Radiative, Chiroptical & Thermal effects at Illuminated Nanoprobes
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Radiative, Chiroptical & Thermal effects at Illuminated Nanoprobes

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Light-based scanning-probe microscopy (SPM) uses a sharp tip with an apex of nanoscopic dimensions for enabling probing at the nanoscale. The region near the tip’s apex is called the nanoprobe. When the nanoprobe is brought in close proximity to the sample and it is illuminated, several electrodynamics effects are at play in the tip-sample nanojunction, andthese effects can be used to unveil numerous physical and chemical properties of the sample. Even in the absence of any target sample, the electromagnetic interaction at illuminated nanoprobe can bring out interesting properties of the incident light at a scale that is beyond the reach of conventional optical microscopy tools. Whereas the goal of nanoscopic probing is to reveal properties of the sample, the intrinsic response of the nanoprobe itself can sometimes overwhelm the sample’s response. This dissertation highlights the above aspects by studying several electrodynamic effects that take place near the illuminated nanoprobe. The first part of the dissertation discusses the near-field enhancement and far-field radiation properties of nanoprobes in tip enhanced Raman spectroscopy (TERS). We numerically design and optimize gold tips decorated with vertical grooves at the tip apex. The proposed designs constitute a feasible route toward a tip fabrication process using focused ion beam (FIB) milling that promises ∼ 10 fold stronger TERS signal compared to a conventional TERS tip. In the second part of the dissertation, we theoretically investigate the photo-induced force on an illuminated nanoprobe with nonzero chirality above a bare glass substrate . We find the differential force due to left circularly polarized (LCP)/right circularly polarized (RCP) is directly related to the chirality of the illuminating light, also known as the helicity density. Under realistic experimental conditions, including the illumination intensity, tip dimension, and the chirality parameter of the tip, we predict that force values can reach several hundreds of fN, just above the noise floor of common force-based SPM techniques, including photo-induced force microscopy (PiFM). Our findings show that a direct characterization of optical chirality at the nanoscale is possible, which may have implications for chiro-optical applications such as enantiomer sorting. In the third and final part of the dissertation, we investigate the thermal expansion of the illuminated nanoprobe and determine the expected relaxation dynamics at the nanoprobe’s apex due to light modulation. Finally, we also explore the effect of the tip expansion in PiFM measurements. Our analysis provides important information on the thermal response of the illuminated nanoprobe as well as its impact on PiFM’s sensitivity to the thermal response of the sample.

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