Transient Nonlinear Optics of Two-Dimensional Materials

Two-dimensional (2D) layered materials with remarkable optical and electronic properties, are promising for a variety of applications in the area of electronics, photonics, and optoelectronics. Recently, increasing efforts are targeted toward the nonlinear optical properties of 2D materials, which are not only fascinating in terms of fundamental physics, but also intriguing for various potential applications. Beneficial from their atomically thin nature, 2D materials are the ideal material to address the subjects on how the materials respond to the light excitation without considering phase-matching effects. In this thesis, I devote main efforts to studying the relation between carrier dynamics and nonlinear optical responses in 2D materials and to developing potential applications in the area of ultrafast nonlinear optics. Details are expressed as below: 1. Transient linear optical absorption of monolayer MoS2 is studied in the sub-bandgap region from 860 nm to 1400 nm. A significant absorption enhancement of up to 4.2% is observed due to the pump-induced absorption at the excited carrier states. 2. Transient second-harmonic generations in transition metal dichalcogenide monolayers are investigated in a broadband energy range. The pump modified excitonic oscillation strength enables to enhance the SHG at dark exciton states and bleach the SHG at bright exciton states. The enhancement of SHG is achieved as high as 386 times. Further, the SHG modulation has been realized with a modulation depth of up to 60% and a response time of ~600 fs. 3. Transient higher-order harmonic generations are finally studied. Broadband third-harmonic spectroscopy displays a series of electronic states in monolayer MoS2 with high signal contrast up to 800 times. The ultrafast photocarrier dynamics of electronic states in MoS2 are explored by transient third-harmonic generation. Further, the all-optical modulation of high-harmonic generation in monolayer MoS2 is demonstrated with a modulation depth up to 90%, offering a promising platform for the ultrafast optical signal processing of nonlinear optics.