Study On The Focusing Performance Of2D Metallic Nanoslit Array

Surface plasmon polaritons are a special kind of electromagnetic field excited bythe interactions of light and free-electrons on the metal surface. It is greatly confined onthe metal surface and could propagate in subwavelength scale, and thus attracted manyresearch interest. The application of SPP enables the realization of integrated photonicdevices beyond the difraction limit. Much efort has been paid to develop SPP-basedsubwavelength devices in recent years because of the increasing demand for miniaturiza-tion and integration of modern optical device. Among them, the metallic nanoslit arrayhas received special attention due to its unique ability in spatial field modulation and sub-wavelength focusing. Based on the basic theory of SPP and using numerical simulationmethods, we investigated the focusing performance of several kinds of typical2D metallicslit lenses which are designed based on diferent mechanisms. The main content of thisthesis are summarized as follows:First, the focusing performance of small f-number2D metallic lens with depth-modulated slits is studied. Metallic slits filled with dielectric are designed to producedesired optical phase retardation. Numerical simulation is performed through the finite-diference time-domain (FDTD) method. The focal spot size, the difraction efciency,the real focal position, and the normalized transmitted power are presented. Diferentfrom the conventional dielectric lens, the metallic lens can be used as a pure difractiveelement without energy loss brought by the light refraction at curved surfaces and totalinternal reflection. For the traditional dielectric lens, decreasing the f-number results insmaller radius of curvature. The energy loss brought by the light refraction and inner totalreflection will increase consequently. The difraction efciency of the designed metallicnanoslit lens with a small f-number is larger than that of dielectric lens with the sameshape.Second, a theoretical analysis based on scalar2D difraction theory about the recent-ly reported focal-shift phenomena in planar metallic nanoslit lenses is given. An axial in-tensity formula is obtained under Fresnel approximation and used for the analysis of focalperformance in the far field zone of lens. It turns out that the relative focal shift is depen-dent with the Fresnel number only. The influences of the lens size, preset focal lengthand incident wavelength are all come from the change of Fresnel number. The total phase diference of the lens is almost equal to the Fresnel number multiplied by π. Numericalsimulations are performed using FDTD method and show agreement with the theoreticalanalysis. In addition, using the theoretical formula assisted by simple numerical method,we provide calculation results about the real focal position for the previous literatures.The results show great agreement with the experimental measurements provided by theliteratures.Third, we propose a binary plasmonic lens based on metallic nanoslit array with d-iferent fillings. The phase range of π is achieved by changing the refractive indices ofthe filling materials in the slits. These index-modulated slits are demonstrated to have u-nique advantages in phase delay compared with the width-modulated ones. The proposedbinary structures can keep the advantage in phase modulation and reduce the difculty infabrication. Lenses with diferent functions, such as focusing light to one or two points orcorrecting oblique incident light, are designed using simulated annealing algorithm andchecked by finite-diference time-domain method.Fourth, a gradient index (GRIN) diverging lens is embedded into a metallic slit arrayto form a gradient index metallic lens. Although the index distribution is opposite to thatof the traditional gradient index converging lens, the dispersive relations are similar. Wedemonstrate numerically that such gradient index metallic lens can converge a incidentplane wave into a deep-subwavelength focus. The focusing behavior is investigated andcompared with the traditional converging GRIN lens. In addition, the external focusing isalso investigated both numerically and analytically.Fifth, a curved metallic slit array constructed by alternately stacking metal and di-electric arcs is considered. The light rays do not propagate parallel to the slits but deflectto inner boundary of the array. The conformal transformation is used to study the dis-persive relation of the curved metallic nanoslit array. The dispersive relation is similar tothat of a monotonically gradient index lens, which demonstrates the focusing ability ofthe curved nanoslit array. For a incident plane wave, we verified numerically that a focusof deep-subwavelength can be achieved in the inner slit. The numerical simulation resultsshow that the focusing position along the arc can be controlled by altering the incidentwavelength or the angle of incidence.