Tip Enhanced Raman Spectroscopy (TERS) has proven to be an important tool for nanoscale imaging with chemical specificity. In contrast to TERS where the molecule/s under study are excited in the optical near-field, but the scattered photons are detected in the far-field by using the sharp metal probe as an optical antenna, Photon-Induced Force Microscopy (PIFM) is an all-near-field measurement. We present recent advances in stimulated Raman nanoscopy and spectroscopy of molecules in a plasmonic gap, excited without resonant electronic enhancement, measured using near-field photon-induced forces, eliminating the need for far-field optical detection. Photon-induced interaction force between the sharp metal coated silicon tip of an Atomic Force Microscope (AFM) and a sample resulting from stimulated Raman excitation was detected. The molecules were imaged by vibrating an AFM cantilever on its second flexural eigenmode enabling the tip to be controlled much closer to the sample, thereby improving the detected signal-to-noise ratio when compared to vibrating the cantilever on its first flexural eigenmode. We present the measured Raman vibrational spectra of azobenzene thiol and l-phenylalanine which agreed well with measured data from bulk materials. We imaged 30 nm diameter gold nanoparticles functionalized with a self-assembled monolayer of 4-nitrobenzenethiol molecules. The maximum number of molecules detected by the gold-coated nano-probe at the position of maximum field enhancement could be fewer than about 42 molecules. We demonstrate the implementation of stimulated Raman nanoscopy measured using photon-induced force with non-collinear pump and stimulating beams which could have applications in polarization dependent Raman nanoscopy and spectroscopy and pump-probe nano-spectroscopy. We also discuss using PIFM as a technique to sort and select best performing metal coated tips for further use in tip-enhanced experiments. The use of ultrafast pulsed lasers and a rough vacuum to operate the AFM could push the detection sensitivity towards the single molecule limit.<br/>

Advances in stimulated Raman nanoscopy and spectroscopy measured using photon-induced forces

2019 Fall National Meeting

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