Electronic properties and third-order optical nonlinearities in tetragonal chalcopyrite AgInS2, AgInS2/ZnS and cubic spinel AgIn5S8, AgIn5S8/ZnS quantum dots

Abstract

Comprehensive studies on the electronic properties and spectral dependencies of the third-order nonlinear optical properties of quantum-confined tetragonal chalcopyrite AgInS₂ and its non-stoichiometric cubic spinel AgIn₅S₈ with reference to their corresponding Zn²⁺ alloyed compounds i.e. AgInS₂/ZnS and AgIn₅S₈/ZnS are presented in this work. Nonlinear refraction and nonlinear absorption in the quantum-confined nanocrystals were measured in a wide range of wavelengths (550–1200 nm) using the Z-scan technique. The results revealed the presence of strong two-photon absorption bands in the near-infrared range for both compounds. The impact of Zn²⁺ ion alloying was also investigated for both compounds revealing a significant increase of the cubic nonlinearity for the chalcopyrite quantum dots and a negligible change for the spinel-like quantum dots which was discussed as a consequence of different surface reconstruction mechanisms of the tetragonal and cubic nanocrystals. The cubic nonlinearity of the quantum dots is discussed in terms of electronic properties and linear dielectric function dispersions of both systems obtained within the density functional theory. Kinetics of the two-photon excited state recombination was investigated with the femtosecond time-resolved photoluminescence spectroscopy revealing the complex character of electronic relaxation in both systems. The results indicate that the optical properties of the tetragonal AgInS₂ and cubic AgIn₅S₈ quantum dots are much better suited for the steady-state as well as time-resolved multiphoton fluorescence techniques utilized commonly for bio-sensing applications than their CuIn_xS_y homologues.