Ultra-dispersive anomalous diffraction from Pancharatnam-Berry metasurfaces

Achieving ultra-dispersive diffractions is fundamentally important to improve the chromatic resolution of spectrometers for numerous applications, such as Raman measurements, atom and molecule identification, and so on. Gratings, as traditionally widely used diffraction elements, disperse chromatic light into different angles according to the phase matching condition, which resorts to the transverse reciprocal vectors of the grating lattice. Mathematically, gratings show higher diffraction dispersion for larger diffraction angles. Either increasing grating line frequencies or steepening the groove angles is adopted to enlarge the deflection angles. However, all of them cause problems of near-zero diffraction efficiency and complicated fabrication. Here, we realize an ultra-dispersive diffraction in the framework of metasurfaces using an alternative phase matching strategy, in which the contributions of both the reciprocal vectors of the lattice and the local wave vectors arisen by the phase gradient are considered simultaneously. The diffraction angle of more than 80° is achieved with the resulting dispersion 4 times larger than the Littrow grating counterparts.