A Practical Apertometer for Solid Immersion Objective

Abstract

A solid immersion objective is generally composed of a truncated solid ball and a commercial objective lens. It features higher numerical aperture (NA) than oil-immersion objectives and thus shows capabilities in some applications requiring higher NA or spatial resolutions. Different from commercial products of oil-immersion objectives and their pre labeled NA values, solid immersion objective is generally home-designed/developed, and the theoretical NA is taken as its practical value, which is not suggested. So far, there has been no proper and commercial apparatus to measure this NA value especially for bare solid immersion objectives. In this work, we propose an extremely simple but very practical approach for the measurement, which inherits the concept of Fourier optics but offers some specific features. Furthermore, we specifically propose an algorithm of multipoint reference to achieve high accuracy of NA value. Detailed comparisons between the proposed algorithm and the previous algorithm of single-point reference are elaborated. An experimental prototype of apertometer for solid immersion objective is built up to verify the proposed approach and algorithm. To the best of the authors’ knowledge, this is the simplest, cheapest, and most practical approach and apparatus for the NA measurement of bare solid immersion objective.