R. J. Warburton

Hybridization of electronic states in quantum dots through photon emission

The self-assembly of semiconductor quantum dots has opened up new opportunities in photonics. Quantum dots are usually described as ‘artificial atoms’, because electron and hole confinement gives rise to discrete energy levels. This picture can be justified from the shell structure observed as a quantum dot is filled either with excitons (bound electron–hole pairs) or with electrons. The discrete energy levels have been most spectacularly exploited in single photon sources that use a single quantum dot as emitter. At low temperatures, the artificial atom picture is strengthened by the long coherence times of excitons in quantum dots, motivating the application of quantum dots in quantum optics and quantum information processing. In this context, excitons in quantum dots have already been manipulated coherently. We show here that quantum dots can also possess electronic states that go far beyond the artificial atom model. These states are a coherent hybridization of localized quantum dot states and extended continuum states: they have no analogue in atomic physics. The states are generated by the emission of a photon from a quantum dot. We show how a new version of the Anderson model that describes interactions between localized and extended states can account for the observed hybridization.

Excitons in self-assembled quantum ring-like structures

Abstract A remarkable morphological change of self-assembled InAs quantum dots takes place during growth if a pause is introduced after overgrowing the dots with a few nm of GaAs. Atomic force microscopy indicates that the shape of the dots changes lens-like to ring-like. We report here the results of capacitance and interband transmission experiments on such ring-like structures embedded in a GaAs matrix. In particular, we compare the electronic properties of conventional dots with those of the rings. Significant changes are found which qualitatively support a quantum ring model.

Zero-field spin splitting in InAs-AlSb quantum wells revisited

We present magnetotransport experiments on high-quality InAs-AlSb quantum wells that show a perfectly clean single-period Shubnikov-de Haas oscillation down to very low magnetic fields. In contrast to theoretical expectations based on an asymmetry induced zero-field spin splitting, no beating effect is observed. The carrier density has been changed by the persistent photo conductivity effect as well as via the application of hydrostatic pressure in order to influence the electric field at the interface of the electron gas. Still no indication of spin splitting at zero magnetic field was observed in spite of highly resolved Shubnikov- de Haas oscillations up to filling factors of 200. This surprising and unexpected result is discussed in view of other recently published data.

Storage of electrons and holes in self-assembled InAs quantum dots

We report spectroscopic measurements of charge-tunable quantum dots. The samples contain vertically aligned double dots which we can fill with electrons from a back contact. We show how we can also accumulate holes in the dots by illuminating the samples with below band gap radiation when a large negative bias is applied. We argue that this is possible through a large disparity in the electron and hole tunneling times. Interband spectroscopy reveals a strong reduction in the quantization energy for the dots in the second layer.

Optical emission from single, charge-tunable quantum rings

Abstract We have succeeded in preparing excitons with a specific charge in single semiconductor quantum rings. Buried InAs quantum rings are loaded with electrons from a reservoir through a tunneling barrier and an additional electron–hole pair is generated by optical excitation. Single rings are addressed with nano-optical techniques. We observe abrupt shifts in the emission energy as electrons are added one by one. Furthermore, the experiments provide unique insights into the interaction of electrons in semiconductor nano-islands with their environment.

Collective effects in intersubband transitions

We present experiments on the intersubband resonance (ISR) in InAs=AlSb quantum wells with the aim of understanding the linewidth. We nd that uctuations in the well width dominate the scattering right up to temperatures well beyond room temperature. ISR with two occupied subbands is used to gain insight into these phenomena. We nd clear evidence for Landau damping and we argue generally that Landau damping represents the crucial scattering mechanism for all ISRs. The argument is strengthened by considering the intrasubband plasmon, where we also nd evidence for Landau damping, in this case in a magnetic eld. ? 2000 Elsevier Science B.V. All rights reserved.

Interband absorption on self-assembled InAs quantum dots

Abstract We have studied the interband excitations of an ensemble of InAs self-assembled quantum dots by detecting absorption directly in transmission experiments. The dots are embedded in a MISFET structure, allowing the dots’ electron occupation to be controlled with a gate voltage. We show how Coulomb blockade in the device’s C−Vg characteristic corresponds to Pauli blocking of optical transitions in transmission. Furthermore, the second absorption peak of the dots shifts by some 20xa0meV and weakens when the first electron level is filled with two electrons, evidence of an exciton–electron interaction. The results also provide a direct measurement of the oscillator strength of the dots.

CYCLOTRON RESONANCE OF ELECTRON-HOLE SYSTEMS IN InAs/GaSb/AlSb

We present cyclotron resonance measurements on electron-hole systems in the crossed gap system InAsGaSb with additional confinement from AlSb barriers. A large, ∼2.5 meV, splitting has been observed in the electron cyclotron resonance when holes are also present. We focus on the pronounced filling factor dependence of the effect to argue that we have a nonparabolicity-induced splitting, enhanced through an additional energy dependence of the g factor. This enhancement could be caused by a spin-orbit effect, with the holes apparently allowing the two transitions to be observed through a decoupling of the two cyclotron resonance transitions.

Dynamics of Bright and Dark Excitons in a Self‐Assembled Quantum Dot

We report on a spin‐flip process in a charge tunable quantum dot in which a non‐radiative dark exciton, with angular momentum L = 2, becomes a radiative bright exciton, L = 1, through interactions with a Fermi sea in an n‐doped back contact. This process presents itself by a strongly bias dependent secondary lifetime in radiative lifetime measurements of the neutral exciton. By studying dots with different emission energies the spin‐flip process is shown to have a strong dependence on the tunneling rate between the dot and the back contact. An Anderson‐like Hamiltonian is used to model a coherent spin swap process between an electron in the dot and electrons in the back contact and gives very good agreement with the experiment.

Interband optics of charge-tunable quantum dots

Self-assembled quantum dots have been incorporated into a field-effect structure allowing the dots to be filled sequentially with electrons. Results of interband spectroscopy on charge-tunable structures are presented. For InAs dots in GaAs and for InAs dots in InP characterisation of the devices with capacitance and transmission spectroscopies yields detailed energy level diagrams. Furthermore, we have measured the oscillator strengths of the various interband transitions. Both these systems have a band gap in an awkward spectral range for single dot experiments. Instead, we present photoluminescence experiments on single quantum rings. Charging of the rings with single electrons is detected by pronounced shifts in the photoluminescence energy.