Velocity-selective double resonance in Doppler-broadened rubidium vapor

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We study resonantly enhanced cascaded two-photon transitions in Doppler-broadened rubidium vapor, induced by an optical frequency comb. Cascaded two-step excitation of atoms from the ground state 5S{}1/2$ to the final 5$D5/2 state via an intermediate 5P{}3/2$ state as well as the influence of the frequency comb repetition rate on the two-photon fluorescence signal have been investigated. It is shown that the careful choice of ${f}R$ leads to enhancement of the two-photon signal with considerable increase of the final $5{D}5/2$-state population due to velocity-selective double resonance. Since the double-resonance condition for two different rubidium isotopes is fulfilled for different ${f}R$, the possibility of isotope-selective two-photon excitation is explored. The dependence of the two-photon signal on external magnetic field values and different repetition rates of the optical frequency comb is also investigated.