of the hybridized states in dots offset along their stacking direction enables qubit rotation to be driven optically, allowing for an all-optical qubit control scheme. Increasing the magnitude of this spin mixing is important for optical quantum control protocols. To enhance the tunnel coupling and spin-mixing across the dots, we introduce Bi in the GaAs inter-dot barrier. Previously, we showed how to model InAs/GaBiAs in an atomistic tight-binding formalism, and how the dot energy levels are affected by the alloy. In this paper, we discuss the lowering of the tunnel barrier, which results in a three fold increase of hole tunnel coupling strength in the presence of a 7% alloy. Additionally, we show how an asymmetric strain between the two dots caused by the alloy shifts the resonance. Finally, we discuss device geometries for which the introduction of Bi is most advantageous.Comment: RevTex 4-2, 11 pages, 9 figure