Efficient frequency conversion of ultrashort pulses from the visible (0.55 $\mu$) to the infrared (1.5 $\mu$) in barium vapor

The frequency of $\lambda_p\approx0.55\,\mu$ radiation of ultrashort duration was converted to the infrared ($\lambda=1.5\,\mu$) in Ba vapor in the presence of an Ar buffer gas whose pressure was varied between 1 and 300 Torr. The conversion quantum efficiency was 30%. An investigation was made of the mechanisms for population of the $^1P^0_1$Ba level. It was found that when the detuning of the pump radiation from the appropriate transition was large, the only mechanism for population of the $^1P_0^1$ level (followed by lasing at $\lambda=1.5\,\mu$) was “optical collisions”: Ba $(^1S_0)+$ Ar $+\hbar\omega_H\to$ Ba $(^1P_1^0)+$ Ar. For small detunings and low Ar pressures other mechanisms for population of the $^1P^0_1$ level were also found. Measurements were made of the temporal characteristics of the $\lambda=1.5\,\mu$ radiation. Possible reasons for the absence of electronic stimulated Raman scattering are discussed.