Doppler-free two-photon resonances for atom detection and sum frequency stabilization
Abstract
We investigate the excitation of the 5D5/2 level in Rb atoms using counter-propagating laser beams, which are nearly resonant to the one-photon 5S1/2 - 5P3/2 and 5P3/2 - 5D5/2 transitions, ensuring that a sum of the optical frequencies corresponds to the 5S1/2 - 5D5/2 transition. The excitation produced by two-photon and step-wise processes is detected via spontaneously emitted fluorescence at 420 nm arising from the 6P3/2 - 5S1/2 transition. The dependences of blue fluorescence intensity on atomic density and laser detuning from the intermediate 5P3/2 level have been investigated. The sensitivity of the frequency detuned bi-chromatic scheme for atom detection has been estimated. A novel method for sum frequency stabilization of two free-running lasers has been suggested and implemented using two-photon Doppler-free fluorescence and polarization resonances.