Enhanced sensitivity of photonic crystal slab transducers by oblique-angle layer deposition.

Photonic crystal slabs (PCS) are one of the major transducers for label-free, optical biosensing applications. In this paper we present oblique-angle layer deposition of the high index slab material as a method to improve the PCS sensitivity. In simulations and experiments we consider PCSs composed of a high index silicon monoxide layer on a nanostructured resist layer on a glass substrate. By mounting the substrate at an oblique angle with respect to the evaporation source, the high index material distribution on the nanostructured surface is modified due to shadowing effects. Finite-difference time-domain (FDTD) simulations were performed to predict bulk and surface sensitivities. In order to verify the simulation results we fabricated PCSs at various deposition angles using nanoimprint lithography to replicate a linear grating nanostructure into the resist layer and thermal evaporation for a 60-nm silicon monoxide deposition. The bulk sensitivities of these structures were measured using water-glycerol dilutions. A sensitivity improvement of 281% was obtained for PCSs fabricated at 45° deposition angle compared to normal incidence deposition.