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Physical Review A

Nonlinear optical generation in a hydrogen discharge

Dennis W. Tokaryk, G. Z. Zhang, Boris Peter Stoicheff

Published Apr 1, 1999• 8 citations
Publisher

Abstract

Sum-frequency generation with electromagnetically induced transparency (EIT) has been examined in a hydrogen discharge. EIT is demonstrated in the $3p\ensuremath{-}1s$ and $4p\ensuremath{-}1s$ 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 $7p\ensuremath{-}1s$ and $8p\ensuremath{-}1s)$ 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.

DOI
10.1103/physreva.59.3116

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Sum-frequency generation with electromagnetically induced transparency is investigated experimentally as the generation of coherent Lyman-(Beta) radiation (103 nm) in atomic hydrogen. We demonstrate that with the application of a strong laser field to couple the 2 s and 3 p levels, the resonant Lyman-(Beta) transition becomes transparent, and the generated wave continues to grow without suffering resonant absorption. At a density-length product NL of 3 X 1015 cm-2, a conversion efficiency of 2 X 10-4 is obtained for the coherent 103 nm radiation.
Second harmonic generation (SHG) in free atoms is one of the unsolved problems in nonlinear optics. The most probable mechanism is due to Stark mixing between nearby excited states through an electric field caused by charge separation through the process of three-photon ionization. To test the mechanism, atomic hydrogen appears to be the best candidate because of its well-known electronic structure and large dipole moments of its excited states. In this work we report on the observation of SHG in atomic hydrogen in the Lyman-α region. Pulsed dye laser radiation was doubled and tuned to 243 nm for excitation of the 2s ← 1s transition at 121.6 nm by a two-photon resonance process. A cw atomic beam was used with hydrogen atoms generated in a dc discharge of H2 gas. The laser beam was focused ~1 mm below the nozzle where the hydrogen atom density was estimated to be 1013 cm−3. The second harmonic radiation was strongly enhanced by adding an external dc field of up to 10 kV/cm. Stark splitting was clearly observed at higher fields.