Focusing Beyond Diffraction Limit Based On Surface Plasmonic Effects

Surface plasmon polaritons (SPP) wave is an oscillation of free electrons excited by electromagnetic wave at the interface between metal and dielectric. On one hand. SPP wave has shorter wavelength than electromagnetic wave in free space. On the other hand, the notable local field enhancement makes SPP find potential application in many fields, such as biosensing, molecular detection, super-resolution imaging, nanolithography, optical dense storage, and so forth.In this thesis, the focusing structures based on plasmonic lenses and nanoantennas are designed and optimized through theoretical analysis and numerical simulation. Besides, we analyze the generation efficiency of SPP at metal nanoslit, propose a near field focusing lens with breaking through the theoretical diffraction limit, and improve the focusing performances of classical focusing structures.Firstly, the fundamental principle and propagation of SPP, as well as the finite-difference-time-domain (FDTD) method are introduced. Taking the noble metals of Au and Ag as an example, several dispersion models of the metal materials are simply introduced.Secondly, a plasmonic focusing lens constructed by annular metal grating and coupling slots is proposed. We investigate the focusing performances with different depths of the slots and number of the rings. The proposal achieves a far-field focusing with performances enhanced simultaneously.Subsequently, a focusing structure composed of metal nanoantennas is designed based on Fer mat’s Law and Snell’s Principle. The phase shifts from 0-27r rad are realized by different geometries of the nanoantennas, which harness the transmission beam to form a far-field focusing. The proposed structure breaks the diffraction limit. Furthermore, the tri-layered focusing structure can achieve a subwavelength focusing. The focusing performances under different spaces between adjacent layers are also analyzed.Finally, based on the excitation principle of SPP at metal nanoslit, the generation efficiency of SPP at the nanoslit is analyzed and a near-field focusing lens with two semiannular nanoslits is proposed, SPPs are generated at the nanoslits and leave a super-resolution focal spot at the center of the lens. Also, some proposed classical focusing structures are improved to obtain higher efficiency and enhanced focusing performances.