Giant two-photon absorption in MXene quantum dots

Abstract

Looking for materials with compelling nonlinear optical (NLO) response is of great importance for next-generation nonlinear nanophotonics. We demonstrate an escalated two-photon absorption (TPA) in ultrasmall niobium carbide quantum dots (Nb2C QDs) that is induced by a two-even-parity states transition. The TPA response of Nb2C QDs was observed in the near-infrared band of 1064–1550 nm. Surprisingly, at 1064 nm, Nb2C QDs shows an enhanced TPA response than other wavelengths with a nonlinear absorption coefficient up to a value of 0.52 ± 0.05 cm/GW. Additionally, the nonlinear optical response of Nb2C changes to saturable absorption when the incident wavelength is between 400–800 nm wavelength. Density functional theory (DFT) validates that TPA, induced by two even-parity states transition, breaks the forbidden single-photon transition, enabling a tremendous TPA response in Nb2C QDs at 1064 nm. It offers the possibility of manipulating the NLO response of Nb2C via morphology or surface termination.