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Broadband vectorial ultra-flat optics with experimental efficiency up to 99% in the visible via universal approximators

Integrating conventional optics into compact nanostructured surfaces is the goal of flat optics. Despite the enormous progress of this technology, there are still critical challenges for real world applications due to a limited efficiency in the visible, on average lower than $60\%$, which originates by absorption losses in wavelength thick ($\approx 500$nm) structures. Another issue is the realization of on-demand optical components for controlling vectorial light at visible frequencies simultaneously in both reflection and transmission, and with a predetermined wavefront shape. In this work, we developed an inverse design approach that allows the realization of highly efficient (up to $99\%$) ultra-flat (down to $50$nm thick) optics for vectorial light control and broadband input-output responses on a desired wavefront shape. The approach leverages on a hidden network of universal approximators, which exist in the physical layer of suitably engineered semiconductor nanostructures. Near unity performance results from the ultra-flat nature of these surfaces, which reduces absorption losses to almost negligible values. Experimentally, we discuss polarizing beam splitters, comparing their performances with the best results obtained from both direct and inverse design techniques, and new flat-optics components represented by dichroic mirrors and the basic unit of a flat optics display that creates full colors by using only two sub-pixels, overcoming the limitations of conventional LCD/OLED technologies that require three sub-pixels for each composite color. Our devices are manufactured with a complementary metal-oxide-semiconductor (CMOS) compatible process, making them scalable for mass production at inexpensive costs.