Light Scattering Effects Of Plasmonic And Magnetic Nanoparticles And Their Arrays

The history of light scattering of small particles can be traced back to the ancient time.At that time,people already started to study the origin of the rainbow and the color of the sky.Today,this field continues to generate exciting theoretical and experimental advances such as optical nanoantennas and Fano-resonance,leading to many new applications in biology,photonics and information technologies.Meanwhile,the fabrication tools for complex nanoparticle systems has become accessible to researchers,such as the "top-down" lithography methods and the "bottom-up" selfassembly technologies.The emergence of advanced optical characterization tools,e.g.objective lens with large numerical aperture,detectors with high sensitivity and new imaging methods,also makes the study on the light scattering of complex nanoparticle system become possible.This thesis mainly focuses on the light scattering effects of single nanoparticles and nanoclusters/arrays composed of nanoparticles.The optical response of the nanoparticle systems to the external light field is analyzed using both theoretical models and microscopic spectrographs.The purpose is to provide theoretical and scheme support for the applications of nanoparticles in actual scenes.In the first part of the thesis,the theoretical model,numerical tools and experimental methods for light scattering of nanoparticles is systematically introduced.Although nanoparticles with different materials,shapes and sizes have different scattering effects,their inner physics can be described using a unified theoretical framework.These theoretical model and experimental methods lay the foundation for study of the light scattering effects of complex nanoparticle systems in the later parts of this thesis.In chapter 2,the polarization effects of a single spherical plasmonic nanoparticle in light scattering were studied.It is generally believed that the depolarization effect in light scattering of a nanostructure is mainly caused by its anisotropy,and in the case of an isotropic structure,e.g.a nanosphere,the depolarized signal will be too weak to be detected.We experimentally demonstrate that even a totally symmetric gold nanosphere exhibits sophisticated depolarization effects.The scattering image is not only dependent on the detailed excitation-observation polarization configuration but also related to the numerical aperture of the observation system.The depolarization effect of a single gold nanosphere was also confirmed with a reflective polarized light microscope.This is contrary to the commonly used image interpretation theory in polarized light microscopy that the image contrast is solely caused by the anisotropy of the sample.In chapter 3,we investigated the light scattering of an individual nanocluster composed of gold nanoparticles assembled on a nanostructured substrate,and specifically studied how the substrate geometry influence on the scattering spectrum.An explicit model is built with the help of the Green's tensor theory,showing that there are two distinct types of substrate effects,namely,the interferometric scattering caused by the local corrugation and the spectral modulation caused by the global features(i.e.,stratified substrate in this case).The result predicted by the model agrees with the experimental results well,providing a simple yet quantitative tool for the spectral interpretation of plasmonic nanostructures with complex substrates.The last system investigated in this thesis is a 1D tunable optical antenna consisting of an array of vertically aligned magnetic nanoparticles.Here,magnetic nanoparticles was assembled into a chain in the external magnetic field and anchored on a patterned substrate.The scattering signal of a single magnetic nanoparticle chain was characterized by the dark-field microscope.The theoretical analysis shows that the scattering patterns of the antenna is sensitive to the experimental parameters,including the polarization,wavelength,incident angle of the excited light and the interparticle distance between the magnetic particles which is determined by the strength of the external magnetic field.The parameter-dependent scattering phenomena were indeed observed in angular pattern measurements of single antennas with different wavelengths and external magnetic fields.We believe that these phenomena can be potentially used in compact sensing and light modulation devices.