Ultrafast Photo-induced Force Microscopy
Photo-induced force microscopy is a novel technique where mechanical detection with a cantilevered probe replaces the detection of photons to investigate optically induced processes and states. A theoretical and experimental analysis is performed here of the forces present in photo-induced force microscopy operated in tapping mode, which reveals two dominant optically induced forces, the gradient force and the scattering force. Force-distance curves are reconstructed from experimental amplitude and phase information for glass, gold nanowires and molecular clusters of silicon naphtalocyanine samples. The scattering force is shown to be insensitive to both nano-scale tip-sample distances and sample polarizability and is dependent on the form of the tip. The gradient force demonstrates a z-4 tip-sample distance dependence, localized to a few nanometers, and is strongly dependent on the polarizability of the sample which enables spectroscopic imaging through force detection. The different distance-dependence and polarizability-dependence of the gradient and scattering forces give rise to a complex force-distance curve which determines imaging contrast along with the cantilever set-point, knowledge of which is essential for image interpretation.
Photoinduced force microscopy is then extended to ultrafast pump-probe measurements of the material's nonlinear polarization. It is demonstrated that the photo-induced force is sensitive to the same excited state dynamics as measured in optically-detected experiments. Ultrafast pump-probe force microscopy brings nanoscale spatial resolution with non-optical detection to time-resolved studies of excited state dynamics with sensitivities approaching the single molecule limit.