Abstract

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.

© 1989 Optical Society of America