Response Function Theory for Many-Body Systems Away from Equilibrium: Conditions of Ultrafast-Time and Ultrasmall-Space Experimental Resolution

Abstract

A response function theory and scattering theory applicable to the study of physical properties of systems driven arbitrarily far removed from equilibrium, specialized for dealing with ultrafast processes, and in conditions of space resolution (including the nanometric scale) are presented. The derivation is done in the framework of a Gibbs-style nonequilibrium statistical ensemble formalism. The observable properties are shown to be connected with time- and space-dependent correlation functions out of equilibrium. A generalized fluctuation-dissipation theorem, which relates these correlation functions with generalized susceptibilities, is derived. The method of nonequilibrium-thermodynamic Green functions, which proves useful for calculations, is also presented. Two illustrative applications of the formalism, which study optical responses in ultrafast laser spectroscopy and Raman scattering of electrons in III-N semiconductors (of “blue diodes”) driven away from equilibrium by electric fields of moderate to high intensities, are described.