Metalens for Detection of a Topological Charge

In this paper, we considered design of the complex metalens with high numerical aperture. It proposed to be manufactured in a thin film of silicon nitride. The element represents two slanted sectored metalenses, every sector of which is binary sub-wavelength gratings set. The diameter of the proposed lens is 14 µm. Numerical modelling by applying the finite difference time-domain method demonstrated that the proposed element was able to detect laser vortex beams with different topological charges –2 and –1. Moreover, it can operate over the whole visible range of light. The proposed metalens is able to discern beams with different wavelengths since they are focused in different focal planes. The change of wavelength in a 1 nm produces the shift of focal spot in about 4 nm. In the case of a Gaussian incident beam with left circular polarizing, this metalens simultaneously forms two vortex beams with topological charges one and two at the focal length of 6 μm.