A Transition of Saturable and Reverse Saturable Absorption between Monolayer and Bilayer/Multilayer of TMDCs

Abstract: The nonlinear absorption of two-dimensional molybdenum disulfide was analyzed using the Z-scan technique. Sample size corresponding to a number of layer determines the band gap in the materials which play the key role for nonlinearity. Exciton dipole transition from ground state to excited state for monolayer and bilayer/multilayer is determined by the excitation source, ESA and GSA values. If the band gap is wider than the excitation source, two-photon excitation process is dominant which requires the larger ESA than GSA. For narrower bandgap, one-photon excitation is favorable because of the larger GSA than ESA. In addition, defects, the temperature may alter the band gap. Therefore, the transition from RSA to SA and vice versa is determined by the number of layers in materials, temperature, and defects for a given excitation energy.