arXiv subjects

Paweł Machnikowski

Publications and source records attributed to Paweł Machnikowski.

At least 73 records · Page 4Linked to original sources

arXiv2013

Dynamics of dissipative multiple exciton generation in nanocrystals

The population dynamics of single- and bi-exciton states in semiconductor nanocrystals is modeled numerically in the presence of Coulomb coupling between single- and two-exciton states and a dissipation channel in order to study the transient bi-exciton population that occurs in an optically excited semiconductor nanocrystal. The results show that the system evolution strongly changes if the dissipation is included. In a certain range of parameters, the growth of the exciton number (MEG process) is fast (on picosecond time scale) and the following decay (Auger process) is much slower (hundreds of picoseconds). In some cases, the maximum occupation of the bi-exciton state increases when dissipation is included. The dynamics of an ensemble of nanocrystals with a certain size dispersion is studied by averaging over the energy of the bi-exciton state which can be different for each single nanocrystal. The validity of Markov and secular approximation is also verified.

cond-mat.mes-hall↗

arXiv2012

Intraband absorption in finite, inhomogeneous quantum dot stacks for intermediate band solar cells: limitations and optimization

We present a theoretical analysis of intraband optical transitions from the intermediate pseudo-band of confined states to the conduction band in a finite, inhomogeneous stack of self-assembled semiconductor quantum dots. The chain is modeled with an effective Hamiltonian including nearest-neighbor tunnel couplings and the absorption under illumination with both coherent (laser) and thermal radiation is discussed. We show that the absorption spectrum already for a few coupled dots differs from that of a single dot and develops a structure with additional maxima at higher energies. We find out that this leads to an enhancement of the overall transition rate under solar illumination by up to several per cent which grows with the number of QDs but saturates already for a few QDs in the chain. The decisive role of the strength of inter-dot coupling for the stability of this enhancement against QD stack inhomogeneity and temperature is revealed.

cond-mat.mes-hall↗

arXiv2012

Enhanced spontaneous emission from inhomogeneous ensembles of quantum dots is induced by short-range couplings

We study theoretically the spontaneous emission from an inhomogeneous ensemble of quantum dots in the weak excitation limit. We show that collective, superradiance-like effects lead to an enhanced emission rate in the presence of sufficiently strong coupling between the dots in agreement with experimental observations, which means that the quantum dot sample cannot be treated as an ensemble of individual emitters. We demonstrate also that the collective behavior of the quantum dot system relies on short-range interactions, while long-range dipole couplings are too weak to have any impact on the emission dynamics for a system with a realistic degree of inhomogeneity.

cond-mat.mes-hall↗

arXiv2012

Vacuum-induced coherence in quantum dot systems

We present a theoretical study of vacuum-induced coherence in a pair of vertically stacked semiconductor quantum dots. The process consists in a coherent excitation transfer from a single-exciton state localized in one dot to a delocalized state in which the exciton occupation gets trapped. We study the influence of the factors characteristic of quantum dot systems (as opposed to natural atoms): energy mismatch, coupling between the single exciton states localized in different dots, different and non-parallel dipoles due to subband mixing, as well as coupling to phonons. We show that the destructive effect of the energy mismatch can be overcome by an appropriate interplay of the dipole moments and coupling between the dots which allows one to observe the trapping effect even in a structure with technologically realistic energy splitting on the order of milli-electron-Volts. We also analyze the impact of phonon dynamics on the occupation trapping and show that phonon effects are suppressed in a certain range of system parameters. This analysis shows that the vacuum induced coherence effect and the associated long-living trapped excitonic population can be achieved in quantum dots.

cond-mat.mes-hall↗

arXiv2012

Coulomb matrix elements for the impact ionization process in nanocrystals: the envelope function approach

We propose a method for calculating Coulomb matrix elements between exciton and biexciton states in semiconductor nanocrystals based on the envelope function formalism. We show that such a calculation requires proper treatment of the Bloch parts of the carrier wave functions which, in the leading order, leads to spin selection rules identical to those holding for optical interband transitions. Compared to the usual (intraband) Coulomb couplings, the resulting matrix elements are additionally scaled by the ratio of the lattice constant to the nanocrystal radius. As a result, the Coulomb coupling between exciton and biexciton states scale as 1/R^2. We present also some statistical estimates of the distribution of the coupling magnitudes and energies of the coupled states The number of biexciton states coupled to exciton states form a certain energy range shows a power-law scaling with the ratio of the coupling magnitude to the energy separation. We estimate also the degree of mixing between exciton and biexciton states. The amount of biexciton admixture to exciton states at least 1 eV above the multiple exciton generation threshold can reach 80% but varies strongly with the nanocrystal size.

cond-mat.mes-hall↗

arXiv2012

Electronic and optical properties in non-uniformly shaped QDashes

We theoretically study the optical properties and the electronic structure of highly elongated quantum dots (quantum dashes) and show how carrier trapping due to geometrical fluctuations of the confining potential affects the excitonic spectrum of the system. We focus on the study of the optical properties of a single exciton confined in the structure. The dependence of the absorption and emission intensities on the geometrical properties (depth and position) of the trapping center of a quantum dash is analyzed and the dependence of the degree of linear polarization on these geometrical parameters is studied in detail.

cond-mat.mes-hall↗

arXiv2012

Collective spontaneous emission from a system of quantum dots

We study the spontaneous emission from a regular lateral array or a randomly distributed ensemble of quantum dots under strong excitation (full inversion) conditions. We focus on the similarities and differences between the cases of random and regular arrangement of the dots and show that there is very little difference between the evolution of luminescence in these two cases, both for identical dots and for a realistically inhomogeneously broadened ensemble. This means that the enhanced emission or superradiance effect is not due to accidental clustering of pairs of dots. Moreover, we point out that observation of an enhanced emission under weak excitation does not prove that true superradiance will develop in a fully inverted system.

cond-mat.mes-hall↗

arXiv2012

Carrier trapping in a quantum dash: optical signatures

We theoretically study the optical properties and the electronic structure of highly elongated quantum dots (quantum dashes) and show how geometrical fluctuations affect the excitonic spec- trum of the system. The dependence of the absorption intensities on the geometrical properties (depth and length) of the trapping center in a quantum dash is analyzed and the dependence of the degree of the linear polarization on these geometrical parameters is studied.

cond-mat.mes-hall↗

arXiv2012

Phonon-assisted relaxation between hole states in quantum dot molecules

We study theoretically phonon-assisted relaxation and inelastic tunneling of holes in a double quantum dot. We derive hole states and relaxation rates from kp Hamiltonians and show that there is a finite distance between the dots where lifetimes of hole states are very long which is related to vanishing tunnel coupling. We show also that the light hole admixture to hole states can considerably affect the hole relaxation rates even though its magnitude is very small.

cond-mat.mes-hall↗

arXiv2012

Polaron contributions to the biexciton binding energies in self-assembled quantum dots

The contribution to the biexciton binding energy in quantum dots resulting from the interaction with longitudinal optical phonons is estimated by performing the configuration--interaction calculation of the few-particle states in a simple model of the confining potential and including the phonon corrections by means of a perturbation theory. It is found that the polaron contribution tends to compensate the Coulomb-related biexciton shift (binding energy) and reduces its value by several to even 30%, depending on the material parameters of the system.

cond-mat.mes-hall↗

arXiv2011

Tunneling transfer protocol in a quantum dot chain immune to inhomogeneity

We propose a quantum dot (QD) implementation of a quantum state transfer channel. The proposed channel consists of N vertically stacked QDs with the nearest neighbor tunnel coupling, placed in an axial electric field. We show that the system supports high-fidelity transfer of the state of a terminal dot both by free evolution and by adiabatic transfer. The protocol is to a large extent insensitive to inhomogeneity of the energy parameters of the dots and requires only a global electric field.

cond-mat.mes-hall↗

arXiv2011

Collective spontaneous emission from pairs of quantum dots: long-range vs. short-range couplings

We study the spontaneous emission from a coherently delocalized exciton state in a double quantum dot as a function of the distance between the dots, focusing on the similarities and differences between the cases of radiative (long-range, dipole) and tunnel coupling between the excitons in the dots. We show that there may be no qualitative difference between the collective emission induced by these two coupling types in spite of their essentially different physical properties.

cond-mat.mes-hall↗

arXiv2011

Intermediate band formation and intraband absorption for electrons in an inhomogeneous chain of quantum dots

We study the electron states of a chain of non-identical, vertically stacked quantum dots (QDs). We discuss how the pseudo-band formed of the ground states confined in the QDs disintegrates upon increasing the inhomogeneity of the electron energies and analyze the impact of localization on the intraband absorption from the pseudo-band to extended (bulk) states. We describe also the dependence of the intraband absorption spectrum on the QD size.

cond-mat.mes-hall↗

arXiv2010

Second-order polaron resonances in self assembled quantum dots

We theoretically study the optical properties of an InAs/GaAs quantum dot (QD) near the area of the second-order resonance between an electron confined in the QD and two longitudinal optical phonons. We present the absorption spectra of an inhomogeneously broadened QD ensemble and show that the minimal model needed for an accurate description of such a system needs to account for 3-phonon states. We study also the influence of the QD height to width ratio on the optical properties of the polaron system. The dependence of the width of the resonance and the position of the second-order resonant feature on the height to width ratio is presented.

cond-mat.mes-hall↗

arXiv2010

Theory of the time-resolved Kerr rotation on trapped holes

We formulate a model of the time-resolved Kerr rotation experiment on a single hole in a semiconductor nanostructure (e.g., a quantum dot) or on an ensemble of trapped holes (e.g., in a quantum well) in a tilted magnetic field. We use a generic Markovian description of the hole and trion dephasing and focus on the interpretation of the time-resolved signal in terms of the microscopic evolution of the spin polarization. We show that the signal in an off-plane field contains components that reveal both the spin relaxation rate and the spin coherence dephasing rate. We derive analytical formulas for the hole spin polarization, which may be used to extract the two relevant rates by fitting to the measurement data.

cond-mat.mes-hall↗

arXiv2010

Two-phonon polaron resonances in self-assembled quantum dots

We study the second-order polaronic resonance between 2-LO-phonon states and p-shell electron states in a quantum dot. We show that the spectrum in the resonance area can be quantitatively reproduced by a theoretical model using only standard material constants, without any free parameters. We propose also a perturbative approach to the problem based on a quasi-degenerate perturbation theory. This method not only considerably reduces the numerical complexity without considerable loss of accuracy but also gives some insight into the structure and origin of the resonance spectrum.

cond-mat.mes-hall↗

arXiv2010

Phonon-assisted relaxation and tunneling in self-assembled quantum dot molecules

We study theoretically phonon-assisted relaxation processes in a system consisting of one or two electrons confined in two vertically stacked self-assembled quantum dots. The calculation is based on a k.p approximation for single particle wave functions in a strained self-assembled structure. From these, two-particle states are calculated by including the Coulomb interaction and the transition rates between the lowest energy eigenstates are derived. We take into account phonon couplings via deformation potential and piezoelectric interaction and show that they both can play a dominant role in different parameter regimes. Within the Fermi golden rule approximation, we calculate the relaxation rates between the lowest energy eigenstates which lead to thermalization on a picosecond time scale in a narrow range of dot sizes.

cond-mat.mes-hall↗

arXiv2010

Hole subband mixing and polarization of luminescence from quantum dashes: a simple model

In this paper, we address the problem of luminescence polarization in the case of nanostructures characterized by an in-plane shape asymmetry. We develop a simple semi-qualitative model revealing the mechanism that accounts for the selective polarization properties of such structures. It shows that they are not a straightforward consequence of the geometry but are related to it via valence subband mixing. Our model allows us to predict the degree of polarization (DOP) dependence on the in-plane dimensions of investigated structures assuming a predominantly heavy hole character of the valence band states, simplifying the shape of confining potential and neglecting the influence of the out-of plane dimension. The energy dependence modeling reveals the importance of different excited states in subsequent spectral ranges leading to non-monotonic character of the DOP. The modeling results show good agreement with the experimental data for an ensemble of InAs/InP quantum dashes for a set of realistc parameters with the heavy-light hole states separation being the only adjustable one. All characteristic features are reproduced in the framework of the proposed model and their origin can be well explained and understood. We also make some further predictions about the influence of both the internal characteristics of the nanostructures (e.g. height) and the external conditions (excitation power, temperature) on the overall DOP.

cond-mat.mes-hall↗