Coherence Effects and Time Dependences of the Optical Response of Surfaces and Interfaces of Optically Absorbing Materials

Components in the optical spectra of surfaces and interfaces that are related to energy and lifetime derivatives of the bulk dielectric response can be understood if final-state coherence effects that are ignored in conventional quasistatic approaches are retained. The theory shows that the finite penetration depth of light in optically absorbing materials generates wave packets that are localized to the same region as the photon field. This eliminates the energy-conservation and causality violations of the conventional picture and in particular allows the surface or interface potential to influence energy gaps and broadening parameters. Since the localized packets are only stable dynamically, optical absorption is an intrinsically nonlocal process. The formulation also provides a quantitative physical basis for Lorentzian broadening, which is usually treated phenomenologically.