Two-dimensional lattice solitons

Through a balance between linear and nonlinear processes, periodic nonlinear systems can produce a self-localized state: a lattice soliton. Such systems include, for example, “breather” states in biological α-helixes and quantum vortex pairs in linear arrays of current-driven Josephson junctions. Now, Mordechai Segev (Technion-Israel Institute of Technology), Demetri Christodoulides (University of Central Florida), and their colleagues have created such a system in two dimensions. The physicists used light both as the means for creating the lattice—by interfering pairs of plane waves within a photorefractive, anisotropic crystal—and as the “probe” beam to form the optical lattice soliton. The degree of nonlinearity was set by a tunable electric field and by adjusting the ratio between the probe beam and the lattice waves. At a low voltage, the probe propagated linearly through the 6-mm-long crystal and showed a “discrete diffraction” pattern (left image, above). At a higher voltage, however, self-trapping occurred and a...