It is widely known that a monochromatic electro-magnetic wave can travel forever without losing information. It is similarly common knowledge among experimentalists that after a material has been photo-excited an additional, induced, electric field gets super-imposed to the external probe. This field is generated by the electronic charge oscillations. The key point is that this induced field appears only after the excitation. The system, at rest, is not able to produce it. 

 

What makes the electrons to oscillate in phase and produce a macroscopic field is coherence?

 

In this talk I will discuss the role of coherence in out-of-equilibrium phenomena. I will describe some particularly striking examples of how a photo-excited material can reveal coherent phenomena like: magnetic phase transitions, coherent phonon oscillations and the formation of a spontaneous excitonic condensate.

 

I will also discuss the peculiarities of these phenomena taking the equilibrium perspective. From which it will appear evident that the system reacts is non-perturbative. A dynamics extremely challenging to describe theoretically.

 

The final message of my talk will be that concepts taken from the equilibrium regime are, often inadequate to describe out-of-equilibrium systems. A striking example will be the Exciton. Can we really say that excitons exist as real population of bound electron-hole pairs?