Generating a three dimensional dark focus from a single conically   refracted light beam
  We report here the generation of a three dimensional dark focus from a single focused monochromatic Gaussian beam that undergoes conical refraction when it propagates along one of the optic axes of a biaxial crystal. We study the resulting ring intensity pattern behind the crystal as a function of the ratio between the ring radius and the beam waist and derive the particular parameter values for which a three dimensional dark focus with null intensity at the ring center is formed. We have performed experiments with a ${\rm KGd(WO_4)_2}$ biaxial crystal reporting the generation of a bottle beam in full agreement with our theoretical investigations. 
Yu. V. Loiko, A. Turpin, T. K. Kalkandjiev, E. U. Rafailov, and J.   Mompart
physics.optics