Large focal length planar focusing of Dyakonov polaritons in hyperbolic metamaterial

Achieving subwavelength optical focusing is of great importance in nanophotonics. However, achieving focusing with both a small focal spot size and a large focal length remains elusive so far. Here, a large focal length planar focusing device is presented, utilizing highly oriented Dyakonov polaritons in hyperbolic metamaterial with periodic silver rings as the excitation source. Experimental results show that by controlling the size of the excitation sources, the focal length can reach 6λ0（where λ0 is the illumination wavelength）, and the focal spot size can be reduced to λ0/10. This method provides new prospects for planar polariton optical applications.