Abstract
We propose a novel trench-assisted circular metal nano-slit (CMNS) structure implementable on a fiber platform for the generation of a low-noise cylindrical surface plasmon (CSP) hotspot. We design trench structures based on a multi-pole cancellation method in order that a converging surface plasmon signal is well separated from co-propagating non-confined diffracted light (NCDL) at the hotspot location. In fact, the secondary radiation by the quasi-pole oscillation at the edge of the trench cancels the primary NCDL, thereby enhancing the signal-to-noise ratio (SNR) of the CSP hotspot. In particular, we investigate two types of trench structures: a rectangular-trench (RT) structure and an asymmetric-parabolic-trench (APT) structure, which are considered for the sake of the simplicity of fabrication and of the maximal enhancement of the SNR, respectively. In comparison with a conventional CMNS having no trenches, we highlight that the mean SNR of the CSP hotspot is enhanced by 6.97 and 11.89 dB in case of the optimized RT and APT CMNSs, respectively. The proposed schemes are expected to be useful for increasing the SNR of plasmonic devices that are interfered by NCDL, such as various types of nano-slits for generating high-resolution plasmonic signals, for example.
Highlights
Surface plasmons, which are collective oscillations of free electrons propagating along the interface of metal and dielectric, have extensively been studied because of their fascinating properties that cannot be obtained with conventional optical approaches [1,2,3,4,5,6,7,8,9]
Among them circular metal nano-slit (CMNS) structures patterned on a thin metal film are of great interest because of their high capability of generating focused cylindrical surface plasmons (CSPs), i.e., plasmonic hotspots, via relatively simple fabrication methods [15,16,17,18,19]
Chen et al combined the previous two techniques, thereby applying radially polarized light to a multiring structure for further enhancements [18]. While such a variety of structures and schemes were extensively investigated for intensifying the plasmonic hotspots, the impact of the disturbance of non-confined diffracted light (NCDL) that normally accompanies surface plasmons has little been discussed to an extensive level [23,24]
Summary
Surface plasmons, which are collective oscillations of free electrons propagating along the interface of metal and dielectric, have extensively been studied because of their fascinating properties that cannot be obtained with conventional optical approaches [1,2,3,4,5,6,7,8,9]. Chen et al combined the previous two techniques, thereby applying radially polarized light to a multiring structure for further enhancements [18] While such a variety of structures and schemes were extensively investigated for intensifying the plasmonic hotspots, the impact of the disturbance of non-confined diffracted light (NCDL) that normally accompanies surface plasmons has little been discussed to an extensive level [23,24]. The fiber platform format allows for much simplified procedures to excite CSPs because a radially polarized optical mode can readily be excited or generated through an optical fiber [35] This integrated scheme can offer a great merit of localizing and intensifying the resultant plasmonic hotspot in a compact and efficient form.
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