  We demonstrate a metalens with NA = 0.98 in air, a bandwidth (FWHM) of 274 nm
and a focusing efficiency of 67% at 532 nm wavelength, which is close to the
transmission performance of a TiO2 metalens. Moreover, and uniquely so, our
metalens can be front-immersed into immersion oil and achieve an ultra-high NA
of 1.48 experimentally and 1.73 theoretically, thereby demonstrating the
highest NA of any metalens in the visible regime reported to the best of our
knowledge.
