Metallic Nanoparticles on Waveguide Structures: Effects on Waveguide Mode Properties, and the Promise of Sensing Applications
We consider a waveguide structure overcoated with metallic nanoparticles, either in an ordered array or randomly distributed, with the size of the particles and the distances between them small compared to the wavelength of light. The presence of the nanoparticles will modify the "self-wavenumber" of the waveguide mode, in general adding a real part associated with a shift in wavenumber, and an imaginary part associated with absorptive loss. We develop a simple strategy for calculating these shifts and decay rates. The problem can be simplified and reduced to one where at most a numerical calculation is necessary to treat only longitudinal electric fields; transverse field components can then be included in an analytic way because they vary little over nanoparticles and the distances between them, and we treat them with a simple transfer matrix formalism. Since in the plane of the nanoparticles the longitudinal electric fields are short-range, the required numerical calculation is not problematic. As an even simpler first step we consider the limit of nanoparticles treated in the point dipole approximation, where everything can be done analytically. We find that nanoparticle-covered waveguide modes offer a number of interesting possibilities for sensing applications.