Noble metal nanoparticles show specific optical properties due to the excitation of localized surface plasmons that make them attractive candidates for highly sensitive bionanosensors. The underlying physical principle is either an analyte-induced modification of the dielectric properties of the medium surrounding the nanoparticle or an increase of the excitation and emission rates of an optically active analyte by the resonantly enhanced plasmon field. Either way, besides the nanoparticle geometry the dielectric properties of the metal and nanoscale surface roughness play an important role for the sensing performance. As the underlying principles are however not yet well understood, we aim here at an improved understanding by analyzing the optical characteristics of lithographically fabricated nanoparticles with different crystallinity and roughness parameters. We vary these parameters by thermal annealing and apply a thin gold film as a model system to retrieve modifications in the dielectric function. We investigate, on one hand, extinction spectra that reflect the far-field properties of the plasmonic excitation and, on the other hand, surfaceenhanced Raman spectra that serve as a near-field probe. Our results provide improved insight into localized surface plasmons and their application in bionanosensing.
In this study the applicability of an interface procedure for combined ray-tracing and finite difference time domain (FDTD) simulations of optical systems which contain two diffractive gratings is discussed. The simulation of suchlike systems requires multiple FDTD↔RT steps. In order to minimize the error due to the loss of the phase information in an FDTD→RT step, we derive an equation for a maximal coherence correlation function (MCCF) which describes the maximum degree of impact of phase effects between these two different diffraction gratings and which depends on: the spatial distance between the gratings, the degree of spatial coherence of the light source and the diffraction angle of the first grating for the wavelength of light used. This MCCF builds an envelope of the oscillations caused by the distance dependent coupling effects between the two diffractive optical elements. Furthermore, by comparing the far field projections of pure FDTD simulations with the results of an RT→FDTD→RT→FDTD→RT interface procedure simulation we show that this function strongly correlates with the error caused by the interface procedure.
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