Electromagnetic exploration of focusing properties of high-numerical-aperture micro-Fresnel zone plates

Focusing properties of high-numerical-aperture micro-Fresnel zone plates (FZP) have been rigorously investigated at the wavelength from deep ultraviolet to visible based on the combination of the vectorial angular spectrum theory and three-dimensional finite-difference time-domain method. New insights are found. The axial spot size evaluated by full-width-at-half-maximum is in direct proportion to the inverse of the total annulus number. Linearly negative focal shift phenomenon can be observed due to the wavelength dispersion over a finite range of tens of nanometers. A proper wavelength deviation can lead to the maximum focal intensity. A simple method has been demonstrated to probe either multi-focus or a light needle using a composite microstructure based on high-NA FZPs. The findings of this study expand the common understandings of the classic widely-used FZPs.