Simultaneous realization of high sensing sensitivity and tunability in plasmonic nanostructures arrays

Abstract

A plasmonic nanostructure (PNS) which integrates metallic and dielectric media within a single structure has been shown to exhibit specific plasmonic properties which are considered useful in refractive index (RI) sensor applications. In this paper, the simultaneous realization of sensitivity and tunability of the optical properties of PNSs consisting of alternative Ag and dielectric of nanosphere/nanorod array have been proposed and compared by using three-dimensional finite element method. The proposed system can support plasmonic hybrid modes and the localized surface plasmonic resonances and cavity plasmonic resonances within the individual PNS can be excited by the incident light. The proposed PNSs can be operated as RI sensor with a sensitivity of 500 nm/RIU (RIU = refractive index unit) ranging from UV to the near-infrared. In addition, a narrow bandwidth and nearly zero transmittance along with a high absorptance can be achieved by a denser PNSs configuration in the unit cell of PNS arrays. We have demonstrated the number of modes sustained in the PNS system, as well as, the near-field distribution can be tailored by the dielectric media in PNSs.

Figure 1

(a) The truncated view of a 2-D periodic array of PNSs. (b) The unit cell of the two proposed cases, i.e., (c) case 1: an all-metal PNS with a combination of a metal nanosphere and a metal nanorod and (d) case 2: a combination of a metal nanosphere and a core-shell nanorod (metal-shell with a dielectric nanorod in core region), respectively. The axis shows the propagation direction and polarization of the incident EM wave where the period, height of the nanorod, radius of the nanorod, radius of the nanophere, thickness of the Ag nanoshell and radius of the dielectric nanorod in Ag nanoshell are denoted by Λ, h, r, R, t and d, respectively. The environmental refractive index is set to be n = 1.0 for air.

Figure 2

(a) Near-field intensity and (b) transmittance spectra of case 2 with tuning ε (ε = 1.0, 2.25, 5.0 and 9.0, where ε is the permittivity of dielectric media in PNSs). Case 1 is also displayed for comparison. (c) Distributions of the electric field intensity |E _T| (in the x-y and x-z plane), and (d) Magnetic field intensity |H _T| (in the x-y and x-z plane) of case 2 (with ε  = 2.25 at λ_res = 670 nm).

Figure 3

(a) Transmittance spectrum of case 2 (with ε = 2.25 (dashed lines) and 3.1 (solid lines)) in the index-matching liquid from n = 1.30 to n = 1.42 RIU (RIU = refractive index unit), where n is the surrounding medium of PNSs; and (b) Relation between the wavelength shift and the full width at half maximum (FWHM) versus RI variation.

Figure 4

(a) Near-field intensity and (b) transmittance spectra of case 2 with t = 9 nm and varied ε (ε = 1.0, 3.0, 5.0, 7.0 and 9.0, where ε is the permittivity of dielectric media in PNSs). Dependence of (c) near-field intensity and (d) transmittance spectra on the refractive index with dielectric media in the range of 1.0–3.3 in case 2 with t = 9 nm. (e) The x-z sectional plane E-field intensity distribution (left) and half-body surface charge density distribution (right) for case 2 (t = 9 nm) with ε = 3.1, showing selected wavelengths (340, 420, 510, and 770 nm) chosen as representatives of the modes 1–4.

Figure 5

(a) Transmittance spectra of case 2 (t = 9 nm) with ε = 3.1 exposed to air as well as three other fluids (i.e., water (n = 1.33), isopropanol (n = 1.37) and optical oil (n = 1.63)). Comparison of (b) E-field intensities (including electric force lines, green lines), (c) electric energy density (including electric energy flows, green arrows), and (d) magnetic energy density (including magnetic energy flows, green arrows) of the surrounding medium in the case of t = 9 nm exposed to air (n = 1.0, left panel) and optical oil (n = 1.63, right panel).

Figure 6

(a) A truncated diagram of case 2 (t = 9 nm, ε = 2.25, left panel) in the PNS array with 4 × 4 PNSs in a unit cell (right panel). The gap among the PNSs is set to be g = 20 nm. The other parameters are the same as those used in Fig. 4. (b) Transmittance (T), absorptance (A) and reflectance (R) spectra of case 2 (t = 9 nm) with ε = 2.25 exposed to air (n = 1.00) and a surrounding medium ranging in 1.00 < n < 1.70 in step of 0.1.

Figure 7

The comparison of E-field intensity and magnetic field intensity (including the 3-D profiles of electric/magnetic force lines (green lines) and energy flows (pink arrows) of case 2 (t = 9 nm, ε = 2.25) exposed to (a) air (n = 1.00) at λ_res = 690 nm, and (b) a surrounding medium (n = 1.30) at λ_res = 800 nm.