The recent paper by Zablocky and Engheta [J. Opt. Soc. Am. A 10, 740 (1993)] on the propagation of a unitstep- function-modulated electromagnetic signal in a linear, causal, dispersive chiral medium with singleresonance dispersion in the limit of zero material damping is found to be only partially valid in that singular limiting case. Of particular interest is their use of an overly restrictive definition of signal arrival in such lossless dispersive medium. The correct, modern asymptotic description of signal arrival in dispersive pulse propagation is readily available in the literature [J. Opt. Soc. Am. B 5, 817 (1988)] and is carefully reviewed here for a lossy dispersive dielectric as described by the single-resonance Lorentz model. This general definition provides an unrestricted description of signal arrival in the limit of zero damping that yields results equivalent to those of Zablocky and Engheta in regions of normal dispersion.

© 1995 Optical Society of America

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