An Ultrahigh Numerical Aperture Microwave Metalens With Ultrathin Thickness

Ultrahigh numerical aperture (NA) metalenses can produce a tight focusing spot with highest resolution very close to diffraction limit. So far, several ultrahigh metalenses have been presented, but mostly with dielectric metasurfaces for experimentally reported NA < 1 and with bulky device thickness compared to its wavelength λ. In this letter, an ultrahigh NA metalens (spot size is as small as 0.5λ, NA = 1) by using multilayer plasmonic metasurface with only 0.24λ thickness is demonstrated both numerically and experimentally. Notably, this measured NA = 1 is the highest level among all of the reported ultrahigh NA metalenses in the free space. Besides, the measured focusing efficiency can reach a large 31%, which can also be compared to the state-of-the-art ultrahigh NA metalenses. Such an ultrahigh resolution focusing in axial also produces a tight longitudinal depth of focus (DOF) of 2.3λ and 3λ according to simulations and measurements, respectively. The presented metalens can find potential applications such as ultrahigh resolution microwave imaging, compact wireless power harvesting or transfer systems, miniaturized communication or sensing systems, and many others.