A Varifocal Graphene Metalens for Broadband Zoom Imaging Covering the Entire Visible Region.

The ever-increasing demand for miniaturized optical systems has placed stringent requirements on the core element: lenses. Developing ultrathin flat lenses with a varifocal capability and broadband spectral response is critical for diverse applications, but remains challenging and has been the focus of intensive research. The recent demonstration of tunable focal length for a single wavelength with metalenses marked an important milestone for transforming the complex and bulky tunable lens kit into a single flat lens. However, achieving color imaging with desired tunability over the entire visible spectrum essential for practical applications still remains elusive. Here we propose and demonstrate experimentally a broadband varifocal graphene metalens (250 nm in thickness) covering the entire visible spectrum. It is able to simultaneously tune the focal lengths for different wavelengths continuously. By laterally stretching the lens, an over 20% focal length tuning range can be achieved for red (650 nm), green (550 nm), and blue (450 nm) light as three example wavelengths. Zoom imaging of different objects located along the axial direction has been demonstrated at these wavelengths by simply controlling the stretch ratio of the graphene metalens. This broadband graphene zoom lens enables enormous applications in miniaturized imaging devices such as cell phones, wearable displays, and compact optical or communication systems with multi-color-channel functionalities.