  Metalens research has made major advances in recent years. These advances
rely on the simple design principle of arranging meta-atoms in regular arrays
to create an arbitrary phase and polarization profile. Unfortunately, the
concept of equally spaced meta-atoms reaches its limit for high deflection
angles where the deflection efficiency decreases. The efficiency can be
increased using nano-antennas with multiple elements, but their polarization
sensitivity hinders their application in metalenses. Here, we show that by
designing polarization-insensitive dimer nano-antennas and abandoning the
principle of equally spaced unit cells, polarization-independent ultrahigh
numerical aperture (NA=1.48) oil-immersion operation with an efficiency of 43%
can be demonstrated. This represents a significant improvement on other
polarization-independent designs at visible wavelength. We also use this single
layer metalens to replace a conventional objective lens and demonstrate the
confocal scanning microscopic imaging of a grating with a period of 300 nm at
532 nm operating wavelength. Overall, our results experimentally demonstrate a
novel design concept that further improves metalens performance.
