Light propagation properties of the Bi2ZnOB2O6 acentric biaxial crystal: Angular orbital momentum from conical diffraction

We have studied a slab of an acentric biaxial Bi2ZnOB2O6 single crystal grown by the Kyropoulos method under conditions of low temperature gradients, oriented perpendicularly to the optical axis. We focused slightly behind the sample the Gaussian beam from a HeNe laser at 632.8 nm and we made magnified images of the near field intensity on a CCD camera. A reference beam was taken from the entrance and redirected on the CCD screen to obtain a controlled wedge fringe pattern visualising the phase of the wave. The specific rotatory power was measured to be 2.74 rad/cm. We present results of conoscopic patterns obtained launching through the crystal slab a left-circular polarized beam and projecting the output beam on a right-circular polarization state or on a linear polarization state at −45° from the horizontal axis. All the experimental results (optical activity, intensity and phase of the output wave and vortices) are well described by a theoretical model based on the bi-anisotropy of the BZBO crystal. The orbital angular momentum is calculated to be 0.996 ℏ/photon for the output circular projection and 0.456 ℏ/photon for the linear projection.