Mechanism Behind Angularly Asymmetric Diffraction in Phase-Gradient Metasurfaces

Asymmetric absorptivity has recently been demonstrated in phase-gradient metasurfaces. Such an effect varies from an integer m, which is the number of discrete unit cells in a supercell. There is still lack of analytical formulas to reveal the relationship between the absorptivity and the value of m. We enrich the previous studies by considering a subwavelength metallic grating at optical frequencies and present a simple and intuitive semianalytical approach to explain the observed asymmetric absorptivity. Our theory clearly indicates that the phenomenon of m dependent asymmetric absorptivity is very robust and can be observed in any grating with a phase gradient. The mechanism is that the phase gradient ensures an additional process of multiple total internal reflections inside the gratings, leading to more dissipation and an asymmetric diffraction response.