Electromagnetic fields 4 External electromagnetic field 2PI effective action 2 Band structure 2 Band structure effects 2 Crystallographic orientations 2 Current conservation 2 Driven system 2 Electromagnetic field effects 2 Electron emission 2 Electronic systems 2 Expectation values 2 Gauge transformation 2 Generalized Ward-Takahashi identities 2 Grazing incidence 2 Mean values 2 Nonlinear control systems 2 Nonlinear driven transport 2 Nonlinear response theory 2 Photoelectron spectroscopy 2 Photoelectron spectrum 2 Photoelectrons 2 Photons 2 Response functions 2 Surface interactions 2 Time-dependent 2 Ultrashort Laser 2 Ultrashort pulses 2 Valence electron 2