Improvement of Sensing and Trapping Efficiency of Double Nanohole Apertures via Enhancing the Wedge Plasmon Polariton Modes with Tapered Cusps

In the past few years, double nanohole (DNH) apertures in a gold film have been used extensively to trap and sense biological and artificial dielectric nanoparticles. Using numerical simulations we show that the conical shape of a DNH, milled by a focused ion beam into a thin gold layer, which is an inherent property of the fabrication process, plays a critical role in the sensitivity of the DNHs, and is beneficial to the optical sensing and trapping applications. The slope of the metallic wedges in an appropriately designed DNH leads to 2D nanofocusing of gap surface plasmons (GSPs) and couples them to the wedge plasmon polaritons (WPPs), creating “hot spots” required for trapping. The transmission variations due to the trapping polystyrene nanoparticles of radii 11 ± 1 nm by particularly designed DNHs, measured at the wavelength near the corresponding wedge mode resonance, are shown to be in good agreements with numerical results using conically modeled DNHs. This observation highlights the extreme sensitivity of aperture assisted trapping, specifically with regard to the DNH structure. These findings open up new routes toward the design and optimization of efficient aperture structures for trapping and sensing applications.