Zn, Cd and Hg doping of AgInS₂ quantum dots - efficient strategy to modify nonlinear absorption

This work aims to investigate a possible strategy to enhance the nonlinear optical absorption in a well-known group of defect-rich ternary AgInS₂ quantum dots. In this work, we propose that hosting of Zn²⁺, Cd²⁺ and Hg²⁺ ions in the AgInS₂ structure can be an effective way to tune optical nonlinearities in the quantum dots of the I-II-VI system. Non-linear refraction and non-linear absorption in the AgInS₂:Zn²⁺, AgInS₂:Cd²⁺ and AgInS₂:Hg²⁺ quantum dots were measured over a wide range of wavelengths (550-1800 nm) using the Z-scan technique. The results showed that the changes caused by the doping of AgInS₂ quantum dots with heavier Zn homologues are of different nature. The observed differences are discussed here in terms of changes in the electronic structure of defects under the influence of Zn, Cd and Hg doping. Distinct spectral changes induced by hosting Zn²⁺, Cd²⁺ and Hg²⁺ ions are also well resembled in the nonlinear excitation regime. It is shown that Cd²⁺ doping and Hg²⁺ ions have much greater impact on the two-photon (σ₂) and three-photn (σ₃) absorption cross section enhancement than zinc doping. The results indicate that the two- and three-photon absorption cross section values can be significantly increased by the adopted doping strategy, and the AgInS₂ quantum dots may be even better suited not only to the two-photon but also to the three-photon fluorescent techniques commonly used for bio-sensing applications.