The problem of Förster resonance energy transfer ͑FRET͒ between two molecules in the vicinity of a metallic nanoparticle such as a nanoshell is studied within a phenomenological model which takes into account the nonlocal optical response of the metal. This model allows for arbitrary locations and orientations of the two molecular dipoles with respect to the metal particle which can be of ultrasmall sizes ͑Ͻ10 nm͒ and for which nonlocal effects are of high significance. In particular, for the nanoshell case, the molecules can be located both outside, both inside, or one inside and one outside the shell. Also, the case with a metallic spherical particle studied mostly in the literature can be obtained in the limit of zero inner radius for the nanoshell. Particular focus will be on the enhancement of this FRET process due mainly to the surface plasmon excitation of the free metallic electrons, and the nonlocal effects on this will be studied with reference to a number of factors including the molecular locations and orientations, the transition frequency of the donor and acceptor;…etc. Numerical results show that the resonances in the enhanced FRET rate will be dominated by the multipolar bonding and antibonding cross-coupled plasmonic modes of the nanoshell; and that the nonlocal effects will generally lead to blueshifted resonances, as well as diminution of the enhancement for the low-frequency portions of both modes. Such information will be useful for future application of plasmonic enhanced FRET using metallic particles of ultrasmall sizes.

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We present a theoretical model for describing light scattering from randomly distributed Au nanoparticles on a substrate, including the clustering effect. By using the finite-element Green's function method and spherical harmonic basis functions, we are able to calculate the polarization-dependent reflectivity spectra of the system (modeled by randomly distributed nanoparticles coupled with clusters) efficiently and accurately. The calculated ellipsometric spectra of the system with clusters can adequately describe the experimental data for the whole frequency range. We find that the clustering effect leads to some prominent features in the low frequency range of the ellipsometric spectra, which are attributed to plasmonic resonances associated with the coupling of Au nanoparticles and clusters.

Recent studies have established certain specific results concerning the optical reciprocity of source and observer in the presence of dielectric medium with nonlocal response (i.e. spatial dispersion). These include the case of a linear dielectric response with dependence on the electric field gradient; as well as the case of a general anisotropic nonlocal medium in the long wavelength quasistatic limit. Here we present a more rigorous study of this problem by extending the previous results to the most general case of anisotropic nonlocal medium in the context of exact electrodynamics with reference to the symmetry of the Green dyadic. It is established that for nonmagnetic materials within linear optics, reciprocity will remain valid as long as the dielectric tensor satisfies the condition . Possible applications of the results are briefly discussed. ( ) (1

The concept of reciprocity symmetry for matter-wave propagation is established for nonrelativistic quantum mechanics with previous results in the literature extended to include nonlocal interactions. Examples are given for cases with both local and nonlocal potentials, where we show in particular that reciprocity can be violated for the motion of a charged particle in an external electromagnetic field. In addition, this symmetry is applied to interpret a recent analysis ͓Phys. Rev. A 64, 042716 ͑2001͔͒ on the symmetry of transmission through one-dimensional complex potentials, with the emphasis that the validity of reciprocity can go beyond that of time-reversal symmetry, such as in the presence of absorption in which the latter symmetry breaks down.