Elsevier

Optics Communications

Volume 502, 1 January 2022, 127441
Optics Communications

Plasmonic Fano-type nanocavity for double resonance enhanced SERS and optical sensing

https://doi.org/10.1016/j.optcom.2021.127441Get rights and content

Abstract

A highly tunable design for obtaining double resonance enhanced surface-enhanced Raman spectroscopy (SERS) substrates with ultrasmall vertical nanogaps is proposed by utilizing the plasmonic Fano resonances resulted from the strong coupling between the broad gap plasmon mode of the Au nanocavity and the narrow surface plasmons polaritons. A sandwiched plasmonic nanocavity of Au nanostrip grating upon a gold film separated by a dielectric spacer of nanopillar with ultrasmall nanogap is designed and simulated to obtain the double resonances at the laser excitation and Raman scattering frequencies for Raman signals detection. We demonstrate that by tuning the geometrical and material parameters of the proposed nanocavity, the double resonance frequencies and the SERS enhancement can be readily controlled. Our proposed plasmonic nanocavity exhibits very high SERS enhancement factor up to 1.1×1010, which are promising for detect specific signals of sport doping drugs and medical diagnoses.

Introduction

In recent years, due to the demand for high sensitivity detection and identification of the specific molecules, biosensors based on plasmonic resonance have attracted much appeal for use in medical diagnoses, food safety and sport doping drugs monitoring [1], [2], [3], [4]. Among different types of the biosensors, surface-enhanced Raman scattering (SERS) is one powerful analytical tool for detection and identification of material information at molecular level, showing great potential in chemical and biological sensing applications [5], [6]. Since the discovery of SERS by Fleischmannin 1974 [7], with the development of micro–nano processing technology, various metallic nanostructures have been used for SERS substrates in the scientific and industrial applications ranging from ultra-violet (UV) to near-infrared (NIR) region owing to the electric field enhancement originating from the excitation of the localized surface plasmon (LSP) resonance [8], [9], [10], [11], [12].

Up to now, it is the widely accepted that the electric field enhancement resulting from the near-field interactions between metal nanoparticles with small nanogap is stronger than that of a single nanoparticle [13], [14]. Such the small nanogaps are called “hotspots”, which is important for achieving higher SERS enhancements [15]. Extensive experimental researches on closely spaced metallic nanoparticles have demonstrated that the shape, size, and gap distance of the nanoparticles could significantly influence the performance of the SERS substrates [16], [17], [18]. Another effective method of tuning the plasmonic resonance property is the plasmon hybridization, which results from the interference between two or more plasmonic modes [19]. Fano resonances, arising from the plasmon hybridization, are defined by their optical response with its characteristic asymmetric lineshape and reduced extinction cross section within a narrow spectral window in many physical systems, due to the coupling between their plasmonic broad bright mode and narrow dark mode [20]. Plasmonic Fano resonances have been realized in various sorts of complex metallic nanostructures such as ring-disks [21], dolmen nanostructures [22], metal nanoshells [23], waveguide structures [24], graphene and semimetal metamaterials  [25], [26], [27]. The dark mode of metallic Fano type nanostructure can significantly enhance the local electric field intensity within the hotspots due to its capability to trap energy within the gap between the nanoparticles [28]. Consequently, plasmonic Fano type nanostructures can provide large SERS enhancements when the position of Fano resonance is tuned near the excitation frequency and the Raman scattering frequencies of the targeted molecules.

Recently, the double-resonance based plasmonic substrates at the laser excitation and Raman scattering frequencies were reported, which shows that they provide higher SERS enhancement than the similar structures with a single plasmonic resonance [29], [30], [31]. It is due to the SERS enhancement factor (EF) is proportional to the product of the local electric field intensity (E2) enhancements both at the excitation frequency and the Raman scattering frequencies  [29]. For a single LSP resonance, it has been demonstrated that the maximum SERS enhancement can be achieved when such single LSP resonance is located between the excitation and the Raman scattering wavelength. Nevertheless, owing to the excitation and Raman scattering wavelengths could be more than 200 nm away from each other in the NIR-SERS range, a single resonance structure could not realize EF enhancement at both excitation and scattering wavelengths [29]. This can be circumvented in a plasmonic double resonance structure. Metallic grating structure is one of the well-known structures to exhibit efficient surface plasmons polaritons (SPP). There are various reports on taking the advantage of the SPP effect of metallic grating for SERS applications [32], [33]. However, there are just a handful of reports on the studies on the double plasmonic effect realized by the coupling between the LSP mode and the SPP mode in the metallic grating structure with small nanogap [34].

In this work, we present a structure consisting of a sandwiched plasmonic nanocavity with Au nanostrip grating upon a gold film separated by a dielectric spacer of nanopillar to generate a Fano resonance based plasmon induced absorption phenomenon due to the strong coupling between the broad gap plasmon mode of the Au nanocavity and the narrow surface plasmons polaritons propagating on the Au film. It is found that through suitable structural design, the SERS enhancement at both excitation and scattering wavelengths is realized. Moreover, by lifting the Au nanostrip into air to reduce the dielectric substrate effect, the local electric fields are mainly distributed into the air space of the plasmonic nanocavity, which increases the interaction of the enhanced electric field with the background analyte within the vertical ultrasmall nanogap. In addition, coupling effects with different structural parameters also are presented in detail. Our proposed Fano-type double resonance plasmonic nanocavity exhibits a high SERS EF up to 1.1×1010, which is able to provide significant application in the detection of sport doping drugs and medical diagnoses.

Section snippets

Methods

Fig. 1(a) schematically exhibits the designed metallic nanocavity structure composed of Au nanostrip grating standing on Au substrate separated by a silica spacer of nanopillar with refractive index (ng) of 1.45. The magnified front views of a unit cell of the designed metallic nanocavity is shown in Fig. 1(b). The width (w) and height (h) of the Au nanostrip, the width (wg) and height (hg) of the dielectric nanopillar and the period (P) of the metallic nanocavity are set to be 60 nm, 60 nm,

Results and discussion

Fig. 2(a) shows the calculated absorption spectra of the designed array of Au nanocavity structure under a normal incident transverse magnetic (TM) wave. Here the structural parameters of a single Au nanocavity are the same as shown in Fig. 1. For the period (P) being 800 nm, as can be clearly seen, there are two distinct narrow absorption peaks at 791.5 nm and 856.5 nm, labeled as peak I and II with a steep absorption dip in between (solid blue line) shown in Fig. 2(a). This is a typical

Conclusions

In this work, we demonstrated a highly tunable metallic grating based plasmonic nanocavity with ultrasmall nanogap to support a double-resonance SERS. The double resonances of our proposed plasmonic nanocavity arise from the plasmonic Fano resonance effect by the spectra overlap of the narrow surface plasmons polaritons mode propagating on the Au film and the broad gap plasmon resonance mode of an individual Au nanocavity. We demonstrate that by lifting the Au grating into air to reduce the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Funding

This work was financially supported by the Project of Philosophy and Social Science Research in Colleges and Universities in Jiangsu Province [2021SJA0124]; the National Natural Science Foundation of China (NSFC) [11704184 and 12074151]; Open fund by Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control [KHK2007]; Science Innovation Foundation for Young Scientists of Nanjing Forestry University [CX2019024]; and the Natural Science Foundation of Zhejiang Province [

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