Giant two-photon absorption in MXene quantum dots

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 (Nb 2 C QDs) that is induced by a two-even-parity states transition. The TPA response of Nb 2 C QDs was observed in the near-infrared band of 1064-1550 nm. Surprisingly, at 1064 nm, Nb 2 C 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 Nb 2 C 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 Nb 2 C QDs at 1064 nm. It offers the possibility of manipulating the NLO response of Nb 2 C via morphology or surface termination.