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Optics Letters

Second-harmonic generation at Lyman-α in atomic hydrogen

L. Marmet, K. Hakuta, Boris Peter Stoicheff

Published Feb 15, 1991• 7 citations
Publisher

Abstract

Second-harmonic generation in atomic hydrogen is investigated with and without an applied electric field, by tuning laser radiation at 243.1 nm through the 2 (2)S((1/2))-1(2)S((1/2)) two-photon resonance. Even at zero applied field, secondharmonic radiation at 121.6 nm was observed. The measured intensity is shown to arise from a charge-separation field caused by ions produced in three-photon ionization. calculated value based on the measured ion density.

DOI
10.1364/ol.16.000261

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Second-harmonic generation in atomic hydrogen is investigated with and without an applied electric field, by tuning laser radiation at 243.1 nm through the 2 (2)S((1/2))-1(2)S((1/2)) two-photon resonance. Even at zero applied field, secondharmonic radiation at 121.6 nm was observed. The measured intensity is shown to arise from a charge-separation field caused by ions produced in three-photon ionization. calculated value based on the measured ion density.
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.
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.
The interaction between a metastable H(2s) atomic hydrogen beam and an external electric field leads to the emission of the Lyman-α line. It originates in the Stark mixing of the near-degenerate 2s1/2 and 2p1/2 levels separated by the Lamb shift. The quenched radiation proportional to the square of the electric field amplitude is recovered in vacuum by using such an atomic probe beam. We observe the strong enhancement of the signal when the field is oscillating at the Lamb shift frequency. This technique is applied in a plasma, offering an alternative way to measure weak electric fields by direct and non-intrusive means.