Quantum interference in the fluorescence of a molecular system - art. no. 013818

It has been observed experimentally [H.R. Xia, C.Y. Ye, and S.Y. Zhu, Phys. Rev. Lett. 77, 1032 (1996)] that quantum interference between two molecula r transitions can lead to a suppression or enhancement of spontaneous emiss ion. This is manifest in the fluorescent intensity as a function of the det uning of the driving field from the two-photon resonance condition. Here we present a theory that explains the observed variation of the number of pea ks with the mutual polarization of the molecular transition dipole moments. Using master equation techniques we calculate analytically as well as nume rically the steady-state fluorescence, and find that the number of peaks de pends on the excitation process. If the molecule is driven to the upper lev els by a two-photon process, the fluorescent intensity consists of two peak s regardless of the mutual polarization of the transition dipole moments. L f the excitation process is composed of both a two-step, one-photon process and a one-step, two-photon process, then there are two peaks on transition s with parallel dipole moments and three peaks on transitions with antipara llel dipole moments. This latter case is in excellent agreement with the ex periment.