Cascade Conical Diffraction

Summary form only given. A beam of light propagating along the optic axis of a biaxial crystal will be transformed to a cone of light and emerge from the crystal as a hollow cylinder as predicted by W. R. Hamilton in 1832 [1] and experimentally observed shortly thereafter by H. Lloyd [2]. Cascade Conical Diffraction refers to a conical diffraction system consisting of two or more conical diffraction crystals (CDCs) in series./n this contribution, we report on (i) the observed patterns produced using cascade configurations of two, three and four crystals in the ring plane; (ii) the cross sectional evolution of a two-crystal cascade system, and (iii) the effect of the handedness of circularly polarized light on the patterns produced away from the ring plane for a three-crystal cascade. /n addition the effects of polarisation on the ring plane patterns was investigated Using cascade configurations of two, three and four crystals with a rotation angle y of π/2 between each crystal the patterns produced are composed of two, four and eight concentric rings respectively as shown in Figure 1. This is in keeping with the prediction of M.V. Berry that the ring pattern produced by an N-crystal 2N-1 cascade is a superposition of rings [3, 4].For the two crystal cascade configuration, the cross section of the beam evolution was recorded. The resulting image was reminiscent of the cross section for single crystal conical diffraction, with both rings being produced by the intersection of two cones and collapsing to a Raman spot with highest axial intensity at a distance z. /n the cascade case the variable z is the sum of the z value for the individual CDCs. The effect of leftand rightcircularly polarised light on the ring pattern produced away from the focal image plane of the system by a three crystal cascade was also investigated. /mages and intensity distributions showed there to be discrepancies between the two cases, with variations in intensity, peak resolution and - he appearance of an axial spot in the case of left-handed circularly polarized light. This was in keeping with the prediction of M.V. Berry in [3] that the observed patterns would differ. 2N-1 The observations presented here, that a cascade of N crystals will produce concentric rings in the ring plane and that circularly polarised light of different handedness leads to different patterns away from the ring plane, are in agreement with the recently advised paraxial theory and the predictions therein. The recorded cross section illustrated the evolution of the beam in free space and highlights that the ring plane pattern is produced by the intersection of two oppositely propagating cones.