In this case, gradually increases, transits to a small plateau around the central region, and then gradually decreases. A physical interpretation can be given by considering the contributions of nonlinear coupling from different regions of the medium, namely, the left region and the right region which are separated by the virtual mirror. When the mirror is moved leftward, the length of the left region decreases and thus the angle of the sideband beams increases. The opposite case is shown by the dotted curve, where only the contribution of the right region was taken into account. The solid line agrees well with the dashed and dotted curves except for the central part.
Figure shows the experimental setup.
The feedback mirror was placed behind lens 2, following the concept of Fig. . Here, L was determined by the distance between the feedback mirror and point P', which was the image of point P where the beam axis crossed the back surface of the KNbO 3 crystal. Between the mirror positions for obtaining stable regular hexagons, squares, and squeezed hexagons, we observed the competition of different patterns. For example, when the mirror was placed between the two positions which favored regular hexagons and squares, respectively, regular hexagons and squares appeared alternately with time. The transition from one pattern to another appeared in an irregular manner.