Experimental demonstration of near-field focusing of a phase micro-Fresnel zone plate (FZP) under linearly polarized illumination

A high numerical aperture binary phase micro-Fresnel zone plate (FZP) is designed and fabricated on a glass substrate by using a focused ion beam technique. Focusing characteristics of the phase micro-FZP are measured by a near-field scanning optical microscope using linearly polarized light as an illumination source. It is found that an asymmetric spot with subwavelength beam width and elongated depth of focus can be obtained from the phase micro-FZP. Furthermore, the measurement is shown to be consistent with the calculation result. Further, the tolerance in fabrication errors like tilt of side walls on focusing is discussed with numerical simulations.