Terahertz quasi-perfect vortex beam with integer-order and fractional-order generated by spiral spherical harmonic axicon

Abstract

We have proposed a new method to generate terahertz perfect vortex beam with integer-order and fractional-order. A new optical diffractive element which is composed of the phase combination of a spherical harmonic axicon and a spiral phase plate is designed, and called spiral spherical harmonic axicon. A terahertz Gaussian beam passes through the spiral spherical harmonic axicon to generate a terahertz vortex beam. When only increasing the number of topological charges carried by spiral spherical harmonic axicon, the ring radius of terahertz vortex beam increases slightly, so it is a terahertz quasi-perfect vortex beam. Importantly, the terahertz quasi-perfect vortex beam can carry not only integer-order but also fractional-order topological charges. This is the first time that vortex beam and quasi-perfect vortex beam with fractional-order has been successfully realized in terahertz domain and experiment.