Increased Refractive Index Sensitivity by Circular Dichroism Sensing through Reduced Substrate Effect

An optical sensor based on the localized surface plasmon resonance (LSPR) of chiral Au nanohooks with increased refractive index (RI) sensitivity via circular dichroism (CD) measurements is presented. Programmed control of sample rotation combined with angled physical vapor deposition is applied to hole-mask colloidal lithography to provide in process modification of the hole-masks and generate arrays of chiral nanostructures with an adjustable optical response. Extinction spectra with unpolarized light as well as circular dichroism measurements are compared for left- and right-handed hook structures. Analysis of the LSPR peak shift of the substrate-attached nanostructures revealed the CD measurements to be twice as sensitive as the measurements with unpolarized light (304 and 146 nm RIU-1 , respectively) and close to the maximum predicted for LSPR sensing at this spectral region (similar to 700 nm). Finite-difference time-domain simulations with different substrate materials show that the difference in RI sensitivity can be attributed to the limiting effect of the substrate for the unpolarized extinction measurements, while CD-based sensing retains a high sensitivity, unaffected by the limiting effect of the substrate. CD-based readout could provide a complementary and improved sensitivity for substrate-bound LSPR sensor formats.