Failure of the group-velocity description for ultrawideband pulse propagation in a causally dispersive, absorptive dielectric

The accuracy of the group-velocity description of dispersive pulse propagat ion in a double-resonance Lorentz model dielectric is shown to decrease mon otonically as the propagation distance increases, whereas the accuracy of t he asymptotic description increases monotonically as the propagation distan ce increases above a single absorption depth in the medium at the pulse car rier frequency. (C) 1999 Optical Society of America [S0740-3224(99)03009-X] .