Purely geometric spin–orbit Laguerre–Gauss waveplates from 3D laser-nanostructured silica glass

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. It 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 determine the extent to which the modal behavior remains of good quality when operating conditions deviate from the nominal specifications.