Manipulation and Removal of Defects in Spontaneous Optical Patterns

Defects play an important role in a number of fields dealing with ordered structures. We demonstrate theoretically and experimentally the possibility of an active manipulation of defects in terms of an externally induced motion. We focus on the spontaneous formation of two-dimensional spatial structures in a nonlinear-optical system, a liquid crystal light valve under single optical feedback. For a particular parameter setting, a spontaneously formed hexagonal intensity pattern contains several dislocation-type defects. A scheme based on Fourier filtering allows us to restore spatial order in a selectable part of the pattern. Starting without control, the controlled area is progressively expanded, such that defects are swept out of the pattern.

Figure 1

Scheme of the experimental setup; details are given in the text.

Figure 2

Experimental sweeping of defects at the times indicated. Each panel contains the intensity distribution on the left and the extracted defect locations on the right. The gray line indicates the front separating the controlled from the uncontrolled part. Different marker types correspond to different hexagonal modes, where □, +, and ⊳ indicate one charge, ◇, ×, and ⊲ indicate the opposite topological charge, respectively.

Figure 3

Left panel: All defect locations, detected during the sequence of the moving control front (cf. Fig. 2). Right panel: Experimental example of a penta-hepta defect, where two rolls of different modes end close to each other (as indicated).

Figure 4

Numerical simulation of progressive removal of defects at times $t=0$ (a), $t=28$ (b), $t=44$ (c), and $t=80$ (d), where $t$ has been normalized by the LCLV response time. The vertical bar signals the position of the control mask moving from left to right. Each panel contains the intensity distribution on the left and the location of defects on the right.