Resonant sum-difference frequency mixing enhanced by electromagnetically induced transparency in krypton

We report experiments that investigate the generation of coherent VUV radiation at 123.6 nm via a resonant sum-difference frequency mixing scheme in Kr. One of the mixing fields at 212.55 nm was in two-photon resonance with the transition between the ${4p}^6{}^{1}S$ ground state and the ${4p}^{5}5p[0,\frac{1}{2}]$ excited state $\left(94\mathrm{}093.7\hspace{0.5em}{\mathrm{cm}}^{-1}\right).\mathrm{}$ The second field (the coupling field) at 759 nm was in single-photon resonance with the transition between the ${4p}^{5}5p[0,\frac{1}{2}]$ state and the ${4p}^{5}5s[1,\frac{1}{2}]$ state at $80917.6\hspace{0.5em}{\mathrm{cm}}^{-1}.$ The presence of the coupling field leads to electromagnetically induced transparency (EIT) at the wavelength of the generated field, 123.6 nm. This is predicted to enhance the four-wave-mixing efficiency by a large factor. We have studied the dependence of the four-wave mixing process on the detuning and strength of the coupling field. The efficiency for four-wave mixing was found to be enhanced by a significant factor (>5) by the EIT effect when the resonant coupling field strength exceeded about half the Doppler width $\left(0.1\mathrm{}\hspace{0.5em}{\mathrm{cm}}^{-1}\right).\mathrm{}$ A calculation for monochromatic fields and a uniform slab of gas provided qualitative agreement with the results of experiment. The relative conversion efficiency for this resonant mixing scheme is found to be ${10}^4$ times greater than that for a similar scheme where the coupling laser is tuned far $\left(1270\mathrm{}\hspace{0.5em}{\mathrm{cm}}^{-1}\right)$ off resonance. A substantial part of this resonant enhancement can thus be attributed to the EIT effect.