Controllable Spectral Blueshifts of Intense Femtosecond Pulses upon Reflection from Metal Surfaces*

Controllable spectral blueshifts of ~5 nm are observed upon reflection of an intense femtosecond mid-IR pulses from metal surfaces and are absent in the case of reflection from dielectric surfaces. Thes shifts are attributed to the interaction of the ultrashort pulse with an emerging layer of free-electron gas hovering above the surface. The electrons are expelled from the metal via laser-induced fast thermionic emission and form a transient capacitor structure with the positively charged surface. We have developed a model description of interaction of a short pulse with such transient capacitor that is evolving on a characteristic timescale comparable with the pulse duration. The model predicts a blue shift of the maximum of the pulse spectral density, which depends on the pulse duration and on the material characteristics of the metal.. The magnitude of the shifts is in a good agreement with the experimental observations. The discovered controllable blueshift in the spectrum of an intense femtosecond laser pulse upon interaction with a metal surface carries promises for both the surface diagnostics and the pulse shape control.