Planar metal/dielectric single-periodic multilayer ultraviolet flat lens

In 1968, Veselago predicted that a thin film composed of a material with a negative refractive index would act as a flat lens [Sov. Phys. Usp. 10, 509 (1968)]. Hyperbolic metamaterials have been designed that show negative refraction, but the condition of an isotropic refractive index 𝑛=−1 required to realize a diffraction-limited focus cannot be realized using the hyperbolic dispersion. Here, we present an experimental realization of a metamaterial design in which the dispersion is described by spherical rather than hyperbolic isofrequency wavevector contours, emulating an isotropic refractive index 𝑛=−1. We fabricate a flat lens that operates in the UV at 𝜆=364  nm using a silver/titanium dioxide single-periodic thin-film multilayer structure. Using confocal microscopy, we find a clear focus positioned only 350 nm away from the flat lens surface, in very good agreement with analytical calculations and numerical simulations.