  We analyze the interaction of a triple quantum dot molecules controlled by
the tunneling coupling instead of coupling laser. A general analytic expression
for the steady-state linear susceptibility for a probe-laser field is obtained
and we show that the system can exhibit two transparency windows. The group
velocity of the probe-laser pulse is also analyzed. By changing the tunneling
couplings, two laser pulses with different central frequency can propagate with
the same group velocity. And the group velocity can be as low as 300 m/s in our
system. We extend our analysis to the case of multiple quantum dot molecules
(the number of the quantum dots is N) and show that the system can exhibit at
most N-1 transparency windows. And at most N-1 laser pulses with different
central frequencies can be slowed down.
