Progress in design, nanofabrication and performance of metalenses

Metalens has been shown to overcome the diffraction limit of conventional optical lenses to achieve sub-wavelength resolution. Due to its planar structure and lightweight, metalens has the potential applications in the manufacture of flat lenses for cameras and other high resolution imaging optics. However, currently reported metalenses have low focusing efficiencies: 26% - 68% in THz and GHz range, 1% - 91% in near infrared range (NIR), and 5% - 91.6% in the visible range. Far field imaging in the visible light is essential for use in camera and mobile phones, which requires a complex metalens structure with multi-layers of alternating metal and dielectric layers. Most of the reported metalenses work in a single wavelength, mainly due to the high dispersion characteristics of the diffractive metalenses. It remains a challenge to realize high resolution imaging for a wide wavelength band in particular in the visible range. In this review, we report the state-of-the-art in metalens design principle, types of nanoscale structures, and various fabrication processes. We introduce femtosecond laser direct writing based on two-photon polymerization as an emerging nanofabrication technology. We provide an overview of the optical performance of the recently-reported metalenses and elaborate the major research and engineering challenges and future prospects.