Broadband and Tunable-Focus Flat Lens with Dielectric Metasurface

Metasurface-based flat lens have great potentials for applications in compact and portable cameras. However, the properties of optical metasurfaces are strongly limited by the intrinsic ohmic loss of metal and the fixed working distance. Here, we propose a highly efficient all-dielectric metasurface lens working in near-infrared frequency range. By exploiting silicon nanoblock as meta- atoms, the silicon metasurface produces phase changes covering 0 to 2 pi and near-unity reflection, inducing the focusing of light over a broad wavelength range. Interestingly, the focal length of such an ultrathin flat lens can be dynamically controlled by incorporating tunable materials such as liquid crystals. By utilizing the phase transition or electrical alignment of liquid crystals, the focal length has been tuned up to 10 %. We believe our research will be important to accelerate the applications of optical metasurfaces.