Fundamental challenges induced by phase modulation inaccuracy and optimization guidelines of geometric phase metasurfaces with broken rotation symmetry.

Geometric phase metasurfaces feature complete phase manipulation of light at the nanoscale. While a majority of prior works assume the structure rotation in a fixed lattice of unit cells as equivalent to the element rotation required by the geometric phase principle, we argue that this assumption is fundamentally challenged for many current schematics which induce phase modulation inaccuracy. Here we take the dielectric nanobar type geometric phase metasurfaces as an example and perform an in-depth analysis about the physical origins of the phase modulation inaccuracy: imperfect structure rotation, resonance, tilted incidence and aperiodic arrays. We clarify the trade-off in phase modulation accuracy, efficiency, broadband property and wide angle acceptance. Furthermore, we present several examples of geometric phase metasurface devices to evaluate the performance degradation under different applications. Finally, based on the research, we provide a set of practical design and optimization guidelines to outperform the present devices of geometric phase metasurface.