Spatiotemporal dynamics of photoexcited quasiparticles in two-dimensional crystals studied by ultrafast laser techniques

Abstract

Layered materials in which atomic sheets are stacked together by weak van der Waals forces can be used to fabricate two-dimensional systems. They represent a diverse and rich, but largely unexplored, source of materials. Atomically-thin structures derived from these materials possess a number of interesting electrical, optical, and mechanical properties, and are attractive for new nanodevices. For their applications in semiconductor industry, it is necessary to understand the dynamics of photoexcited quasiparticles that occur on ultrafast time scales of less than one nanosecond. In this dissertation, I discuss ultrafast optical experimental techniques and results from various two-dimensional materials, which provide information about electronic dynamics. First, a second harmonic generation technique that can be used to find the crystalline orientation, thickness uniformity, layer stacking, and single-crystal domain size is discussed, with results presented on exfoliated and chemical vapor deposition MoS2 samples. Second, a third harmonic generation technique is discussed, which can be used to explore nonlinear optical properties of materials, and results are presented on graphene and few-layer graphite films. Third, a spatially resolved femtosecond pump-probe is described, which can be used to study hot carrier and photoexcited phonon dynamics and results are presented on Bi2 Se3 sample. Then, exciton dynamics in MoS2 and MoSe2 are explored by using transient absorption microscopy with a high spatiotemporal resolution. Finally, a polarization-resolved femtosecond transient absorption spectroscopy that can be used to study valley and spin dynamics is discussed, with results presented on monolayer, few-layer, and bulk MoSe2 samples.