Surface states controlled broadband enhancement of two-photon absorption

We demonstrate the controllable broadband enhancement of two-photon absorption in a wide spectral range from 710 nm to 960 nm by controlling the surface states of aqueous Co2+ doped CdTe quantum dots, which is consistent with the measurement results of surface potential and fluorescence decay. The enhancement can be tuned in the range between 1 and 1.7 by changing the dopant concentrations that determine the surface states.