Variable Pitch Crossed Surface Relief Gratings for Point-of-Care Sensing

Abstract

Nanostructures consisting of crossed surface relief gratings (CSRGs) support surface plasmon resonances (SPRs) that are compatible with biosensing applications [1], [2]. At normal incidence, surface plasmons are excited between a metal and a dielectric at a light wavelength ( $\lambda_{\text{SPR}}$ ) given by $\lambda_{SPR}=n\Lambda[\sqrt{\varepsilon_{m}/(n^{2}+\varepsilon_{m})}]$ , where $n$ is the index of refraction of the dielectric, and $\varepsilon_{m}$ is the real part of the permittivity of the metal and $\Lambda$ is the grating pitch. When placed between crossed linear polarizers, CSRGs allow for plasmonic energy exchange between the two superimposed gratings that eliminate any incident polychromatic light, except for the narrow SPR bandwidth where polarization conversion occurs. The result is an effective transmission of the SPR signal with a high signal-to-noise ratio. The single pitch of CSRGs, however, limit their operation to a specific plasmonic resonance wavelength. Here, we present a plasmonic sensor based on variable-pitch CSRG that allows plasmonic resonances at different wavelength bandwidth depending on the illuminated region. The variable-pitch CSRGs (VP-CSRGs) are fabricated on azobenzene-functionalized films through a simple two-step procedure. Fig. 1a presents an actual AFM image of the surface of a nanofabricated VP-CSRGs. The plasmonic response of the VP-CSRG sensor was evaluated using Finite-Difference Time-Domain (FDTD) simulations to show the e-field intensity distribution on the gold-coated VP-CSRG surface. Electric field enhancement and distribution, due to the plasmonic conversion, was assessed through the change of the RI of the dielectric medium in contact with the metallic crossed gratings, and through the spectral diversity of the light source to excite the surface plasmons. For the simulations, the surface of the CSRGs with pitches of 520, 540 and 560 nm were modeled using the function, $f(x, y)=G(\cos[(2\pi/p)x]+\cos[(2 \pi/p)y])$ , where $G$ is the amplitude and $p$ the period of the structure (Fig. 1a). The simulations were used to obtain the e-field intensity distribution, normalized with respect to the incident plane wave. Periodic boundary conditions in both $x$ and $y$ directions and a perfectly matched layer (PML) in the $z$ direction were used for the analysis region. A uniform mesh size of 3 nm was used for the envelope of the nanostructure, comprising the azobenzene layer, the gold film and the dielectric medium, in all the directional axes. A plane wave, polarized along the $y$ -axis and orthogonal to the $x-y$ plane, was employed to induce a SPR in the structure. Fig. 1b demonstrates the plasmonic excitation in the $x$ -direction, when using p-polarized light and the absence of plasmonic excitation in the $y$ -direction. However, when s-polarized light is used, the nanostructures are excited in the $y$ -direction. These results present an evidence on the unique plasmonic energy transfer between the crossed gratings that has been hypothesised before [1]. Experimentally, these results can be used to confirm that the transmitted light acquired in a collinear setup using VP-CSRGs between two orthogonal polarizers corresponds, only, to the plasmonic signature of the nanostructure.