SURFACE-PLASMONS ENHANCE OPTICAL-TRANSMISSION THROUGH SUBWAVELENGTH HOLES

Optically thick metal films perforated with a periodic array of subwav elength holes show exceptional transmission properties. The zero-order transmission spectra exhibit well-defined maxima and minima of which the positions are determined by the geometry of the hole array. We sho w that the minima are the collection of loci for Wood's anomaly, which occurs when a diffracted beam becomes tangent to the film, and that t he maxima are the result of a resonant excitation of surface plasmons (SP's). SP's from both surfaces of the metal film are apparent in the dispersion diagram, independent of which side of the him is illuminate d, indicating an anomalously strong coupling between the two sides. Th is leads to wavelength-selective transmission with efficiencies that a re about 1000 times higher than that expected for subwavelength holes.