High numerical aperture singular beam scanning microscopy: preliminary experimental results

High sensitivity and a high speed nanoscale measurement becomes an important subject in modern industry, when analysis of high speed moving nanoscale objects on a surface is required. Recently we have shown a possible approach to this problem using singular beam microscopy. Singular beam microscopy performs scanning by utilization of the relative movement between the focused singular beam and the investigated object. This allows collection of sufficient information for evaluation of various features of the investigated object. Our theoretical results were in good agreement with the experimental results, which showed 20nm experimentally proven sensitivity under a moderate numerical aperture of 0.4. One of the possibilities for sensitivity improvement is increasing the numerical aperture. In this publication we report our progress with a numerical aperture of 0.55. This transition requires rigorous numerical analysis and a more accurate experimental setup. In order to approach the problem of modeling tight singular beam focusing, we developed an extension of the existing Richards-Wolf method which allows evaluation of a 3D tightly focused optical field. The modeling of scattering of tightly focused singular beams can be done by existing electro-magnetic methods. In this work we will present experimental results of detection and evaluation of a phase step in different experimental conditions. The investigated equivalent phase step heights were down to 10nm.