Spatial filtering of light by chirped photonic crystals
  We propose an efficient method for spatial filtering of light beams by propagating them through 2D (also 3D) longitudinally chirped photonic crystals, i.e. through the photonic structures with fixed transverse lattice period and with the longitudinal lattice period varying along the direction of the beam propagation. We prove the proposed idea by numerically solving the paraxial propagation equation in refraction index-modulated media, and we evaluate the efficiency of the process by plane-wave-expansion analysis. The technique can be applied to filter (to clean) the packages of atomic waves (Bose condensates), as well improve the directionality of acoustic and mechanical waves. 
Kestutis Staliunas and Victor J. Sanchez-Morcillo