Imageless microscopy of surface patterns using optical waveguides

Recent experiments using a grating coupled low-index nanoporous silica supported monomode waveguide have demonstrated that living cells can cause heavy distortion of the grating coupler resonance lines, in some cases even leading to a separation into two resonance peaks. These findings stand in contrast to previously reported data, where simple peak broadening was observed during cell attachment and spreading using less sensitive waveguide designs. In order to explain these observations, we apply the local interference method to simulate the effects of inhomogeneity patterns on the surface of grating coupled planar optical waveguides and obtain the resonant peaks for the modes. It is shown that analyte inhomogeneities affect both the position and shape of the resonant peaks. Depending on the deposited cell or domain size, refractive index contrast and waveguide design, peak shift, peak deformations or peak splitting can be observed. On the basis of simulation data, characteristic parameters of the resonant peaks such as peak width at half maxima, overall width, central position and peak integral are connected for the first time to quantitative parameters of the inhomogeneity patterns; like analyte covered sensor area, surface averaged effective refractive index and domain size. Our results indicate that by careful investigations of the incoupling resonant peaks, quantitative information about sample inhomogeneities at the micrometer scale can be obtained, thus allowing for a new generation of simple, low cost, label free and imageless optical sensors, which are well suited for high throughput screening applications.