In many of the materials and systems in which resonance energy transfer occurs, the individual chromophores are embedded within a superstructure of significantly different chemical composition. In accounting for the influence of the surrounding matter, the simplest and most widely used representation is commonly cast in terms of a dependence on local refractive index. However, such a depiction is a significant oversimplification, as it fails to register the electronic and local geometric effects of material specifically in the vicinity of the chromophores undergoing energy transfer. The principal objective of this study is to construct a detailed picture of how individual photon interaction events are modified by vicinal, non-absorbing chromophores. A specific aim is to discover what effects arise when input excitation is located in the neighborhood of other chromophores that have a slightly shorter wavelength of absorption; this involves a passive effect exerted on the transfer of energy at wavelengths where they themselves display no significant absorption. The theory is based on a thorough quantum electrodynamical analysis that allows the identification of specific optical and electronic chromophore attributes to expedite or inhibit electronic energy transfer. The Clausius-Mossotti dispersion relationship is then deployed to elicit a dependence on the bulk refractive index of the surroundings. A distinction is drawn between cases in which the influence on the electromagnetic coupling between the donor and the acceptor is primarily due to the static electric field produced by a polar medium, and converse cases in which the mechanism for modifying the form of energy transfer involves the medium acquiring an induced electric dipole. The results provide insights into the detailed quantum mechanisms that operate in multi-chromophore systems, pointing to factors that contribute to the optimization of photosystem characteristics. © 2013 AIP Publishing LLC. [http://dx
Third-harmonic scattering is a nonlinear optical process that involves the molecular second-hyperpolarizability, γ. This work presents a rigorous quantum electrodynamical analysis of the scattering process, involving a partially index-symmetric construction of the fourth-rank γ tensor-dispensing with the Kleinman symmetry condition. To account for stochastic molecular rotation in fluids, methods of isotropic averaging must be employed to relate the molecular properties to accessible experimental quantities such as depolarization ratio. A complete eighth-rank tensor rotational average yields results for observable third-harmonic scattering rates, cast as a function of the natural-invariant γ components, and the polarization geometry of the experiment. Decomposing the tensor γ into irreducible weights allows specific predictions to be made for each molecular point group, allowing greater discrimination between the results for different molecular symmetries.
The achievement of optimum conversion efficiency in conventional spontaneous parametric downconversion requires consideration of quantum processes that entail multisite electrodynamic coupling, actively taking place within the conversion material. The physical mechanism, which operates through virtual photon propagation, provides for photon pairs to be emitted from spatially separated sites of photon interaction; occasionally pairs are produced in which each photon emerges from a different point in space. The extent of such nonlocalized generation is influenced by individual variations in both distance and phase correlation. Mathematical analysis of the global contributions from this mechanism provides a quantitative measure for a degree of positional uncertainty in the origin of down-converted emission. DOI: 10.1103/PhysRevLett.118.133602 Spontaneous parametric down-conversion (SPDC) is a process in which light passing through an optically nonlinear medium can generate double-wavelength output. At a fundamental level, the process entails the conversion of pump photons into conascent, phase-matched pairs, executed by material interactions that entail the secondorder nonlinear optical susceptibility: a third-order electric dipole response. Each generated pair of photons has a combined energy and momentum equal to that of the corresponding annihilated photon, and they also exhibit correlated polarization. When the emergent photons equally share the energy of the input, the process is known as degenerate down-conversion (DDC); an alternative perspective is to regard the conversion as an exact time reversal of second-harmonic generation [1,2]. Much work has been carried out on correlated photon pairs, including their generation [3][4][5], manipulation [6][7][8][9], and application [10][11][12]. The entanglement of the quantum states in each pair has important applications in quantum computing and communication [13], and potential utilization clinically [14].In this Letter we develop an expanded theory of SPDC, highlighting and quantifying an important contribution from nonlocalized couplings. While SPDC is one of the main sources of entangled photon pairs [15], an exact location for the creation of each output photon cannot of course be inferred by direct observation-although pump photon annihilation and down-converted photon emission are generally assumed to be colocated. Of course, the diffuse nature of atomic and molecular orbitals precludes exact identification of the location for any photon creation event; equally, even the emission of two correlated photons cannot be considered as precisely colocated in origin. However, the spatial extent of the region from within which a pair of down-converted photons may emerge is considerably larger than may usually be supposed.In fact, there is a possibility that for each input photon the process may entail correlated photon interactions at two separate locations, creating one down-converted photon at each as indicated in Fig. 1. Accounting for such delocalized interact...
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