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We experimentally investigated the contrast mechanism of infraredphotoinduced force microscopy (PiFM) for recording vibrational resonances.Extensive experiments have demonstrated that spectroscopic contrast in PiFM ismediated by opto-mechanical damping of the cantilever oscillation as theoptical wavelength is scanned through optical resonance. To our knowledge, thisis the first time opto-mechanical damping has been observed in the AFM. Wehypothesize that this damping force is a consequence of the dissipativeinteraction between the sample and the vibrating tip; the modulated lightsource in PiFM modulates the effective damping constant of the 2nd eigenmode ofthe cantilever which in turn generate side-band signals producing the PiFMsignal at the 1st eigenmode. A series of experiments have eliminated othermechanisms of contrast. By tracking the frequency shift of the PiFM signal atthe 1st cantilever eigenmode as the excitation wavenumber is tuned through amid-infrared absorption band, we showed that the near-field optical interactionis attractive. By using a vibrating piezoelectric crystal to mimic samplethermal expansion in a PiFM operating in mixing mode, we determined that theminimum thermal expansion our system can detect is 30 pm limited by systemnoise. We have confirmed that van der Waal mediated thermal-expansion forceshave negligible effect on PiFM signals by detecting the resonant response of a4-methylbenzenethiol mono molecular layer deposited on template-stripped gold,where thermal expansion was expected to be < 3 pm, i.e., 10 times lower thanour system noise level. Finally, the basic theory for dissipative tip-sampleinteractions was introduced to model the photoinduced opto-mechanical damping.Theoretical simulations are in excellent agreement with experiment.

Observation of Nanoscale Opto-Mechanical Molecular Damping; Origin of Spectroscopic Contrast in Photo Induced Force Microscopy