Shape and polarization distribution of non-circular conical diffraction beams from conjugate cascades.

Abstract

Peculiar non-circularly shaped vector type beams can be obtained naturally by the conical diffraction phenomenon if specific manipulations in wavevector space are performed between optically biaxial crystals arranged in a cascade. We analyze in detail this situation by focusing on the general shapes and the polarization distribution. Both are shown to be correlated to the values of structure parameters introduced in this work. These control parameters depend on the conical diffraction cone aperture angle, on the crystal lengths, and on the magnification values due to x- and y-oriented cylindrical lenses placed between the crystals and coupling common conjugate planes. The local polarization is found to be always linear with the exception of regions where structures composing the pattern intersect or overlap, where elliptical or circular polarization can occur. The way in which the obtained patterns depend on the orientation of individual crystal samples around the common optical axis and on an eventual polarization filtering at various stages of the cascade is discussed as well. Theoretical and experimental findings agree well, as verified for the case of a cascade of two crystals.