Plasmonic dynamic screening in a gold film by intense femtosecond laser light

It has been found experimentally that in intense femtosecond laser fields the surface plasmon dispersion has an oscillatory character as a function of the exciting laser intensity. It has been interpreted as the result of the dynamic screening of electrons by the strong laser field. A simple model is described in addition to the experimental results, being in good agreement with these findings. The results imply an electron effective mass of around 10 percent lighter than the free electron mass. The effective mass decreases with increasing laser intensity.