Nonlinear optical generation in a hydrogen discharge

Sum-frequency generation with electromagnetically induced transparency (EIT) has been examined in a hydrogen discharge. EIT is demonstrated in the $3p-1s$ and $4p-1s$ transitions at 102.6 and 97.3 nm, respectively, with increased efficiency of generation arising from higher H-atom density, although competing effects due to the presence of ${\mathrm{H}}_2$ distort the line shapes of these spectra. Only for shorter-wavelength transitions (such as $7p-1s$ and $8p-1s)$ that show very large shifts and distortions is there a decrease in intensity in comparison with generated radiation from H atoms external to the discharge.