Many-body interactions in semiconductors revealed by two-dimensional electronic spectroscopy

Place: Hinarejos room. 

Abstract: 

Semiconductors and layered van-der-Waals materials are extremely promising candidates for high-speed optoelectronic and functional devices as well as nonlinear optics applications. At the very core, their development relies on the profound fundamental understanding and control of the optical properties of matter on ultrafast – that is, femtosecond (10^-15 s) – timescales. Especially the investigation of Coulomb interactions between and among free charge carriers (FCs) and quasiparticles (such as, e.g., excitons) is of vital importance in condensed matter physics because these many-body phenomena commonly dominate the nonlinear optical response. Particularly the separation of exciton- and FC-driven effects is desirable, not least because the carrier species (exciton or FC) many provide an additional control knob to modify the optical response, but extremely challenging from an experimental point of view. After introducing the general concepts and limitations of ultrafast transient absorption spectroscopy, we will explore the benefits of more sophisticated two-dimensional (2D) spectroscopy techniques to tackle these challenges. In a first example, experiments on bulk gallium selenide separate the room temperature exciton dissociation from dominant free-carrier-driven many-body contributions through the excitation energy information that is inherent to the 2D technique. Additional experimental results on gallium arsenide (GaAs) highlight the capability of the technique to distinguish many-body interactions from energy migration pathways, which yield a cluttered response in standard ultrafast spectroscopy schemes. The measurements, and in particular the 2D approach, pave the way towards the detailed study of energetic correlations in semiconductors with unprecedented information content.