Transitions In Semiconductor Quantum Dots Research Articles

Enhancement of Spontaneous Photon Emission in Inverse Photoemission Transitions in Semiconductor Quantum Dots.

Inverse photoemission (IPE) is a radiative electron capture process where an electron is transiently captured in the conduction band (CB) followed by intraband de-excitation and spontaneous photon emission. IPE in quantum dots (QDs) bypasses optical selection rules for populating the CB and provides insights into the capacity for electron capture in the CB, the propensity for spontaneous photon emission, intraband transition energies where both initial and final states are in the CB, and the generation of photons with frequencies lower than the bandgap. Here, we demonstrate using time-dependent perturbation theory that judicious application of electric fields can significantly enhance the IPE transition in QDs. For a series of CdSe, CdS, PbSe, and PbS QDs, we present evidence of field-induced enhancement of IPE intensities (188% for Cd54Se54), field-dependent control of emitted photon frequencies (Δω = 0.73 eV for Cd54Se54), and enhancement of light-matter interaction using directed Stark fields (103% for Cd54Se54).

Optical Second Harmonic Generation in GaAs/Ga 1− x Al x As Quantum Dots with Screened Kratzer Potential

We theoretically study the second harmonic generation (SHG) associated with intersubband transitions in semiconductor quantum dots (QDs) emphasizing on the effects of the screened Kratzer (SK) potential parameters and regulating factors, such as temperature and the depth of potential. We apply the Nikiforov-Uvarov-Functional-Analysis (NUFA) method for solving the radial Schrödinger equation. Then, we derive the formulae for the SHG coefficient using the compact-density-matrix method. The results display that the SK potential parameters and the regulating factors play a key role in both the peak and position changes of the SHG in semiconductor QDs system. This article is protected by copyright. All rights reserved.

The Effect of Ligands and Solvents on Nonradiative Transitions in Semiconductor Quantum Dots (A Review)

Data on quantum yields and photoluminescence decay times of quantum dots have been collected. Photoprocesses that occur in quantum dots are compared with photoprocesses occurring in complex organic molecules in the condensed phase. The review consists of the introduction, three parts, and conclusions. The first two parts are devoted to quantum dots that are formed by indirect-gap semiconductors. The first part is devoted to data on the photoluminescence quantum yields and decay times of carbon quantum dots, and Table 1 presents selected values and short comments to these data. Table 2 of the same part presents data on fast relaxation processes in the same objects. In the second part, Tables 3 and 4, as well as the following text, contain similar information about silicon quantum dots. Data on photoprocesses in quantum dots formed by direct-gap semiconductors are collected in the third part. Data on the photoluminescence yields, decay times, and relaxation processes are listed in Tables 5 and 6. Particular attention in the present review is given to the effect that a change in the frequency of vibrations in the environment of a quantum dot has on the photoluminescence yields and the rate of relaxation processes between electronic levels in bands, which indicates that the inductive resonance mechanism of nonradiative transitions is applicable to these systems.

Multi-wave coherent control of a solid-state single emitter

Coherent control of individual two-level systems (TLSs) is at the basis of any implementation of quantum information. An impressive level of control is now achieved using nuclear, vacancies and charge spins. Manipulation of bright exciton transitions in semiconductor quantum dots (QDs) is less advanced, principally due to the sub-nanosecond dephasing. Conversely, owing to their robust coupling to light, one can apply tools of nonlinear spectroscopy to achieve all-optical command. Here, we report on the coherent manipulation of an exciton via multi-wave mixing. Specifically, we employ three resonant pulses driving a single InAs QD. The first two induce a four-wave mixing (FWM) transient, which is projected onto a six-wave mixing (SWM) depending on the delay and area of the third pulse, in agreement with analytical predictions. Such a switch enables to demonstrate the generation of SWM on a single emitter and to engineer the spectro-temporal shape of the coherent response originating from a TLS. These results pave the way toward multi-pulse manipulations of solid state qubits via implementing the NMR-like control schemes in the optical domain.

Scattering-induced dephasing of many-particle transitions in semiconductor quantum dots

We study the homogeneous linewidth of transitions between many-particle states in semiconductor quantum dots due to scattering processes. Carrier–density-dependent scattering rates due to carrier–carrier Coulomb interaction and carrier–LO-phonon interaction are obtained on a non-perturbative level and connected to a von Neumann–Lindblad equation for the quantum-dot many-particle configurations, allowing to identify the dephasing of transitions between many-particle states. For different dot geometries, we discuss implications of energetic quantum-dot wetting-layer separation on Coulomb and LO-phonon contributions.

Spontaneous emission from dipole-forbidden transitions in semiconductor quantum dots

We theoretically investigate the multipolar effects on the dipole-forbidden transitions of a semiconductor quantum dot. An approximated expression for the decay rate of these transitions is derived. Unlike the general theory of the spontaneous emission beyond the dipole approximation, the distinct roles of the emitter and the vacuum electric field in the transition rate are here clearly recognizable and can be separately optimized. We illustrate the potential of this formalism by calculating the spontaneous emission decay rate of an InAs/GaAs quantum dot embedded into two realistic nanostructures---an L3 photonic crystal cavity and a plasmonic dimer antenna. The obtained results show that, although the two structures provide an enhancement of the same order of magnitude, the plasmonic antenna constitutes a more promising candidate for the experimental observation of the dipole-forbidden transitions of a quantum dot.

Efficient control of coulomb enhanced second harmonic generation from excitonic transitions in quantum dot ensembles.

In this work, the second harmonic generation from excitonic transitions in semiconductor quantum dots is computationally studied. By integrating a density matrix treatment with a partial configuration interaction approach, we obtain the second order susceptibility as a function of externally applied electric and magnetic fields for highly confined neutral and charged excitons. Our results show an enhancement in the nonlinear response with respect to analogous optical processes based on intraband transitions, and predict their efficient tunability by taking advantage of the interplay between Coulomb effects and field-driven wave function manipulation.

Radiative coupling of quantum dots in photonic crystal structures

We derive a general formalism to model the polariton states resulting from the radiation-matter interaction between an arbitrary number of excitonic transitions in semiconductor quantum dots and photon modes in a photonic crystal structure in which the quantum dots are embedded. The Maxwell equations, including the linear nonlocal susceptibility of the exciton transitions in the quantum dots, are cast into an eigenvalue problem, which can be applied to any structure whose photon modes can be computed with reliable accuracy, and in addition naturally allows for disorder effects to be taken into account. We compute realistic photon modes using Bloch-mode expansion. As example systems, we study typical InGaAs quantum dots in a GaAs photonic crystal structures---an $Ln$ cavity or a $\mathit{W}\mathit{1}$ waveguide. For a single dot, we reproduce known analytical results, while for the two-dot case we study the radiative excitation transfer mechanism and characterize its strength, the dependence on the detuning between quantum dot and photon modes, and the dependence on interdot distance. We find in particular that the interdot radiative coupling strength can reach $100\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}$eV in a short cavity, and its decay with distance in longer cavities and waveguides is determined by the group velocity of the exchanged photons and their radiative lifetime. We also show that, for an $Ln$ cavity of increasing length, the radiative excitation transfer mechanism is subject to a crossover from a regime where a single photon mode is dominating, to a multimode regime---occurring around $\mathit{n}$ $=$ 150 for the system under study.

Intraband optical transitions in semiconductor quantum dots: Radiative electronic-excitation lifetime

Intraband optical transitions in semiconductor quantum dots (QDs) in the forms of a parallelepiped, sphere, and cylinder have been considered. It is shown that the size dependence of the matrix elements of electron-photon interaction, which includes the intraband transitions, differs in the E · r and A · p representations of electron-photon coupling, which are widely used to describe various optical processes. The radiative intraband relaxation rates of QD electron excitations have been calculated, depending on the QD size and shape and the parameters of the QD material. It is shown that the radiative intraband transition rate may reach 109 s−1.

Spatial weak infrared-light bright and dark solitons in semiconductor quantum-dot nanostructures

The formation of spatial weak infrared-light solitons through interband and intersubband transitions in semiconductor quantum dots (QDs) is theoretically investigated with feasible parameters. Our analysis is shown using GaAs–Al–GaAs QD structures that allow operation at communication wavelength. Such a three-level QD solid-state system for the infrared-light-soliton operation is much more practical than that in gaseous media as a result of its flexible design and the wide adjustable parameters and thus facilitate more practical applications in optical signal processing and telecommunication.

Intersublevel transitions in self-assembled quantum dots

Intersublevel transitions in semiconductor quantum dots are transitions of a charge carrier between quantum dot confined states. In InAs/GaAs self-assembled quantum dots, optically active intersublevel transitions occur in the mid-infrared spectral range. These transitions can provide a new insight on the physics of semiconductor quantum dots and offer new opportunities to develop mid-infrared devices. A key feature characterizing intersublevel transitions is the coupling of the confined carriers to phonons. We show that the effect of the strong coupling regime for the electron–optical phonon interaction and the formation of mixed electron–phonon quasi-particles called polarons drastically affect and control the dynamical properties of quantum dots. The engineering of quantum dot relaxation rates through phonon coupling opens the route to the realization of new devices like mid-infrared polaron lasers. We finally show that the measurement of intersublevel absorption is not limited to quantum dot ensembles and that the intersublevel ultrasmall absorption of a single quantum dot can be measured with a nanometer scale resolution by using phonon emission as a signature of the absorption. To cite this article: P. Boucaud et al., C. R. Physique 9 (2008).

Dynamic photon emission from multiphoton-excited semiconductor quantum dots

The dynamic process of multiphoton optical transitions in semiconductor quantum dots (QDs) has been studied by a Monte Carlo scheme. The scheme includes optical transitions of all electrons, initially occupying the valence-band confined states in the QD, among the confined states in valence and conduction bands. The optical transition probabilities are calculated by the time-dependent Schrödinger equation, and nonradiative phonon scattering processes have been included. Assisted by a two-photon excitation by a continuous-wave laser (one photon energy equals half of the QD energy band gap), an assembly of the QDs shows an emission peak around the band gap in the optical emission spectrum, while an ultrafast pulsed laser, whose photon energy is below the QD band gap, also induces a similar narrow but weaker emission peak, which results in a nonstrict multiphoton excitation condition for many potential applications including biophotonics. Extension of the theoretical study to the spherical CdS∕Cd0.5Zn0.5S∕ZnS-multicoated CdSe QD has reproduced the experimental absorption and multiphoton emission spectra.

Elastic spin-relaxation processes in semiconductor quantum dots

Electron spin decoherence caused by elastic spin-phonon processes is investigated comprehensively in a zero-dimensional environment. Specifically, a theoretical treatment is developed for the processes associated with the fluctuations in the phonon potential as well as in the electron procession frequency through the spin-orbit and hyperfine interactions in the semiconductor quantum dots. The analysis identifies the conditions (magnetic field, temperature, etc.) in which the elastic spin-phonon processes can dominate over the inelastic counterparts with the electron spin-flip transitions. Particularly, the calculation results illustrate the potential significance of an elastic decoherence mechanism originating from the intervalley transitions in semiconductor quantum dots with multiple equivalent energy minima (e.g., the X valleys in SiGe). The role of lattice anharmonicity and phonon decay in spin relaxation is also examined along with that of the local effective field fluctuations caused by the stochastic electronic transitions between the orbital states. Numerical estimations are provided for typical GaAs and Si-based quantum dots.

Theory of quantum dot luminescence from acoustically excited intersubband transitions

AbstractIn this paper a theory of acoustically driven intersubband transitions in semiconductor quantum dots is developed. Well off‐resonant acoustic excitation leads to subsequently emitted THz radiation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nonradiative multiphonon transitions in semiconductor quantum dots

A mathematical formalism suitable for a treatment of nonradiative multiphonon transitions in semiconductor quantum dots is developed. An analytical expression for the temperature dependence of the transition rate is derived.

Optical spectra of quantum dots: effects of non‐adiabaticity

It is shown that in many cases an adequate description of optical spectra of semiconductor quantum dots requires a treatment beyond the commonly used adiabatic approximation. We have developed a theory of phonon-assisted optical transitions in semiconductor quantum dots, which takes into account non-adiabaticity of the exciton–phonon system. Effects of non-adiabaticity lead to a mixing of different exciton and phonon states that provides a key to the understanding of surprisingly high intensities of phonon satellites observed in photoluminescence spectra of quantum dots. A breakdown of the adiabatic approximation gives an explanation also for discrepancies between the serial law, observed in multi-phonon optical spectra of some quantum dots, and the Franck–Condon progression, prescribed by the adiabatic approach.

Enhancedprobabilities of phonon-assisted optical transitions insemiconductor quantum dots

A theory of phonon-assisted optical transitions insemiconductor quantum dots is developed which takes into accountthe non-adiabaticity of the exciton-phonon system. The role ofnon-adiabaticity is shown to be of paramount importance inspherical quantum dots, where the lowest one-exciton state canbe degenerate or quasi-degenerate. In quantum dots of lowersymmetry, the effects of non-adiabaticity reveal themselvesmainly due to the phonon-induced mixing of states which belongto different energy levels. Our approach is applied to explainthe optical spectra of several quantum-dot structures: ensemblesof spherical CdSe, CdSexS1-x and PbS quantum dots,self-assembled InAs/GaAs and CdSe/ZnSe quantum dots,brick-shaped InAs/GaAs quantum dots and CdS/HgS/CdS quantum dotheterostructures.

Third-harmonic generation in InAs/GaAs self-assembled quantum dots

We have observed third-harmonic generation associated with intraband transitions in semiconductor quantum dots. The frequency tripling $(12\mathrm{}\mathrm{\ensuremath{\mu}}\stackrel{\ensuremath{\rightarrow}}{m}4\mathrm{}\mathrm{\ensuremath{\mu}}\mathrm{m})$ occurs in the valence band of InAs/GaAs self-assembled quantum dots. We show that the third-harmonic generation is enhanced due to the achievement of the double resonance condition between intraband transitions. A third-order nonlinear susceptibility $|{\ensuremath{\chi}}_{3\ensuremath{\omega}}^{(3)}|$ as large as $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}(\mathrm{m}/\mathrm{V}{)}^{2}$ is measured for one dot plane.

The absorption cross section for bound–free transitions in semiconductor quantum dots

The absorption cross sections for intraband transitions in spherical semiconductor quantum dots are considered, with the self-consistent effects taken into account. Electronic states are described by the effective mass Schrödinger equation, and the influence of accumulated electronic charge on the electronic structure is accounted for by the conventional numerical self-consistent procedure, i.e. by solving the Schrödinger and the Poisson equations iteratively. The self-consistent Hartree potential is found to push the wave function out of the dot “core” and thus may significantly influence the transition matrix elements, particularly in cases where the upper state is shallow or becomes a resonant state upon including the self-consistency.

Photoluminescence of spherical quantum dots

In order to interpret the phonon-assisted optical transitions in semiconductor quantum dots, a theory is developed comprising the exciton interaction with both adiabatic and Jahn-Teller phonons and also the external nonadiabaticity (pseudo-Jahn-Teller effect). The effects of nonadiabaticity of the exciton-phonon system are shown to lead to a significant enhancement of phonon-assisted transition probabilities and to multiphonon optical spectra that are considerably different from the Franck-Condon progression. The calculated relative intensity of the phonon satellites and its temperature dependence compare well with the experimental data on the photoluminescence of CdSe quantum dots, both colloidal and embedded in glass.