Conical refraction produces the well-known ring profile when circularly polarised light is incident on a biaxial crystal.
Conical diffraction of linearly polarised light in a biaxial crystal produces a beam with a crescent-shaped intensity
profile. Rotation of the plane of polarisation of the incident light produces the unique effect of spatially moving the
crescent-shaped beam around a ring. We use this effect to trap microspheres and white blood cells and to position them
at any angular position on the ring. Continuous motion around the circle is also demonstrated by rotating the input linear
polarisation. The ability to spatially locate a beam and an associated trapped object simply by varying the polarisation of
light suggests that this optical process should find application in the manipulation and actuation of micro- and nano-scale
physical and biological objects.
Internal conical refraction leads to the formation of zero (J<sub>0</sub>) and first order (J<sub>1</sub>) Bessel beams in superposition. The
(J<sub>0</sub>) beam retains the input circular polarisation and the (J<sub>1</sub>) has opposite polarisation but with a single phase change
around the beam axis giving it &barh; optical angular momentum per photon. This results in the conical beam having ½ &barh;
net optical angular momentum per photon. This provides a simple system in which a beam of 0, ½ and &barh; optical
angular momentum can be easily generated and selected with use of only a circular polariser. In the far field the
characteristic Bessel beam structures are formed and can be made non-diverging with use of a lens. We report the
formation of non-diverging Bessel beam of core diameter (a) of 5.7μm over a maximum non-diverging core length of
1(±.05)mm. However due to the fine structure of the conical beam at its beam waist position two cores are produces
and are of opposite phase.