Observation of electromagnetically induced transparency in six-level Rb atoms and theoretical simulation of the observed spectra

We report the observation of electromagnetically induced transparency (EIT) in a six-level Lambda-type system in atomic Rb vapor containing both Rb-87 and Rb-85. The experimental observation includes five velocity selective optically pumped (VSOP) absorption dips for both Rb-87 and Rb-85. The EIT signal appears on the background of one such VSOP absorption dips. The measured EIT linewidth (Gamma(t)) shows sub-natural (Gamma(t) < Gamma) values for both lower and higher values of pump Rabi-frequencies. The density matrix based theoretical model for the six-level system is developed and solved numerically by taking into account the Doppler broadening. A complete analytical solution (non perturbative) for a three level Lambda-type system has been obtained and compared with the experimentally observed sub-natural EIT linewidth. The simulated spectra are in good agreement with the experimental findings.