arXiv subjects

Michał Gawełczyk

Publications and source records attributed to Michał Gawełczyk.

27 records · Page 2Linked to original sources

arXiv2022

Beyond the four-level model: Dark and hot states in quantum dots degrade photonic entanglement

Entangled photon pairs are essential for a multitude of photonic quantum applications. To date, the best performing solid-state quantum emitters of entangled photons are semiconductor quantum dots operated around liquid-helium temperatures. To favor the widespread deployment of these sources, it is important to explore and understand their behavior at temperatures accessible with compact Stirling coolers. Here we study the polarization entanglement among photon pairs from the biexciton-exciton cascade in GaAs quantum dots at temperatures up to 65 K. We observe entanglement degradation accompanied by changes in decay dynamics, which we ascribe to thermal population and depopulation of hot and dark states in addition to the four levels relevant for photon pair generation. Detailed calculations considering the presence and characteristics of the additional states and phonon-assisted transitions support the interpretation. We expect these results to guide the optimization of quantum dots as sources of highly entangled photons at elevated temperatures.

quant-ph↗

arXiv2022

Bardeen's tunneling theory applied to intraorbital and interorbital hopping integrals between dopants in silicon

We utilize Bardeen's tunneling theory to calculate intra- and interorbital hopping integrals between phosphorus donors in silicon using known orbital wave functions. While the two-donor problem can be solved directly, the knowledge of hoppings for various pairs of orbitals is essential for constructing multi-orbital Hubbard models for chains and arrays of donors. To assure applicability to long-range potentials, we rederive Bardeen's formula for the matrix element without assuming non-overlapping potentials. Moreover, we find a correction to the original expression allowing us to use it at short distances. We also show that accurate calculation of the lowest donor-pair eigenstates is possible based on these tunnel couplings, and we characterize the obtained states. The results are in satisfactory quantitative agreement with those obtained with the standard Hückel tight-binding method. The calculation relies solely on the wave functions in the barrier region and does not explicitly involve donor or lattice potentials, which has practical advantages. We find that neglecting the central correction potential in the standard method may lead to qualitatively incorrect results, while its explicit inclusion raises severe numerical problems, as it is contained in a tiny volume. In contrast, using wave functions obtained with this correction in the proposed method does not raise such issues. Nominally, the computational cost of the method is to calculate a double integral along the plane that separates donors. For donor separation in directions where valley interference leads to oscillatory behavior, additional averaging over the position of the integration plane is needed. Despite this, the presented approach offers a competitive computational cost as compared to the standard one. This work may be regarded as a benchmark of a promising method for calculating hopping integrals in lattice models.

cond-mat.mes-hall↗

arXiv2021

Droplet epitaxy symmetric InAs/InP quantum dots for quantum emission in the third telecom window: morphology, optical and electronic properties

The rapidly developing quantum communication technology requires deterministic quantum emitters that can generate single photons and entangled photon pairs in the third telecom window, in order to be compatible with existing optical fiber networks and on-chip silicon photonic processors. InAs/InP quantum dots (QDs) are among the leading candidates for this purpose, due to their high emission efficiency in the required spectral range. However, fabricating versatile InAs/InP QD-based quantum emitters is challenging, especially as these QDs typically have asymmetric profiles in the growth plane, resulting in a substantial bright-exciton fine structure splitting (FSS). This hinders the generation of entangled photon pairs and thus, compromises the versatility of InAs/InP QDs. We overcome this by implementing droplet epitaxy (DE) synthesis of low surface density (2.8$\times$10$^8$ cm$^{-2}$) InAs QDs on an (001)-oriented InP substrate. The resulting QDs are located in etched pits, have concave bases, and most importantly, have symmetric in-plane profiles. We provide an analytical model to explain the kinetics of pit formation and QD base shape modification. Our theoretical calculations of electronic states reveal the properties of neutral and charged excitons and biexcitons confined in such QDs, which agree with the optical investigations of individual QDs. The optical response of QD ensembles suggests that FSS may indeed be negligible, as reflected in the vanishing degree of linear polarization. However, single QD spectrum gathered from an etched mesa shows moderate FSS of (50$\pm$5) $μ$eV that we link to destructive changes made in the QD environment during the post-growth processing. Finally, we show that the studied DE QDs provide a close-to-ideal single-photon emission purity of (92.5$\pm$7.5) $\%$ in the third telecom window.

cond-mat.mes-hall↗

arXiv2021

Tunneling-related electron spin relaxation in self-assembled quantum-dot molecules

We study theoretically spin relaxation during phonon-assisted tunneling of a single electron in self-assembled InAs/GaAs quantum-dot molecules formed by vertically stacked dots. We find that the spin-flip tunneling rate may be as high as 1% of the spin-conserving one. By studying the dependence of spin relaxation rate on external fields, we show that the process is active at a considerable rate even without the magnetic field, and scales with the latter differently than the relaxation in a Zeeman doublet. Utilizing a multiband $\boldsymbol{k}\cdot\boldsymbol{p}$ theory, we selectively investigate the impact of various spin-mixing terms in the electron energy and carrier-phonon interaction Hamiltonians. As a result, we identify the main contribution to come from the Dresselhaus spin-orbit interaction, which is responsible for the zero-field effect. At magnetic fields above $\sim$ 15 T, this is surpassed by other contributions due to the structural shear strain. We also study the impact of the sample morphology and determine that the misalignment of the dots may enhance relaxation rate by over an order of magnitude. Finally, via virtual tunneling at nonzero temperature, the process in question also affects stationary electrons in tunnel-coupled structures and provides a Zeeman-doublet spin relaxation channel even without the magnetic field.

cond-mat.mes-hall↗

arXiv2020

Optical and electronic properties of symmetric InAs/InGaAlAs/InP quantum dots formed by a ripening process in molecular beam epitaxy: a promising system for broad-range single-photon telecom emitters

We present a detailed experimental optical study supported by theoretical modeling of InAs quantum dots (QDs) embedded in an InAlGaAs barrier lattice-matched to InP(001) grown with the use of a ripening step in molecular beam epitaxy. The method leads to the growth of in-plane symmetric QDs of low surface density, characterized by a multimodal size distribution resulting in a spectrally broad emission in the range of $1.4-2.0$ $μ$m, essential for many near-infrared photonic applications. We find that, in contrast to the InAs/InP system, the multimodal distribution results here from a two-monolayer difference in QD height between consecutive families of dots. This may stem from the long-range ordering in the quaternary barrier alloy that stabilizes QD nucleation. Measuring the photoluminescence (PL) lifetime of the spectrally broad emission, we find a nearly dispersionless value of $1.3\pm0.3$ ns. Finally, we examine the temperature dependence of emission characteristics. We underline the impact of localized states in the wetting layer playing the role of carrier reservoir during thermal carrier redistribution. We determine the hole escape to the InAlGaAs barrier to be a primary PL quenching mechanism in these QDs.

cond-mat.mes-hall↗

arXiv2018

Controllable electron spin dephasing due to phonon state distinguishability in a coupled quantum dot system

We predict a spin pure dephasing channel in electron relaxation between states with unequal Zeeman splittings, exemplified by a spin-preserving electron tunneling between quantum dots in a magnetic field. The dephasing is caused by a mismatch in electron $g$-factors in the dots leading to distinguishability of phonons emitted during tunneling with opposite spins. Combining multiband $\boldsymbol{k}{\cdot}\boldsymbol{p}$ modeling and dynamical simulations via a Master equation we show that this fundamental effect of spin measurement effected by the phonon bath may be widely controlled by the size and composition of the dots or on demand, via tuning of external fields. By comparing the numerically simulated degree of dephasing with the predictions of general theory based on distinguishability of environment states, we show that the proposed mechanism is the dominant phonon-related spin dephasing channel and may limit spin coherence time in tunnel-coupled structures at cryogenic temperatures.

cond-mat.mes-hall↗

arXiv2018

Dominant role of the shear strain induced admixture in spin-flip processes in self-assembled quantum dots

We study theoretically the spin-flip relaxation processes for a single electron in a self-assembled InAs/GaAs quantum dot, using an 8-band kp theory in the envelope function approximation. We show that the dominating channel of spin relaxation is spin admixture induced by symmetry-breaking shear strain, which can be mapped onto two effective spin-phonon terms in a conduction band (effective mass) Hamiltonian that have a similar structure and interfere constructively. Unlike the Dresselhaus coupling that dominates spin relaxation in larger, unstrained dots, the shear strain contribution cannot be modeled by a unique standard term in the Hamiltonian but rather relies on the actual strain distribution in the quantum dot.

cond-mat.mes-hall↗

arXiv2018

Spin dynamics and magneto-optical response in charge-neutral tunnel-coupled quantum dots

We model the electron and hole spin dynamics in an undoped double quantum dot structure, considering the carrier tunneling between quantum dots. Taking into account also the presence of an in-plane or tilted magnetic field, we provide the simulation of magneto-optical experiments, like the time resolved Kerr rotation measurement, which are performed currently on such structures to probe the temporal spin dynamics. With our model, we reproduce the experimentally observed effect of the extension of the spin polarization life time caused by the spatial charge separation, which may occur in structures of this type. Moreover, we provide a number of qualitative predictions concerning the necessary conditions for observation of this effect as well as about possible channels of its suppression, including the spin-orbit coupling, which leads to tunneling of carriers accompanied by a spin-flip. We consider also the impact of the magnetic field tilting, which results in an interesting spin polarization dynamics.

cond-mat.mes-hall↗

arXiv2018

Optical initialization of hole spins in p-doped quantum dots: orientation efficiency and loss of coherence

We study theoretically the recently proposed hole spin initialization scheme for $p$-doped quantum well or dot systems via coupling to trion states with sub-picosecond circularly polarized laser pulses. We analyze the efficiency of spin initialization an predict the intrinsic spin coherence loss due to the pulse excitation itself as well as the phonon-induced spin dephasing, both taking place on the timescale of the driving laser pulse. We show that the ratio of the degree of dephasing to the achieved orientation effect does not depend on the pulse area but is sensitive to the temperature and detuning. The optimal excitation parameters are identified.

cond-mat.mes-hall↗