Tunable focusing by a flexible metasurface

An efficient reflective metasurface with elastically tunable focal length is proposed and experimentally demonstrated. The metasurface consists of electric resonators embedded in a stretchable elastic substrate which allows continuous elongation of the system. Our theory and numerical simulations predict how the focal length is controlled by means of the stretching, which we experimentally verify. By performing phase-sensitive measurements of the scattered field, we are able to differentiate the true focus, where all scattered waves are in phase, from the point of maximum amplitude. These phase measurements further enable us to characterise an axial aberration in the stretched structure, due to rays projected from distinct parts of the structure converging at different focal lengths. Additionally, we characterise the efficiency of our structure, showing that 78-95% of the incident power is reflected, depending on the degree of tuning. We also quantify the fraction of incident power which is directed into the beam waist. Our results demonstrate that metamaterials integrated with engineered elastic structures are an effective platform for functional devices.