The DFG Emmy Noether grant (SE2558/2-1) that funds our group is dedicated to the theoretical description of pump-probe spectroscopies in solids. Michael became interested in working on such problems during his postdoctoral appointment at SLAC and Stanford University (2011-2014). At the time, the free-electron laser (FEL) at SLAC had just produced the first results and it was clear that the future was bright for ultrafast movies of the electronic motion inside solids with femtosecond time resolution. One femtosecond equals 0.000 000 000 000 001 seconds, which is really hard to imagine for us humans. If one stretches one femtosecond to become one second, then under the same stretching of time one second becomes a time that is similar to the age of the universe.
The key idea behind pump-probe spectroscopy is best illustrated with the analogy of the running horse. Eadweard Muybridge filmed the motion of a galopping horse back in the late 1870s. One of his goals was to figure out whether the horse has all four of its legs up in the air at the same time at any point in time, and the answer enabled by his stop-action movies was yes. In the pump-probe analogy, the cameras that he used to create the movie are the probe. The pump is the thing that sets the horse in motion in the first place. Obviously, it would not make too much sense to use a sub-second time resolution movie camera to film a sleeping horse. That is why we need a pump to observe interesting dynamics, at least for classical objects.
Spectroscopy from quantum objects has been a tool of the trade for more than a century (see, for example, spectral emission series of atoms). In solids, we use their spectral properties …
Nonequilibrium dynamics of condensed matter
Theory of pump-probe spectroscopy
Efficient numerical methods for real-time dynamics
Light-matter interactions for materials engineering