Plasmonic Film with Diluted Nanostructures for Light Energy Harvesting and Sensing

Harvesting the electromagnetic light field in spatially dispersed nanostructures and focusing the sensing events at those locations enables a gain of several orders of magnitude in sensitivity. It significantly reduces the amount of target material needed to produce a measurable signal. This paper establishes that, contrary to common belief, at very low target concentrations, increasing the periodicity of a nanoparticle array instead of diminishing it significantly lowers the limit of detection (LOD) of such structured plasmonic sensors. We demonstrate this numerically on a thin gold film covered by an array of 50 nm diameter nanocylinders with periodicity ranging from 400 to 3000 nm. We found a minimal capture target volume per surface unit of a few tens of nm³/μm², which is an improvement of orders of magnitude from about 10⁴ nm³/μm² necessary for obtaining the same minimal signal using a conventional propagative-based plasmon sensor, or about 10³ nm³/μm² for a similar plasmonic biochip structure but with a typical 200-400 nm periodicity array range. Two key mechanisms are involved in achieving such a significant breakthrough. First, energy harvesting is enhanced by incorporating the underlying thin film, which enables the coupling of electromagnetic field energy into the localized plasmon mode of the nanoparticles. This harvesting coupling effect is shown to be limited by the film's plasmon propagation attenuation length and, consequently, its appropriate thickness. Second, target binding events must be precisely positioned in areas of enhanced optical field intensity.