  Self-consistent simulations of the ultrafast electron dynamics in thin metal
films are performed. A regime of nonlinear oscillations is observed, which
corresponds to ballistic electrons bouncing back and forth against the film
surfaces. When an oscillatory laser field is applied to the film, the field
energy is partially absorbed by the electron gas. Maximum absorption occurs
when the period of the external field matches the period of the nonlinear
oscillations, which, for sodium films, lies in the infrared range. Possible
experimental implementations are discussed.
