High-resolution absorption spectroscopy in laser-cooled rubidium atoms with bichromatic laser excitation

We are studying a new spectroscopic method using laser-cooled rubidium atoms that allows us to lock the carrier frequency of modulated laser in the coherent population trapping (CPT) clock experiment. This method uses counter-propagation of the bichromatic laser field produced by the modulated laser. The counter-propagation cancels the CPT effect near the optical resonance and gives rise to enhanced Doppler-free absorptive resonances with high-contrast away from the transition. We demonstrate the CPT clock operation by locking the modulated laser using this resonance. We also describe a theoretical model that we have developed based on density-matrix formalism to simulate this spectroscopic scheme for accurately predicting the lineshape and amplitude of resonances under different excitation conditions. Our study suggests that this spectroscopic method is also quite suiatble for developing cold-atom based optical frequency standard.