Coherent blue emission generated by Rb two-photon excitation using diode and femtosecond lasers

The coherent blue light generated in rubidium vapor due to the combined action of an ultrashort pulse train and a continuous wave diode laser is investigated. Each step of the two-photon transition 5S–5P${}_{3/2}$–5D is excited by one of the lasers, and the induced coherence between the 5S and 6P${}_{3/2}$ states is responsible for generating the blue beam. Measurements of the excitation spectrum reveal the frequency comb structure and allow us to identify the resonant modes responsible for inducing the nonlinear process. Further, each resonant mode excites a different group of atoms, making the process selective in atomic velocity. The signal dependency on the atomic density is characterized by a sharp growth and a rapid saturation. We also show that for high intensity of the diode laser, the Stark shift at resonance causes the signal suppression observed at low atomic density. (paper)