  Slow light and rainbow trapping attract many attentions in last twenty years,
and in most of the previous works, the researchers achieve the slow light and
rainbow trapping with complicate configurations or techniques, for example,
metamaterial techniques. In this paper, we propose a simple waveguide consisted
with semiconductor and dielectric layers, and we find that the slow-light peaks
appear in the dispersion curves. Besides, the cutoff frequency with $v_g = 0$
($v_g$ is the group velocity) in the slow-light peaks is depending on the
thicknesses of the semiconductor and dielectric. Then, we design a tapered,
horizontally symmetric structure. By using software COMSOL and finite
difference time domain method as well, we achieve double directional rainbow
trapping and more importantly, releasing the rainbow in such structure. This
approach is promising for optical isolator, optical buffer, optical switch and
other optical functional devices in miniaturization optical integrated circuit.
