Giant Extra-Ordinary Near Infrared Transmission from Seemingly Opaque Plasmonic Metasurface: Sensing Applications

Abstract

In the present study, we report giant extra-ordinary transmission of near infrared (NIR) light, more than 90%, through a seemingly opaque plasmonic metasurface, which consists of two metal nano-slits arrays (MNSAs) with alternate opening arrangements. By using perfect coupling of the plasmonic modes formed between the sharp edges of the upper and lower MNSAs of silver, a giant, wavelength selective transmission could be obtained. The study is accompanied by optimization of electromagnetic (EM) field coupling for different interlayer spacings and lateral overlap between the two MNSAs to understand their significance in light transmission through the metasurface. The interlayer spacing between the MNSAs works as the transmitting channel for light. The optimization of performance with different fill factors and plasmonic metals was performed as well. Because of the excitation of extended surface plasmons (ESPs) generated at both the MNSAs, the metasurface can be used for refractive index (RI) sensing as one of its applications by using a transparent and flexible polymer, such as polydimethylsiloxane (PDMS), as substrate. The maximum sensitivity which could be achieved for the optimal configuration of the metasurface was 1435.71 nm/RIU, with a figure of merit (FOM) of 80 RIU^-1 for 90.45% optical transmission of light for the refractive index variation of analyte medium from 1.33 to 1.38 RIU. The present study strengthens the concept of light funneling through subwavelength structures due to plasmons, which are responsible for light transmission through this seemingly opaque metasurface and finds use in highly sensitive, flexible, and cost-effective EOT-based sensors.

Keywords: Coupled plasmonic mode; Extra-ordinary optical transmission; Metasurfaces; Nano-slits; Sensor; Surface plasmon.

Fig. 1

Schematic of the proposed metasurface consisting of two MNSAs separated by “H” distance

Fig. 2

(a, b) Transmission spectra for various thicknesses of Ag metal nano-slits; (c–e) electric field profiles for 20 nm, 60 nm, and 100 nm Ag film thicknesses at respective resonance wavelengths, respectively. The insets in (c–e) show the zoomed in view of the field profile near edges

Fig. 3

Transmission spectra for (a) decreasing and (b) increasing interlayer vertical distance (H) between upper and lower MNSAs; (c-f) are the electric field profile at shifting resonance wavelength for H = 25 nm, 55 nm, 75 nm, and 95 nm, respectively. The insets of (c–f) show the zoomed in view of the EM field enhancement at the edges of both the MNSAs

Fig. 4

(a) Transmission spectrum for increasing value of g. (b-d) are the electric field profiles at resonance wavelengths for 20%, 60%, and 90% lateral overlap, respectively. The insets of figures (b-d) show enlarged views of the EM field profiles confined in the hot spot between both the MNSAs

Fig. 5

Transmittance spectra for different–(a) values of fill factor f; (b) different noble metals

Fig. 6

(a) Electric and (b) magnetic field profile of the optimal plasmonic metasurface at the resonance wavelength = 1641.2 nm

Fig. 7

(a) Transmission spectra for varying $n_a$ over the metasurface based flexible sensor and (b) characteristic response curve of the sensor

Fig. 8

Transmission spectra for (a) constant value of n and varying k values and (b) both n and k varying. The corresponding insets show the quantitative variation of the resonance wavelength and peak transmittance with changes in the complex RI