Gold Nanopost-Shell Arrays Fabricated by Nanoimprint Lithography as a Flexible Plasmonic Sensing Platform

Abstract

Plasmonic noble metal nanostructured films have a huge potential for the development of efficient, tunable, miniaturized optical sensors. Herein, we report on the fabrication and characterization of gold-coated nanopost arrays, their use as refractometric sensors, and their optimization through photonics simulations. Monolithic square nanopost arrays having different period and nanopost size are fabricated by nanoimprint lithography on polymer foils, and sputter-coated by gold films. The reflectivity of these gold nanopost-shell arrays present dips in the visible range, which are efficient for refractometric sensing. By finite-difference time-domain (FDTD) simulations we reproduce the experimental spectra, describe the electric fields distribution around the nanopost-shells, and then explain their good sensitivity, around 450 nm/RIU. Furthermore, we determine by simulations the influence of several geometrical parameters, such as array period, nanopost width, gold film thickness, and nanopost side coverage on both reflectivity spectra and sensing capabilities. Fully coated nanoposts provide an extremely deep reflectivity minimum, approaching zero, which makes the relative reflectivity change extremely high, more than two orders of magnitude higher than for partially coated nanoposts. These results contribute to the understanding of the plasmonic properties of metal coated nanopost arrays, and to the development of efficient platforms for sensing and other surface plasmon based applications.

Narrowband Light Reflection Resonances from Waveguide Modes for High-Quality Sensors

“…To investigate the potential applications of high-quality sensors, which are based on the narrowband light reflection resonances from waveguide modes, in Figure 6 a–d we calculate the reflection spectra for the varied refractive index n of the environment medium surrounding the Au spheres. As the refractive index n varies from 1.0 to 1.3 in steps of 0.1, two reflection peaks from the TM and TE waveguide modes red-shift significantly, which move from 775.2 and 831.9 nm to 798.7 and 855.7 nm, as presented in Figure 6 e. So, we can achieve a conventionally defined sensitivity [ 31 , 53 , 54 , 55 , 56 , 57 ], S = Δ λ /Δ n = 78.3 and 79.3 nm/RIU, respectively. For the refractive index n = 1.0, the full width at half maximum FWHM of two reflection peaks is 2.4 and 3.2 nm, respectively.…”

“…For the refractive index n = 1.0, the full width at half maximum FWHM of two reflection peaks is 2.4 and 3.2 nm, respectively. Then, the conventionally defined figure of merit [ 31 , 53 , 54 , 55 , 56 , 57 ] FOM = S / FWHM = 32.6 and 24.8, respectively. FOM can not only indicate the sensitivity of the reflection peaks to the change of n , but can also indicate the bandwidths of reflection peaks.…”

Perfect Absorption and Refractive-Index Sensing by Metasurfaces Composed of Cross-Shaped Hole Arrays in Metal Substrate

“…Figure 7b shows the dependence of the position of the absorption peak on the refractive index. Conventionally, two important parameters, sensitivity (S) and figure of merit (FOM), are widely used to estimate the sensing performance [80][81][82][83][84]. S and FOM can be defined as:…”

“…Figure 7 b shows the dependence of the position of the absorption peak on the refractive index. Conventionally, two important parameters, sensitivity ( S ) and figure of merit ( FOM ), are widely used to estimate the sensing performance [ 80 , 81 , 82 , 83 , 84 ]. S and FOM can be defined as: where Δλ is the spectral shift of the absorption peak, Δ n is the change of the refractive index, and FWHM is the full width at half maximum of the perfect absorption peak.…”

Latest Novelties on Plasmonic and Non-Plasmonic Nanomaterials for SERS Sensing

“…The key point in order to obtain zones of strong electric field (called hotspots) is a precise control of the shape, size, and spatial organization of plasmonic nanostructures. The control of these parameters is enabled and realized thanks to a great number of lithographies such electron beam lithography [ 24 , 25 , 26 , 27 ], optical lithographies [ 28 , 29 , 30 ], nanosphere lithography [ 31 , 32 , 33 ], and nanoimprint lithography [ 34 , 35 , 36 ]. Several groups examined a broad number of designs as plasmonic nanodisks, nanodimers, and nanorods, which have reached important EF values (EF = 10 6 –10 9 ) [ 37 , 38 , 39 ].…”