The diffraction by a small aperture

We study the diffractions by one small circular aperture, and numerically calculate the diffraction intensity distribution with the help of the finite-difference time-domain (FDTD) technique. The diffraction patterns are presented, the variation of the near-field diffraction with the radius and the propagation distance can be observed clearly. The one-dimensional distributions on the axis, along the boundary shadow and the transverse direction are discussed too. These results are also compared with to ones obtained according to Rayleigh–Sommerfeld diffraction and the Fresnel approximation. We think that this work does not only enrich the diffraction theory of the circular aperture, but it is also helpful for the applications of the small holes in near field.