Can the point source method be used for design of sub-wavelength surface plasmon devices?

Integrating optical components into electronic chips needs compressing the dimensions of optical components, which are now nearly thousand times larger than those of electronic components. Surface plasmon-based devices may offer a solution to this size-compatibility problem. Shi et al. introduced a new method, called point source method, to design sub-wavelength surface plasmon devices. To test the validity of this method, we calculated the light transmission through sub-wavelength structures by the finite-difference time-domain (FDTD) method and point source method. Comparing the two results, we found that the correct light distribution through slits can be obtained by the point source method only in the comparatively far zone and the slit widths have to be selected carefully. Phase distributions obtained by the two methods are not the same and the phase difference is not constant for slits with different widths.