Optical Activity in Helically Twisted PCF

01.01.2014, 00:00

Newsletter 7

In a recent paper we showed theoretically and experimentally that continuously twisted solid-core photonic crystal fibre displays optical activity, the magnitude of the associated circular birefringence increasing linearly with twist rate. This is rather unexpected since there is no linear birefringence or anisotropy in the structure. A theoretical model based on symmetry properties and perturbation theory confirms that this is a non-resonant geometrical effect, which can only be understood if both spin and orbital angular momentum are considered. It turns out that the degenerate left- and right-circularly polarised modes of the untwisted PCF are not 100% circularly polarised but carry a small amount of orbital angular momentum caused by the interaction between the core mode and the hollow channels. The non-zero orbital angular momentum density close to the hollow channels in the vicinity of the fibre core (see figure) leads to a non-negligible contribution to the total angular momentum density and becomes dominant when circular birefringence is considered. Optically active twisted fibres are useful in applications such as current sensing and polarization control.

Contact: gordon.wong(at)mpl.mpg(dot)de
Group: Russell Division
Reference: X. M. Xi et al., Phys. Rev. Lett. 110, 143903 (2013).