Purely geometric Laguerre-Gauss spin-orbit waveplates from silica optics

We present the experimental demonstration of laser nanostructured silica modal waveplates that allow the generation of pure Laguerre-Gauss modes whose azimuthal order sign l is controlled by the spin state of an incident circularly polarized Gaussian beam of fixed radial order p. This overcomes the previous limitation of laser nanostructuring technology associated with the loss of modality for one of the two spin states due to unavoidable dynamic phase modulation from spatially varying retardance. This allows the spin-orbit nature of the fabricated optical element to be fully exploited. This is made possible by extending the usual 2D structuring strategy for laser-induced anisotropic optical properties to a 3D strategy. In addition, although the resulting modal waveplates are designed to operate for predetermined values of incident beam size and wavelength, we experimentally and theoretically show that the modal behaviour survives up to marginal deviations when the operating conditions do not match the nominal specifications, which allows us reporting on polychromatic modal beam shaping.