Four-wave parametric mixing in optically inverted barium ions

Optically inverted barium ions in the $6p_{\left(\frac{1}{2}\right)}$ excited state served as the nonlinear medium for doubly resonant four-wave parametric generation of coherent radiation at 167 nm. The excited barium ions were produced via selective autoionization. Stimulated-emission photons at 650 nm corresponding to the $6p_{\left(\frac{1}{2}\right)}-5\mathrm{}{d}_{\left(\frac{3}{2}\right)}$ transition were mixed with input 451-nm laser photons turned to the $6p_{\left(\frac{1}{2}\right)}-6\mathrm{}{f}_{\left(\frac{5}{2}\right)}$ two-photon resonance to produce the 167-nm output photons.