Abstract
The coupling of the surface plasmon near-field into the sensing medium is key to the sensitivity of surface plasmon-based sensing devices. A low-index dielectric is necessary for the sensing medium to support a highly-penetrating surface plasmon evanescent field that extends well into the dielectric medium. The air-like refractive index, n, of an aerogel substrate provides another dimension for ultralow-index plasmonic devices. In this paper, we experimentally observed an angular surface plasmon resonance dip at 74° with the ultralow-index aerogel substrate, as was expected from theory. We also demonstrated the comparatively high-sensitivity surface plasmon resonance wavelength, λ, while the change in Δλ/Δn with different substrates was studied in detail. A 740 nm-period metal grating was imprinted on aerogel (n = 1.08) and polydimethylsiloxane (PDMS; n = 1.4) substrates. The ultraviolet–visible–near-infrared spectra were observed in the reflection mode on the grating, resulting in sensitivities of 740.2 and 655.9 nm/RIU for the aerogel and PDMS substrates, respectively. Numerical simulations were performed to understand the near-field of the surface plasmon, which demonstrated resonances well correlated with the experimentally observed results. The near-field due to excitation of the surface plasmon polaritons is observed to be more confined and to penetrate deeper into the sensing medium when a low-index substrate is used.
Highlights
A surface plasmon polaritons (SPP) induce strong confinement and intense optical fields, resulting in high-performance label-free biosensing platforms
In plasmonic refractive sensors made of metallic nanostructures on a dielectric substrate, an antisymmetric SPP mode is generated across the metal thin film and the optical field is mainly concentrated at the substrate/ metal interface[14,15]
Total internal reflection was observed beyond the critical angle of 67.1°, which is consistent with the ultralow refractive index of 1.08
Summary
The surface plots of the norm of the electric field are shown in Fig. 5(b,c) for the aerogel and PDMS substrates, respectively, where the caption atop each figure shows the refractive index of the superstrate analyte and the corresponding SPP resonance wavelength. This is somewhat the reverse of the result observed for the case of aerogel substrate, where the field strength in the analyte is high for n = 1.4176 This simulation study shows the explicit coupling and localization of the surface plasmon near-field with the two different substrate media with varying analytes, where the near-field favors localization in the low-index material at the metal interface. These results show that the use of an aerogel substrate provides significant benefits for plasmonic sensing
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