Excitons In Self-assembled Quantum Dots Research Articles

Atomistic theory of dark excitons in self-assembled quantum dots of reduced symmetry

We use an atomistic model to consider the effect of shape symmetry breaking on the optical properties of self-assembled InAs/GaAs quantum dots. In particular, we investigate the energy level structure and optical activity of the lowest energy excitons in these nanostructures. We compare between quantum dots with twofold rotational and two reflections $({C}_{2v})$ symmetry and quantum dots in which this symmetry was reduced to one reflection only $({C}_{s})$ by introducing a facet between the quantum dots and the host material. We show that the symmetry reduction mostly affects the optical activity of the dark exciton. While in symmetric quantum dots, one of the dark exciton eigenstates has a small dipole moment polarized along the symmetry axis (growth direction) of the quantum dot, in nonsymmetric ones, the two dark excitons' dipole moments are predominantly cross-linearly polarized perpendicular to the growth direction and reveal pronounced polarization anisotropy. Our model calculations agree quantitatively with recently obtained experimental data.

Atomistic theory of emission from dark excitons in self-assembled quantum dots

We present atomistic theory of emission from dark excitons in self-assembled quantum dots. Using the atomistic tight-binding model coupled with the configuration-interaction approach, we calculate the emission spectrum of a dark exciton confined in a single, self-assembled quantum dot. We demonstrate that the dark exciton state is optically active and that it emits photons in the plane of the dot, polarized along its vertical axis. This finite radiative lifetime is due to the mixing of hole subbands and can be tuned over several orders of magnitude by engineering the size and shape of the quantum dot.

In-plane radiative recombination channel of a dark exciton in self-assembled quantum dots

We demonstrate evidence for a radiative recombination channel of dark excitons in self-assembled quantum dots. This channel is due to a light hole admixture in the excitonic ground state. Its presence was experimentally confirmed by a direct observation of the dark exciton photoluminescence from a cleaved edge of the sample. The polarization resolved measurements revealed that a photon created from the dark exciton recombination is emitted only in the direction perpendicular to the growth axis. Strong correlation between the dark exciton lifetime and the in-plane hole g-factor enabled us to show that the radiative recombination is a dominant decay channel of the dark excitons in CdTe/ZnTe quantum dots.

Eliminating the fine structure splitting of excitons in self-assembled InAs/GaAs quantum dots via combined stresses

Eliminating the fine structure splitting (FSS) of excitons in self-assembled quantum dots (QDs) is essential to the generation of high quality entangled photon pairs. It has been shown that the FSS has a lower bound under uniaxial stress. In this letter, we show by a extended two-level model that the FSS of excitons in a general self-assembled InGaAs/GaAs QD can be fully suppressed via combined stresses along the [110] and [010] directions. The results of the model Hamiltonian are confirmed by atomic empirical pseudopotential calculations. For all the QDs we calculated, the FSS can be tuned to be vanishingly small (<0.1 μeV), which is sufficient small for high quality entangled photon emission.

Probing long-lived dark excitons in self-assembled quantum dots

Long-lived dark exciton states are formed in self-assembled quantum dots due to the combination of the angular momentum of electrons and holes. The lifetime of dark excitons are determined by spin-flip processes that transfer dark excitons into radiative bright excitons. We employ time-resolved spontaneous emission measurements in a modified local density of optical states to unambiguously record the spin-flip rate. Pronounced variations in the spin-flip rate with the quantum dot emission energy are observed demonstrating that the exciton storage time can be extended by controlling the quantum dot size. The energy dependence is compared to a recent model from the literature, in which the spin flip is due to the combined action of short-range exchange interaction and acoustic phonons. We furthermore observe a pronounced enhancement of the spin-flip rate close to semiconductor-air interfaces, which illustrates the important role of interfaces for quantum dot based nanophotonic structures.

Frequency dependence of the radiative decay rate of excitons in self-assembled quantum dots: Experiment and theory

We analyze time-resolved spontaneous emission from excitons confined in self-assembled InAs quantum dots placed at various distances to a semiconductor-air interface. The modification of the local density of optical states due to the proximity of the interface enables unambiguous determination of the radiative and nonradiative decay rates of the excitons. From measurements at various emission energies, we obtain the frequency dependence of the radiative decay rate, which is only revealed due to the separation of the radiative and nonradiative parts. It contains detailed information about the dependence of the exciton wave function on quantum dot size. We derive the quantum optics theory of a solid-state emitter in an inhomogeneous environment and compare this theory to our experimental results. Using this model, we extract the frequency dependence of the overlap between the electron and hole wave functions. We furthermore discuss three models of quantum dot strain and compare the measured wave-function overlap to these models. The observed frequency dependence of the wave-function overlap can be understood qualitatively in terms of the different compressibility of electrons and holes originating from their different effective masses and binding energies.

Photoexcitation of excitons in self-assembled quantum dots

Using an approach of combining Fourier transform infrared spectroscopy with resonant illumination from a secondary external light source, we have measured the photocurrent (PC) for multiple layers of self-assembled InAs dots embedded in a matrix of InP. Without external illumination, we observe photoexcitation of electrons from bound states in the dots to the InP barrier. By additional illumination from the external light source, a strong broadening of the PC signal is observed. We interpret this broadening in terms of photoexcitation of electrons in the presence of additional holes in the dots created by the external light source. We extract the spectral distribution of the photoexcitation process at 6 and 77K, respectively, and show by comparison with theoretical calculations that it is consistent with an exciton binding energy of 20meV.

Kondo excitons in self-assembled quantum dots

We describe excitons in quantum dots by allowing for an interaction with a Fermi sea of electrons. We argue that these excitons can be realized very simply with self-assembled quantum dots, using the wetting layer as host for the Fermi sea. We show that a tunnel hybridization of a charged exciton with the Fermi sea leads to two striking effects in the optical spectra. First, the photoluminescence lines become strongly dependent on the vertical bias. Second, if the exciton spin is nonzero, the Kondo effect leads to peculiar photoluminescence line shapes with a linewidth determined by the Kondo temperature.

Charged Excitons in Self-assembled Quantum Dots

ABSTRACTWe have succeeded in generating highly charged excitons in InAs self-assembled quantum dots by embedding the dots in a field-effect heterostructure. We discover an excitonic Coulomb blockage: over large regions of gate voltage, the exciton charge remains constant. We present here a summary of the emission properties of the charged excitons.

Kondo-excitons and Auger processes in self-assembled quantum dots

ABSTRACTWe describe theoretically novel excitons in self-assembled quantum dots interacting with a two-dimensional (2D) electron gas in the wetting layer. In the presence of the Fermi sea, the optical lines become strongly voltage-dependent. If the electron spin is nonzero, the width of optical lines is given by kBTK, where TK is Kondo temperature. If the spin is zero, the exciton couples with the continuum due to Auger-like processes. This leads to anticrossings in a magnetic field. Such states can be called Kondo-Anderson excitons. Some of the described phenomena are observed in recent experiments.