Optical Properties Of Metal Nanostructures

Noble metal (such as Ag and Au) nanostructures have gained wide-spread interest for their strong surface plasmon (SP), which is a collective oscillation of the conduction electrons and can be coupled with incident optical waves in the form of propagating wave or localized excitations. The SP resonance frequency and induced electric field enhancement depend strongly on the size and shape of the nanostructures and the surrounding dielectric environment. For the unique optical properties of SPs, noble metal nanostructures are promising for biosensors, super-resolution imaging, and sub-wavelength optical communication. In this thesis, we have studied optical properties of three Ag nanostructures, and the main results are summarized as follows: (1) The far-field scattering property of a single Ag nanoring is investigated. Under oblique excitation, two-focus scatterings with distinct intensities were observed. We show that the two-focus scatterings result from the interference of far-field scattering light from the ring circumference and that the local field enhancement effect of SPs plays the key role in the focus intensity. By finite difference time domain and numerical integer methods, we calculated far-field scattering and surface plasmons distributions. The results are in good agreement with the experiment.(2) Resonance modes and Q value of a CdS nanowire Fabry-Pérot cavity modulated by single Ag nanoparticle was studied. By placing a single Ag nanoparticle (NP) nearby a CdS NW to form an optical cavity and by adjusting their relative positions, we show that the wavelength and relative intensity of the resonance modes in NW cavity can be systematically tuned. We further show that the NP can enhance the Q factor (by 56%) and induce aÏ€-phase shift of the propagation waves.(3) Surface plasmon polariton (SPP) propagation and surface-enhanced Raman scattering (SERS) induced by SPP was investigated. Light can only be coupled in and out of Ag nanowire at hot-spots such as ends and NP absorption sites. With confocal optical technique, we have successfully detected SERS signal of monolayer molecule on Ag nanowire induced by SPP propagation. The SERS intensity decays in a roughly exponential fashion as a function of SPP propagation distance, and the ability to transfer SERS can be over 10μm, which are in good agreement with theory and previous studies. The development of ultra-high vacuum technique, which provides an ideal environment for high quality sample growth and characterization, has greatly promoted fundamental surface physics and chemistry. In this thesis, we constructed an in-situ optical spectroscopy setup on an ultra-high vacuum system molecule beam epitaxy-scanning tunneling microscope-angle-resolved photoemission spectroscopy system. Raman spectra of clean Si(111)-7×7 was obtained, which is in good agreement with previous works. The reflection spectra of topological insulator Bi₂Te₃ thin films grown by molecule beam epitaxy were measured as a function of thickness (1– 5 nm). A sudden change was observed as the thickness increases from 1 QL to 2 QL. It may be attributed to the topological surface states formation in the films.