Imaging with flat optics: metalenses or diffractive lenses?

Abstract

Recently, there has been an explosion of interest in metalenses for imaging. The interest is primarily based on their subwavelength thicknesses. Diffractive gratings have been used as thin optical elements since the late 19th century. Here, we show that multilevel diffractive lenses (MDLs), when designed properly, can exceed the performance of metalenses. Furthermore, MDLs can be designed and fabricated with larger constituent features, making them accessible to low-cost, large-area volume manufacturing, which is generally challenging for metalenses. The support substrate will dominate overall thickness for all flat optics. Therefore, the advantage of a slight decrease in thickness (from $\sim 2\lambda$ to $\sim \lambda /2$) afforded by metalenses may not be useful. We further elaborate on the differences between these approaches and clarify that metalenses have unique advantages when manipulating the electromagnetic fields, rather than intensity.