Compound meta-optics: there is plenty of room at the top.

Metasurfaces have been widely exploited in imaging and sensing, holography, light-matter interaction, and optical communications in free space and on chip, thanks to their CMOS compatibility, versatility and compact form. However, as this technology matured from novelty to performance, stringent requirements on diffraction efficiency, scalability, and complex light control have also emerged. For instance, the limited thickness of single-layer meta-optics poses fundamental constraints on dispersion engineering and lossless transmission over large-scale devices, whereas in-plane symmetry limits the polarization transformations that can be realized. Cascaded and multi-layer flat optics can alleviate these constraints, offering new possibilities for realizing high-efficiency devices, full polarization control, and achromatic response. In this perspective, recent advances in multi-layer metasurfaces including inherent challenges and opportunities will be discussed. Compound meta-optics hold the promise for enabling complex optical systems with enhanced performance and unprecedented functionality for a diverse set of applications in sensing, imaging, high-capacity communications, and beyond.