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) S-1 ground state and the 4p(5) 5p[0,1/2] excited state (94 093.7 cm(-1)). The second field (the coupling field) at 759 nm was in single-photon resonance with the transition between the 4p(5) 5p[0,1/2] state and the 4p(5) 5s[1,1/2] state at 80 917.6 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 (0.1 cm(-1)). 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 (1270 cm(-1)) off resonance. A substantial part of this resonant enhancement can thus be attributed to the EIT effect. [S1050-2947(98)00511-3].