Taming topological defects with light

Real crystals are rarely perfect; they tend to contain the occasional defect, such as the one in the colloidal packing depicted here. The packing is visualized as a Voronoi diagram, with each 2-µm-diameter colloidal particle represented by a polygonal cell. In what’s known as a dislocation, two extra rows of particles, indicated by dashed yellow lines, have been squeezed into the hexagonal lattice, leaving one particle (white) with only five nearest neighbors and another (gray) with seven. (For more on colloidal crystals, see Physics Today, September 2010, page 30 , and December 1998, page 24 .) A form of topological defect, dislocations are common in both colloidal and atomic crystals. Understanding how they emerge and interact is key to modeling the kinetics of melting and of solid-to-solid phase transitions. But experimentalists have lacked the means to reproducibly control such defects in the lab. Now William Irvine (University of Chicago), Paul Chaikin (New York University), and their coworkers have demonstrated a way to manipulate colloidal-lattice dislocations using arrays of holographically generated optical traps. Dubbed topological tweezers, the arrays can capture dozens of particles simultaneously—each particle in its own potential well—such that clusters of particles can be pushed or pulled in unison. By strategically nudging selected clusters, the researchers can create, steer, and even induce fission of lattice dislocations. They can also rotate a cluster to create a grain boundary. But curiously, reversing the rotation doesn’t return a pristine lattice; it only causes the boundary to expand. (W. T. M. Irvine et al., Proc. Natl. Acad. Sci. USA 110, 15544, 2013.)