Abstract
Due to their unique optical properties, metallic nanoparticles offer a great potential for important applications such as disease diagnostics, demanding highly integrated device solutions with large refractive index sensitivity. Here we introduce a new type of monolithic localized surface plasmon resonance (LSPR) waveguide sensor based on the combination of an adiabatic optical fiber taper and a high-density ensemble of immobilized gold-reinforced silver nanoprisms, showing sensitivities up to 900 nm/RIU. This result represents the highest value reported so far for a fiber optic sensor using the LSPR effect and exceeds the corresponding value of the bulk solution by a factor of two. The plasmonic resonance is efficiently excited via the evanescent field of the propagating taper mode, leading to pronounced transmission dips (−20 dB). The particle density is so high (approx. 210 particle/μm2) that neighboring particles are able to interact, boosting the sensitivity, as confirmed by qualitative infinite element simulations. We additionally introduce a qualitative model explaining the interaction of plasmon resonance and taper mode on the basis of light extinction, allowing extracting key parameters of the plasmonic taper (e.g., modal attenuation). Due to the monolithic design and the extremely high sensitivity we expect our finding to be relevant in fields such as biomedicine, disease diagnostics, and molecular sensing.
Highlights
Metallic nanoparticles can show a very distinct optical response in the visible and near infrared part of the spectrum in case their diameters fall into the nanoscale domain
We introduce a new type of monolithic localized surface plasmon resonance (LSPR) waveguide sensor based on the combination of an adiabatic optical fiber taper and a high-density ensemble of immobilized gold-reinforced silver nanoprisms, showing sensitivities up to 900 nm/RIU
It is important to note that in the case of the plasmonic taper the nanoprisms are immobilized on glass and a spectral shift of the LSPR is expected compared to the measured resonance of the NPs in solution, i.e., justifies using λR as fit parameter (σ = σt)
Summary
Metallic nanoparticles (meNPs) can show a very distinct optical response in the visible and near infrared part of the spectrum in case their diameters fall into the nanoscale domain. Many of the currently employed plasmonic colloidal systems rely on noble metals such as gold or silver due to low intrinsic ohmic damping, which yields in strong LSPRs in the visible and near infrared.[4] Up to date, the most widely used colloidal NPs are gold nanospheres due to long-term stability and straightforward synthesis pathway These nanospheres reveal comparably low RI sensitivity (of the order of 80 nm/RIU),[8] making them unattractive in case very small amounts of species are to be detected. We present simulations from a qualitative scattering model, showing that interparticle interaction leads to a substantial increase in RI sensitivity in the case of high particle densities
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