Coherent population trapping at low light levels.

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A three-level lambda ($\ensuremath{\Lambda}$) system driven by two coherent fields at the Raman resonance condition evolves to a nonabsorbing eigenstate of the Hamiltonian called the coherent population trapping (CPT) state. We study the various factors dictating the dynamical evolution of the coherences and the populations to the CPT state. We demonstrate the formation of the CPT state even at low intensities, though it takes much longer to form such a state. For the case of unequal decay rates we demonstrate an interesting sharp dip in the steady-state response of the medium at the Raman resonance condition.