Scientists have been familiar with the phenomenon of birefringence ever since Christian Huygens characterized the anisotropy of calcite in the late 1600s. Now a group from University of Southampton in the UK and Toshiba Research Europe Ltd. in Cambridge, UK, has reported optical trirefringence. In the Feb. 19 issue of Physical Review Letters, the researchers describe the reflectance of silicon-based photonic crystal waveguides with a triangular lattice of air columns, which they placed in a Fabry-Perot cavity. They performed the measurements with a white-light laser, revealing six principal polarization axes, and suggest that tailored photonic crystals will enable the production of novel optical components with multirefringent properties.