Abstract

We experimentally demonstrate double EIT resonances in a four-level tripod system, formed within the Zeeman sublevels of the $F = 2 \to F^{\prime} = 1$ transition of $^{87}{\rm Rb}$ with co-propagating, phase coherent pump and probe beams. We observe two EIT peaks with unequal peak heights in the probe transmission signal. The larger peak exhibits linear power broadening with varying pump power and longitudinal magnetic field, while the smaller EIT peak, influenced by the earth’s non-zero transverse magnetic field, maintains a constant linewidth with power variation and shows mixed behavior with longitudinal magnetic field variation. Our analysis involves decomposing the tripod into two interacting $\Lambda$ systems, highlighting the impact of this interaction on the weaker $\Lambda$ system and its resulting immunity to power broadening. Theoretical simulations, based on steady-state solutions of density matrix equations, align well with experimental observations. We reveal that, in higher magnetic fields, the stronger $\Lambda$ system dominates the weaker one, affecting its normal behavior, while in lower magnetic fields, power broadening is observed. This study has important applications in atom-based magnetometry and precision measurement. Further, the narrower linewidth of the smaller EIT peak can help with longer storage times in an atomic medium compared to the usual stronger $\Lambda$ system.

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