Measurement of high conversion efficiency to 123.6-nm radiation in a four-wave-mixing scheme enhanced by electromagnetically induced transparency

We report measurements of the absolute yield of 123.6-nm vacuum ultraviolet (VUV) radiation produced in a resonant four-wave-mixing scheme in krypton enhanced by electromagnetically induced transparency. One of the mixing fields, in the ultraviolet (UV) at 212.55 nm, was in two-photon resonance with the ${4p}^6{}^1{S}_0–{4p}^{5}5p[0,\frac{1}{2}]$ transition of krypton, while a second field (the coupling field) at 759 nm was resonant with the ${4p}^{5}5p[0,\frac{1}{2}]{-4p}^{5}5s[1,\frac{1}{2}]$ transition. Electromagnetically induced transparency has been predicted to be induced by this coupling field and to enhance the efficiency of the generation of the field at 123.6 nm, on the ${4p}^{5}5s[1,\frac{1}{2}]$ to ${4p}^6{}^1{S}_0$ transition. This was confirmed by measurements of the absolute VUV radiation yield using a calibrated photodiode in the limit of a large density-length product. Energies of ∼30 nJ were produced, which gave a conversion efficiency, from the coupling field energy of ∼1%. Higher yields are thought to be achievable by increasing the coupling laser intensity, the path length of the medium, and by the use of a transform-limited UV pulse of the same duration as the coupling field.