Abstract
Abstract We propose a vortex beam generator based on a nanometer spiral slit and explore the propagation rule of the topological charge. Compared to the common methods of generation of a vortex beam with a fixed topological charge, the optical vortex generated by the proposed vortex beam generator has the topological charge varying with the propagation distance. The value of topological charge can be modulated by the geometric charge of the spiral slit and the propagation distance. Theoretical analysis predicts the variation rule of the topological charge of vortex beam in the near field, and numerical simulations and experimental measurement verify the proposed scheme. Discussion on the shape and structure of the spiral slit is also presented. This work provides the theoretical foundation for the generation of a vortex field with variable topological charge. The simple geometry of the vortex beam generator and the flexible modulation of the topological charge must inspire applications of the vortex beam.
Highlights
A vortex beam refers to a light beam possessing a helical phase front and can be described by a transverse phaseThe common methods to generate vortex beams include a spiral phase plate [7], a spatial light modulator [8], a cylindrical lens [9], and a metasurface [10]
We propose a vortex beam generator based on a nanometer spiral slit and explore the propagation rule of the topological charge
We study essentially the propagation of the vortex beam generated by a nanometer spiral slit etched on a metal film and explore the evolution of the topological charge (TC) with increase in the propagation distance
Summary
A vortex beam refers to a light beam possessing a helical phase front and can be described by a transverse phase. We study essentially the propagation of the vortex beam generated by a nanometer spiral slit etched on a metal film and explore the evolution of the TC with increase in the propagation distance. Theoretical analysis provides the expression of the phase front as a function of the propagation distance, chirality of the incident polarization light, and the rotation direction of the spiral slit. The influence of the shape and structure of spiral slits on the formation of TC is discussed This new study on the vortex beam with variable TC is expected to expand flexible applications of the vortex beam to many fields including optical manipulation, microscopy imaging, and quantum information processing [14, 15]. The orbit of a nanometer spiral slit can be expressed in polar coordinates as [16]
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