Ultrahigh Numerical Aperture Metalens at Visible Wavelengths.

Subwavelength imaging requires the use of high numerical aperture (NA) lenses together with immersion liquids in order to achieve the highest possible resolution. Following exciting recent developments in metasurfaces that have achieved efficient focusing and novel beam-shaping, the race is on to demonstrate ultrahigh-NA metalenses. The highest NA that has been demonstrated so far is NA = 1.1, achieved with a TiO2 metalens and back-immersion. Here, we introduce and demonstrate a metalens with a high NA and high transmission in the visible range, based on crystalline silicon (c-Si). The higher refractive index of silicon compared to TiO2 allows us to push the NA further. The design uses the geometric phase approach also known as the Pancharatnam-Berry (P-B) phase, and we determine the arrangement of nanobricks using a hybrid optimization algorithm (HOA). We demonstrate a metalens with NA = 0.98 in air, a bandwidth (full width at half-maximum, 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 ultrahigh 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. The fabricating process is fully compatible with microelectronic technology and therefore scalable. We envision the front-immersion design to be beneficial for achieving ultrahigh-NA metalenses as well as immersion metalens doublets, thereby pushing metasurfaces into practical applications such as high resolution, low-cost confocal microscopy and achromatic lenses.