Transmission enhancement of bow-tie nano-aperture with double-layered structure

A double-layered nano-aperture on metallic film composed of a large square aperture under a small bow-tie aperture is proposed. Numerical analysis of the proposed structure by using 3-dimensional finite difference time domain method showed that the transmission enhancement factor reached about 245 at 20 nm away from the metal surface compared to the incident intensity. Compared to the single-layered bow-tie aperture, the proposed structure had intensity about three times higher than that of single-layered one at the same resonant wavelength of 1200 nm. In addition, a near-field full width at half-maximum intensity spot size was about 56 nm in the x-direction and 64 nm in the y-direction. The enhancement factor of the proposed structure was mainly depending on the size of the additional square aperture. These properties can be adopted for nano-optical applications such as scanning near-field optical microscopy, optical data storage, nano-lithography, and bio-sensing.