Optical Metasurfaces: Evolving from Passive to Adaptive

Optical metasurfaces are nanoengineered architectures aiming at the functional control of light propagation. In recent years, the performance of metasurfaces has been significantly improved and with the availability of such 2D metamaterials, different optical elements, such as flat achromatic lenses or compact spectrometers have been demonstrated. However, to a large extent, such demonstrations have focused on the realization of entirely passive (nonadaptive) metasurface properties. Adaptivity and active tuning of the optical function of a metasurface after fabrication is highly desirable, because it enables a host of more advanced applications such as beam scanning and dynamic holographic displays. Here, the evolution of the state of the art of metasurfaces toward attaining the adaptivity property and the resulting active control over the wavefront of light is reviewed. Starting from an overview of passive metasurfaces, the different approaches targeting adaptive metasurfaces are discussed, concluding with identifying crucial challenges and opportunities in this future-oriented field.