Optical nonlinearities and two-photon excited time-resolved luminescence in colloidal quantum-confined CuInS2/ZnS heterostructures

Abstract

Comprehensive studies on the spectral dependencies of third-order nonlinear optical properties and one/two-photon excited time-resolved luminescence of quantum-confined ternary CuInS₂/ZnS core/shell heterostructures with reference to their corresponding uncoated CuInS₂ cores are presented in this work. Nonlinear refraction and nonlinear absorption in the quantum confined type-I core/shell CuInS₂/ZnS and their corresponding CuInS₂ cores were measured over a wide range of wavelengths (550–1500 nm) using the Z-scan technique. The results revealed the presence of size-dependent two-photon absorption bands in the red and near-infrared spectral range. The cubic nonlinearity of core/shell CuInS₂/ZnS quantum heterostructures is discussed in terms of both the CuInS₂ core's optical transitions and the impact of the wide-gap ZnS shell layer. Kinetics of recombination of the single/two-photon excited states was investigated with femtosecond time-resolved photoluminescence spectroscopy revealing the complex character of electronic relaxation. A general statistical model involving normalized discrete distribution of decay rates was used for the analysis of luminescence decay curves. Different relaxation paths including the excitonic relaxation, donor–acceptor pair recombination and free-to-bound state recombination are discussed. The results indicate that optical properties of the CuInS₂/ZnS and CuInS₂ quantum dots are well suited for the steady-state as well as time-resolved single/multiphoton fluorescent techniques utilized commonly for the bio-sensing issues.