Coherent resonances in a dipole-broadened profile of selective reflection from the transparent dielectric–atomic rubidium vapour interface

We discuss coherent narrow resonances in a homogeneously broadened profile of selective reflection from interface between a cell window and high-density atomic rubidium vapour, where the dipole broadening is greater than the Doppler width of unresolved components of the rubidium D₂ line. The coherent resonances are produced by coherent scattering of probe and pump optical fields at induced pulsations of the ground state and excited state populations at a beat frequency. The spectral width of each resonance depends on a decay rate of the population difference. Using a simple theoretical model we find the conditions for observing resonances with the Lorentz spectral shapes. The experimentally recorded resonances are fitted by the Lorentz function with adjustable amplitude, width and spectral baselines. In the limit of very small saturation, the measured half-width at half-maximum (HWHM) γ_res/2π is near 52 MHz, which value is remarkably higher than the radiative decay rate of the rubidium excited state 5P_3/2.