Ultrafast and nonlinear optical investigations on van der Waals materials

Abstract

Van der Waals (vdW) materials, due to their highly anisotropic nature, have become a novel platform for creating flat 2-dimensional layers and investigating the 2-dimensional properties of the material. Research on these materials has further expanded through the realization of novel emergent phenomena in heterostructures and superlattices made by artificial stacking, where weakly bonded interlayer structure plays a key role. Therefore, understanding the properties of materials based on their structure is pivotal for this field of research. Within this context, we investigated various properties of vdW materials and interpreted them from a structural perspective using ultrafast and nonlinear optical methods. We characterize the thermal transport properties of TMPS3 (TM = Mn, Ni, Fe) using time-domain thermoreflectance method and interpreted them with their structural disarrangement. The multiferroicity of NiI2 was traced using optical second harmonic generation, and we demonstrated the possible existence of multiferroicity down to the bilayer limit, where interlayer interaction is largely suppressed. We also applied hydrostatic pressure using a diamond anvil cell. Under the hydrostatic pressure application, we discovered that transition metal ditellurides undergo interlayer structural phase transitions at low pressure and intralayer structural phase transitions at high pressure. We discuss various physical properties emerging in the high-pressure phase based on their structural changes. Furthermore, we discovered that the stacking angle of bi layer transition metal ditelluride, WSe2, can be continuously controlled with the application of hydrostatic pressure. These characterizations of vdW materials from a structural perspective will contribute to the advancement of research based on vdW materials.