It seems inevitable. If you send a short, narrow pulse of light through a dispersive medium, the light’s component colors, traveling at different speeds, will string out along the propagation direction; diffraction will spread the pulse laterally; and what begins as a tight, white bullet of light becomes a blurred, rainbow-colored streak.
But it’s possible, under certain circumstances, to thwart the attenuating effects of dispersion and diffraction. Last year, Paolo Di Trapani of the University of Insubria in Como, Italy, and his collaborators discovered they could send short, intense laser pulses through a transparent crystal of lithium triborate without the pulses’ typical spreading. 1
In the Como experiment, the pulses emerged from the laser aperture with the usual Gaussian profile. But when they encountered the dispersive medium of the crystal, nonlinear interactions spontaneously transformed them into a new shape that in longitudinal cross section looks like an X. Figure 1...
