Manipulating Far-field Optical Responses in Plasmonic Nanostructures.

摘要

Metallic nanostructures can confine and manipulate electromagnetic fields at the subwavelength scale through the excitation of surface plasmons. Depending on the size and shape of the nanostructures, localized or propagating surface plasmons can be excited on the metal surface. The geometry of nanostructures is crucial in the manipulation and amplification of both the far-field responses and the near-field field enhancements. This dissertation explores, both experimentally and theoretically, the manipulation of different aspects of far-field optical responses of plasmonic nanostructures including: (1) spectral responses, (2) nonlinear absorption, and (3) 3D light patterns. Through the control of different geometric parameters such as thickness, width, periodicity and particle shapes, we demonstrated tunable far field behaviors in different nanostructures. First, we provide direct evidence that both out-of-plane lattice plasmon and surface plasmon polaritons can be excited in a 1D nanograting and the two types of resonances can be controlled through the grating thickness and the line width. We further demonstrated that out-of-plane lattice plasmon is a type of Fano interference between diffraction modes and the localized surface plasmon in the thickness direction. Out-of-plane lattice plasmon can be tuned over a broad wavelength range from visible to near-infrared by manipulating the diffraction modes through the grating periodicity and azimuthal angle of the excitation. In addition, lattice plasmon is also sensitive to the out-of-plane localized surface plasmon mode which can be controlled through the grating thickness and the dielectric environment. Using nanograting with appropriate thickness, we demonstrated out-of-plane coupling in an index-mismatched environment. Moreover, we studied the nonlinear responses of different shaped nanoparticles with similar linear properties using Z-scan and pump-probe techniques. We discovered shape-dependent nonlinear spectral and time responses in the far-field induced by hot-electrons. Finally, we investigated the 3D light patterns generated from periodic hole array and we demonstrated that the ratio between the periodicity and wavelength is crucial to the 3D light patterns. These studies not only explore the unconventional plasmonic properties of different plasmonic nanostructures but also provide a comprehensive understanding on how to tune the far-field responses through the control of geometric parameters, which could be applied to chemical sensing, laser-triggered drug release and 3D lithography.