Nonlinear Optical Interactions and Relaxation in 2D Layered Transition Metal Dichalcogenides Probed by Optical and Photoacoustic Z‑Scan Methods

Atomically thin 2D materials, currently being at the forefront of scientific and technological interest, can be categorized as metallic, semimetallic, semiconducting, insulating, or superconducting, depending on their chemical composition and structural configuration. They also exhibit, in some cases, a transition from an indirect to a direct bandgap alignment when bulk materials are scaled down to monolayers. An important class of 2D materials is layered transition metal dichalcogenides (TMDs) with a tunable bandgap, because photogenerated optical excitations and subsequent excitation dynamics, which produce energy migration and photogenerated charge carrier transport, make them promising candidates for a variety of optoelectronic devices, including solar cells, photodetectors, light-emitting diodes, and phototransistors. In this work, we probe the excitation dynamics following nonlinear optical absorption/scattering in two unexplored TMDs, metallic NbS₂ and semimetallic ZrTe₂, using a combination of the standard optical Z-scan and photoacoustic Z-scan techniques, and compare them with semiconducting MoS₂. The comparison of optical Z-scan (OZ-scan), which depends on the contributions of both nonlinear scattering and nonlinear absorption, with photoacoustic Z-scan (PAZ-scan), which depends only on nonlinear absorption due to local heating from nonradiative relaxation, allows us to separate these contributions from the total nonlinear response. In addition, these studies also allow us to look at the nature of nonlinear absorption as to whether it is due to saturable absorption (SA) of a one-photon transition, reverse saturable absorption (RSA) derived from two-photon excitation processes, or any combination thereof. In MoS₂, NbS₂, and ZrTe₂, we observed both SA and RSA. The relevant nonlinear absorption coefficient parameters were obtained. Density functional theory modeling provides an insight onto possible underlying physical processes.