  For Doppler-broadened media operating under double-double electromagnetically
induced transparency (EIT) conditions, we devise a scheme to control and reduce
the probe-field group velocity at the center of the second transparency window.
We derive numerical and approximate analytical solutions for the width of EIT
windows and for the group velocities of the probe field at the two distinct
transparency windows, and we show that the group velocities of the probe field
can be lowered by judiciously choosing the physical parameters of the system.
Our modeling enables us to identify three signal-field strength regimes (with a
signal-field strength always higher than the probe-field strength), quantified
by the Rabi frequency, for slowing the probe field. These three regimes
correspond to a weak signal field, with the probe-field group velocity and
transparency window width both smaller for the second window compared to the
first window, a medium-strength signal field, with a probe-field group velocity
smaller in the second window than in the first window but with larger
transparency-window width for the second window, and the strong signal field,
with both group velocity and transparency window width larger for the second
window. Our scheme exploits the fact that the second transparency window is
sensitive to a temperature-controlled signal-field nonlinearity, whereas the
first transparency window is insensitive to this nonlinearity.
