An Inverted Honeycomb Plasmonic Lattice as an Efficient Refractive Index Sensor

Abstract

We present an efficient refractive index sensor consisting of a heterostructure that contains an Au inverted honeycomb lattice as a main sensing element. Our design aims at maximizing the out-of-plane near-field distributions of the collective modes of the lattice mapping the sensor surroundings. These modes are further enhanced by a patterned SiO2 layer with the same inverted honeycomb lattice, an SiO2 spacer, and an Au mirror underneath the Au sensing layer that contribute to achieving a high performance. The optical response of the heterostructure was studied by numerical simulation. The results corresponding to one of the collective modes showed high sensitivity values ranging from 99 to 395 nm/RIU for relatively thin layers of test materials within 50 and 200 nm. In addition, the figure of merit of the sensor detecting slight changes of the refractive index of a water medium at a fixed wavelength was as high as 199 RIU−1. As an experimental proof of concept, the heterostructure was manufactured by a simple method based on electron beam lithography and the measured optical response reproduces the simulations. This work paves the way for improving both the sensitivity of plasmonic sensors and the signal of some enhanced surface spectroscopies.

Imaging of Antiferroelectric Dark Modes in an Inverted Plasmonic Lattice

“…In this case, most of the excited charge is found to spread inside the hexagons, relatively far from the slits forming the array (see Figure 3 h for FDTD results with linearly polarized light). The two opposite sign charge distributions in the upper and lower halves of each hexagon cause a large electric field with an out-of-plane component that extends hundreds of nanometers from the structure 28 (see Figure S1 in the Supporting Information). For unpolarized radiation with normal incidence, the charge distribution will oscillate from the center of the hexagon to the outer part following a kind of breathing mode and giving rise to the EELS signal and electric-field distribution depicted in Figure 3 g and i.…”

“…Such an out-of-plane electric field enhancement is crucial for their integration in sensing and advanced spectroscopic architectures such as refractive index sensors. 28 The implementation of any design pathway for enhanced plasmonic nanoarchitectures requires of an understanding of the plasmonic near-field response. A possible route to obtain a clear picture of the plasmonic resonances supported by a particular structure is to use electron energy loss spectroscopy (EELS).…”

“…Such an out-of-plane electric field enhancement is crucial for their integration in sensing and advanced spectroscopic architectures such as refractive index sensors. 28 …”

“…Here we show that the manufacture of a honeycomb array of slits fosters the formation of out-of-plane hotspots related to the SLR that present large enhancement factors of the electric field, even hundreds of nanometers away from the 2D array into the surrounding medium. Such an out-of-plane electric field enhancement is crucial for their integration in sensing and advanced spectroscopic architectures such as refractive index sensors …”