Comparison of Electromagnetically Induced Transparency (EIT) Spectra for Six-level Lambda \(\Lambda\) and Five-level V-type Systems

Electromagnetically induced transparency (EIT) is experimentally studied in a rubidium vapour cell (without buffer gas, both \(^{87}\)Rb and \(^{85}\)Rb present according to their natural abundance) kept within two-layers of \(\mu(\mu)\)-metal shields to avoid the effect of Earth's magnetic field on the energy levels of atomic rubidium. An external cavity diode laser (ECDL), used as a low power probe laser, is locked to the hyperfine cross-over peak of \(F = 2 \rightarrow  F' = 2, 3\) transitions of \(^{87}\)Rb. The frequency of another ECDL, the pump laser, is set to scan the \(F=1 \rightarrow  F' = 0, 1, 2\) transitions of \(^{87}\)Rb. These form the  \(\Lambda\)-type six level system. In the V-type system, both the pump and the probe lasers share the same \(F = 2\) ground level. The probe beam coming out of the cell is detected by a low noise fast photodetector. The resulting spectra show signature of EIT in the "peak" for the  \(\Lambda\)-type system and in the "dip" for the V-type system. Numerical calculation based simulated spectra are also compared with the experimental spectra. In both the cases very narrow EIT linewidth \((\Gamma_{t} < \Gamma)\) is observed even at high value of pump Rabi-frequency \((\Omega_{c}\gg \Gamma)\). Narrower value of EIT linewidth is due to Doppler averaging phenomena.