Optical Properties of Interstellar Dust from Cosmic Dust Analogs Studied in the Lab

In the past years, cosmic dust research has taken great advantage from the laboratory efforts to investigate the properties of analog materials such as amorphous magnesium/iron silicates and various forms of carbon. Optical properties are particularly important as they provide the link to observations and need to be incorporated into models in the form of dust opacities. Laboratory investigation of dust opacities has to cover a wide wavelength range from the UV to the millimeter wavelength range and to study the temperature dependence of the electromagnetic excitation processes governing the optical response. For understanding the underlying physics, a detailed knowledge of the microscopic structure is required, which in turn may allow predictions for dust in cosmic environments, being subject to continuous structural modifications. Given the complexity of amorphous structures, our understanding of the optical properties of interstellar dust is still incomplete. In particular, the opacity outside the strong fundamental excitations of the electron and phonon system are not yet very well constrained, but even some of the strong bands such as the π -π ⋆ transition band of carbonaceous dust and its relation to the interstellar 217.5 nm extinction feature still require closer investigation. In this respect, it is crucial to develop further the structural analysis of such amorphous structures, and to find the relation to optical properties. This review intends to summarize the current state of these efforts.