Optical elements with refractive index distribution structured at subwavelength scale, provide an expansive optical design space that can be harnessed for demonstrating multifunctional free-space optical devices. First, I discuss our recent work on dielectric optical metasurfaces that provide sophisticated functionality not easy to achieve with conventional optics. Then I discuss 3D dielectric elements, designed to be placed on top of the pixels of image sensors, that sort and focus light based on its color and polarization with efficiency significantly surpassing 2D absorptive and diffractive filters. The devices are designed via iterative gradient-based optimization to account for multiple target functions while ensuring compatibility with existing nanofabrication processes, and they are experimentally validated using a scaled device that operates at microwave frequencies. This approach combines arbitrary functions into a single compact element, even where there is no known equivalent in bulk optics, enabling novel integrated photonic applications. I further discuss capabilities enabled by volumetric meta-optical devices for compact on-chip polarimetry and discuss implementations using multi-layer nanofabrication.
