A lithography-free approach to polaritonic Luneburg lenses.

Planar polaritonic lenses, achieved by precisely controlling interface polaritons, hold significant promise for subwavelength light focusing. While most existing designs rely on polariton interference or refraction, creating gradient-index polaritonic lenses remains a substantial challenge due to the lack of practical and cost-effective fabrication methods. Here, we introduce a lithography-free approach for producing polaritonic lenses with gradient effective refractive indices by engineering the dielectric environment of polaritons. This method involves in situ transformation of polymer microspheres into spherical caps through controlled melting, allowing the fabrication of polaritonic Luneburg lenses with focal spots as small as λ/18, where λ is the free-space wavelength. Our approach could also apply to other gradient-index polaritonic lenses, such as Maxwell fisheye lenses and Mikaelian lenses, based on a wide range of in-plane isotropic polariton modes. By circumventing traditional nanofabrication constraints, this versatile and cost-effective technique offers a promising platform for developing polaritonic devices, which are essential for future polaritonic systems and integrated circuits.