Exciton Fine-structure Splitting Research Articles

Control of fine-structure splitting and excitonic binding energies in selected individual InAs∕GaAs quantum dots

A systematic study of the impact of annealing on the electronic properties of single InAs∕GaAs quantum dots (QDs) is presented. Single QD cathodoluminescence spectra are recorded to trace the evolution of one and the same QD over several steps of annealing. A substantial reduction of the excitonic fine-structure splitting upon annealing is observed. In addition, the binding energies of different excitonic complexes change dramatically. The results are compared to model calculations within 8-band k∙p theory and the configuration interaction method, suggesting a change of electron and hole wave function shape and relative position.

Control of structural and excitonic properties of self-organized InAs/GaAs quantum dots

A systematic dependence of excitonic properties on the size of self-organized InAs/GaAs quantum dots is presented. The bright exciton fine-structure splitting changes from negative values to more than 0.5 meV, and the biexciton binding energy varies from antibinding to binding, as the height of truncated pyramidal dots increases from 2 to above 9 InAs monolayers. A novel mode of metalorganic vapor phase epitaxy was developed for growing such quantum dots with precise shape control. The dots consist of pure InAs and feature heights varying in steps of complete InAs monolayers. Such dot ensembles evolve from a strained, rough two-dimensional layer with a thickness close to the critical value for the onset of the 2D–3D transition. Dots with a common height represent subensembles with small inhomogeneous broadening. Tuning of subensemble emission energy is achieved by varying the mean lateral extension of the respective QDs. Detailed knowledge of the structural properties of individual dots enable realistic k·p calculations to analyze the origin of the observed excitonic properties. The binding energies of charged and neutral excitons increase due to correlation by the gradually increasing number of bound states for increasing dot size. The monotonously increasing magnitude of the fine-structure splitting with dot size is shown to be caused by piezoelectricity. The identification of key parameters allows to tailor exciton properties, providing a major step towards the development of novel applications.

Effect of uniaxial stress on excitons in a self-assembled quantum dot

The fine structure of the neutral exciton in a single self-assembled InGaAs quantum dot is investigated under the effect of an applied uniaxial stress. The spectrum of the excitonic Rayleigh scattering was measured in reflectivity using high-resolution laser spectroscopy while the sample was submitted to a tunable uniaxial stress along its [110] crystal axis. We show that using this stretching technique, the quantum dot potential is elastically deformable such that the exciton fine structure splitting can be substantially reduced.

Quantum-dot size dependence of exciton fine-structure splitting

A systematic variation of the exciton fine-structure splitting with quantum dot size in single MOCVD-grown self-organized InAs/GaAs quantum dots is observed, ranging from several tens to as much as 520 μ eV , thus covering more than one order of magnitude. Piezoelectricity is identified to be the dominant factor governing the observed trend. A change in sign of the fine-structure splitting is reported for the first time, originating from quantum dots with confinement potentials elongated in the [ 1 1 ¯ 0 ] and [ 1 1 0 ] crystal direction, respectively.

Size-Dependent Fine-Structure Splitting in Self-OrganizedInAs/GaAsQuantum Dots

A systematic variation of the exciton fine-structure splitting with quantum dot size in single quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from to as much as with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.

Exciton fine structure splitting of single InGaAs self-assembled quantum dots

We show how the resonant absorption of the ground state neutral exciton confined in a single InGaAs self-assembled quantum dot can be directly observed in an optical transmission experiment. A spectrum of the differential transmitted intensity is obtained by sweeping the exciton energy into resonance with laser photons exploiting the voltage induced Stark-shift. We describe the details of this experimental technique and some example results which exploit the ∼1 μeV spectral resolution. In addition to the fine structure splitting of the neutral exciton and an upper bound on the homogeneous linewidth at 4.2 K , we also determine the transition electric dipole moment.

Triggered polarization-correlated photon pairs from a single CdSe quantum dot

We report on the generation of polarization-correlated photon pairs by the radiative biexciton (XX)-exciton (X) cascade of single CdSe quantum dots (QDs). Under nonresonant optical pulsed excitation (76 MHz) at low temperature (4 K), a high collinear correlation degree of 74.5% was observed from cross-correlation measurements between single XX and X emissions, which reflects an asymmetry-induced exciton fine-structure splitting. In consideration of the excitonic radiative lifetime (250 ps) this effect allows for direct conclusions about the relaxation time (T⩾480 ps) between the corresponding sublevels. Our results also suggest that the biexciton-exciton cascade in a CdSe QD is well suited for triggered single photon and/or photon pair generation rates above 1 GHz.

Theory of Paramagnetic Excitons in Solid Free Radicals

A linear chain of S=½ molecules with alternating intermolecular exchange integrals is used as a model to investigate the properties of paramagnetic excitons in solid free radicals from a theoretical point of view. The spin exchange Hamiltonian for the problem is H=Σν(JS2ν·S2ν+1+J′S2ν·S2ν−1),when J>J′>0. In the limiting case that J≫J′, the exciton bandwidth is simply J′. For this case we have calculated (a) the fine-structure splitting of the paramagnetic resonance of isolated (noninteracting) excitons, (b) wave functions for a free exciton gas of arbitrary excitation density, (c) the magnetic susceptibility of the exciton gas, and (d) first-order exchange interactions between excitations. In the case that thermal energies are much larger than the exciton bandwidth, exchange interactions between excitations are simply proportional to the excitation concentration, but at temperatures corresponding to thermal energies less than the bandwidth, there is a ``Fermi hole'' type repulsion between the excitons that reduces the exchange interaction. In fact, the excitons behave somewhat like Fermi particles, except their number is not constant (depending on the temperature), and only one exciton may occupy a given momentum state, irrespective of spin. Also, when the exciton bandwidth is comparable to or larger than kT, it is predicted that the exciton fine-structure splitting will be temperature dependent. When J′ becomes comparable to J, then the elementary exciton takes on a complicated, distributed spin structure.