Gallium nitride-based geometric and propagation metasurfaces for vortex beam emissions

This work experimentally demonstrates the highly -efficient geometric and propagation metasurfaces for vortex beam emissions. These metasurfaces are respectively composed of high -aspect -ratio fin -like and cylindrical gallium nitride (GaN) meta -atoms. Remarkably, the optimized configuration of the fin -like GaN meta -atoms achieves a cross -polarization transmission efficiency of up to 99 %. Similarly, the cylindrical GaN meta -atoms exhibit an average co -polarization transmission efficiency of 97 %. Both metasurfaces, designed for vortex beam emission, exhibit annular intensity converging capabilities at distinct wavelengths in the visible. Notably, the geometric metasurface shows achromatic annular intensity distributions over a continuous wavelength range up to 100 nm, in sharp contrast to the propagation metasurface, which is subject to inherent wavelength dispersion limitations.