Electrically Tunable Bifocal Metalenses Mediated by Liquid Crystals

Tunable optical devices powered by metasurfaces provide a new path for functional planar optics. In particular, lenses with tunable focal lengths could play a key role in various fields with applications in imaging, displays, and augmented and virtual reality devices. Here, we present a method for designing electrically controllable varifocal metalenses at visible wavelengths by incorporating a metasurface designed to focus light at two different focal lengths with liquid crystals to actively manipulate the focal length through the application of an external bias. At a single wavelength in the visible spectrum, using an optimized low-loss a:Si-H we successfully demonstrate a bifocal metalens with a focusing efficiency of ~45% at both focal points, as well as showing proof of electrically tunable imaging. Furthermore, we present a method for designing an RGB achromatic bifocal metalens using a computational algorithm and experimentally realize it through a scalable nanoimprinting technique using a TiO2 nanoparticle embedded resin.