Kinematic saturation of the electromagnetically-induced-transparency peak in thermal atomic vapors

We demonstrate that, in thermal atomic vapors, the electromagnetically-induced-transparency (EIT) transmission peak exhibits saturation. Investigating the amplitude of the velocity-selected double-resonance transmission peak-a purely kinematic effect arising from finite wavevector mismatch-we identify a threshold atomic column density Ns. For column densities N below Ns, the amplitude of the transmission peak maximum increases with column density. Once it exceeds Ns, the peak amplitude maximum remains constant, while the corresponding detuning shifts away from resonance and scales as the square root of ln(N over Ns). Furthermore, We find that the operating point for optimum photon-shot-noise-limited sensitivity deviates from that of the transmission peak maximum. Our study provides guidance for optimizing EIT readout in vapor cell and for precision measurements built upon this scheme.