Ultrafast Dynamics of Photoexcited Insulators Probed by Time- and Angle-Resolved Photoemission

Photoexcitation above the band gap of insulators or semiconductors may lead to non-equilibrium processes on ultrafast timescales. Depending on excitation density their dynamics are governed by exciton formation and electron-phonon scattering or more complex phenomena leading to phase transitions. These processes typically occur on ultrafast (femto- to picosecond) time scales. We employ femtosecond time- and angle-resolved photoemission spectroscopy (trARPES) to study surface excition formation as well as ultrafast insulator-to-metal (IM) transitions in several materials.

On ZnO surfaces we observe the formation surface-bound excitons within only 200 fs after photoexcitation, which are stable in the presence of a charge accumulation layer caused by hydrogen adsorption. Strong excitation close to the Mott limit enhances screening of the Coulomb interaction and reduces the exciton formation probability [1]. On the other hand, for the correlated electron material VO₂, strong photoexcitation leads to an quasi-instantaneous band gap collapse upon photo-excitation, followed by hot carrier relaxation within 200 fs. In conjunction with many body theory, these results show that the photoinduced phase transition is caused by doping of valence band with photoholes, leading to significantly enhanced screening of the Coulomb interaction and drastic band gap renormalization [2]. Furthermore, we have investigated the mechanism of the photoinduced IM transition in the prototypical charge-density wave (CDW) system (RTe₃, R=Te, Dy), where at low temperatures a periodic lattice distortion leads to the opening of an electronic gap at the Fermi surface. trARPES allows for probing directly the transient evolution of the electronic structure and collective phonon dynamics through their influence on the electronic band structure. This enables a systematic study of the opening and closing of the CDW gap and the dynamics of the CDW amplitude mode during the photo-induced non-equilibrium phase transition [3].

1. J.-C. Deinert, et al, Phys. Rev. Lett. 113, 057602 (2014)
2. D. Wegkamp, et al., Phys. Rev. Lett. 113, 216401 (2014)
3. L. Rettig, et al, Faraday Discuss. 171, 299 (2014).