Femtosecond terahertz studies of excitons

Excitons, Coulomb bound pairs of one electron with one hole, are among the most important elementary excitations in condensed matter physics. They are often seen as analogous to the hydrogen atom. However, a complex phase diagram due to intricate many-body interactions sets limits to this picture. Up to now, excitons have almost exclusively been investigated by optical techniques resonant to the band gap. Since these studies involve generation or annihilation of electron-hole pairs, they are only indirectly sensitive to existing populations. In contrast, femtosecond terahertz (THz) technology (1 THz ap 4.1 meV) has been advanced in recent years to directly trace the complex-valued dielectric response of low-energy resonances in extreme non-equilibrium systems (Huber et al., 2005). The internal quantum fine structure of excitons has thus become accessible (Kaindl et al., 2003). We exploit THz spectroscopy to test the quasi-particle concept of excitons in a direct, time-resolved way and explore new quantum optical processes.