  Subwavelength planar structured interfaces, also known as metasurfaces, are
ultra-thin optical elements modulating the amplitude, phase, and polarization
of incident light using nanostructures called meta-atoms. The optical
properties of such metasurfaces can be controlled across wavelengths by
selecting geometries and materials of the meta-atoms. Given recent
technological developments in optical device miniaturization, components for
beam splitting and beam combining are sought for use within these devices as
two quintessential components of every optical setup. However, realizing such
devices using metasurfaces typically leads to poor uniformity of diffraction
orders and narrow-band operation. Using a modified version of particle swarm
optimization, we propose and numerically demonstrate a broadband, reciprocal
metasurface beam combiner/splitter with uniformity>97% and diffraction
efficiency>90% in the continuous band from {\lambda}=1525 nm to {\lambda}=1575
nm. The proposed approach significantly extends the current state of the art of
metasurfaces design in terms of uniformity, bandwidth, and efficiency and opens
the door for devices requiring high power or near-unit uniformity.

Optics