Divergence features of laser beams with angular momentum

Abstract

The physical features of the divergence of laser beams with angular momentum are investigated. The beams are presented in the form of a coherent superposition of two modes without angular momentum. Analytical formulas are used that satisfy Maxwell's equations for all components of the electric and magnetic fields of the initial modes. This allows one to describe the superposition of modes in terms of the components of the Umov–Poynting vector. The relationship between the beam divergence and its angular momentum, caused by the dependence of the radial and azimuthal components of the Umov–Poynting vector on the longitudinal components of the fields, is demonstrated. The components of this vector are analysed for an annular beam obtained using the superposition of azimuthally and radially polarised modes at various phase shifts between them. According to the analysis, beams with angular momentum can propagate without divergence. A method for generating such beams is discussed.