Size-Dependent Nonlinear Optical Properties of Atomically Thin Transition Metal Dichalcogenide Nanosheets

Abstract

Size-dependent nonlinear optical properties of modification-free transition metal dichalcogenide (TMD) nanosheets are reported, including MoS2 , WS2 , and NbSe2 . Firstly, a gradient centrifugation method is demonstrated to separate the TMD nanosheets into different sizes. The successful size separation allows the study of size-dependent nonlinear optical properties of nanoscale TMD materials for the first time. Z-scan measurements indicate that the dispersion of MoS2 and WS2 nanosheets that are 50-60 nm thick leads to reverse saturable absorption (RSA), which is in contrast to the saturable absorption (SA) seen in the thicker samples. Moreover, the NbSe2 nanosheets show no size-dependent effects because of their metallic nature. The mechanism behind the size-dependent nonlinear optical properties of the semiconductive TMD nanosheets is revealed by transient transmission spectra measurements.

2D Saturable Absorbers for Fibre Lasers

“…After initial successes demonstrating the effectiveness of MoS 2 [30] as a saturable absorber in pulsed fibre lasers, recent studies have considered other members of the TMD family of 2D materials, including MoSe 2 , WS 2 and WSe 2 , revealing complementary optical properties and suitability as a high-potential photonic platform. Strong intensity-dependent absorption has been measured, with the magnitude of the nonlinear optical response being found to depend on flake size and thickness, suggesting that few-layer TMD-based devices could be engineered for specific applications [60,62].…”

“…Additionally, it was shown that thermal effects under nanosecond or longer optical pulse excitation can lead to effects, including the formation of micro-bubbles, which cause nonlinear scattering. Nonlinear scattering manifests as an optical limiting behaviour similar to RSA [33,60].…”

“…Indeed, for many nonlinear absorption studies on TMD materials, the effect was not monotonic: under sufficiently high intensity, the material exhibited an increase in absorption for increasing intensity (i.e., reverse saturable absorption, RSA, or optical limiting) [33,42,54,56,60]. This RSA effect has been attributed to two-photon absorption (TPA), where under intense illumination, two photons can transfer their energy to excite a single electron over the bandgap, explaining why this effect has been observed for excitation wavelengths corresponding to photon energies less than the material bandgap, but greater than half the gap energy.…”

“…Zhou et al compared flakes of >100 nm, with ∼50 nm lateral size (of constant thickness), and showed that excited state absorption (ESA) occurred in the smaller flakes. They proposed that a greater number of edge-states existed in the bandgap for smaller flakes, which were quenched at high incident intensities [60]. The ratio of edge to surface area has also been attributed to sub-bandgap saturable absorption [32].…”

Emerging 0D Transition‐Metal Dichalcogenides for Sensors, Biomedicine, and Clean Energy

“…Tailoring the size and dimension of nanomaterials has become one of the most fascinating routes to endow them with novel properties and further broaden their applications . Of significance, transformation from 2D to 0D morphologies occurs when the planar sizes of 2D materials are reduced to below 100 nm for both x‐y dimensions.…”