Institute for Nuclear Problems, Беларусь

As is well known, introducing astigmatism into an optical vortex beam is one of the simplest and most effective ways to remove the degeneracy of an n-fold degenerate Laguerre–Gaussian beam. In particular, when a thin astigmatic Fresnel zone plate (aFZPs) is illuminated by a plane wave carrying an optical vortex, the resulting intensity distribution at the focal plane consists of a series of isolated intensity minima arranged along a straight line inclined at an angle of ±π/4 with respect to the zone plate axes. The number of intensity minima is equal to the absolute value of the topological charge, while the orientation of the line determines the sign of the charge. In this work, a novel liquid-crystal aFZP-based optical element was investigated both theoretically and experimentally. It was demonstrated that the operating state of the fabricated LC-FZP can be electrically switched by varying the applied voltage, enabling selective operation either in the beam-transmission mode, in which the incident optical vortex propagates predominantly without focusing, or in the focusing mode, in which the beam is efficiently diffracted into the desired diffraction order for topological charge identification. Notably, the twist–planar design of the proposed element enables a first-order diffraction efficiency approaching the theoretical limit of 40.5% for binary phase FZP. The experimental results are in excellent agreement with the theoretical predictions and demonstrate the potential of the proposed device for applications in electrically tunable optical tweezers and microparticle manipulation systems, as well as in optical communications and quantum information technologies. This work was supported by the Belarussian Republican Foundation for Fundamental Research under the grant no F25ME-005 and State Program for Scientific Research “Convergence 2030” (Task 3.1.04.2).