Fluorescence emission at dielectric and metal-film interfaces

Abstract

It is well known that the classical optical properties of a bare or metal-film-coated dielectric surface significantly the emission pattern of a fluorophore in close proximity to it. Most previous classical calculations of this perturb model the fluorophore as a continuous fixed-amplitude dipole acting as a simple radiator. However, for effect modeling steady-state excitation, a fixed-power dipole is more appropriate. This modification corresponds to normalizing fixed-amplitude dipole intensities by the total dissipated power, which is itself dependent on fluorophore orientation and proximity to the surface. The results for the fixed-power model differ nontrivially from the fixed-amplitude model. Using the fixed-power dipole model, we calculate the observation-angle-dependent intensity as a function of the fluorophore’s orientation and distance from the surface. The surface can have an intermediate layer of arbitrary thickness on it, which is used to model a metal-film-coated dielectric. In addition, general expressions are derived for the emission power as observed through a circular-aperture collection system (such as a microscope objective) located on either side of the interface. These expressions are applied to several common cases of fluorophore spatial and orientational distributions at bare glass–water and metal-film-coated glass-water interfaces. The results suggest practical experimental approaches for measuring the spatial and orientational distribution of fluorophores adsorbed at a surface, utilizing the distance-dependent fluorescence near a metalized surface and optimizing the collection efficiency from a well-defined volume near a quenching surface.