Optical properties and far field radiation of periodic nanostructures fed by an optical waveguide for applications in fluorescence and Raman scattering

Various systems based on nanostructures built on optical waveguides have recently appeared in literature, since this configuration guarantees an efficient optical feeding to nano-elements and/or the possibility to manipulate guided signals. In this work, we present the analysis of the optical scattering properties of such type of structures, more specifically a periodic array of Au nano-cylinders or nano-domes fabricated upon an ion-exchanged waveguide, an integrated system considered for fluoroscopy and Raman spectroscopy. Absorption, scattering and extinction were calculated by means of Finite Difference Time Domain (FDTD) simulations and Scattering Theory. We took advantage of some particular adaptations to apply the classical scattering theory to this context, and used it to extract the far-field radiation pattern. We show that the rectangular periodicity allows to obtain highly collimated beams and negligible irradiation in a wide angle range, which enables the possibility to steer away the undesirable excitation signals from the detectors, avoiding a mixing with emitted photons from molecules. Our analysis suggests that, in the context of this setup and applications, the modeling of the nanostructures, which aims at the tuning of the plasmonic enhancement of the emission, can be relatively separated from the definition of spatial filtering through the periodic array. Our results add new considerations to the list of potentialities of this configuration for the development of efficient integrated surface enhanced spectroscopic setups.