Abstract
Improving surface sensitivities of nanostructure-based plasmonic sensors is an important issue to be addressed. Among the SPR measurements, the wavelength interrogation is commonly utilized. We proposed using blue-shifted surface plasmon mode and Fano resonance, caused by the coupling of a cavity mode (angle-independent) and the surface plasmon mode (angle-dependent) in a long-periodicity silver nanoslit array, to increase surface (wavelength) sensitivities of metallic nanostructures. It results in an improvement by at least a factor of 4 in the spectral shift as compared to sensors operated under normal incidence. The improved surface sensitivity was attributed to a high refractive index sensitivity and the decrease of plasmonic evanescent field caused by two effects, the Fano coupling and the blue-shifted resonance. These concepts can enhance the sensing capability and be applicable to various metallic nanostructures with periodicities.
Highlights
Where m is the bulk sensitivity, i.e. a wavelength shift divided by a refractive index change, na the adsorbate biolayer refractive index, ns the refractive index of the bulk solution, ld the decay length of evanescent field and d the biolayer thickness
The Bloch wave surface plasmon polariton (BW-surface plasmon polariton (SPP)) is split into two modes: the forward (+kSPR) and backward (−kSPR) propagating modes
We proposed the use of blue-shifted Fano couplings with oblique-angle incidence to increase the surface sensitivities of periodic metallic nanostructures with longer periodicities (900 nm and 1,200 nm)
Summary
Where m is the bulk (refractive index) sensitivity, i.e. a wavelength shift divided by a refractive index change, na the adsorbate biolayer refractive index, ns the refractive index of the bulk solution, ld the decay length of evanescent field and d the biolayer thickness. This work combines the Fano coupling effect and blue-shifted resonance in capped silver nanoslits with a longer periodicity, which takes the advantages of large refractive index sensitivity due to long periodicity and short decay length due to short wavelength SPR and Fano coupling effect. The idea of blue-shifted surface plasmon resonance mode (angle-dependent) in a long-periodicity nanostructure coupled with localized SPR mode (angle-independent) can enhance the wavelength sensitivity and be applicable to various metallic nanostructures with periodicities
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