Generating a three-dimensional dark focus from a single conically refracted light beam

We report here the generation of a three-dimensional (3D) 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 3D dark focus with null intensity at the ring center is formed. We have performed experiments with a KGd(WO4)2 biaxial crystal, reporting the generation of a bottle beam in full agreement with our theoretical investigations

A Mesoscopic Blue-detuned Light Ring for Bose-Einstein Condensates Based on Conical Refraction

Conical refraction (CR) is an optical phenomenon observed in biaxial crystals (BCs) that has been a subject of renewed interest since the last decade [1], despite being predicted by W.R. Hamilton in 1832. CR occurs when a light beam propagates along one of the optic axes of a biaxial crystal with birefringence R0 = l·α, where l is the length of the crystal and α is its conicity [2]. Under such conditions, an input focused Gaussian beam with beam waist radius w0, is transformed into two concentric bright rings at the focal image plane of the system, as long as the condition R0 >> w0 is satisfied. These bright rings are separated by a thin ring of null intensity known as the Poggendorff dark ring (PDR), see Fig. 1