Chapter 4 - Optical properties of two-dimensional materials

Abstract

Several analysis methods are introduced to explore the optical properties of two-dimensional (2D) materials such as the topological insulator, the transition-metal dichalcogenide, the transition-metal monochalcogenide, and the MXene. Raman spectroscopy characterizes the number of layers and structures of the 2D materials by comparison of AFM and optical microscope measurement results. The characteristic Raman peak varies for the number of layers giving accurate information of the 2D material sample. Electronic band structures are analyzed to investigate electronic and optical bandgaps, which determine the operational spectrum wavelength of the 2D material. Especially for mid-infrared or THz wave operation, the optical spectrum must be large implying small or zero bandgap, which can be obtained by metallic or semimetallic 2D materials. Linear and nonlinear absorption is used to characterize saturable absorbers (SAs) that are used to obtain mode-locked ultrashort laser pulses. Nonlinear absorption shows the saturation-absorption characteristics for the operation as an SA.