High performance metalenses: numerical aperture, aberrations, chromaticity, and trade-offs

Metalenses consist of nanostructures that locally control the optical phase. They offer many degrees of freedom for manipulating a wavefront, which gives a number of advantages over bulk lenses, such as the straightforward elimination of spherical aberrations and an ultra-thin dimension. Here, we compare the phase profiles of metalenses made of different dielectric materials and note the advantage of high refractive index materials. Higher refractive index materials such as silicon afford more degrees of freedom in terms of design and fabrication and are the basis for high performance metalenses, even in the visible. Nevertheless, the imaging performance of metalenses is still limited by coma and chromatic aberrations. This limitation is exacerbated by high numerical apertures and large areas. We review the challenges and trade-offs between numerical aperture, field of view, coma, chromatic aberration and size. We also evaluate different phase engineering approaches to address these problems. We trust that this review will help to guide future developments in high numerical aperture metalenses towards wide-field and high-resolution imaging, enabling novel applications such as wearable optics, ultra-lightweight displays, and high-end miniature imaging systems.