Seminars Archive

Pump and probe experiments from first principles

Abstract
The development of ultra--short laser pulses has opened the opportunity to investigate the dynamics of electrons on the fs time--scale (1 fs=10^-15 seconds). After the photo--excitation with such lasers pulses, electrons are in a regime which is highly out--of--equilibrium. Here we present a novel numerical approach, based on the merging of the out--of--equilibrium Green's function method with the \ai\, Density--Functional--Theory, to describe this regime in semi--conductors. Silicon is used as reference material to show the physical process involved. The simulations are also compared with recent two photon photo--emission and transient--reflectivity measurements.
In the 2PPE experiment we show that different processes take place: (i) scattering between degenerate states, activated by the pump--pulse induced symmetry breaking, (ii) L --> X inter--valley scattering, and, finally, (iii) the relaxation towards the thermal equilibrium. In the TR experiment we underline the key role of optical--gap renormalization induced by the pump pulse, combined with bleaching, needed to explain the experimental signal.
Moreover, we discuss how the same approach is able to capture coherent excitons in materials with strong electron-hole interaction. This ensures a correct description of the interaction with the pump pulse. We consider as an example MoS₂ and discuss transient absorption and transient Kerr in the system. Finally we consider a model system with strong lectron-hole interaction and discuss how the approach should be extended to capture the formation of non-coherent excitons populations and describe time resolved photoemission in this case.