Although a large sensitivity is important in biosensor design, a sharp and distinct resonance will enhance the minimum detectable

shift for an improved VRT752271 research buy detection limit. Therefore, in the design of the step and gradient YH25448 order profile structures, a tradeoff between sensitivity of the resonance position to small changes in refractive index and the resonance intensity was considered. A very small step or gradient refractive index change leads to a very large BSSW sensitivity. However, similar to a WG, the resonance intensity and mode confinement are reduced with a small refractive index contrast between H and L layers due to the reduced mirror strength of the multilayer. For very large refractive index changes within the multilayer, field confinement is increased, resulting in a sharp and distinct resonance; however, BSSW sensitivity decreases as a result of decreased surface area for molecular capture [22]. PX-478 Figure 3 shows both the

simulated (RCWA) and experimental angle-resolved reflectance spectra of an optimized grating-coupled step and gradient index BSW/BSSW sensor. In Figure 3a, the BSW resonance is located at approximately 21° and the single step BSSW mode is located at approximately 25°. In Figure 3b, the BSW mode is located at approximately 15° and the remaining peaks correspond to the different BSSW orders created by the gradient index profile. The different resonance angles are a result of the different refractive index

step and gradient depth profiles used in the optimization. Good agreement is observed between the simulations and experiment. Minor deviations are likely a result of a nonlinear refractive index gradient or step caused by the KOH etch [8]. Both the step and gradient BSW/BSSW designs are suitable for size-selective sensing applications. However, the step index sensor has a higher detection sensitivity due to the single well-confined BSSW resonance, as shown in the field profile in Figure 1b, while the gradient index until sensor with multiple BSSW modes spatially distributed within several high index layers of the multilayer allows for the determination of the depth of infiltration of molecules within the multilayer. Figure 3 Simulated and experimental reflectance spectra of optimized (a) step and (b) gradient index PSi BSW/BSSW sensor in air. The resonance at the lowest angle for each sensor corresponds to the BSW mode while the other resonances are BSSW modes. Simulations show good agreement with experiment, with small error derived from nonlinear refractive index changes within the PSi multilayer. In order to demonstrate the sensing capabilities of the step and gradient index BSW/BSSW, small APTES molecules that bind primarily within the porous matrix and large nanospheres that may only bind onto the surface of the PSi are exposed to the sensors (Figure 4a).