Optical Property And Its Application Of Film-coupled Metal Nanoparticles In A Thin-film System

In today’s boom of nanomaterials research, noble metal nanoparticles are widely used in applications of bio-chemical sensor, surface enhanced Raman spectroscopy, biomedical sciences, optical data storage and nanophotonics. Localized surface plasmon resonance (LSPR) of metal nanoparticles is excited by external irradiation, due to the resonant oscillation of conduction electrons. It can concentrate electromagnetic (EM) field into an ultra-small volume, as a result, localized field is significantly enhanced. Besides, LSPR is very sensitive to dielectric function of the surrounding environment. When metal nanoparticles are placed near a metal film, owing to the hybridization of surface plasmon resonances, EM field is further confined in the gap (typical several nanometers) between nanoparticle and metal film, which is also called plasmonic gap resonance. As a consequence, localized field is dramatically enhanced and gap resonance is extremely sensitive to the gap distance, which provides us a good platform for studying strong plexcitonic coupling process, fluorescence emission enhancement and surface enhanced Raman scattering. As the gap distance increases, gap resonance becomes weak. Instead the interference is dominating, which leads to obvious changes of optical property of such nanoparticle-film (NP-film) system, including scattering and absorption characteristics. It will contribute to our study of photo-thermal conversion. In addition to single nanoparticle, highly-efficient broadband absorbers in the near-infrared region based on multilayers gold nanorods coupled to a metal film can also be realized.In the first chapter of this thesis, we briefly introduce the backgrounds of LSPR of metal nanoparticles and the optical property of metal NP-film system, including the preparation of gold nanorod and nanocube, LSPR’s characteristic and its potential applications in strong coupling and absorbers, etc.In the second chapter of this thesis, we experimentally and theoretically investigate the plasmonic resonances in individual gold nanorods coupled to a gold film with different gap spacing. The spectral widths, wavelengths, and optical polarizabilities of the peak in measured single-nanoparticle scattering spectra are significantly dependent on the gap distance (in the sub-20 nm domain). When comparing the experimental data with numerical simulations, it reveals that these modifications are the result of the complex hybridization of several dipolar and multipolar plasmon modes strongly localized at the gap. These plasmon gap resonances show distinct resonant and spatial characteristics due to near-field interaction between the nanorods and the gold film. In addition, the excitation of these gap resonances is highly dependent on the gap distance.In the third chapter of this thesis, under different conditions, we simulated and measured the scattering spectra and intensity of gold nanorods placed at varied distances (>50nm) above gold films spaced by a dielectric layer. It shows that scattering property of the nanorod-film (NR-film) system is highly dependent on illumination conditions. It also reveals that the illumination-dependent properties of the NR-film system are caused by the interference by studying the surrounding electric fields of nanorods. Both simulations and experiments show that through optimising the NR-film distance, scattering magnitudes can be greatly enhanced up to about 20 times for certain illumination conditions.In the fourth chapter of this thesis, we demonstrate the strong plexcitonic coupling on single film-coupled nanocube cavities that can confine light fields within an ultra-narrow nanogap (several nanometers). This deep sub-wavelength mode volume enables a considerable enhancement of light-matter interaction, significantly modifying far-field scattering patterns of the cavity, indicating that the spatial characteristics of gap resonances can be altered in the strong coupling regime. Simulation results agree well with experimental findings. We believe that our work may pave the way for studying the strong coupling process, with potentials to be applied in cavity Quantum Electrodynamics.In the fifth chapter of this thesis, we demonstrate a highly-efficient ultra-broadband absorber for the 900-1,600 nm wavelength range. Here we employ an inexpensive droplet evaporation method to create patterns of gold nanorods dispersed on a gold film spaced by a thin dielectric layer. The major factor for the broad absorption band arise from the diversity of the complicated random stacking of the chemically-synthesized gold nanorods. Such a metamaterial absorber may open up a new perspective for cost- effective manufacture of large-area black metasurfaces.At last, in the sixth chapter of this thesis, we summarize the main work in this thesis and provide perspectives to the future research works.