Spiral modulated groove position gratings for the generation of two focused optical vortices with equal intensity

Abstract

By combining a spiral zone plate (SZP) and a modulated groove position grating (MGPG) through a single logic XOR operation, we propose and experimentally demonstrate a single optical element, termed as spiral modulated groove position gratings (SPG), to generate two tightly focused optical vortices with approximately equal intensity. The third-order diffraction light intensity can be reduced from 21.12% of the required first-order diffraction intensity to less than the intensity of background, and the dependence of the unwanted zero-order extinction and high-order diffraction suppression on the period, maximum movement distance and aperture size is investigated. The experimental results are also presented, agreeing well with the theoretical predictions, and confirm that the output beam has the helical phase structures with multiple topological charges. The unique characteristics of the proposed SPG make it attractive for many applications such as particle trapping and optical imaging.