Electromagnetically induced transparency and dark fluorescence in a cascade three-level Lithium molecule

We observed electromagnetically induced transparency (EIT) and dark fluorescence in a cascade three-level diatomic Lithium system using Optical-Optical Double Resonance (OODR) spectroscopy. When a strong coupling laser couples the intermediate state $A^{1}\Sigma^{+}_{u}(v=13, J=14)$ to the upper state $G^{1}\Pi_{g}(v=11, J=14)$ of $^7Li_2$, the fluorescence from both $A^{1}\Sigma^{+}_{u}$ and $G^{1}\Pi_{g}$ states was drastically reduced as the weak probe laser was tuned through the resonance transition between the ground state $X^{1}\Sigma^{+}_{g}(v=4, J=15)$ and the excited state $A^{1}\Sigma^{+}_{u}(v=13, J=14)$. The strong coupling laser makes an optically thick medium transparent for the probe transition. In addition, The fact that fluorescence from the upper state $G^{1}\Pi_{g}(v=11, J=14)$ was also dark when both lasers were tuned at resonance implies that the molecules were trapped in the ground state. We used density matrix methods to simulate the response of an open molecular three-level system to the action of a strong coupling field and a weak probe field. The analytical solutions were obtained under the steady-state condition. We have incorporated the magnetic sublevel (M) degeneracy of the rotational levels in the lineshape analysis and report $|M|$ dependent lineshape splitting. The theoretical calculations are in excellent agreement with the observed fluorescence spectra. We show that the coherence is remarkably preserved even when the coupling field was detuned far from the resonance.