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 ultra-high 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 ultra-high NA metalens (spot size is as small as 0.5λ, NA = 1) by using multi-layer 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 ultra-high 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 ultra-high NA metalenses. Such an ultra-high resolution focusing in axial also produce 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 ultra-high resolution microwave imaging, compact wireless power harvesting or transfer systems, miniaturized communication or sensing systems, and many others.