Exotic femtosecond nonlinear optical properties of laser ablated MoS2 quantum dots

Abstract

Molybdenum disulfide (MoS2) quantum dots (QDs) of sizes 2.05–4.40 nm were synthesized using the femtosecond pulse laser ablation in liquid (PLAL) method. The optical and ultrafast nonlinear optical (NLO) properties of MoS2 QDs were comprehensively studied using photoluminescence and femtosecond Z-scan method. We synthesized these QDs in ethanol (Et) and DI water (DW) with varying ablation times of 20 min and 10 min; the sample names are Et20 and Et10, DW20, and DW10, respectively. The nonlinear absorption (NLA) coefficients [three-photon absorption (3PA)], nonlinear refractive index, etc., of these QDs, were studied using the fs Z-scan method. Interestingly, Et20 possessed saturable absorption (SA) followed by reverse saturable absorption (RSA) due to the free carrier absorption (FCA) with FCA cross-section values of – (0.61–0.89) × 10−18 cm2. While for Et10, DW20, and DW10, we found 3PA, a rare finding for MoS2 QDs. The estimated values of the second hyperpolarizability of these samples were found to be ∼10−30 esu. Due to the strong NLA phenomena, these samples have the optical limiting (OL) onset values in the range of ∼ (5.11–8.43) mJ/cm2, proving the optical limiting application potential of these QDs. The obtained optical, NLO parameters, and Kane energy values of laser-ablated MoS2 QDs suggest that these can be suitable material systems for optoelectronic, NLO switching, logic gates, and other photonic applications.