Modified luneburg lens for achromatic subdiffraction focusing and directional emission

Broadband achromatic subdiffraction focusing is in urgent need in application of imaging. However, the previously published broadband achromatic metalenses are restricted by diffraction effect or limited efficiency. In this communication, a new gradient refractive index (GRIN) lens with a modified refractive index (RI) profile of classical Luneburg lens is proposed and designed based on a radial gradient periodic structure. Highly efficient achromatic subdiffraction focusing with full width at half maximum (FWHM) around 0.36λ and large numerical aperture (NA) of 1.26 has been accomplished. The sample of the proposed GRIN lens is manufactured by 3-D printing technology. Experimental results of the near- and far-field are well consistent with those of theoretical predictions and numerical simulations. This GRIN lens could yield subdiffraction focusing spot with high focusing efficiency (above 74%) from 8 to 16 GHz. Directive far-field radiation (side lobes below -7 dB over an ultrabroadband frequency range with bandwidth ratio of 120% (4-16 GHz)) is also achieved. The demonstrated GRIN lens has a great potential to be applied in the subdiffraction imaging system in the future.