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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Signatures of Förster and Dexter transfer processes in coupled nanostructures for linear and two-dimensional coherent optical spectroscopy

Judith F. Specht, Marten Richter

Published Mar 13, 2015• 0 citations
Publisher

Abstract

In this manuscript, we study the impact of the two Coulomb induced resonance energy transfer processes, Förster and Dexter coupling, on the spectral signatures obtained by double quantum coherence spectroscopy. We show that the specific coupling characteristics allow us to identify the underlying excitation transfer mechanism by means of specific signatures in coherent spectroscopy. Therefore, we control the microscopic calculated coupling strength of spin preserving and spin flipping Förster transfer processes by varying the mutual orientation of the two quantum emitters. The calculated spectra reveal the optical selection rules altered by Förster and Dexter coupling between two semiconductor quantum dots. We show that Dexter coupling between bright and dark two-exciton states occurs.

DOI
10.1117/12.2076586

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In this manuscript, we study the impact of the two Coulomb induced resonance energy transfer processes, Förster and Dexter coupling, on the spectral signatures obtained by double quantum coherence spectroscopy. We show that the specific coupling characteristics allow us to identify the underlying excitation transfer mechanism by means of specific signatures in coherent spectroscopy. Therefore, we control the microscopic calculated coupling strength of spin preserving and spin flipping Förster transfer processes by varying the mutual orientation of the two quantum emitters. The calculated spectra reveal the optical selection rules altered by Förster and Dexter coupling between two semiconductor quantum dots. We show that Dexter coupling between bright and dark two-exciton states occurs.
We investigate the optical properties of a Coulomb-coupled double-quantum dot system excited by coherent light pulses. Basic effects of Coulomb coupling regarding linear and nonlinear optical processes are discussed. By numerically solving the Heisenberg equation of motion we are able to present the temporal evolution of the system's density matrix for a wide range of coupling parameters. The two main coupling effects in dipole approximation, biexcitonic shift and F\"orster energy transfer, are investigated and their qualitative and quantitative influence on absorption spectra, Rabi oscillations, and single- and two-pulse excitation is discussed. We present simulated differential transmission spectra to allow for comparison with recent experimental studies.
The theory of coherent energy transfer in nanocrystal (NC) systems is generalised for multi-exciton states. We study the effect of the Forster coupling between two individual semiconductor nanocrystals on the pump and induced Rabi oscillations (ROs). The relevant excitonic states in an isolated NC can be described by an effective four-state system, consisting of the ground state, the exciton state, the biexciton state and the triexciton state. The dynamics of a single donor-acceptor pair is described using a density matrix approach that incorporates dipole coupling to multi-exciton states. Our study shows that for short pulse the multi-exciton states play an important role in the dynamics. We also show that the amplitude and the period of the ROs in both dots are heavily influenced by Forster coupling strength. The quality factor of the induced ROs in the acceptor yields information on the energy transfer rate between the two dots. We present result for a variety of sample materials and realistic structure parameters.
We study the dynamics of excitonic states in one dimensional, dimer and trimer, and two dimensional arrangements of colloidal quantum dots using a density-matrix approach.The dots are coupled via a dipole-dipole interaction akin to the Förster mechanism.Coherent oscillations of tuned donor dots are shown to appear as plateaus in the acceptor dots, and therefore in their optical response.This behavior provides one with an interesting and unique handle to monitor the quantum state of clusters in an eavesdroping arrangement.A trimer cluster in a symmetrical loop shows steady states in a shorter characteristic time than the typical radiative lifetime of the dots.Breaking the symmetry of the loop results again in damping oscillatory states in the donor dots and plateaus in the eavesdropping/acceptor dot.The use of realistic parameters allows direct comparison with recent experiments and indicates that coherent state monitoring is possible in real experiments.Estimates of the Förster coupling V F are obtained from a microscopic description of the exciton levels in the quantum dot.We present results for different materials CdS, CdSe, GaAs, and InP as function of dot features.V F shows a nonmonotonic variation with dot sizes and a maximum value that depends on the specific dot sizes, material parameters, and dot separation.We also study the effect of the orientation factor on coherent energy transfer from a donor quantum dot to an acceptor quantum dot by calculating the polarization of the acceptor dot.We study how the measurement of the ac-ceptor polarization can be used to determine the relative orientation between the donor-acceptor pair.We also find that Förster coupling between dots results in a polarization memory transfer.
Quantum dots have been the focus of both fundamental and applied research due to their atomic-like properties. The delta-function-like density of states along with strong nonlinear optical properties have made quantum dots the focus of novel opto-electronic devices and of quantum computing proposals. The key issue for implementing quantum dots in devices such as quantum computers is the ability to coherently entangle separately localized quantum dot states. Using continuous wave coherent nonlinear optical spectroscopy techniques, this thesis focuses on creating coherent super-position states built from separately localized quantum dot states. The narrow bandwidths and mutual coherence of the applied laser fields allow for the creation of radiative and nonradiative coherences which can be understood from the coherent nonlinear optical response. The quantum dots under study are formed due to the width fluctuations of a thin quantum well. By applying nondegenerate mutually coherent fields on the sample in the traditional pump and probe geometry and homodyne detecting the nonlinear signal field, nonradiative coherences between two laterally coupled quantum dots are created and the coupling between their states examined. The coherent nonlinear optical response is compared with theoretical predictions based on the density matrix formalism for the interacting two exciton model. From this comparison, binding energies of the two-exciton bound state created with separately localized excitons is found to be on the order of 20 mueV for like-polarized excitons and on the order of 100 mueV for unlike-polarized excitons. A second order coherence between the ground and two-exciton state is also noted in the experimental data after comparison with theory. Further work attempting to study a vertically coupled quantum dot system was unsuccessful. The spectrum of a quantum dot is further understood from the comparison of the linear and nonlinear excited state spectra. Excited states which appear in both spectra imply a Coulomb interaction between them and their respective ground states. Coherent nonlinear optical signatures of excited states correlated with their linear excitation spectra are presented, and the importance of such states for a quantum computer read-out is described.
Abstract The analytical solution of the system consisting of donor-acceptor double quantum dots of approximately the same size, located inside a high-quality cavity is studied. The double quantum dots interact with the same phonon fields within the cavity, as well as with a single-mode electromagnetic field. The effect of the modified Förster interaction strength coefficient on the atomic occupation probabilities, ρ11(t), ρ22(t), the atomic population inversion ρz(t), the von Neumann entropy, the information entropies H(σx), H(σy) and H(σz), quantum Fisher information (FI ) and quantum coherence (Qc) of this system are investigated. The results clearly show that the modified Förster interaction strength coefficient (Ω) plays a pivotal role in controlling the dynamics of the coupled quantum dot system, where the interaction between quantum dots accelerates energy transfer and information exchange processes within the system. Hence, under the influence of the Förster interaction, which occurs when the donor’s emission spectrum overlaps with the acceptor’s absorption spectrum, a quantum tunneling current is generated between the two quantum dots using photons from negatively charged excitons. Also, the ability to control the degree of entanglement through Ω provides a mechanism to tune the properties of the quantum system.