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VOLUME 53 (1991) | ISSUE 4 | PAGE 188
Time-varying diffraction effects in the propagation of an electromagnetic pulse in vacuum
A new optical effect is predicted: As an electromagnetic field pulse with a steep leading edge propagates in vacuum, the transverse distribution and strength of the field at the leading edge remain constant. This effect is linked with the existence of a time-dependent "vacuum dispersion." An analogy is drawn between this effect and Sommerfeld and Brillouin's problem of the propagation of a signal in a dispersive medium and also with the coherent propagation of a small-area pulse (a Οπ pulse) in a gas of resonantly absorbing two-level particles.