When the light is illuminated to the tip-sample geometry, the thermal and the dipole responses are simultaneously created. Each of the response shows exactly different spectral behavior. The thermal response shows non-dispersive spectrum because of its absorptivity but the dipole response shows dispersive spectra because it is based on the refractive index. There is an interesting debating in this field by developing the photo-induced force microscopy (PiFM). The PiFM measures the force interaction between the sharp metal tip and the sample under a light illumination condition. Because both of the thermal and the dipole interaction may be contributed to the system, the origin of the chemical selectivity of the PiFM is not clear.
Here, we address that the origin of PiFM is contributed by both of the thermal and the dipole response but they can be distinguished with respect to their experimental conditions: pulse width, sample size and tip size. We provide the rigorous theoretical modeling and experimental demonstration. Our understanding can be extend to the recent advances of the opto-mechanical nanoscopy and spectroscopy such as Photo-induced force microscopy, Photothermal infrared microscope (PTIR) and the Peak force infrared microscope (PFIR). We suggest the general theory to understand these opto-mechanical microscopy by using simple effective stiffness concept. Finally we provide the experimental demonstration for the each of the chemical imaging mode.
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.
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.