The directional emission sensitivity of photonic crystal waveguides to air hole removal

To obtain highly directional light output from photonic crystal waveguides (PCWs), the emission characteristics of the narrow-width waveguide structures are investigated by tailoring the geometry of the exit sides. The local structural deformations in the form of air hole removal from the triangular-lattice photonic crystal (PC) show the effectiveness of the previously proposed approach that was implemented by us for another type of PC. The spatial broadening of the beam is greatly suppressed. With the modified waveguide exits, highly directional emissions with small side lobes are achieved. The frequency dependency of the directional emissions is evaluated. We show that the divergence angles of the beams depend linearly on the wavelength for a regular type of PCW but the modified PCW exits have local minima with respect to wavelength in terms of the divergence angle. The present work may prove to be helpful in the design of couplers and edge-emitting lasers and in the implementation of free-space optical communications.