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

TL;DR

This study investigates how the nonlinear optical absorption in MoS2 changes from monolayer to multilayer forms, revealing layer-dependent transitions between saturable and reverse saturable absorption influenced by band gap, defects, and temperature.

Contribution

It provides new insights into the layer-dependent nonlinear absorption mechanisms in TMDCs, highlighting the transition between RSA and SA based on physical and environmental factors.

Findings

Layer number determines the band gap and nonlinear absorption behavior.

Transition from RSA to SA depends on layer thickness, temperature, and defects.

Two-photon excitation dominates in wider bandgap monolayers, while one-photon excitation is favored in narrower bandgap multilayers.

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.