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.
Junghoon Jahng, "Tip-enhanced thermal expansion and dipole interaction in tip-sample geometry (Conference Presentation)," Proc. SPIE 10549, Complex Light and Optical Forces XII, 1054910 (Presented at SPIE OPTO: February 01, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2291603.5751542201001.
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