Localized surface-plasmon resonances in periodic nondiffracting metallic nanoparticle and nanohole arrays

We compare the optical response of periodic nondiffracting metallic nanoparticle and nanohole arrays. Experimental data from both structures show a pronounced minimum in their wavelength-dependent transmittance that, through numerical modeling, we identify as being due to the excitation of localized surface-plasmon resonances associated with the nanoparticles/nanoholes. Our main finding is that, while the optical response of the nanoparticle arrays is largely independent of interparticle separation, the response from nanohole arrays shows a marked dependence on interhole separation. We attribute this effect to coupling between localized surface-plasmon resonances mediated by the symmetric surface plasmon-polaritons associated with the metal film. Further numerical modeling supports this view.

Figure 1

Experimental transmittance spectra are compared against simulated transmittance and absorbance spectra obtained for $8\mu \text{m}$ square arrays of 90 nm diameter holes in a (a) 20 nm thick Au film and (b) 90 nm diameter, 20 nm height cylindrical Au particles with periodicity 200 nm, illuminated at normal incidence in air. The dashed gray curve in both (a) and (b) shows the experimental transmittance spectrum obtained from a 20 nm Au film illuminated at normal incidence in air.

Figure 2

(Color online). Field profiles are shown at the absorbance maximum of the hole and particle array structures. The upper profiles show the instantaneous scattered electric field vector for (a) the hole and (b) particle, taken at the same instant in phase for each structure. The arrows above (a) and (b) indicate the vector of the incident electric field. The middle plots show the time-averaged scattered electric field profiles for (c) the hole and (d) particle. In (e) and (f), a line plot has been taken through the center of the structure along the dashed lines shown in (c) and (d), and the time-averaged electric field magnitude is shown as a function of position across the unit cell.

Figure 3

Comparison between experimental and simulated transmission spectra for [(a) and (c)] 90 nm diameter hole and [(b) and (d)] particle arrays with periodicity 200, 225, and 250 nm.