  We demonstrate experimentally and theoretically that a nanoscale hollow
channel placed centrally in the solid glass core of a photonic crystal fiber
strongly enhances the cylindrical birefringence (the modal index difference
between radially and azimuthally polarized modes). Furthermore, it causes a
large split in group velocity and group velocity dispersion. We show
analytically that all three parameters can be varied over a wide range by
tuning the diameters of the nanobore and the core.
