Anomalous light propagation and diffraction control in waveguide arrays

The understanding of light propagation primarily derives from studies of isotropic media. The law of refraction predicts that the tilt of a beam traversing an interface between two media will monotonously grow with the angle of incidence. The law of diffraction predicts beam spreading being completely determined by the ratio of wavelength and width, only slightly affected by the refractive index and independent of the tilt. In this paper, we demonstrate anomalies in light refraction and diffraction in evanescently coupled waveguide arrays ('discrete' refraction and diffraction). We have studied the propagation of beams in these arrays. It turned out that refraction and diffraction exhibit strong anomalies as they depend periodically on the initial beam tilt. In contrast to isotropic systems we found that transverse energy transport cannot exceed a certain maximum velocity and that the diffractive spreading depends on the direction of propagation, i.e., by varying the angle of incidence, size and sign of diffraction can be controlled and it can even be arrested. For particular initial tilts the array can undo beam spreading. The experiments were performed on homogeneous arrays of 75 waveguides in an inorganic-organic polymer on thermally oxidized silicon wafers. The 6 cm long samples were fabricated by UV-lithography on 4" wafers. Each waveguide provided low loss single mode waveguiding (<0.5 dB/cm) at lambda = 633 nm. The uniform separation of adjacent guides was chosen for efficient evanescent coupling. The theoretical explanation of the measured effects was done based on coupled mode theory.