P. T. Leung

Effects of extraneous surface charges on the enhanced Raman scattering from metallic nanoparticles.

Motivating by recent experiments on surface enhanced Raman scattering (SERS) from colloidal solutions, we present here a simple model to elucidate the effects of extraneous surface charges on the enhanced Raman signal. The model is based on the well-established Gersten-Nitzan model coupled to the modified Mie scattering theory of Bohren and Hunt in the long wavelength approximation. We further introduce corrections from the modified long wavelength approximation to the Gersten-Nitzan model for the improvement of its accuracy. Our results show that the surface charge will generally lead to a blueshift in the resonance frequency and greater enhancements in the SERS spectrum. Possible correlations with the recent experiments are elaborated.

Guided Plasmon Modes of a Graphene-Coated Kerr Slab

We study analytically propagating surface plasmon modes of a Kerr slab sandwiched between two graphene layers. We show that some of the modes that propagate forward at low field intensities start propagating with negative slope of dispersion and positive flux of energy (fast-light surface plasmons) when the field intensity becomes high. We also discover that our structure supports an additional branch of low-intensity fast-light guided modes. The possibility of dynamically switching between the forward and the fast-light plasmon modes by changing the intensity of the excitation light or the chemical potential of the graphene layers opens up wide opportunities for controlling light with light and electrical signals on the nanoscale.

Förster Resonance Energy Transfer Between Molecules in the Vicinity of Graphene-Coated Nanoparticles

The recent demonstration of the plasmonic-enhanced Förster resonance energy transfer (FRET) between two molecules in the vicinity of planar graphene monolayers is further investigated using graphene-coated nanoparticles (GNP). Due to the flexibility of these nanostructures in terms of their geometric (size) and dielectric (e.g., core material) properties, greater tunability of the FRET enhancement can be achieved employing the localized surface plasmons. It is found that while the typical characteristic graphene plasmonic enhancements are manifested from using these GNPs, even higher enhancements can be possible via doping and manipulating the core materials. In addition, the broadband characteristics are further expanded by the closely spaced multipolar plasmon resonances of the GNPs.

Equivalence between the mechanical model and energy-transfer theory for the classical decay rates of molecules near a spherical particle

In the classical modeling of decay rates for molecules interacting with a nontrivial environment, it is well known that two alternate approaches exist which include: (1) a mechanical model treating the system as a damped harmonic oscillator driven by the reflected fields from the environment; and (2) a model based on the radiative and nonradiative energy transfers from the excited molecular system to the environment. While the exact equivalence of the two methods is not trivial and has been explicitly demonstrated only for planar geometry, it has been widely taken for granted and applied to other geometries such as in the interaction of the molecule with a spherical particle. Here we provide a rigorous proof of such equivalence for the molecule-sphere problem via a direct calculation of the decay rates adopting each of the two different approaches.

Theoretical study of nonlocal effects in the optical response of metallic nanoshells

The optical response of metallic nanoshell which includes coated dielectric particles is investigated in the longwavelength limit in which the nonlocal response of the metal is taken into account. Using our recent formalism which calculates the nonlocal multipole polarizability of such a shell, we have studied how these nanoshells interact with both the far field and near field of a light source. For the far field case, both absorption and scattering cross sections for an incident plane wave are calculated in the dipole approximation. For the near field case, we study how the fluorescence properties of an emitting molecule in the vicinity of a nanoshell are affected, by including all higher multipolar response of the shell. It is found that the nonlocal effects are most prominent for higher order multipoles, and hence for the description of molecules in close proximity interacting with the nanoshell.

A theoretical model for the temperature-dependent sensitivity of the optical sensor based on surface plasmon resonance

A theoretical model is developed for the temperature-dependent sensitivity of the optical sensor based on surface plasmon resonance. From the numerical results, the angular interrogation approach seems to have the preferred stability against temperature variations.