Coherent blue emission induced by a combination of diode and femtosecond   lasers
  We report the investigation of a collimated blue light generated in rubidium vapor due to the combined action of an ultrashort pulse train and a cw diode laser. 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, indicating that each individual mode is responsible for inducing a nonlinear process. The strong signal dependency on the atomic density is characterized by a sharp growth and rapid saturation. 
Jesus P. Lopez, Marco P. M. de Souza, Marcio H. G. de Miranda, Sandra   S. Vianna
physics.atom-ph physics.optics