Absorption-Based Spectroscopy and Microscopy on Nanostructures

Spectroscopy and microscopy on nanostructures has been a heated research area for the past few decades. Among the different strategies, absorption-based techniques attracted a large amount of attention. Since absorption effect scales with volume while scattering effect scales with volume squared, for particles with sizes in the nanometer range, absorption is usually stronger than scattering. There are two techniques that are mainly discussed in this thesis, Spatial Modulation Spectroscopy (SMS) and Photothermal Heterodyne Imaging (PHI). In the first study, optical trapping was integrated with SMS to investigate the absorption behavior of gold nanoparticles in a homogenous aqueous environment at the single particle level. The linewidths of the absorption spectra were studied carefully and large electron-surface scattering effect was observed, possibly due to chemical interface damping. In the second studies, PHI was applied in the mid-infrared range so that the spatial resolution of mid-infrared imaging was improved by more than ten times, from the current state-of-the-art 5 mm to what was presented in this thesis of 0.3 mm. This mid-infrared photothermal imaging technique (MIR-PHI) has been demonstrated on a variety of soft matter systems, including polystyrene nanoparticles, photoresist polymer patterns, and single bacterium cells. High sensitivity (signal-to-noise >100) and flexible field-of-view (5 x 5 mm to 100 x 100 mm) was also achieved. In order to understand the fundamental mechanism that gives rise to the MIR-PHI signal, finite element analysis (FEA) was performed with Comsol Multiphysics. Simulations revealed that the two mechanisms that contribute to the overall signal, refractive index change and thermal expansion, counteract with each other. For polystyrene, refractive index change is more prominent than thermal expansion. Also, the medium effect that was the dominating effect in conventional photothermal experiments is not as important in this setting anymore, especially for the larger particles. MIR-PHI might find its potential in stain-free histology, as well as other biomedical applications.