Our FDTD simulations show slight changes in maximum intensity of the evanescent for different points of incidence (see Figure ). For incidence close to the center of the groove (IN1) the intensity is higher than at incidence further from the groove (IN2). This can be explained by different propagation losses at different pathlengths in Si. The difference is mainly due to different propagation loss of different pathlength. Calculated for excitation at 1601 cm -1 . The lateral intensity distribution of evanescent field does not change with distance from the surface (see Figure ).