Ultrathin nanostructured resonators on surfaces, i.e., metasurfaces, have shown great potential for the miniaturization of advanced photonic technologies such as aberration-free lenses, beam shapers, and holograms. These metasurface designs focus strongly on obtaining desired optical properties through a careful optimization of the shape, the relative orientation, and the composition of structures. However, once manufactured the resulting optimized properties cannot change, preventing the use of metasurfaces in the implementation of displays or wearables, needed for applications involving augmented and virtual reality.
Recently, there has been a lot of interest in manipulating the properties of metasurfaces by mechanical deformation using an electrical current, an optical beam, or heat. A particularly successful design of optomechanical metasurfaces is based on elastic resonators with nontrivial mechanical and optical properties. The position, distance, or relative orientation of elastic resonators is changed by making use of an optical beam exerting an optical force. The optical properties of the surfaces change due to the reconfiguration of these resonators, depending on the the power of the pump. Therefore, changes in power may control the optical properties of metasurfaces and allow for their use in reconfigurable photonic devices.
Now, writing in Physical Review Letters, scientists from Chalmers University of Technology (Sweden), Vrije Universiteit Brussel (Belgium) and Harvard University (USA), point out that the reconfiguration process of these surfaces, i.e., the motion of elastic resonators towards their equilibrium configuration, crucially determines the refresh rate of optomechanical metasurfaces. For simple reconfiguration processes, the refresh rate is ultimately limited by the nonlinear interactions between elastic elements and the optical beam, rendering the process much too slow for practical applications. The results suggest that the current approach towards optomechanically reconfigurable metasurfaces needs to be reconsidered with a specific focus on the nonlinear dynamics of the system.
Image caption: The figure illustrates the interaction of two beams with a reconfigurable metasurface: the red beam reconfigures the metasurface, the purple beam is modified by the surface. The unit cells are visualized as hourglasses, representing the unexpected long reconfiguration time of each unit cell, fundamentally limited by the nonlinear transient dynamics.