arXiv subjects

D. E. Reiter

Publications and source records attributed to D. E. Reiter.

35 records · Page 2Linked to original sources

arXiv2017

Systematic study of the influence of coherent phonon wave packets on the lasing properties of a quantum dot ensemble

Coherent phonons can greatly vary light-matter interaction in semiconductor nanostructures placed inside an optical resonator on an ultrafast time scale. For an ensemble of quantum dots as active laser medium phonons are able to induce a large enhancement or attenuation of the emission intensity, as has been recently demonstrated. The physics of this coupled phonon-exciton-photon system consists of various effects, which in the experiment typically cannot be clearly separated, in particular because a rather complex strain pulse impinges on the quantum dot ensemble. Here we present a comprehensive theoretical study how the laser emission is affected by phonon pulses of various shapes as well as by ensembles with different spectral distributions of the quantum dots. This gives insight into the fundamental interaction dynamics of the coupled phonon-exciton-photon system, while it allows us to clearly discriminate between two prominent effects: the adiabatic shifting of the ensemble and the shaking effect. This paves the way to a tailored laser emission controlled by phonons.

cond-mat.mes-hall↗

arXiv2017

Reading the orbital angular momentum of light using plasmonic nanoantennas

Orbital angular momentum of light has recently been recognized as a new degree of freedom to encode information in quantum communication using light pulses. Methods to extract this information include reversing the process by which such twisted light was created in the first place or interference with other beams. Here, we propose an alternative new way to directly read out the extra information encoded in twisted light using plasmonic nanoantennas by con- verting the information about the orbital angular momentum of light into spectral information using bright and dark modes. Exemplarily considering rotation-symmetrical nanorod nanoan- tennas we show that their scattering cross-section is sensitive to the value of the orbital angular momentum combined with the polarisation of an incident twisted light beam. Explaining the twist-dependence of the excited modes with a new analytical model our results pave the way to twisted light nanoplasmonics, which is of central importance for future on-chip communication using orbital angular momentum of light.

cond-mat.mes-hall↗

arXiv2017

Time-resolved pump-probe signals of a continuously driven quantum dot affected by phonons

The interaction of a light field with a quantum-mechanical system can be studied in an optically controlled semiconductor quantum dot. When driven by a continuous light field switched on instantaneously, the quantum dot occupation performs Rabi oscillations. Unlike an atomic system, the quantum dot is coupled to phonons, which leads to a damping of the Rabi oscillations. Here we model the time-resolved probe spectra to monitor these dynamics and study the influence of phonons on the spectra. The spectra consist of up to three peaks, similar to the Mollow-triplet known from quantum optics. We develop analytical equations within a rate equation model and show that they agree excellently with a numerical solution using a well established correlation expansion approach.

cond-mat.mes-hall↗

arXiv2016

Impact of phonons on dephasing of individual excitons in deterministic quantum dot microlenses

Optimized light-matter coupling in semiconductor nanostructures is a key to understand their optical properties and can be enabled by advanced fabrication techniques. Using in-situ electron beam lithography combined with a low-temperature cathodoluminescence imaging, we deterministically fabricate microlenses above selected InAs quantum dots (QDs) achieving their efficient coupling to the external light field. This enables to perform four-wave mixing micro-spectroscopy of single QD excitons, revealing the exciton population and coherence dynamics. We infer the temperature dependence of the dephasing in order to address the impact of phonons on the decoherence of confined excitons. The loss of the coherence over the first picoseconds is associated with the emission of a phonon wave packet, also governing the phonon background in photoluminescence (PL) spectra. Using theory based on the independent boson model, we consistently explain the initial coherence decay, the zero-phonon line fraction, and the lineshape of the phonon-assisted PL using realistic quantum dot geometries.

cond-mat.mes-hall↗

arXiv2016

Formulation of the twisted-light--matter interaction at the phase singularity: beams with strong magnetic fields

The formulation of the interaction of matter with singular light fields needs special care. In a recent article [Phys.~Rev.~A {\bf 91}, 033808 (2015)] we have shown that the Hamiltonian describing the interaction of a twisted light beam having parallel orbital and spin angular momenta with a small object located close to the phase singularity can be expressed only in terms of the electric field of the beam. Here, we complement our studies by providing an interaction Hamiltonian for beams having antiparallel orbital and spin angular momenta. Such beams may exhibit unusually strong magnetic effects. We further extend our formulation to radially and azimuthally polarized beams. The advantages of our formulation are that for all beams the Hamiltonian is written solely in terms of the electric and magnetic fields of the beam and as such it is manifestly gauge-invariant. Furthermore it is intuitive by resembling the well-known expressions in the dipole-electric and dipole-magnetic moment approximations.

physics.optics↗

arXiv2016

Dynamics of excitons in individual InAs quantum dots revealed in four-wave mixing spectroscopy

A detailed understanding of the population and coherence dynamics in optically driven individual emitters in solids and their signatures in ultrafast nonlinear-optical signals is of prime importance for their applications in future quantum and optical technologies. In a combined experimental and theoretical study on exciton complexes in single semiconductor quantum dots we reveal a detailed picture of the dynamics employing three-beam polarization-resolved four-wave mixing (FWM) micro-spectroscopy. The oscillatory dynamics of the FWM signals in the exciton-biexciton system is governed by the fine-structure splitting and the biexciton binding energy in an excellent quantitative agreement between measurement and analytical description. The analysis of the excitation conditions exhibits a dependence of the dynamics on the specific choice of polarization configuration, pulse areas and temporal ordering of driving fields. The interplay between the transitions in the four-level exciton system leads to rich evolution of coherence and population. Using two-dimensional FWM spectroscopy we elucidate the exciton-biexciton coupling and identify neutral and charged exciton complexes in a single quantum dot. Our investigations thus clearly reveal that FWM spectroscopy is a powerful tool to characterize spectral and dynamical properties of single quantum structures.

cond-mat.mes-hall↗

arXiv2016

Fast and selective phonon-assisted state preparation of a quantum dot by adiabatic undressing

We investigate theoretically the temporal behavior of a quantum dot under off-resonant optical excitation targeted at fast acoustic phonon-assisted state preparation. We demonstrate that in a preparation process driven by short laser pulses three processes can be identified: a dressing of the states during the switch on of the laser pulse, a subsequent phonon-induced relaxation and an undressing at the end of the pulse. By analyzing excitation scenarios with different pulse shapes we highlight the decisive impact of an adiabatic undressing on the final state in short pulse protocols. Furthermore, we show that in exciton-biexciton systems the laser characteristics such as the pulse detuning and the pulse length as well as the biexciton binding energy can be used to select the targeted quantum dot state.

cond-mat.mes-hall↗

arXiv2015

Direct optical state preparation of the dark exciton in a quantum dot

Because of their weak coupling to the electromagnetic field dark excitons in semiconductor quantum dots possess extremely long lifetimes, which makes them attractive candidates for quantum information processing. On the other hand, preparation and manipulation of dark states is challenging, because commonly used optical excitation mechanisms are not applicable. We propose a new, efficient mechanism for the deterministic preparation of the dark exciton exploiting the application of a tilted magnetic field and the optical excitation with a chirped, i.e., frequency modulated, laser pulse.

cond-mat.mes-hall↗

arXiv2014

Formulation of the twisted-light--matter interaction at the phase singularity: the twisted-light gauge

Twisted light is light carrying orbital angular momentum. The profile of such a beam is a ring-like structure with a node at the beam axis, where a phase singularity exits. Due to the strong spatial inhomogeneity the mathematical description of twisted-light--matter interaction is non-trivial, in particular close to the phase singularity, where the commonly used dipole-moment approximation cannot be applied. In this paper we show that, if the polarization and the orbital angular momentum of the twisted-light beam have the same sign, a Hamiltonian similar to the dipole-moment approximation can be derived. However, if the signs of polarization and orbital angular momentum differ, in general the magnetic parts of the light beam become of significant importance and an interaction Hamiltonian which only accounts for electric fields, as in the dipole-moment approximation, is inappropriate. We discuss the consequences of these findings for twisted-light excitation of a semiconductor nanostructures, e.g., a quantum dot, placed at the phase singularity.

physics.optics↗

arXiv2014

The role of phonons for exciton and biexciton generation in an optically driven quantum dot

For many applications of semiconductor quantum dots in quantum technology a well controlled state preparation of the quantum dot states is mandatory. Since quantum dots are embedded in the semiconductor matrix, the interaction with phonons plays often a major role in the preparation process. In this review, we discuss the influence of phonons on three basically different optical excitation schemes which can be used for the preparation of exciton, biexciton, and superposition states: a resonant excitation leading to Rabi rotations in the excitonic system, an excitation with chirped pulses exploiting the effect of adiabatic rapid passage, and an off-resonant excitation giving rise to a phonon-assisted state preparation. We give an overview over experimental and theoretical results showing the role of the phonons and compare the performance of the schemes for state preparation.

cond-mat.mes-hall↗

arXiv2014

Energy transport and coherence properties of acoustic phonons generated by optical excitation of a quantum dot

The energy transport of acoustic phonons generated by the optical excitation of a quantum dot as well as the coherence properties of these phonons are studied theoretically both for the case of a pulsed excitation and for a continuous wave (cw) excitation switched on instantaneously. For a pulsed excitation, depending on pulse area and pulse duration, a finite number of phonon wave packets is emitted, while for the case of a cw excitation a sequence of wave packets with decreasing amplitude is generated after the excitation has been switched on. We show that the energy flow associated with the generated phonons is partly related to coherent phonon oscillations and partly to incoherent phonon emission. The efficiency of the energy transfer to the phonons and the details of the energy flow depend strongly and in a non-monotonic way on the Rabi frequency exhibiting a resonance behavior. However, in the case of cw excitation it turns out that the total energy transferred to the phonons is directly linked in a monotonic way to the Rabi frequency.

cond-mat.mes-hall↗

arXiv2013

Adiabatic rapid passage in quantum dots: phonon-assisted decoherence and biexciton generation

We study the evolution of a quantum dot controlled by a frequency-swept (chirped), linearly polarized laser pulse in the presence of carrier-phonon coupling. The final occupation of the exciton state is limited both due to phonon-induced transitions between the adiabatic spectral branches and because of phonon-assisted transitions to the biexciton state. When the biexciton shift is large enough, the quantum dot can be modeled as a two-level system, which corresponds to excitation with circularly polarized light. For this case, we compare different methods of simulations: (i) a time convolutionless method, (ii) correlation expansion and (iii) path integrals. We show that results obtained from these methods agree perfectly at low temperatures.

cond-mat.mes-hall↗

arXiv2013

Optical signals of spin switching using the optical Stark effect in a Mn doped quantum dot

The optically induced spin dynamics of a single Mn atom embedded into a single semiconductor quantum dot can be strongly influenced by using the optical Stark effect. The exchange interaction gives rise to simultaneous spin flips between the quantum dot electron and Mn. In the time domain these flips correspond to exchange induced Rabi oscillations, which are typically off-resonant. By applying a detuned laser pulse, the states involved in the flipping can be brought into resonance by means of the optical Stark effect increasing the amplitude of the Rabi oscillations to one. In this paper we study theoretically how this spin dynamics can be monitored in time-resolved spectroscopy. In the spectrum the exchange interaction leads to a splitting of the exciton line into six lines, each corresponding to one of the six Mn spin states. The dynamical behavior of the Mn spin is reflected by the strength of the individual lines as a function of time. When an off-resonant optical pulse is applied the spectral positions of the lines shift, but still the flipping dynamics is visible.

cond-mat.mes-hall↗

arXiv2013

Phonon Effects on Population Inversion in Quantum Dots: Resonant, Detuned and Frequency-swept Excitations

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 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.

cond-mat.mes-hall↗

arXiv2013

Fluctuation properties of acoustic phonons generated by ultrafast optical excitation of a quantum dot

We study theoretically the fluctuation properties of acoustic phonons created in a semiconductor quantum dot after ultrafast optical excitation. An excitation with a single ultrafast pulse creates an exciton confined to the quantum dot, which is coupled to longitudinal acoustic phonons. This leads to the formation of a polaron in the quantum dot accompanied by the emission of a phonon wave packet. We show that the fluctuations of the lattice displacement associated with the wave packet after a single laser pulse excitation in resonance with the exciton transition are always larger than their respective vacuum values. Manipulating the exciton with a second pulse can result in a reduction of the fluctuations below their vacuum limit, which means that the phonons are squeezed. We show that the squeezing properties of the wave packet strongly depend on the relative phase and the time delay between the two laser pulses.

cond-mat.mes-hall↗

arXiv2012

Biexciton state preparation in a quantum dot via adiabatic rapid passage: comparison between two control protocols and impact of phonon-induced dephasing

We investigate theoretically under which conditions a stable and high-fidelity preparation of the biexciton state in a quantum dot can be realized by means of adiabatic rapid passage in the presence of acoustic phonon coupling. Our analysis is based on a numerically complete real-time path integral approach and comprises different schemes of optical driving using frequency-swept (chirped) pulses. We show that depending on the size of the biexciton binding energy, resonant two-photon excitations or two-color schemes can be favorable. It is demonstrated that the carrier-phonon interaction strongly affects the efficiency of the protocols and that a robust preparation of the biexciton is restricted to positive chirps and low temperatures. A considerable increase of the biexciton yield can be achieved realizing temperatures below 4 K.

cond-mat.mes-hall↗

arXiv2012

Dephasing in the adiabatic rapid passage in quantum dots: the role of phonon-assisted biexciton generation

We study the evolution of an exciton confined in a quantum dot adiabatically controlled by a frequency-swept (chirped) laser pulse in the presence of carrier-phonon coupling. We focus on the dynamics induced by a linearly polarized beam and analyze the decoherence due to phonon-assisted biexciton generation. We show that if the biexciton state is shifted down by a few meV, as is typically the case, the resulting decoherence is strong even at low temperatures. As a result, efficient state preparation is restricted to a small parameter area corresponding to low temperatures, positive chirps and moderate pulse areas.

cond-mat.mes-hall↗