Ja. Gaspar et P. Halevi, LINEAR PULSE-PROPAGATION IN AN ABSORBING MEDIUM - EFFECT OF FILM THICKNESS, Physical review. B, Condensed matter, 49(15), 1994, pp. 10742-10744
We compute the peak velocity of a broad Gaussian pulse propagating in
an absorbing thin film modeled with a one-oscillator dielectric functi
on. The exact numerical calculation is done for a GaP:N film in the fr
equency region of the bound A exciton. For absorption lengths greater
than the film thickness, the pulse velocity essentially coincides with
the group velocity, and with the experimental results of S. Chu and S
. Wong [Phys. Rev. Lett. 48, 738 (1982)]. However, outside the region
of strong absorption the pulse velocity depends sensitively upon the t
hickness of the slab. Our results give a better account of the Chu and
Wong experiment than the calculation based on the group velocity. For
film thickness with smaller than one absorption length the pulse velo
city disagrees with the group velocity even in the strong absorption r
egion, as a consequence of Fabry-Perot reflections of the pulse inside
the film.