Abstract
This work studies the effect of a plasmonic array structure coupled with thin film oxide substrate layers on optical surface enhancement using a finite element method. Previous results have shown that as the nanowire spacing increases in the sub-100 nm range, enhancement decreases; however, this work improves upon previous results by extending the range above 100 nm. It also averages optical enhancement across the entire device surface rather than localized regions, which gives a more practical estimate of the sensor response. A significant finding is that in higher ranges, optical enhancement does not always decrease but instead has additional plasmonic modes at greater nanowire and spacing dimensions resonant with the period of the structure and the incident light wavelength, making it possible to optimize enhancement in more accessibly fabricated nanowire array structures. This work also studies surface enhancement to optimize the geometries of plasmonic wires and oxide substrate thickness. Periodic oscillations of surface enhancement are observed at specific oxide thicknesses. These results will help improve future research by providing optimized geometries for SERS molecular sensors.
Highlights
Surface enhanced Raman spectroscopy (SERS) molecular sensing has seen increased attention in recent years due to its ability to detect analyte molecules, even down to single molecule detection capabilities [1,2,3]
Plasmonic structures have been shown to improve SERS sensors by focusing light into extremely small gap regions where analyte molecules are located, enhancing the signal produced by the molecules, making them more detectable
Previous results have shown that optical enhancement in gap regions increases as the geometries of the grating structure decrease
Summary
Surface enhanced Raman spectroscopy (SERS) molecular sensing has seen increased attention in recent years due to its ability to detect analyte molecules, even down to single molecule detection capabilities [1,2,3]. SERS can be used in a variety of applications such as improved biomedical technologies, substance detection, and experimental chemical sensing [8,9,10]. Raman spectroscopy takes advantage of vibrational modes in analyte molecules, which weakly scatter light, to produce a characteristic spectrum with peaks corresponding to shifts in energy compared to the incident electromagnetic radiation; it is limited by the small signal strength produced by molecules. Plasmonic nanoantennae can vastly improve the signal strength of molecules on SERS substrates by focusing incident light into ultra-small regions that enhance the electric near-field by many orders of magnitude [11,12,13]. Plasmonic nanogratings have been fabricated with geometries optimized in such a way as to produce the maximum possible electric
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