arXiv subjects

Florian Ungar

Publications and source records attributed to Florian Ungar.

18 recordsLinked to original sources

arXiv2019

Phonon-induced enhancement of photon entanglement in quantum dot-cavity systems

We report on simulations of the degree of polarization entanglement of photon pairs simultaneously emitted from a quantum dot-cavity system that demand revisiting the role of phonons. Since coherence is a fundamental precondition for entanglement and phonons are known to be a major source of decoherence, it seems unavoidable that phonons can only degrade entanglement. In contrast, we demonstrate that phonons can cause a degree of entanglement that even surpasses the corresponding value for the phonon-free case. In particular, we consider the situation of comparatively small biexciton binding energies and either finite exciton or cavity mode splitting. In both cases, combinations of the splitting and the dot-cavity coupling strength are found where the entanglement exhibits a nonmonotonic temperature dependence which enables entanglement above the phonon-free level in a finite parameter range. This unusual behavior can be explained by phonon-induced renormalizations of the dot-cavity coupling $g$ in combination with a nonmonotonic dependence of the entanglement on $g$ that is present already without phonons.

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arXiv2019

Spin dynamics of hot excitons in diluted magnetic semiconductors with spin-orbit interaction

We explore the impact of a Rashba-type spin-orbit interaction in the conduction band on the spin dynamics of hot excitons in diluted magnetic semiconductor quantum wells. In materials with strong spin-orbit coupling, we identify parameter regimes where spin-orbit effects greatly accelerate the spin decay and even change the dynamics qualitatively in the form of damped oscillations. Furthermore, we show that the application of a small external magnetic field can be used to either mitigate the influence of spin-orbit coupling or entirely remove its effects for fields above a material-dependent threshold.

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arXiv2019

Emission-frequency separated high quality single-photon sources enabled by phonons

We demonstrate theoretically that the single-photon purity of photons emitted from a quantum dot exciton prepared by phonon-assisted off-resonant excitation can be significantly higher in a wide range of parameters than that obtained by resonant preparation for otherwise identical conditions. Despite the off-resonant excitation the brightness stays on a high level. These surprising findings exploit that the phonon-assisted preparation is a two-step process where phonons first lead to a relaxation between laser-dressed states while high exciton occupations are reached only with a delay to the laser pulse maximum by adiabatically undressing the dot states. Due to this delay, possible subsequent processes, in particular multi-photon excitations, appear at a time when the laser pulse is almost gone. The resulting suppression of reexcitation processes increases the single-photon purity. Due to the spectral separation of the signal photons from the laser frequencies this enables the emission of high quality single photons not disturbed by a laser background while taking advantage of the robustness of the phonon assisted scheme.

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arXiv2019

From strong to weak temperature dependence of the two-photon entanglement resulting from the biexciton cascade inside a cavity

We investigate the degree of entanglement quantified by the concurrence of photon pairs that are simultaneously emitted in the biexciton-exciton cascade from a quantum dot in a cavity. Four dot-cavity configurations are compared that differ with respect to the detuning between the cavity modes and the quantum dot transitions, corresponding to different relative weights of direct two-photon and sequential single-photon processes. The dependence of the entanglement on the exciton fine-structure splitting $δ$ is found to be significantly different for each of the four configurations. For a finite splitting and low temperatures, the highest entanglement is found when the cavity modes are in resonance with the two-photon transition between the biexciton and the ground state and, in addition, the biexciton has a finite binding energy of a few meV. However, this widely used configuration is rather strongly affected by phonons such that other dot-cavity configurations, that are commonly regarded as less suited for obtaining high degrees of entanglement, become more favorable already at temperatures on the order of 10 K and above. If the cavity modes are kept in resonance with one of the exciton-to-ground-state transitions and the biexciton binding energy is finite, the entanglement drastically drops for positive $δ$ with rising temperatures when $T$ is below $\simeq$ 4 K, but is virtually independent of the temperature for higher $T$.

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arXiv2019

Role of excited states in the dynamics of excitons and their spins in diluted magnetic semiconductors

We theoretically investigate the impact of excited states on the dynamics of the exciton ground state in diluted magnetic semiconductor quantum wells. Exploiting the giant Zeeman shift in these materials, an external magnetic field is used to bring transitions between the exciton ground state and excited states close to resonance. It turns out that, when treating the exciton dynamics in terms of a quantum kinetic theory beyond the Markov approximation, higher exciton states are populated already well below the critical magnetic field required to bring the exciton ground state in resonance to an excited state. This behavior is explained by exciton-impurity correlations that can bridge energy differences on the order of a few meV and require a quantum kinetic description beyond the independent-particle picture. Of particular interest is the significant spin transfer toward states on the optically dark $2p$ exciton parabola which are protected against radiative decay.

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arXiv2019

Origins of overshoots in the exciton spin dynamics in semiconductors

We investigate the origin of overshoots in the exciton spin dynamics after resonant optical excitation. As a material system, we focus on diluted magnetic semiconductor quantum wells as they provide a strong spin-flip scattering for the carriers. Our study shows that overshoots can appear as a consequence of radiative decay even on the single-particle level in a theory without any memory. The magnitude of the overshoots in this case depends strongly on the temperature as well as the doping fraction of the material. If many-body effects beyond the single-particle level become important so that a quantum-kinetic description is required, a spin overshoot appears already without radiative decay and is much more robust against variations of system parameters. We show that the origin of the spin overshoot can be determined either via its temperature dependence or via its behavior for different doping fractions. The results can be expected to apply to a wide range of semiconductors as long as radiative decay and a spin-flip mechanism are present.

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arXiv2019

Phonon impact on the dynamics of resonantly excited and hot excitons in diluted magnetic semiconductors

Phonons are well known to be the main mechanism for the coupling between bright and dark excitons in nonmagnetic semiconductors. Here, we investigate diluted magnetic semiconductors where this process is in direct competition with the scattering at localized magnetic impurities. To this end, a recently developed quantum kinetic description of the exciton spin dynamics in diluted magnetic semiconductor quantum wells is extended by the scattering with longitudinal acoustic phonons. A strong phonon impact is found in the redistribution of exciton momenta on the exciton parabola that becomes especially prominent for high temperatures and exciton distributions further away from the exciton resonance which are optically dark. Despite their impact on the energetic redistribution, acoustic phonons virtually do not affected the exciton spin dynamics as the exciton-impurity interaction always dominates. Furthermore, it turns out that the exciton spin lifetime increases by roughly one order of magnitude for nonequilibrium hot exciton distributions and, in addition, pronounced quantum kinetic signatures in the exciton spin dynamics appearing after resonant optical excitation are drastically reduced.

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arXiv2019

Phonon-induced quantum ratchet in the exciton spin dynamics in diluted magnetic semiconductors in a magnetic field

Magnetically doped semiconductors are well known for their giant Zeeman splittings which can reach several meV even in relatively small external magnetic fields. After preparing a nonequilibrium exciton distribution via optical excitation, the spin dynamics in diluted magnetic semiconductor quantum wells is typically governed by spin-flip scattering processes due to the exciton-impurity exchange interaction. Our theoretical calculations show that the giant Zeeman splitting in these materials in combination with the influence of longitudinal acoustic phonons lead to a quantum ratchet-type dynamics, resulting in an almost complete reversal of the carrier spin polarization at very low temperatures. Furthermore, we find that the predictions of a much simpler rate-equation approach qualitatively agree with a more advanced and numerically demanding quantum kinetic description of the spin dynamics for a wide range of temperatures, although quantitative difference are noticeable.

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arXiv2018

Many-body correlations brought to light in absorption spectra of diluted magnetic semiconductors

Diluted magnetic semiconductors are materials well known to exhibit strong correlations which typically manifest in carrier-mediated magnetic ordering. In this Rapid Communication, we show that the interaction between excitons and magnetic impurities in these materials is even strong enough to cause a significant deviation from the bare exciton picture in linear absorption spectra of quantum well nanostructures. It is found that exciton-impurity correlations induce a characteristic fingerprint in the form of an additional feature close to the exciton resonance in combination with a shift of the main exciton line of up to a few meV. We trace back these features to the form of the self-energy and demonstrate that reliable values of the average correlation energy per exciton can be extracted directly from the spectra. Since the only requirement for our findings is sufficiently strong correlations, the results can be generalized to other strongly correlated systems.

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arXiv2018

Path-integral approach for nonequilibrium multi-time correlation functions of open quantum systems coupled to Markovian and non-Markovian environments

Using a real-time path integral approach we develop an algorithm to calculate multi-time correlation functions of open few-level quantum systems that is applicable to highly nonequilibrium dynamics. The calculational scheme fully keeps the non-Markovian memory introduced by the pure-dephasing type coupling to a continuum of oscillators. Furthermore, we discuss how to deal consistently with the simultaneous presence of non-Markovian and Markovian system reservoir interactions. We apply the method to a crucial test case, namely the evaluation of emission spectra of a laser-driven two-level quantum dot coupled to a continuum of longitudinal acoustic phonons, which give rise to non-Markovian dynamics. Here, we also account for the coupling to a photonic environment, which models radiative decay and can be treated as a Markovian bath. The phonon side bands are found on the correct side of the zero phonon line in our calculation, in contrast to known results where the quantum regression theorem is applied naively to non-Markovian dynamics. Combining our algorithm with a recently improved iteration scheme for performing the required sum over paths we demonstrate the numerical feasibility of our approach to systems with more than two levels. Results are shown for the second-order photonic two-time correlation function of a quantum dot-cavity system with seven states on the Jaynes-Cummings ladder taken into account.

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arXiv2018

Comparison of different concurrences characterizing photon-pairs generated in the biexciton cascade in quantum dots coupled to microcavities

We compare three different notions of concurrence to measure the polarization entanglement of two-photon states generated by the biexciton cascade in a quantum dot embedded in a microcavity. We focus on the often-discussed situation of a dot with finite biexciton binding energy in a cavity tuned to the two-photon resonance. Apart from the time-dependent concurrence, which can be assigned to the two-photon density matrix at any point in time, we study single- and double-time integrated concurrences commonly used in the literature that are based on different quantum state reconstruction schemes. We argue that the single-time integrated concurrence can be thought of as the concurrence of photons simultaneously emitted from the cavity without resolving the common emission time, while the more widely studied double-time integrated concurrence refers to photons that are neither filtered with respect to the emission time of the first photon nor with respect to the delay time between the two emitted photons. Analytic and numerical calculations reveal that the single-time integrated concurrence indeed agrees well with the typical value of the time-dependent concurrence at long times, even in the presence of phonons. Thus, the more easily measurable single-time integrated concurrence gives access to the physical information represented by the time-dependent concurrence. However, the double-time integrated concurrence shows a different behavior with respect to changes in the exciton fine structure splitting and even displays a completely different trend when the ratio between the cavity loss rate and the fine structure splitting is varied while keeping their product constant. This implies the non-equivalence of the physical information contained in the time-dependent and single-time integrated concurrence on the one hand and the double-time integrated concurrence on the other hand.

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arXiv2018

Trend reversal in the magnetic-field dependence of exciton spin-transfer rates in diluted magnetic semiconductors due to non-Markovian dynamics

We investigate theoretically the influence of an external magnetic field on the spin dynamics of excitons in diluted magnetic semiconductor quantum wells. To this end, we apply a quantum kinetic theory beyond the Markov approximation which reveals that non-Markovian effects can significantly influence the exciton spin dynamics. If the magnetic field is oriented parallel to the growth direction of the well, the Markovian spin-transfer rate decreases monotonically with increasing field as predicted by Fermi's golden rule. The quantum kinetic theory follows this result qualitatively but predicts pronounced quantitative differences in the spin-transfer rate as well as in the long-time spin polarization. However, for an in-plane magnetic field, where the Markovian spin-transfer rate first drops and then increases again, quantum kinetic effects become so pronounced that the Markovian trend is completely reversed. This is made evident by a distinct maximum of the rate followed by a monotonic decrease. The deviations can be traced back to a redistribution of carriers in energy space caused by correlations between excitons and magnetic dopants. The same effect leads to a finite electron-spin polarization at long times in longitudinal as well as transverse fields which is much larger than the corresponding Markovian prediction.

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arXiv2017

Quantum kinetic equations for the ultrafast spin dynamics of excitons in diluted magnetic semiconductor quantum wells after optical excitation

Quantum kinetic equations of motion for the description of the exciton spin dynamics in II-VI diluted magnetic semiconductor quantum wells with laser driving are derived. The model includes the magnetic as well as the nonmagnetic carrier-impurity interaction, the Coulomb interaction, Zeeman terms, and the light-matter coupling, allowing for an explicit treatment of arbitrary excitation pulses. Based on a dynamics-controlled truncation scheme, contributions to the equations of motion up to second order in the generating laser field are taken into account. The correlations between the carrier and the impurity subsystems are treated within the framework of a correlation expansion. For vanishing magnetic field, the Markov limit of the quantum kinetic equations formulated in the exciton basis agrees with existing theories based on Fermi's golden rule. For narrow quantum wells excited at the $1s$ exciton resonance, numerical quantum kinetic simulations reveal pronounced deviations from the Markovian behavior. In particular, the spin decays initially with approximately half the Markovian rate and a non-monotonic decay in the form of an overshoot of up to $10\,\%$ of the initial spin polarization is predicted.

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arXiv2017

Nonlinear cavity feeding and unconventional photon statistics in solid-state cavity QED revealed by many-level real-time path-integral calculations

The generation of photons in a microcavity coupled to a laser-driven quantum dot interacting with longitudinal acoustic (LA) phonons is studied in the regime of simultaneously strong driving and strong dot-cavity coupling. The stationary cavity photon number is found to depend in a non-trivial way on the detuning between the laser and the exciton transition in the dot. In particular, the maximal efficiency of the cavity feeding is obtained for detunings corresponding to transition energies between cavity-dressed states with excitation numbers larger than one. Phonons significantly enhance the cavity feeding at large detunings. In the strong-driving, strong-coupling limit, the photon statistics is highly non-Poissonian. While without phonons a double-peaked structure in the photon distribution is predicted, phonons make the photon statistics thermal-like with very high effective temperatures $\sim 10^5$ K, even for low phonon temperatures $\sim 4$ K. These results were obtained by numerical calculations where the driving, the dot-cavity coupling and the dot-phonon interactions are taken into account without approximations. This is achieved by a reformulation of an exact iterative path-integral scheme which is applicable for a large class of quantum-dissipative systems and which in our case reduces the numerical demands by 15 orders of magnitude.

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arXiv2017

Nonexponential spin decay in a quantum kinetic description of the D'yakonov-Perel' mechanism mediated by impurity scattering

The electron spin dynamics in an optically excited narrow quantum well is studied, where the electron spins precess in a k-dependent magnetic field while the electrons scatter at localized impurities. For the resulting spin decay, which is commonly known as the D'yakonov-Perel' mechansim, analytical expressions in the strong- and weak-scattering limits are available. It is found by the numerical solution of quantum kinetic equations in a broad range of parameters that, in situations that are typically relevant for ultrafast optical experiments, the dynamics of the total spin polarization significantly deviates from the pertinent analytical results. This is attributed to the broad spectral width of the optically excited spin-polarized electron distribution, which gives rise to a spin dephasing due to inhomogeneous broadening. Furthermore, it is found that the decay of the spin polarization need not be exponential. The notion of a spin decay time becomes ambiguous and different definitions of spin decay times can lead to different outcomes. The long-term dynamics of the decay of the spin polarization is even dominated by an algebraically decaying component. These findings highlight the importance of the effects of the broad spectral distribution of optically excited carriers in ultrashort magneto-optical experiments.

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arXiv2016

Influence of non-magnetic impurity scattering on the spin dynamics in diluted magnetic semiconductors

The doping of semiconductors with magnetic impurities gives rise not only to a spin-spin interaction between quasi-free carriers and magnetic impurities, but also to a local spin-independent disorder potential for the carriers. Based on a quantum kinetic theory for the carrier and impurity density matrices as well as the magnetic and non-magnetic carrier-impurity correlations, the influence of the non-magnetic scattering potential on the spin dynamics in DMS after optical excitation with circularly polarized light is investigated using the example of Mn-doped CdTe. It is shown that non-Markovian effects, which are predicted in calculations where only the magnetic carrier-impurity interaction is accounted for, can be strongly suppressed in the presence of non-magnetic impurity scattering. This effect can be traced back to a significant redistribution of carriers in $\mathbf{k}$-space which is enabled by the build-up of large carrier-impurity correlation energies. A comparison with the Markov limit of the quantum kinetic theory shows that, in the presence of an external magnetic field parallel to the initial carrier polarization, the asymptotic value of the spin polarization at long times is significantly different in the quantum kinetic and the Markovian calculations. This effect can also be attributed to the formation of strong correlations which invalidates the semiclassical Markovian picture and it is stronger when the non-magnetic carrier-impurity interaction is accounted for. In an external magnetic field perpendicular to the initial carrier spin, the correlations are also responsible for a renormalization of the carrier spin precession frequency.

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arXiv2015

Relaxation and coherent oscillations in the spin dynamics of II-VI diluted magnetic quantum wells

We study theoretically the ultrafast spin dynamics of II-VI diluted magnetic quantum wells in the presence of spin-orbit interaction. We extend a recent study where it was shown that the spin-orbit interaction and the exchange sd coupling in bulk and quantum wells can compete resulting in qualitatively new dynamics when they act simultaneously. We concentrate on Hg$_{1-x-y}$Mn$_x$Cd$_y$Te quantum wells, which have a highly tunable Rashba spin-orbit coupling. Our calculations use a recently developed formalism which incorporates electronic correlations originating from the exchange $sd$-coupling. We find that the dependence of electronic spin oscillations on the excess energy changes qualitatively depending on whether or not the spin-orbit interaction dominates or is of comparable strength with the sd interaction.

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arXiv2015

Ultrafast spin dynamics in II-VI diluted magnetic semiconductors with spin-orbit interaction

We study theoretically the ultrafast spin dynamics of II-VI diluted magnetic semiconductors in the presence of spin-orbit interaction. Our goal is to explore the interplay or competition between the exchange $sd$-coupling and the spin-orbit interaction in both bulk and quantum well systems. For bulk materials we concentrate on Zn$_{1-x}$Mn$_x$Se and take into account the Dresselhaus interaction, while for quantum wells we examine Hg$_{1-x-y}$Mn$_x$Cd$_y$Te systems with a strong Rashba coupling. Our calculations were performed with a recently developed formalism which incorporates electronic correlations beyond mean-field theory originated from the exchange $sd$-coupling. For both bulk and quasi-two-dimensional systems we find that, by varying the system parameters within realistic ranges, both interactions can be chosen to play a dominant role or to compete on an equal footing with each other. The most notable effect of the spin-orbit interaction in both types of systems is the appearance of strong oscillations where the exchange $sd$-coupling by itself only causes an exponential decay of the mean electronic spin components. The mean-field approximation is also studied and it is interpreted analytically why it shows a strong suppression of the spin-orbit-induced dephasing of the spin component parallel to the Mn magnetic field.

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