  We investigate the transient response of a $\Lambda$-type system with one
transition decaying to a modified radiation reservoir with an inverse
square-root singular density of modes at threshold, under conditions of
transparency. We calculate the time evolution of the linear susceptibility for
the probe laser field and show that, depending on the strength of the coupling
to the modified vacuum and the background decay, the probe transmission can
exhibit behaviour ranging from underdamped to overdamped oscillations.
Transient gain without population inversion is also possible depending on the
system's parameters.
