Ultrahigh Numerical Aperture Metalens at Visible Wavelengths

Abstract

Subwavelength\nimaging requires the use of high numerical aperture\n(NA) lenses together with immersion liquids in order to achieve the\nhighest possible resolution. Following exciting recent developments\nin metasurfaces that have achieved efficient focusing and novel beam-shaping,\nthe race is on to demonstrate ultrahigh-NA metalenses. The highest\nNA that has been demonstrated so far is NA = 1.1, achieved with a\nTiO₂ metalens and back-immersion. Here, we introduce and\ndemonstrate a metalens with a high NA and high transmission in the\nvisible range, based on crystalline silicon (c-Si). The higher refractive\nindex of silicon compared to TiO₂ allows us to push the\nNA further. The design uses the geometric phase approach also known\nas the Pancharatnam–Berry (P–B) phase, and we determine\nthe arrangement of nanobricks using a hybrid optimization algorithm\n(HOA). We demonstrate a metalens with NA = 0.98 in air, a bandwidth\n(full width at half-maximum, fwhm) of 274 nm, and a focusing efficiency\nof 67% at 532 nm wavelength, which is close to the transmission performance\nof a TiO₂ metalens. Moreover, and uniquely so, our metalens\ncan be front-immersed into immersion oil and achieve an ultrahigh\nNA of 1.48 experimentally and 1.73 theoretically, thereby demonstrating\nthe highest NA of any metalens in the visible regime reported to the\nbest of our knowledge. The fabricating process is fully compatible\nwith microelectronic technology and therefore scalable. We envision\nthe front-immersion design to be beneficial for achieving ultrahigh-NA\nmetalenses as well as immersion metalens doublets, thereby pushing\nmetasurfaces into practical applications such as high resolution,\nlow-cost confocal microscopy and achromatic lenses.