Design Of Metasurface-based Optical Waveplate And Surface Plasmon Coupler

Due to its extraordinary capability on shaping the wavefront of light beams, metasurface has been received widespread atterntion and many significant developments have been achieved in this area. As compared to conventional crystal-based optical components, metasurface optical components have several advantages:First, it can control the shape of wavefront within subwavelength thickness. Second, all optical energy can be concentrated to selectedly single diffraction order. Third, both the electric and magnetic components of light can be controlled independently, so that the impedance of the metasurface can be matched to that of the vacuum, leading to reflectionless and high efficiency optical components. Finally, metasurface can be readily integrated with gain mediums, non-linear optical materials and electrical devices.The basic idea of metasurface is based on the generalized Huygen’s principle (surface equivalence principle). It is frequently composed of "Huygen’s point sources", which are constructed using specific nanostructures supporting optical resonances. By tuning the resonant amplitude and abrupt phase changes (the phase differences between scattered and incident light) introduced in the resonant process from the metasurface, we can control the propagation, polarization and beam profile of the light.Nanostrucutres of noble metals and dielectric mediums with high refractive indexs support the resonances at optical frequencies, giving rise to a strong scattering, absorption and localization of incident light. Therefore, they have been widely used in various areas such as nanophtonics, metamaterials, optoelectronic devices, optical spectroscopy, optical sensor, to name a few. In this dissertation, by harnessing the localized surface plasmon resonances of noble metal nanostructures and Mie resonances supported by the nanostructures of dielectric mediums with high refractive index, we design and investigate high efficieny waveplates operated at visible and near-infrared frequencies and unidirectional surface plasmon polariton coupler. The contents are briefly introduced in the following.In chapter one, we first give an introduction to the basic properties of localized surface plasmon and surface plasmon polariton. We also analyze the scattering characters of noble metal nanostructures, including scattering cross section, angle dependence of scattered light and abrupt phase change introduced in the scattering process. We then briefly review the design principle and the latest developments of metasurfaces. Finally, numerical simulation method based on Finite Difference Time Domain is introduced, followed by several problems in simulations that should be taken into consideration.In chapter two, we design a reflective half-waveplates operated at optical frequencies, using a structure composed of patterned metal nanoarray/insulator/metal film. We find that, by designing and tuning properly the geometric parameters of the structure, the operating bandwidth of the waveplate can cover the whole band of the visible light. Moreover, we show that the bandwidth of the device can be broadened further by a supercell design. We also propose an average response theory to account for this phenomenon.In chapter three, based on the generalized Huygen’s principle, we design a quarter waveplate with high transmission efficiency. The device is composed of a single layer of subwavelength Si nanowires array. We analyze the resonant modes supported by the single Si nanowire, including electric dipole and magnetic dipole resonances. We reveal that both resonant modes play important roles in the metasurface with high transmittivity. Additionally, we show that the performance of the quarter waveplate can in principle be tuned and improved through optimizing the parameters of the nanowire arrays, making the device operating at the telecommunication frequencies with high efficiency.In chapter four, we design a SPP coupler composed of a column of vertical L-shaped antennas. It is shown that, illuminated with circularly polarized light, the coupler can launch unidirectional SPP and SPP’s propagation direction can be controlled by the polarization state of the incident light. We analyze the resonant modes of L-shaped antenna and interpret the roles of the induced electric dipole moments in the unidirectional SPP launching. Moreover, we propose a SPP interference model, which can be used as a guideline to design and optimize the unidirectional SPP device with better performance.In chapter five, we prospect the challenges and the future opportunities in the research area of metasurface.