Observation of Nanoscale Refractive Index Contrast via Photoinduced Force Microscopy

Near-field optical microscopy (NSOM) is a scanning probe technique that allows optical imaging of sample surfaces with nanoscale resolution. Generally, all NSOM schemes rely on illuminating the sample surface and collecting the localized scattered light resulting from the interaction of the microscopes nanoscale probe with the sample surface in the illuminated region. Currently, a new set of nanospectroscopic techniques are being developed using Atomic Force Microscopes to detect optical interactions without detecting any light. One of these approaches is photoinduced force microscopy (PiFM), where local optical forces, originated by the illumination of the tip sample region, are mechanically detected as forced oscillations of the cantilever of an atomic microscope operating in a multifrequency mode. In this article we show high resolution nanoimaging via PiFM with a contrast only related to the local refractive index of a sample specifically designed to unambiguously decouple morphology from optical response at the nanoscale. Imaging lateral resolution better than 10 nm is obtained, and the optimization of the contrast mechanism is described. Our results represent a step forward in understanding the potential of the PiFM technique, showing the possibility of high resolution imaging of the local polarizability of the sample and subsequently using the mechanism to explore complex spectral behavior at the nanoscale.