Sensitivity optimization of injection-molded photonic crystal slabs for biosensing applications

For label-free assays employing photonic crystal slabs (PCSs), the sensitivity is one of the most important properties influencing the detection limit. We investigate the bulk sensitivity and the surface sensitivity of 24 different PCSs fabricated by injection molding of PMMA and subsequent sputtering of a Ta2O5 high-index layer. The duty cycle of the linear grating is varied in steps of 0.1 between 0.2 and 0.7. Four different Ta2O5 layer thicknesses (89 nm, 99 nm, 189 nm, 301 nm) are deposited. Both bulk and surface sensitivity are optimal for a Ta2O5 layer thickness of 99 nm. The maximum bulk sensitivity of 138 nm/RIU is achieved for a duty cycle of 0.7, while the maximum surface sensitivity of 47 nm/RIU is obtained for a duty cycle of 0.5. Good agreement between experimental results and finite-difference time-domain (FDTD) simulations is observed. The PCSs sensitivity is linked to the mode intensity distribution.