Excitons In Quantum Dots Research Articles

Theoretical studies of excitons in type II CdSe/CdTe quantum dots

We present a method for calculating exciton and bi-exciton energies in type-II colloidal quantum dots. Our methodology is based on an 8-band k · p Hamiltonian of the zinc- blend structure, which incorporates the effects of spin-orbit interaction, strain between the core and the shell and piezoelectric potentials. Exciton states are found using the configuration interaction (CI) method that explicitly includes the effects of Coulomb interaction, as well as exchange and correlation between many-electron configurations. We pay particular attention to accurate modelling of the electrostatic interaction between quasiparticles. The model includes surface polarization and self-polarization effects due to the large difference in dielectric constants at the boundary of the QD.

Surface recombination and charged exciton in nanocrystal quantum dots on photonic crystals under two-photon excitation.

In this study, the two-photon excited fluorescence spectra from cadmium selenide quantum dots (QDs) on a silicon nitride photonic crystal (PhC) membrane under femtosecond laser irradiation were investigated. These spectra can be fit to a tri-Gaussian function in which one component is negative in amplitude, and in which the Gaussian components with positive amplitude are assigned to exciton emission and charged-exciton emission and that with negative amplitude is assigned to absorption from surface recombination. The photonic crystal enhance the charged-exciton emission and exciton emission and, at the same time, also the absorption from surface recombination. Both the charged-exciton emission and the surface recombination are related to Auger recombination; therefore, the photonic crystal controls both radiative recombination and non-radiative recombination. The asymmetries of the two-photon excited fluorescence spectra are due to not only the location of the resonant guide mode of the PhC slab but also the enhancement of the absorption from surface recombination by PhC.

Micro-Raman and micro-photoluminescence study of bio-conjugated core–shell CdSe/ZnS nanocrystals

The micro-Raman and micro-photoluminescence spectra of non-conjugated and conjugated with antibody against S6K2 commercial CdSe/ZnS quantum dots (QDs) were investigated under different excitation wavelengths and at different temperatures. In the photoluminescence (PL) spectra, the additional PL band shifted on 0.6–0.65eV to higher energies from the CdSe/ZnS QD exciton PL band is revealed. The relative intensity of this band is found to be several times larger in bio-conjugated QDs, than in the non-conjugated ones. The characteristics of both PL bands (the PL intensity, spectral position and half-width of the PL band) vary similarly under continuous laser light irradiation, storage of the QD samples in the atmospheric ambience as well as during the temperature change. In the Raman spectra recorded under excitation resonant with the high-energy PL band, the additional Raman peaks at about 300cm−1 and 600cm−1, which are close to the frequency of LO and 2LO phonons of bulk CdS, are found. It is proposed that alloyed QDs with chemical composition close to CdS are responsible for the additional high-energy PL band. The possible reasons for the formation of the alloyed QDs are discussed.

Long-time correlation in non-Markovian dephasing of an exciton-phonon system in InAs quantum dots.

We have observed a time-correlated frequency fluctuation in non-Markovian dephasing of excitons in InAs quantum dots using a six-wave mixing technique. In this measurement, the arrival times of the excitation pulses were controlled to eliminate the influence of Markovian dephasing and to measure the pure non-Markovian behavior. The experimental result shows that the time correlation of the frequency fluctuation due to exciton-phonon interactions was maintained in the quantum dots for over 10ps. This long-time correlation is caused by the modification of the phonon coupling distribution.

Exciton-plasmon interaction in hybrid quantum dot/metal cluster structures fabricated by molecular-beam epitaxy

Hybrid structures consisting of InAs/AlGaAs quantum dots and In clusters on the surface separated by an AlGaAs spacer with a specified thickness are investigated. The enhancement of the photoluminescence signal in the long-wavelength region manifesting itself in the appearance of a new narrow emission line is observed. This line is absent in the control structure with a quantum-dot array and no metal clusters. It also disappears with increasing thickness of the dielectric spacer between the layers. These results are explained in terms of a model taking into account the resonance interaction of excitons in quantum dots with localized surface plasmons in metal clusters.

Phonon-induced effects on exciton dynamics and photon emission from a semiconductor quantum dot microcavity: phonon coherent state representation

A microscopic theory to study the combined dynamics of a quantum dot exciton coupled to a single cavity mode as well as acoustic phonons is presented. By considering the phonon subsystem in the coherent state representation, the phonon’s impact on the emitted photons is investigated. The single photon emission probability and phonon effects on this quantity are studied. On the other hand, we illustrate that while a quantum dot interacts with a single cavity mode, the collapse and revival phenomenon will occur. It is demonstrated that acoustic phonons have a pronounced impact on the collapse and revival of the quantum dot exciton. Our finding reveals that at elevated temperatures the phonon interaction decreases the Rabi frequency. Moreover, the cavity mode possesses non-classical features such as sub-Poissonian statistics.

Analytical temperature dependence of the photoluminescence of semiconductor quantum dots

The excitation and relaxation of spatially confined excitons in semiconductor quantum dots have been considered. The temperature dependence of the luminescence of quantum dots in dielectric matrices is described by the model taking into account the singlet-triplet intercombination conversion of spatially confined excitons. The analytical expression describing the temperature dependence of photoluminescence is derived and the physical meaning of the constants involved in this expression is determined. The applicability of the expression to the analysis of the luminescent properties of the quantum dots is demonstrated by the example of silicon nanoclusters in a thin-film SiO2 matrix.

Study of relaxation dynamics of photogenerated excitons in CuInS2 quantum dots

Abstract

Ultrafast Dynamics within the 1S Exciton Band of Colloidal PbSe Quantum Dots Using Multiresonant Coherent Multidimensional Spectroscopy

The simple particle-in-a-sphere model of quantum dot excitons is the basis for understanding the excitonic peak positions, line widths, and relaxation dynamics in many spectroscopic experiments. Recent multiresonant coherent multidimensional spectroscopy (CMDS) with picosecond excitation pulses measured the two-dimensional spectra of PbSe quantum dots and successfully used this simple model of an inhomogeneous distribution of spherically confined exciton and biexciton states and rate constants to describe the dephasing and population relaxation dynamics. The long excitation pulses prevented resolution of faster dynamics. This work reports the development of multiresonant CMDS with femtosecond excitation pulses to resolve the spectra and dynamics associated with the 1S exciton line shape of PbSe quantum dots. The experiments use different combinations of excitation frequencies, excitation pulse time delays, and a monochromator to display and measure correlations between the spectral features and their dynamics. Line-narrowing of the inhomogeneous distribution occurs at short time delays where the excitation excites a subset of the quantum dots within the 1S line shape and the last pulse probes this subset. The line-narrowing disappears at longer delay times. Three pulse photon echo peak shifts (3PEPS) also occur when the line-narrowing is present, but the shifts disappear as the correlation between the first and last coherence frequencies disappears. Wigner plots reveal the spectral dynamics accompanying the peak shift and the disappearance of the line-narrowing. This work shows there is rapid relaxation dynamics occurring within the line profile of the quantum confined excitonic states that is not consistent with current understanding of the excitonic line broadening. The data suggest that the relaxation dynamics play a more dominant role in defining the excitonic line widths than the inhomogeneous broadening of the quantum dot size distribution. These observations are consistent with other spectroscopic experiments on CdSe and PbS quantum dots. The experiments also show the presence of a higher energy feature that lies outside the 1S line shape and undergoes very rapid relaxation.

Single Photons on Demand from Novel Site-Controlled GaAsN/GaAsN:H Quantum Dots

We demonstrate triggered single-photon emission from a novel system of site-controlled quantum dots (QDs), fabricated by exploiting the hydrogen-assisted, spatially selective passivation of N atoms in dilute nitride semiconductors. Evidence of this nonclassical behavior is provided by the observation of strong antibunching in the autocorrelation histogram of the QD exciton emission line. This class of site-controlled quantum emitters can be exploited for the fabrication of new hybrid QD-nanocavity systems of interest for future quantum technologies.

Dramatically enhanced self-assembly of GeSi quantum dots with superior photoluminescence induced by the substrate misorientation

A dramatically enhanced self-assembly of GeSi quantum dots (QDs) is disclosed on slightly miscut Si (001) substrates, leading to extremely dense QDs and even a growth mode transition. The inherent mechanism is addressed in combination of the thermodynamics and the growth kinetics both affected by steps on the vicinal surface. Moreover, temperature-dependent photoluminescence spectra from dense GeSi QDs on the miscut substrate demonstrate a rather strong peak persistent up to 300 K, which is attributed to the well confinement of excitons in the dense GeSi QDs due to the absence of the wetting layer on the miscut substrate.

Generation of Multiple Excitons in Ag2S Quantum Dots: Single High-Energy versus Multiple-Photon Excitation

We explored biexciton generation via carrier multiplication (or multiple-exciton generation) by high-energy photons and by multiple-photon absorption in Ag2S quantum dots (QDs) using femtosecond broad-band transient absorption spectroscopy. Irrespective of the size of the QDs and how the multiple excitons are generated in the Ag2S QDs, two distinct characteristic time constants of 9.6-10.2 and 135-175 ps are obtained for the nonradiative Auger recombination of the multiple excitons, indicating the existence of two binding excitons, namely, tightly bound and weakly bound excitons. More importantly, the lifetimes of multiple excitons in Ag2S QDs were about 1 and 2 orders of magnitude longer than those of comparable size PbS QDs and single-walled carbon nanotubes, respectively. This result is significant because it suggests that by utilizing an appropriate electron acceptor, there is a higher possibility to extract multiple electron-hole pairs in Ag2S QDs, which should improve the performance of QD-based solar cell devices.

Effects of Temperature and Hydrostatic Pressure on Binding Energy of an Exciton in a Spherical Quantum Dot

Effects of Temperature and Hydrostatic Pressure on Binding Energy of an Exciton in a Spherical Quantum Dot

Suppression of Quenching in Plasmon-Enhanced Luminescence via Rapid Intraparticle Energy Transfer in Doped Quantum Dots

We show the suppression of luminescence quenching by metal nanoparticles (MNPs) in the plasmon enhancement of luminescence via fast sensitized energy transfer in Mn-doped quantum dots (QDs). The rapid intraparticle energy transfer between exciton and Mn, occurring on a few picoseconds time scale, separates the absorber (exciton) from the emitter (Mn), whose emission is detuned far from the plasmon of the MNP. The rapid temporal separation of the absorber and emitter combined with the reduced spectral overlap between Mn and plasmonic MNP suppresses the quenching of the luminescence while taking advantage of the plasmon-enhanced excitation. We compared the plasmon enhancement of exciton and Mn luminescence intensities in undoped and doped QDs simultaneously as a function of the distance between MNP and QD layers in a multilayer structure to examine the expected advantage of the reduced quenching in the sensitized luminescence. At the optimum MNP-QD layer distance, Mn luminescence exhibits stronger net enhancement than that of the exciton, which can be explained with a model incorporating fast sensitization along with reduced emitter-MNP spectral overlap. This study demonstrates that materials exhibiting fast sensitized luminescence that is sufficiently red-shifted from that of the sensitizer can be superior to usual luminophores in harvesting plasmon enhancement of luminescence by suppressing quenching.

Properties of Excitons in Quantum Dots with a Weak Confinement

with a Weak Con nement K. Goaasa, M. Molas, M. Goryca, T. Kazimierczuk, T. Smole«ski, M. Koperski, A. Golnik, P. Kossacki, M. Potemski, Z.R. Wasilewski and A. Babi«ski Institute of Experimental Physics, Faculty of Physics, University of Warsaw, Ho»a 69, 00-681 Warsaw, Poland Laboratoire National des Champs Magnetiques Intenses, Grenoble, France Institute for Microstructural Sciences, NRC Canada, Ottawa, Canada

Decay dynamics and exciton localization in large GaAs quantum dots grown by droplet epitaxy

We investigate the optical emission and decay dynamics of excitons confined in large strain-free GaAs quantum dots grown by droplet epitaxy. From time-resolved measurements combined with a theoretical model we show that droplet-epitaxy quantum dots have a quantum efficiency of about 75% and an oscillator strength between 8 and 10. The quantum dots are found to be fully described by a model for strongly-confined excitons, in contrast to the theoretical prediction that excitons in large quantum dots exhibit the so-called giant oscillator strength. We attribute these findings to localized ground-state excitons in potential minima created by material intermixing during growth. We provide further evidence for the strong-confinement regime of excitons by extracting the size of electron and hole wavefunctions from the phonon-broadened photoluminescence spectra. Furthermore, we explore the temperature dependence of the decay dynamics and, for some quantum dots, observe a pronounced reduction in the effective transition strength with temperature. We quantify and explain these effects as being an intrinsic property of large quantum dots owing to thermal excitation of the ground-state exciton. Our results provide a detailed understanding of the optical properties of large quantum dots in general, and of quantum dots grown by droplet epitaxy in particular.

Independent control of exciton and biexciton energies in single quantum dots via electroelastic fields

We investigate the effect of large in-plane strain and vertical electric fields on the binding energies of excitonic complexes confined in single InGaAs/GaAs quantum dots (QDs) and we find that the two independently tunable perturbations modify the interaction energies among electrons and holes in a different manner. By taking advantage of this difference, we frequency-lock the QD fundamental excitation (the neutral exciton) at a predefined value, while the biexciton transition is actively tuned from a binding to an antibinding configuration. Our electrically controlled dual-knob device demonstrates unprecedented control over the electronic properties of the few-particle states in a QD and may be applied to create novel energy-tunable sources of entangled photons using the time-reordering or the time-bin scheme.

Mechanism for controlling the exciton fine structure in quantum dots using electric fields: Manipulation of exciton orientation and exchange splitting at the atomic scale

Using atomistic tight-binding theory with a configuration interaction description of Coulomb and exchange effects, we describe excitons in symmetric quantum dots in a vertical electric field to explain how field-induced manipulation of exciton orientation and phase can eliminate the anisotropic exchange, leading to a drastic reduction of the fine-structure splitting and a 90 degree rotation of polarization, similar to experiment. This reorientation and rephasing of the exciton is done plane-by-plane inside the dot without significant squeezing of the exciton.

Fine structure of light-hole excitons in nanowire quantum dots

Quantum dots with light-hole ground states could find numerous applications including faster quantum bit operations or coherent conversion of photons into electron spins. Typically, however, holes confined in epitaxial quantum dots are of heavy-hole character. I show, by use of atomistic tight-binding theory, that the hole ground state undergoes a transition from heavy holelike to light holelike with increasing height of a nanowire InAs/InP quantum dot. The fine structure of the light-hole exciton consists of a dark ground state and three bright states. Two of the bright states are quasidegenerate and are in-plane polarized, whereas, the third energetically higher bright state is polarized in the perpendicular out-of-plane direction. The light-hole exciton fine structure is robust against alloying.

Hydrostatic pressure effect on photoionization cross section of a trion in quantum dots

It is known experimentally that stable charged excitons can exist in low-dimensional semiconductor nanostructures. Much less is known about the properties of such charged-exciton complexes since three-body problems are very difficult to solve, even numerically. Here, we use the matrix diagonalization method and compact-density approach to investigate the charged excitons in a two-dimensional parabolic quantum dot. With typical semiconducting GaAs based materials, the photoionization cross section has been examined based on the computed energies and wave functions. We find that the photoionization cross section of charged excitons is strongly affected by the confinement frequency, hydrostatic pressure and temperature of QDs. Our results also show that the photoionization cross section of a negatively charged exciton is larger than that of a positively charged exciton.