Theoretical description of pump-probe spectroscopies in solids

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Ultrafast materials science is located at the boundary between condensed matter physics, which investigates the collective physics of many interacting particles in solids, and ultrafast spectroscopy, which investigates light-matter interactions on small length scales and on ultrashort time scales. One takes a complex many-particle system out of its thermal equilibrium by stimulation with a short laser pulse ("pump"), in order to subsequently monitor the excited state dynamics with a second ("probe") pulse. The goal of this research project is to theoretically understand the flow of energy from the pump laser into the solid on a microscopic level. Importantly, this understanding must connect the many-particle materials science language with the non-equilibrium nature of ultrafast science. In particular, I plan to investigate ordering mechanisms and their laser manipulation in ordered states, such as superconductors and charge-density waves. From a different angle, the interaction between the electrons and the ionic lattice can be manipulated by directly driving the lattice motion. I plan to investigate how lattice driving can help disentangle purely electronic ordering mechanisms from electron-lattice coupling driven mechanisms. Finally, I will study how new light-matter coupled states can be created, which enable tuning of the material properties in the presence of the pump laser. To this end, I will employ the Keldysh Green function technique, which naturally connects many-particle and non-equilibrium physics. I will use extensive numerical simulations for pump-probe spectroscopies, building upon existing computer codes, which were developed by us in the past few years. The proposed project will help guide experimental efforts as well as pave the way for further theoretical developments, in order to understand the dynamical interplay of electrons, lattice, and pump laser photons on an equal footing and to use the lessons learned in order to design optimized materials with better functionalities.