We demonstrate experimentally how the local anisotropy of the dispersion relation of surface plasmon modes propagating over periodic metal gratings can lead to an enhancement of the figure of merit of refractive index sensors. Exploiting the possibility to acquire defocused images of the Fourier space of a highly stable leakage radiation microscope, we report a twofold increase in sensing sensitivity close to the band gap of a one-dimensional plasmonic crystal where the anisotropy of the band structure is the most important. A practical sensing resolution of O(10−6) refractive index units is demonstrated.
Because the grating is fabricated on ITO coated substrates, bonding of PDMS to glass by simple oxygen plasma activation could not be used. To achieve covalent bonding between PDMS and the gold surface, (3-Mercaptopropyl)trimethoxysilane (Sigma-Aldrich) was applied from a 2% solution in methanol. The molecule provides a silane endgroup for binding to the PDMS mesh, as well as an opposing thiol group for bonding to the gold substrate.