Near-field focusing properties of zone plates are investigated in the visible regime by a 3-dimensional finite-difference time-domain method. It is shown that Frensel zone plates (FZPs) with metallic coatings can achieve subwavelength focusing in the visible wavelength. The characteristics of subwavelength focusing are found to be independent of the type of metal coatings used. All the FZPs exhibit similar shift in focal length and depth of focus when compared with classical calculations. These results indicate that plasmonic waves do not contribute to subwavelength focusing. Instead the subwavelength focusing characteristic is attributed to the interference of diffracted evanescent waves from a large numerical aperture. It is found that the near-field focusing of FZPs suppresses higher order foci such that the corresponding diffraction efficiency is improved. The use of phase zone plate structured on glass without opaque coating is proposed to improve the diffraction efficiency of subwavelength focusing.

An analytical model is developed to study the subwavelength focusing characteristics of a binary phase Fresnel zone plate (FZP). The model shows that high numerical-aperture phase FZP under the illumination of linear polarized light produces rotationally asymmetric focal spot with beamwidth varying from 0.36λ to 0.79λ, where λ is the wavelength. On the other hand, rotationally symmetric focal spot with minimum beamwidth of 0.39λ can be obtained from the illumination of radial polarized light.

In this paper, we demonstrate enhanced light trapping by self-organized nanoripples on the germanium surface. The enhanced light trapping leading to high absorption of light is confirmed by the experimental studies as well as the numerical simulations using the finite-difference time-domain method. We used gallium ion (Ga+) focused ion beam to enable the formation of the self-organized nanoripples on the germanium (100) surface. During the fabrication, the overlap of the scanning beam is varied from zero to negative value and found to influence the orientation of the nanoripples. Evolution of nanostructures with the variation of beam overlap is investigated. Parallel, perpendicular, and randomly aligned nanoripples with respect to the scanning direction are obtained via manipulation of the scanning beam overlap. 95% broadband absorptance is measured in the visible electromagnetic region for the nanorippled germanium surface. The reported light absorption enhancement can significantly improve the efficiency of germanium-silicon based photovoltaic systems.

Weaving nanostructures with site-specific ion induced bidirectional bending and a typical 3D folded nanostructure in the form of a mesh.