All-Angle Quasi-Self-Collimation Effect in a Rod-Type Silicon Photonic Crystal

By changing the symmetry of a photonic crystal (PC) with a rectangular lattice to straighten one of the isofrequency contours, the PC shows an all-angle quasi-self-collimation (quasi-SC) effect. To investigate the straightness of the isofrequency contour and the quasi-SC effect, we propose a straightness factor L based on the method of least squares. With L ≤ L
₀
(L
₀
= 0.01 is the critical value), the isofrequency contour is sufficiently straight to induce the quasi-SC effect with the beam quasi-collimating in the structure. Furthermore, the efficiency of light coupling to the quasi-SC PC is studied and can be greatly improved by applying a carefully designed antireflection structure. This quasi-SC effect of the PC and the coupling structure may see applications in novel optical devices and photonic circuits.