Defect-mediated carrier dynamics and third-order nonlinear optical response of WS2 quantum dots

In this Letter, we report the third-order absorptive and refractive nonlinear optical response of highly luminescent WS2 quantum dots (QDs) in the off-resonant femtosec-ond and nanosecond pulses, which is beneficial for optical limiting and quantum information processing. For 800 nm femtosecond excitation, QDs show two-photon absorp-tion (beta = (107 +/- 2)x10-3cm/GW) with positive nonlinearity originating from bound carriers. This picture changes sig-nificantly for 532 nm nanosecond excitation, where it shows reverse saturable absorption with negative nonlinearity pri-marily originating from the sequential absorption of two single photons through the shallow defects, creating free carriers. Our results provide a promising route toward low -dimensional optoelectronic devices. (c) 2022 Optica Publishing Group