Controlling orbital angular momentum using forked polarization gratings

We examine a novel method to control the orbital angular momentum (OAM) of lightwaves using forked polarization gratings (FPGs). We significantly improve the fabrication of FPGs and achieve substantially higher quality and efficiency than prior work. This is obtained by recording the hologram of two orthogonally polarized beams with phase singularities introduced by q-plates. As a single compact thin-film optical element, an FPG can control the OAM state of light with higher efficiency and better flexibility than current methods, which usually involve many bulky optical elements and are limited to lower OAM states. Our simulations confirm that FPGs work as polarization-controlled OAM state ladder operators that raise or lower the OAM states (charge l) of incident lightwaves, by the topological charge (lg) on the FPGs, to new OAM states (charge l ± lg). This feature allows us to generate, detect, and modify the OAM state with an arbitrary and controllable charge. An important application of FPGs are the essential state controlling elements in quantum systems based on OAM eigenstates, which may enable extreme high capacity quantum computation and communication.