Ultrabroadband gradient-pitch Bragg-Berry mirrors

The realization of a geometric phase optical device operating over a broad spectral range is usually confronted with intrinsic limitations depending on the physical process at play. Here we propose to use chiral nematic liquid-crystal slabs with helical ordering that varies in three dimensions, namely, gradient-pitch cholesterics endowed with in-plane space-variant angular positioning of the supramolecular helix. By doing so, we show that the recently introduced Bragg-Berry mirrors [M. Rafayelyan and E. Brasselet, Opt. Lett. 41, 3972 (2016).] can be endowed with an ultrabroadband spectral range. Experimental demonstration is made in the case of ultrabroadband optical vortex generation in the visible domain. These results offer a practical solution to the polychromatic management of the orbital angular momentum of light combining the circular Bragg reflection of chiral media with the Berry phase.