Trapping time statistics and efficiency of transport of optical excitations in dendrimers

We theoretically study the trapping time distribution and the efficiency of the excitation energy transport in dendritic systems. Trapping of excitations, created at the periphery of the dendrimer, on a trap located at its core, is used as a probe of the efficiency of the energy transport across the dendrimer. The transport process is treated as incoherent hopping of excitations between nearest-neighbor dendrimer units and is described using a rate equation. We account for radiative and non-radiative decay of the excitations while diffusing across the dendrimer. We derive exact expressions for the Laplace transform of the trapping time distribution and the efficiency of trapping and analyze those for various realizations of the energy bias, number of dendrimer generations, and relative rates for decay and hopping. We show that the essential parameter that governs the trapping efficiency, is the product of the on-site excitation decay rate and the trapping time (mean first passage time) in the absence of decay.Comment: 26 pages, 6 figure

Tailoring optical response of a hybrid comprising a quantum dimer emitter strongly coupled to a metal nanoparticle

We study theoretically the optical response of a nanohybrid comprising a symmetric quantum dimer emitter coupled to a metal nanoparticle (MNP). The interactions between the exitonic transitions in the dimer and the plasmons in the MNP lead to novel effects in the composite's input-output characteristics for the light intensity and the absorption spectrum, which we study in the linear and nonlinear regimes. We fnd that the exciton-plasmon hybridization leads to optical bistability and hysteresis for the one-exciton transition and enhancement of excitation for the two-exciton transition. The latter leads to a signifcant decrease of the field strength needed to saturate the system. In the linear regime, the absortion spectrum has a dispersive (Fano-like) line shape. The spectral position and shape of this spectrum depend on the detuning of the dimer's one-exciton resonance relative to the plasmon resonance. Upon increasing the applied field intensity to the nonlinear regime, the Fano-like singularities in the absorption spectra are smeared and they disappear due to the saturation of the dimer, which leads to the MNP dominating the spectrum. The above effects, for which we provide physical explanations, allow one to tailor the Fano-like shape of the absorption spectrum, by changing either the detuning or the input power

Plasmon-assisted two-photon Rabi oscillations in a semiconductor quantum dot -- metal nanoparticle heterodimer

Tho-photon Rabi oscillations hold potential for quantum computing and quantum information processing, because during a Rabi cycle a pair of entangled photons may be created. We theoretically investigate the onset of this phenomenon in a heterodimer comprising a semiconductor quantum dot strongly coupled to a metal nanoparticle. Two-photon Rabi oscillations in this system occur due to a coherent two-photon process involving the ground-to-biexciton transition in the quantum dot. The presence of a metal nanoparticle nearby the quantum dot results in a self-action of the quantum dot via the metal nanoparticle, because the polatization state of the latter depends on the quantum state of the former. The interparticle interaction gives rise to two principal effects: (i) - enhancement of the external field amplitude and (ii) - renormalization of the quantum dot's resonance frequencies and relaxation rates of the off-diagonal density matrix elements, both depending on the populations of the quantum dot's levels. Here, we focus on the first effect, which results in interesting new features, in particular, in an increased number of Rabi cycles per pulse as compared to an isolated quantum dot and subsequent growth of the number of entangled photon pairs per pulse. We also discuss the destructive role of radiative decay of the excitonic states on two-photon Rabi oscillations for both an isolated quantum dot and a heterodimer.Comment: 11 pages, 19 figure

Nonlinear optical dynamics of 2D super-crystals of quantum Λ-emitters

We study theoretically the optical response of a 2D super-crystal of quantum Λ-emitters which are coupled by their secondary dipole field. The latter introduces a feedback into the system, the interplay of which with the intrinsic nonlinearity of emitters results in an exotic behavior of the system's optical response, such as periodic or quasi-periodic self-oscillations and chaotic dynamics. We argue therefore that these predicted features can be promising for various nanophotonic applications

Quantum metasurfaces of arrays of Λ-emitters for photonic nano-devices

We address exotic optical response of a planar metasurface comprising a monolayer of regularly spaced quantum three-level emitters with a doublet in the ground state (the so-called Λ-emitters). All emitters are coupled by the retarded dipole field which depends on the current state of all emitters. This coupling introduces a feedback into the system. Complex interplay of the latter with the intrinsic nonlinearity of a three-level system results in several remarkable effects in the monolayer's optical response, such as multistability, self-oscillations, and chaos. The peculiarity of the considered system is that some of the predicted nonlinear effects manifest themselves at very low excitation field intensities (on the order of 1 W/cm2), which is advantageous for possible applications: the monolayer can operate as a perfect reflector, a bistable mirror, and a THz or noise generator. It is argued therefore that the proposed system is a promising candidate for a building block for various photonic nano-devices

Intrinsic optical bistability of thin films of linear molecular aggregates: The two-exciton approximation

We generalize our recent work on the optical bistability of thin films of molecular aggregates [J. Chem. Phys. 127, 164705 (2007); arXiv:0707.1264v1 [cond-mat.dis-nn]] by accounting for the optical transitions from the one-exciton manifold to the two-exciton manifold as well as the exciton-exciton annihilation of the two-exciton states via a high-lying molecular vibronic term. We also include the relaxation from the vibronic level back to both the one-exciton manifold and the ground state. By selecting the dominant optical transitions between the ground state, the one-exciton manifold, and the two-exciton manifold, we reduce the problem to four levels, enabling us to describe the nonlinear optical response of the film. The one- and two-exciton states are obtained by diagonalizing a Frenkel Hamiltonian with an uncorrelated on-site (diagonal) disorder. The optical dynamics is described by means of the density matrix equations coupled to the electromagnetic field in the film. We show that the one-to-two exciton transitions followed by a fast exciton-exciton annihilation promote the occurrence of bistability and reduce the switching intensity. We provide estimates of pertinent parameters for actual materials and conclude that the effect can be realized.Comment: 11 two-column pages, 6 figures, to appear in the Journal of Chemical Physic

Plasmon-assisted two-photon absorption in a semiconductor quantum dot -- metallic nanoshell composite

Tho-photon absorption holds potential for many practical applications. We theoretically investigate the onset of this phenomenon in a semiconductor quantum dot -- metallic nanoshell composite subjected to a resonant CW excitation. Two-photon absorption in this system may occur in two ways: incoherent -- due to a consecutive ground-to-one-exciton-to-biexciton transition and coherent -- due to a coherent two-photon process, involving the direct ground-to-biexciton transition in the quantum dot. The presence of the nanoshell nearby the quantum dot gives rise to two principal effects: (i) -- renormalization of the applied field amplitude and (ii) -- renormalization of the resonance frequencies and radiation relaxation rates of the quantum dot, both depending on the the quantum dot level populations. We show that in the perturbation regime, when the excitonic levels are only slightly populated, each of these factors may give rise to either suppression or enhancement of the two-photon absorption. The complicated interplay of the two determines the final effect. Beyond the perturbation regime, it is found that the two-photon absorption experiences a drastic enhancement, which occurs independently of the type of excitation, either into the one-exciton resonance or into the two-photon resonance. Other characteristic features of the two-photon absorption of the composite, emerging from the coupling between both nanoparticles, are bistability and self-oscillations.Comment: 10 two-column pages, 7 figure

Instabilities in the optical response of a semiconductor quantum dot-metal nanoparticle heterodimer:self-oscillations and chaos

We theoretically investigate the nonlinear optical response of a heterodimer comprising a semiconductor quantum dot strongly coupled to a metal nanoparticle. The quantum dot is considered as a three-level ladder system with ground, one-exciton, and bi-exction states. As compared to the case of a two-level quantum dot model, adding the third (bi-exciton) state produces fascinating effects in the optical response of the hybrid system. Specifically, we demonstrate that the system may exhibit picosecond and sub-picosecond self-oscillations and quasi-chaotic behaviour under {\it single}-frequency continuous wave excitation. An isolated semiconductor quantum dot does not show such features. The effects originate from competing one-exciton and bi-exciton transitions in the semiconductor quantum dot, triggered by the self-action of the quantum dot via the metal nanoparticle. The key parameter that governs the phenomena mentioned is the ratio of the self-action strength and the bi-exciton shift. The self-oscillation regime can be achieved in practice, in particular, in a heterodimer comprised of a closely spaced ZnS/ZnSe core-shell quantum dot and a spherical silver nanoparticle. The results may have applications in nanodevices for generating trains of ultrashort optical pulses