Nonlinear optical characterization of two-dimensional materials under pressure

Abstract

Atomically thin two-dimensional (2D) crystals form a fast-growing class of new materials, featuring ultimate 2D platform for studying physics and potential applications. Depending on their various crystal structure and symmetry, 2D materials display contrasting physical properties from band insulator to superconductors. Despite the explosive findings in 2D materials, the understanding of the newly discovered 2D materials particularly in nonlinear optical regime is still at an infantry stage. Second harmonic generation (SHG), a nonlinear process dominated by the second-order susceptibility of the materials is a powerful nonlinear optical tool for the study of crystallographic symmetry and lattice structures. Owing to the atomic thickness of 2D materials, phase matching is no longer required and SHG could be widely used to in-situ characterize crystal structure and crystallographic symmetry for seeking physical insights. This thesis presents a series of experimental studies of 2D crystals under hydrostatic pressure by means of in-situ nonlinear optical spectroscopies. Our aim is to understand the evolution of crystal symmetry of these materials under pressure. In the first part of this thesis, we focus on discussing the structural evolution change of Indium Selenide (InSe) flake by continuously changing pressure using circularly and linearly polarization-resolved SHG spectroscopies. We found a reconfigurable second-order phase transition of layered InSe in experiments. The structural symmetry transition from threefold rotational symmetry to twofold mirror symmetry by applying pressures was revealed by our in-situ nonlinear optical technique. The nonlinear optical calculations further verified that the interlayer translation plays a very significant role in the evolution of polarizations response of SHG signals. This opens new routes toward potential applications of manipulating crystals symmetry of 2D materials. In the second part, we further experimentally study the structural symmetry transition in ultrathin tungsten disulfide (WS2), including 1L, 2L and 3L samples. The layer-dependent response of SHG signal under a hydrostatic pressure justified a controllable interlayer translation freedom for van der Waals force coupled 2D materials characterized by Raman spectroscopy and polarization-resolved SHG spectroscopy. The invariance of a six-petal pattern for monolayer WS2, unobservable signal in bilayer one and variable azimuthal polarization pattern in trilayer one under up to 7.0 GPa justify the interlayer translation dominates the crystals symmetry of 2D materials. We further confirmed an inter-layer sliding between adjacent layers is along zigzag direction by combing the bond additivity model. Our study reveals a feasible method to reengineering manipulation of structure symmetry of 2D materials. Continuous tunability of structural symmetry under hydrostatic pressure can be probed in a noninvasive and useful nonlinear optical technique. This work can be extended to other 2D materials and van der Waals heterostructures.