Optical dispersion in twisted anisotropic materials

We theoretically investigate optical dispersion in twisted anisotropic materials, that is, when the optic axis undergoes a periodic modulation along the propagation axis while staying normal to the propagation direction. In the case of transversely-homogeneous geometries, we show that both group velocity and group velocity dispersion can be largely tuned both in magnitude and sign by changing the medium anisotropy, the maximum rotation angle, the modulation period and the input polarization. In doing that, we demonstrate a new type of optical spin-Hall effect occurring in the temporal domain. Our results introduce a new way to control the temporal profile of ultrashort pulses, automatically encompassing a structured polarization in time.