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

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.