Multifunctional 2.5D metastructures enabled by adjoint optimization

Optical metasurfaces are two-dimensional arrays of meta-atoms that modify different characteristics of light such as phase, amplitude, and polarization. One of their intriguing features that distinguishes them from conventional optical components is their multifunctional capability. However, because of the limited number of degrees of freedom, there is a trade-off between the efficiency and the number of distinct functions a metasurface may provide. Here we show that 2.5D metastructures, which are stacked layers of interacting metasurface layers, provide sufficient degrees of freedom for implementing efficient multifunctional devices. The large number of design parameters and their intricate intercoupling make the design of multifunctional 2.5D metastructures a complex task, and metastructures designed using conventional techniques are suboptimal devices with inferior performance. We address this issue by designing 2.5D metastructures using the adjoint optimization technique. As proof of concept, we experimentally demonstrate a double-wavelength metastructure designed using the adjoint optimization technique that has significantly larger efficiencies than a similar device designed using a simplified approach conventionally used in metasurface design. The 2.5D metastructure architecture empowered by the optimization-based design technique is a general platform for realizing high-performance multifunctional components and systems.