Second-harmonic generation in atomic hydrogen at Lyman-α

Abstract

We have observed second-harmonic generation in atomic hydrogen in the Lyman-α region, with and without an applied dc electric field, Ea. Experiments with a continuous atomic beam were carried out by using a pulsed dye laser tuned to 243 nm for excitation of the n = 2-1 transition. The laser beam was focused 0.4 mm below the H-source nozzle, where the H-atom density was estimated to be 5 × 1013 cm-3. Measurements of second-harmonic generation (SHG) at ~122 nm were made at various electric fields up to ~27 kV/cm. It was shown that the SH signal produced at E a = 0 occurs because of a charge-separation field of ~800 V/cm that is caused by three-photon ionization. Measurements of the SHG and ion current were found to vary as I2 L and I3 L , respectively. At high fields (E a > 10 kV/cm) the SH signal at the center of the Stark-split components decreased somewhat from the peak value, but the ion current (corresponding to photon absorption) approached zero. This observation is a clear demonstration of a similar effect, predicted by Harris et al.1 of efficient nonlinear conversion with induced transparency, and it suggests the possibility of obtaining intense Lyman-a radiation.