Improved extraordinary optical transmission though single nano-slit by nano-defocusing

In this work, by using the finite difference time domain (FDTD) method, we have numerically studied the transmission properties of a composite structure consisting of a de-focusing tapered slit and nano-strip. For a fixed wavelength, the dependence of transmission efficiency of light on the structure parameters is demonstrated in detail. we have found that the extraordinary optical transmission through a single tapered metallic nano-slit is enhanced by placing a metallic nano-strip over it, in which tapered slit with large taper angle is used for de-focusing of light and be avoid of Fabry-Perot resonance in straight metallic nano-slit. It is shown that a suitable tapered slit assisted by nano-cavity-antenna can enhance the transmission by about 50% (compared with that in straight slit case). Furthermore, the parameters of the tapered slit (the taper angle and the taper length) hardly change the resonance in the horizontal nano-cavity. The transmission spectrum of the tapered slit can also be tuned by adjusting the taper angle. Such striking characteristic tapered slit has promising applications in designing optical nano-scale device.