Effect of Quantum Interference on the Absorption Spectrum of a Four-Level Atom in Photonic Crystals

In this paper, the probe absorption spectrum of a four-level atom in photonic crystals has been investigated. A model is used, which assumes the transitions from the two upper levels to one of the lower levels coupled to the modes of the modified reservoir (photonic band gap) and transitions from the two upper levels to the other lower level coupled to the free vacuum modes. The effect of quantum interference in spontaneous emissions from the two upper levels to the lower levels on the absorption spectrum of an atom is investigated in detail. Most interestingly, it is shown that the atom becomes transparent to a probe laser field coupled to the free space transition only in situations where complete quantum interference takes place in both the free space transitions and the modified reservoir transitions.