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Nanoscale IR Spectroscopy Based on Molecular Optomechanical Damping


The quest to beat the diffraction limit and map sample optical properties with nanoscale spatial resolution led to new techniques of performing microscopy and spectroscopy. Among these techniques is scanning probe microscopy (SPM). SPM relies on a mechanical sensor (a cantilever) with a tip terminated by a nanometer radius apex that is interacting with a sample surface via van der Waal’s forces which extends a few nanometers from sample surface; the radius of the apex determines the interaction volume which in turn determines the spatial resolution. Coupling the electromagnetic field to the apex focuses it to a volume with nanoscale dimensions limited by the apex radius, which can be exploited to prob sample chemical information with nanoscale spatial resolution. With the advent of the advanced nanoscale fabrication processes, tips with apexes of a few nanometers radius become commercially available making SPM an ideal alternative for the conventional, diffraction-limited optical microscopy and spectroscopy. Aperture and apertureless scanning near field optical microscopy are the 1st demonstrations that combine the SPM with optical light source, where sample optical properties down to a few tens of nm have been resolved. To perform nanoscale spectroscopy using SPM, the cantilever either acts as a nano-scattering object to convert the evanescent near-field into propagating far-field or as a force sensor to directly measure near-field optical force. The near-field optical force can be probed while the cantilever tip is in contact with the sample or in non-contact. Over the last decade, the trend of using a cantilever as a force sensor for spectroscopic application continues to rise largely due to the simplicity of the experimental setup, high SNR, and high special resolution.

Infrared photoinduced force microscopy (IR-PiFM) is becoming the standard technique to probe the near-field optical force in non-contact mode. Here, sample chemical properties can be resolved with special resolution better than 10 nm and with monolayer sensitivity. Since PiFM invention (2010), the contrast mechanism is still under debate. This is largely due to the simultaneous electromagnetic excitations of the photothermal-induced and dipole-induced forces, and their interplay with van der Waal’s forces. All the proposed mechanisms for IR-PiFM assume near-field tip-sample optical interaction is conservative. However, this assumption has not been validated experimentally. In this dissertation, we experimentally investigate the contrast mechanism of IR-PiFM for recording vibrational resonances. We demonstrate that the optically vibrating molecules damp the oscillating cantilever, where the damping force is dependent on sample complex refractive index. Thus, the measured spectroscopic information of a sample is directly related to the wavelength dependent energy lost in the oscillating cantilever.

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