Beam focusing by metallic nano-slit array containing nonlinear material

A two-dimensional nonlinear finite difference time domain (NFDTD) method is performed to investigate the beam focusing phenomenon in metallic nano-slit array containing nonlinear material. Illuminated by an incident TM-polarized Gauss beam, the arrayed nano-slits transport electro-magnetic energy in the form of surface plasmons (SPs) and provide desired phase retardations for beam focusing, owing to the nonlinear response. The position of focus can be actively controlled by parameters of incident beam, such as the intensity and the beam waist. The physical origin of the focusing effect is discussed by considering SPs mode and nonlinear optical theory.