Optical field enhancement at cusps between adjacent nanoapertures

Optical nonlinear properties of cusps formed at the junction between two circular apertures in a metal film have been studied by scanning confocal microscopy. For gold, both second harmonic and broadband emission are enhanced when the pump polarization is directed across the gap between cusps, similar to the behavior of the recently studied bowtie antennas and apertures. However, field enhancements are also present when the polarization is perpendicular to the gap direction. A quantitative explanation of the observed relative signal intensities requires considering both field concentration near cusps and shape resonances of the entire structure. The occurrence of broadband emission from the cusp region when Al is used instead of Au indicates that broadband emission is the result of additional mechanisms besides pure two-photon luminescence.