Second-harmonic generation in atomic hydrogen induced by a charge-separation field

Abstract

Second-harmonic generation in the absence of an externally appplied electric field was investigated near 121.6 nm in atomic hydrogen. It was shown that three-photon ionization produces a local electric field due to charge separation that in turn induces second-harmonic generation in this otherwise isotropic medium. The magnitude of the charge-separation field was determined from the observed second-harmonic power and was found to be in good agreement with a calculated value based on the measured number of photo-ions, the interaction volume, and the incident laser pulse duration.