Resonant dipole-dipole interaction in a cavity

Fundamental efficiency bound for quantum coherent energy transfer in nanophotonics

We derive a unified quantum theory of coherent and incoherent energy transfer between two atoms (donor and acceptor) valid in arbitrary Markovian nanophotonic environments. Our theory predicts a fundamental bound η _{m a x} =γ _a γ _d+γ _a for energy transfer efficiency arising from the spontaneous emission rates γ_d and γ_a of the donor and acceptor. We propose the control of the acceptor spontaneous emission rate as a new design principle for enhancing energy transfer efficiency. We predict an experiment using mirrors to enhance the efficiency bound by exploiting the dipole orientations of the donor and acceptor. Of fundamental interest, we show that while quantum coherence implies the ultimate efficiency bound has been reached, reaching the ultimate efficiency does not require quantum coherence. Our work paves the way towards nanophotonic analogues of efficiency-enhancing environments known in quantum biological systems.

Fundamental figures of merit for engineering Förster resonance energy transfer

Over the past 15 years there has been an ongoing debate regarding the influence of the photonic environment on Förster resonance energy transfer (FRET). Disparate results corresponding to enhancement, suppression and null effect of the photonic environment have led to a lack of consensus between the traditional theory of FRET and experiments. Here we show that the quantum electrodynamic theory (QED) of FRET near an engineered nanophotonic environment is exactly equivalent to an effective near-field model describing electrostatic dipole-dipole interactions. This leads to an intuitive and rigorously exact description of FRET, previously unavailable, bridging the gap between experimental observations and theoretical interpretations. Furthermore, we show that the widely used concept of Purcell factor variation is only important for understanding spontaneous emission and is an incorrect figure of merit (FOM) for analyzing FRET. To this end, we analyze the figures of merit which characterize FRET in a photonic environment 1) the FRET rate enhancement factor (FET), 2) FRET efficiency enhancement factor (Feff) and 3) Two-point spectral density (SEE) which is the photonic property of the environment governing FRET analogous to the local density of states that controls spontaneous emission. Counterintuitive to existing knowledge, we show that suppression of the Purcell factor is in fact necessary for enhancing the efficiency of the FRET process. We place fundamental bounds on the FRET figures of merit arising from material absorption in the photonic environment as well as key properties of emitters including intrinsic quantum efficiencies and orientational dependence. Finally, we use our approach to conclusively explain multiple recent experiments and predict regimes where the FRET rate is expected to be enhanced, suppressed or remain the same. Our work paves for a complete theory of FRET with predictive power for designing the ideal photonic environment to control FRET.

Dynamic control of Förster energy transfer in a photonic environment

In this study, the effect of modified optical density of states on the rate of Förster resonant energy transfer between two closely-spaced chromophores is investigated. A model based on a system of coupled rate equations is derived to predict the influence of the environment on the molecular system. Due to the near-field character of Förster transfer, the corresponding rate constant is shown to be nearly independent of the optical mode density. An optical resonator can, however, effectively modify the donor and acceptor populations, leading to a dramatic change in the Förster transfer rate. Single-molecule measurements on the autofluorescent protein DsRed using a λ/2-microresonator are presented and compared to the theoretical model's predictions. The observed resonator-induced dequenching of the donor subunit in DsRed is accurately reproduced by the model, allowing a direct measurement of the Förster transfer rate in this otherwise inseparable multichromophoric system. With this accurate yet simple theoretical framework, new experiments can be conceived to measure normally obscured energy transfer channels in complex coupled quantum systems, e.g. in photovoltaics or light harvesting complexes.

A revisitation of the Förster energy transfer near a metallic spherical nanoparticle: (1) Efficiency enhancement or reduction? (2) The control of the Förster radius of the unbounded medium. (3) The impact of the local density of states

The central motivation of this theoretical revisitation comes from the fact that some experimental works about Förster energy transfer report improvement of the Förster efficiency when the donor-acceptor molecular pair is in the vicinity of a metallic particle, while others found efficiency deterioration. In the presence of a nanoscale metallic sphere, we calculate contour plots of the Förster energy transfer rate KF and the Förster efficiency η as a function of the acceptor position rA for a fixed donor position. These contour plots clearly highlight the influence of the sphere on KF and η as the donor position, the orientations of donor and acceptor dipoles, and the particle size are varied; also the impact on KF(rA) and η due to the excitation of surface plasmons is easily noticeable from these contour plots. Moreover, we obtain the enhancement factor KF/KF0 (KF0 refers to the case without sphere) against the donor-surface separation for particular donor-acceptor spatial distributions, several particle sizes, and distinct molecular dipole orientations. Therefore, our calculations provide a systematic analysis of the Förster energy transfer in the presence of a metallic nanosphere. Based on these results, we formulate hypotheses for explaining the aforementioned contradictory experimental results about η. To complement our study, we examine the impact of the local density of states ρ on KF. KF is practically unperturbed by sphere when the intermolecular separation R is ≲ 3 nm, since the direct donor-acceptor electromagnetic interaction is dominant. On the contrary, when R ≳ 3 nm, the nanosphere perturbs KF and this perturbation is stronger if plasmonic resonances are excited. KF/KF0 can greatly be enhanced in certain regions, but these regions coincide with low-efficiency regions, compromising applications involving the Förster process. In the presence of the nanosphere, the high Förster efficiency region (η ≥ 0.5) has the same shape as that for the case without sphere, but its extension (Förster radius Ro) is reduced; this effect is a consequence of the large increase of the donor direct decay rate and Ro depends strongly on donor position. Consequently, the sphere controls Ro that is associated with the efficiency pattern that corresponds to the unbounded medium; this effect can be exploited in the measuring technique of nanoscale displacements of proteins that is based on the fluorescence resonant energy transfer. The functional form of KF(ρ) is determined by the intermolecular separation R, the spatial configuration and the dipole orientations of the molecular pair, and the donor proximity to the nanoparticle.

Effect of plasmonic nanostructures and nanofilms on fluorescence resonance energy transfer

We investigate the effect of silver nanostructures and nanofilms on fluorescence resonance energy transfer (FRET). Thin layers of poly (vinyl alcohol) of 10 or 20 nm with homogenously distributed donor (Cy3) and acceptor (Cy5) molecules were coated on silver islands and continuous silver films. Several geometrical configurations were studied with multiple donor-acceptor (D-A) samples with various acceptor concentrations. The acceptor concentrations were adjusted to yield FRET efficiencies from about 10 to 92% on bare glass surfaces. Time-resolved fluorescence spectroscopy was used to determine the energy transfer efficiency and the relative transfer rate. It was found that the relative transfer rate increased 2-fold in the presence of continuous silver films and about 25-fold in presence of silver island films. The largest enhancements in the energy transfer efficiency, up to 90%, were observed for low acceptor concentrations (large donor-acceptor separations). To a first approximation, observed changes in FRET efficiencies were explained by an increase of the quantum yield of the donor molecules in the presence of silver islands.

Self-consistent quantum mechanical model for the description of excitation energy transfers in molecules at interfaces

In this paper we present a quantum mechanical model to study excitation energy transfers in molecular systems located in the vicinity of an interface. The model is based on an approximate solution of the time-dependent density functional theory equations and solvent effects are introduced in terms of the integral equation formalism version of the polarizable continuum model. A unique characteristic of this model is that environment induced polarizing effects on the interacting molecules and screening effects on their interaction are included in a coherent and self-consistent way. The model is applied to different situations of the ethylene dimer in the vicinity of an air/water interface and compared with an alternative quantum electrodynamics approach.

Effects of metallic silver island films on resonance energy transfer between N,N'-(dipropyl)-tetramethyl- indocarbocyanine (Cy3)- and N,N'-(dipropyl)-tetramethyl- indodicarbocyanine (Cy5)-labeled DNA

Resonance energy transfer (RET) is typically limited to distances below 60 A, which can be too short for some biomedical assays. We examined a new method for increasing the RET distances by placing donor- and acceptor-labeled DNA oligomers between two slides coated with metallic silver particles. A N,N'-(dipropyl)-tetramethylindocarbocyanine donor and a N,N'-(dipropyl)-tetramethylindodicarbocyanine acceptor were covalently bound to opposite 5' ends of complementary 23 base pair DNA oligomers. The transfer efficiency was 25% in the absence of silver particles or if only one slide was silvered, and it increased to an average value near 64% between two silvered slides. The average value of the Forster distance increased from 58 to 77 A. The energy transfer data were analyzed with a model assuming two populations of donor-acceptor pairs: unaffected and affected by silver island films. In an affected fraction of about 28%, the apparent energy transfer efficiency is near 87% and the Forster distance increases to 119 A. These results suggest the use of metallic silver particles to increase the distances over which RET occurs in biomedical and biotechnology assays.

Radiative decay engineering: the role of photonic mode density in biotechnology

Fluorescence detection is a central technology in biological research and biotechnology. A vast array of fluorescent probes are available with diverse spectral properties. These properties were 'engineered' into fluorophores by modification of the chemical structures. Essentially, all present uses of fluorescence rely on the radiation of energy into optically transparent media, the free space which surrounds the fluorophores. In this paper, we summarize an opportunity for novel fluorescence technology based on modification of the photonic mode density around the fluorophore and thus control of its spectral properties. This modification can be accomplished by proximity of fluorophores to metallic particles of gold, silver and possibly others. By engineering the size and shape of the metal particles, and the location of the fluorophores relative to the surfaces, fluorophores can be quenched, display increases in quantum yield, and changes in lifetime. Fluorophore-metal surface combinations can even display directional rather than isotropic emission. We describe recent experimental results and suggest potential biomedical applications of fluorophore-metal particle interactions.

Increased resonance energy transfer between fluorophores bound to DNA in proximity to metallic silver particles

We examined the effects of metallic silver particles on resonance energy transfer (RET) between fluorophores covalently bound to DNA. A coumarin donor and a Cy3 acceptor were positioned at opposite ends of a 23-bp double helical DNA oligomer. In the absence of silver particles the extent of RET is near 9%, consistent with a Forster distance R(0) near 50 A and a donor to acceptor distance near 75 A. The transfer efficiency increased when the solution of AMCA-DNA-Cy3 was placed between two quartz plates coated with silver island films to near 64%, as determined by both steady-state and time-resolved measurements. The apparent R(0) in the presence of silver island films increases to about 110 A. These values of the transfer efficiency and R(0) represent weighted averages for donor-acceptor pairs near and distant from the metallic surfaces, so that the values at an optimal distance are likely to be larger. The increased energy transfer is observed only between two sandwiched silvered slides. When we replaced one silvered slide with a quartz plate the effect vanished. Also, the increased energy transfer was not observed for silvered slides separated more than a few micrometers. These results suggest the use of metal-enhanced RET in PCR, hybridization, and other DNA assays, and the possibility of controlling energy transfer by the distance between silver surfaces.

Effects of metallic silver particles on resonance energy transfer in labeled bovine serum albumin

Resonance energy transfer (RET) is widely used to detect proximity between biomolecules. In transparent solution the maximum donor-to-acceptor distance for RET is about 70 A. We measured the effects of metallic silver island films on RET from the intrinsic tryptophan of a protein to a bound probe as the acceptor. These preliminary experiments revealed a dramatic increase in the apparent Förster distance increasing from 28.6 to 63 A. These results suggest the use of silver island films for detecting long range proximity between biomolecules and for biotechnology applications based on RET.

Förster energy transfer in an optical microcavity

By studying the transfer of excitation energy between dye molecules confined within an optical microcavity, we demonstrate experimentally that Förster energy transfer is influenced by the local photonic mode density. Locating donor and acceptor molecules at well-defined positions allows the transfer rate to be determined as a function of both mutual separation and cavity length. The results show that the Förster transfer rate depends linearly on the donor emission rate and hence photonic mode density, providing the potential to control energy transfer by modification of the optical environment.