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

Abstract

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 NbS 2 and semimetallic ZrTe 2 , using a combination of the standard optical Z-scan and photoacoustic Z-scan techniques, and compare them with semiconducting MoS 2 . 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 onephoton transition, reverse saturable absorption (RSA) derived from two-photon excitation processes, or any combination thereof. In MoS 2 , NbS 2 , and ZrTe 2 , 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.

Second-Order Nonlinear Optical Properties of Monolayer Transition-Metal Dichalcogenides by Computational Analysis

“…These 2D materials exhibit efficient light–matter interactions, having tightly bound excitons and large exciton-binding energies that lead to enhancements of the linear and nonlinear optical properties . In particular, group VI 2D TMDs are semiconductors, and their linear optical properties have been extensively investigated. ,− Furthermore, experimental measurements of the nonlinear optical properties are abound, particularly, for second-harmonic generation (SHG), in MoS 2 , − MoSe 2 , MoTe 2 , WS 2 , , and WSe 2 . − However, group V 2D TMDs display varying metallic and semiconducting behavior and, in general, have fewer experimental measurements of their linear optical properties − and no known SHG measurements. Nevertheless, group V TMDs could afford a number of linear and nonlinear optical applications. , …”

Switchable Nonlinear Optical Absorption of Metal–Organic Frameworks

“…. [38] I SA is the saturation intensity of the SA response. ∆T and T ns are the modulation depth and non-saturable loss of the sample, respectively.…”

Exploiting Nanomaterials for Optical Coherence Tomography and Photoacoustic Imaging in Nanodentistry

“…In ref. [73], the photoacoustic Z-scan was employed to estimate the efficiency of photothermal relaxation, therefore characterizing the metrics for a photoacoustic contrast agent to be used for nonlinear photoacoustic imaging. Besides employing endogenous substances or biocompatible exogenous as contrast agents for PAI, nanoparticles as gold−silica nanoshells [74] or conjugated polymer [75] nanoparticles have also been used as a contrast agent for photoacoustic tomography.…”