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1
Content available remote Two-Photon Coherent Polarization Flipping of Confined Excitons
100%
Machnikowski P.
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nr 2
289-293
EN
A two-photon process leading to coherent transitions between the two circularly polarized exciton states in a quantum dot is studied. It is shown that optical flipping of the exciton polarization is possible with picosecond laser pulses. The process is closely related to two-photon Rabi oscillations of a biexciton but it is much more stable against shifts of the laser frequency.
2
Content available remote Phonon-Induced Pure Dephasing of Two-Electron Spin States in Vertical Quantum Dot Molecules
63%
Roszak K. , Machnikowski P.
Acta Physica Polonica A
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2009
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tom 116
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nr 5
877-878
EN
Phonon-induced dephasing of two-electron spin states in two vertically stacked self-assembled GaAs/InGaAs quantum dots is studied. A pure dephasing process due to elastic phonon scattering is found to dominate at low temperatures. This process is independent of the spin-orbit coupling and does not require the presence of a magnetic field. It relies on interdot tunneling and the Pauli principle, which make the double quantum dot gate for spin based quantum computing possible, and therefore cannot be avoided.
3
Content available remote Collective Spontaneous Emission from a System of Quantum Dots
63%
Abdussalam W. , Machnikowski P.
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nr 6
994-996
EN
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.
4
Content available remote Electron States, Phonon-Assisted Relaxation and Tunneling in Self-Assembled Quantum Dot Molecules in an Electric Field
63%
Gawarecki K. , Machnikowski P.
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nr 5
637-639
EN
We present a theoretical analysis of the phonon-assisted relaxation in a system composed of two self-assembled vertically stacked quantum dots. We construct realistic model, which takes into account the geometry and strain distribution in the system. We calculate phonon-assisted relaxation rates between the two lowest states (in one- and two-electron cases). The relaxation rates and energy levels are studied as a function of external (axial) electric field and geometry of the structure (dot sizes). We show that the relaxation times can be as low as 1 ps but efficient relaxation occurs only for very finely tuned dots.
5
Content available remote Phonon-Assisted Excitation Transfer in Quantum Dot Molecules
63%
Rozbicki E. , Machnikowski P.
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nr 2
197-202
EN
We derive a quantum-kinetic description of phonon-assisted Förster transfer between two coupled quantum dots (a quantum dot molecule). We show that the exciton state decays to the ground state of the quantum dot molecule via a combination of the Rabi rotation and exponential decay. For moderately spaced dots this process takes place on a picosecond timescale.
6
Content available Collective Encoding of Quantum Information in a Quantum Dot Register
63%
Grodecka A. , Machnikowski P.
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tom 110
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nr 2
195-200
EN
We define the collective logical quantum bit as a state on an array of 2, 4, and 8 quantum dots. We show that such an array can be controlled with less phonon-induced pure dephasing than a single dot. We calculate the dephasing accompanying a single-qubit operation on the collective logical qubit by means of perturbative method describing the open system evolution under arbitrary driving.
7
Content available remote Four-Wave Mixing Spectroscopy of Quantum Dot Molecules
63%
Sitek A. , Machnikowski P.
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nr 2
167-172
EN
We study theoretically the nonlinear four-wave mixing response of an ensemble of coupled pairs of quantum dots (quantum dot molecules). We discuss the shape of the echo signal depending on the parameters of the ensemble: the statistics of transition energies and the degree of size correlations between the dots forming the molecules.
8
Content available remote Singlet-Triplet Dephasing in Asymmetric Quantum Dot Molecules
51%
Marcinowski Ł. , Roszak K. , Machnikowski P.
Acta Physica Polonica A
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2009
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tom 116
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nr 5
874-876
EN
We discuss pure dephasing of singlet-triplet superpositions in two-electron double quantum dots due to elastic phonon scattering. We generalize our previous results to a system built of two non-identical dots. We show that the asymmetry must be very strong in order to considerably affect the dephasing rate.
9
Content available remote Collective Luminescence and Phonon-Induced Processes in Double Quantum Dots
51%
Machnikowski P. , Roszak K. , Sitek A.
Acta Physica Polonica A
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2009
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tom 116
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nr 5
818-825
EN
We study the evolution of a quantum state of a double quantum dot system interacting with the electromagnetic environment and with the lattice modes, in the presence of a coupling between the two dots. We propose a unified approach to the simulation of the system evolution under joint impact of the two reservoirs. We discuss the sub- and superradiant radiative decay of the system, the phonon-induced decay of entanglement between the dots, and the transfer of excitation between them.
10
Content available remote Carrier Trapping in a Quantum Dash: Optical Signatures
51%
Kaczmarkiewicz P. , Machnikowski P. , Kuhn T.
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nr 6
997-1000
EN
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 spectrum 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.
11
Content available remote Hole Subband Mixing and Polarization of Luminescence from Quantum Dashes: A Simple Model
38%
Kaczmarkiewicz P. , Musiał A. , Sęk G. , Podemski P. , Machnikowski P. , Misiewicz J.
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nr 5
633-636
EN
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 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 degree of polarization. The modeling results show good agreement with the experimental data for an ensemble of InAs/InP quantum dashes for a set of realistic 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 degree of polarization.
12
Content available remote Phonon Effects on Population Inversion in Quantum Dots: Resonant, Detuned and Frequency-Swept Excitations
38%
Reiter D. , Lüker S. , Gawarecki K. , Grodecka-Grad A. , Machnikowski P. , Axt V. , Kuhn T.
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nr 6
1065-1068
EN
The effect of acoustic phonons on different light-induced excitations of a semiconductor quantum dot is investigated. Resonant excitation of the quantum dot leads to the Rabi oscillations, which are damped due to the phonon interaction. When the excitation frequency is detuned, an occupation can only occur due to phonon absorption or emission processes. For frequency-swept excitations a population inversion is achieved through adiabatic rapid passage, but the inversion is also damped by phonons. For all three scenarios the influence of the phonons depends non-monotonically on the pulse area.
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