Engineering Through Mode Shaping and Lithographical Nanofabrication of Ultrasensitive Nano-plasmonic Sensors for Molecular Detection

Abstract

The resonance change of plasmonic nanostructures to a small variation of the shallow refractive index as induced by the binding of molecules to the metal surface determines the sensitivity of plasmonic sensors. The magnitude of this change is strongly determined by a number of factors including dielectric constant of the metal at the working wavelength, refractive indices of analyte, and surroundings [J Phys Chem B 109:21556-21565, 2005], but also the spatial overlap between the region of local refractive index change and the plasmon mode. In this chapter we discuss how the plasmon modes of lithographically prepared plasmonic nanostructures can be accurately engineered to design bio-chemical sensors with improved sensitivities. We first describe how metal nanostructures can be designed to control the confinement of light modes down to the nanometer scale. Using 3D calculations based on the finite element method, we then discuss the influence on the sensitivity of the nanostructure geometry and location of the sensed molecule. Finally, we present experimental results that demonstrate this enhanced sensitivity to the detection of small molecules in arrays of gold dimers.