Nonlinear X-wave
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In physics, a Nonlinear X-wave (abbr. NLX) is a multi-dimensional wave which can travel without distortion.
At variance with X-waves, a Nonlinear X-wave does exist in the presence of nonlinearity, and in many cases it self-generates from a gaussian (in any direction) wave packet.
The distinctive feature of a NLX is its "biconical" shape, (see figure) which appears as an "X" in any section plain containing the wave peak and the direction of propagation.
So far, Nonlinear X-waves have been only observed in nonlinear optics experiments, and have been predicted in a variety of nonlinear media including Bose-Einstein condensates.