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Scattering scanning near field optical microscopy (s-SNOM) is a useful tool for providing optical resolution well below the diffraction limit with chemical selectivity. s-SNOM relies on recording the scattering light of a scanning probe tip, coupling the near-field interaction to a far field photo-detector. Photo-induced force microscopy (PiFM) is a much newer technique that also provides chemical resolution well below the diffraction limit. In PiFM, the signal arises from measuring the light induced force on a scanning probe tip of a sample interacting with laser light. It measures and records all information in the near field, with no need for a photo-detector. In this presentation, we describe results comparing and contrasting s-SNOM and PiFM displaying the strengths and weaknesses of both methods.

Raman spectroscopy can provide useful chemical information of nanostructures and molecules. We combine Raman spectroscopy with atomic force microscopy, through dual color photo-induced force microscopy (PiFM). In this modality, images with Raman contrast can be generated with a spatial resolution well below 10 nm at ambient temperature and pressure. Here we utilize this technique to visualize molecules on surfaces with high spatial and temporal resolution. Compared to previous Raman sensitive PiFM measurements, we employ femtosecond pulses and show that this technique is highly sensitive to the stimulated Raman scattering interaction in the molecule.

We examine the mechanical eigenmodes of a quartz tuning fork (QTF) for the purpose of facilitat- ing its use as a probe for multi-frequency atomic force microscopy (AFM). We perform simulations based on the three-dimensional finite element method (FEM) and compare the observed motions of the beams with experimentally measured resonance frequencies of two QTF systems. The com- parison enabled us to assign the first seven asymmetric eigenmodes of the QTF. We also find that a modified version of single beam theory can be used to guide the assignment of mechanical eigen- modes of QTFs. The usefulness of the QTF for multi-frequency AFM measurements is demonstrated through photo-induced force microscopy (PiFM) measurements. By using the QTF in different con- figurations, we show that the vectorial components of the photo-induced force can be independently assessed, and that lateral forces can be probed in true non-contact mode.

Photo-induced force microscopy (PiFM) is a new scan probe method that enables imaging with spectroscopic contrast at the nanoscale. The operating principle of PiFM is based on the coupling between a sharp atomic tip and a polarizable object, as mediated by the electromagnetic field in the vicinity of the tip-sample junction. In this contribution, we develop a description of the photo-induced force in the limit where the tip and object can be approximated as dipoles. This description provides an insightful picture of the forces at play in the tip-sample junction in terms of the gradient and scattering forces. We consider various approximations that are relevant to experimental conditions. The theoretical approach described here successfully explains the previous spectroscopic PiFM measurements in the visible and in the near-IR range, and the anticipated spectral information that can be retrieved under mid infrared illumination.

A new method is presented for visualizing the electric field distributions associated with propagating surface-plasmon-polariton (SPP) modes directly in the near-field. The method is based on detecting the photo-induced gradient force exerted by the evanescent field onto a sharp and polarizable tip. Using a photo-induced force microscope (PiFM), images of propagating SPPs are obtained on flat gold surfaces.