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Probing Nonequilibrium Electron Distributions In Gold By Use Of Second Harmonic Generation, K. L. Moore '99, Thomas D. Donnelly

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Second-harmonic radiation is generated at a gold surface by use of a laser pulse that is varied in duration from 14 to 29 fs and in intensity from 10⁹ to 10¹¹W/cm² . At laser intensities below 10¹⁰W/cm² , the second-harmonic signal has the expected quadratic dependence on pump-laser intensity; however, at higher intensities, the dependence is supraquadratic. This difference arises because the leading edge of the laser pulse interacts significantly with the gold electrons to create a nonequilibrium, photoexcited distribution. The second-harmonic generation process occurs before electron–electron or electron–phonon collisions can equilibrate the …