Abstract
The influence of substrate induced coupling on plasmon excitations is probed by means of visible and near-infrared extinction microspectroscopy on various arrays of gold oblate spheroidal particles deposited onto a 20-nm-thick gold film. At zero incidence angle and for an interparticle spacing smaller than 250 nm, the spectra exhibit two bands instead of the single one observed for similar particles but deposited on indium tin oxide coated glass. We suggest that the short-wavelength band proceeds from two simultaneous mechanisms: (i) Excitation of a surface-plasmon resonance localized on the particles. (ii) Generation at the gold-glass interface of a propagating surface-plasmon wave due to the fact that the gold particles can act as a grating coupler. Surface-enhanced Raman-scattering experiments performed on oblate spheroid arrays give arguments in favor of the attribution of the long-wavelength band to surface-plasmon resonance of an ensemble of strongly coupled particles. When increasing the spacing between particles of the array beyond 250 nm, extinction spectra display the emergence of a new band. Calculations suggest that this new band proceeds from the excitation of a surface-plasmon standing wave on the film due to Bragg scattering. This assignment to Bragg scattering is supported by the investigation of the effect of varying the incidence angle under both transverse magnetic and transverse electric polarizations.
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