Beam focusing by tapered metallic nano-slits

In this study, we discussed the influence of tapered metallic subwavelength slit array with variant width of slits on beam focus properties. In order to analyze the phenomenon, a finite-difference time-domain numerical algorithm and phase matching theory were adopted for computational numerical simulation of the nano-structures. The structures were flanked with the penetrated slits through a metal film coated on a quartz substrate. We found that the phase delay of each slit will grow with the decrease of slit width. Optical transmission of each slit can be adjusted by using an optimum choice of the taper angle, taper slit width and the wavelength of incident light. So by optimizing the slit width of the input and output apertures, the trapezoidal slit array can show focusing property. The focal width and focal intensity can be controlled effectively by regulating the aperture width of slits. These results are very encouraging for further study on nano-optics and the applied planar lenses.