(color online) (a) Schematic diagram of a four-level double Ladder-type system with two closely spaced upper levels and strongly coupled via spontaneous emission. (b) Arrangement of the probe (trigger) electric field and the relevant dipole moments and ( and ). (c) An atomic ensemble is filled into a 1D optical lattice of period which is generated by a retroreflected laser beam with wavelength . The density of trapped atoms takes on a Gaussian distribution in each period of the 1D optical lattice. The probe and trigger fields propagate in the z axis and x axis, respectively. The 1D optical lattice is assumed to move with a velocity along its optical axis z.