Multifunctionnal self-collimating mesoscopic photonic crystals

It was first thought that this mesoscopic self-collimation only occurred when the phase index averaged to 0 over a mesoscopic period: <;n>=(n_PhC.l_PhC+n_b.l_b)/(l_b+l_PhC)=0, requiring a negative index n_PhC in the photonic crystal (PhC) slab to compensate for the positive phase index n_b in the bulk material. This stringent condition leaves no room for other optimization or functionality and imposes air as the bulk material (n_b=1). However, we have recently demonstrated that mesoscopic self-collimation condition does not actually rely on phase index in the PhC slab but on a new concept: the curvature index, n_c. This new condition of zero averaged curvature <;C>=n_c/l_PhC+n_b/l_b=0 leaves room for many other optimizations. In particular, we have numerically demonstrated mesoscopic self-collimation in all positive index material with extremely low filling factor in air of 3%. This opens the way to applications of mesoscopic self-collimation to active structures like laser cavities or amplifier. Moreover, as this condition does not impose a particular mesoscopic period L=l_PhC+l_b and loosely constraints the phase index, it is possible to make mesoscopic self-collimating reflectors with arbitrary reflectivity value using appropriately designed slabs. It is thus possible to tune reflectivity anywhere between high reflectivity. This opens the way towards multifunctional mesoscopic structures. We have also shown that mesoscopic self-collimation can be combined with slow light in structures essentially made of high index and potentially active or non-linear materials.