PT-symmetry in honeycomb photonic lattices
  We apply gain/loss to honeycomb photonic lattices and show that the dispersion relation is identical to tachyons - particles with imaginary mass that travel faster than the speed of light. This is accompanied by PT-symmetry breaking in this structure. We further show that the PT-symmetry can be restored by deforming the lattice. 
Alexander Szameit, Mikael C. Rechtsman, Omri Bahat-Treidel, and   Mordechai Segev
physics.optics