Ultrafast THz- and Infrared-Spectroscopy of Strongly Bound Excitons in GaN, ZnO and Transition-Metal-Dichalcogenides

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We propose femtosecond optical-pump-THz-probe and optical-pump-IR-probe experiments (THz/IR-PP) to investigate the ultrafast dynamics of strongly bound excitons in GaN, ZnO and transition-metal dichalcogenides (TMDC). In the first part of the project, the relaxation dynamics of donor- and acceptor-bound excitons in GaN and ZnO will be investigated. Here, ultrafast THz and mid-IR spectroscopy can provide access to transitions between internal exciton levels of the strongly bound pairs - for insight into pair dynamics at large momenta which are only indirectly accessible in interband methods. Furthermore, we will search for stimulated THz-emission via intra-excitonic transitions in the range of 0,8 THz to 11,5 THz after resonant exciton generation. In the course of the second project part, we will study the dynamics of excitons and free carriers in mono- and multi-layers of the TMDCs molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), along with the photo-induced dynamics of low-energy excitations in titanium diselenide (TiSe2) whose intriguing low-temperature phase has been associated with the development of an excitonic insulator phase. Due to their direct band gap, the TMDC monolayers MoS2 and WSe2 are not only of interest for the study of fundamental many-particle interactions but are also promising material systems for the realization of ultrathin transistors and optoelectronic devices in the burgeoning fields of spin- and valleytronics. The dynamics of spin and valley effects will be investigated by time-resolved Kerr and Faraday rotation spectroscopy. Finally, 1T-TiSe2 undergoes a charge density wave (CDW) instability characterized by a 2 × 2 × 2 real-space superstructure beneath a critical temperature TC . It is assumed that the CDW is caused by an excitonic insulator state, but the relative influence of excitonic correlations and lattice distortions is under debate. Here, we will investigate the CDW gap dynamics as well as the THz-frequency phonon excitation spectrum for insight into the mutual coupling of these low-energy degrees of freedom in 1T-TiSe2 on their natural interaction timescales.