High thermal nonlinearity in thin films of Zn-doped CuS QDs dispersed in a PMMA matrix for NLO applications

We report improved thermally stimulated third-order nonlinear optical coefficients (nonlinear refraction and nonlinear absorption) in thin films of Zn-doped CuS quantum dots (QDs) dispersed in a poly (methyl methacrylate) (PMMA) matrix. Undoped and Zn-doped (1, 2, and 3 wt%) CuS QDs were synthesized using a co-precipitation method. Powder samples were characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), ultraviolet-visible (UV-vis) spectroscopy, and open- and closed-aperture z-scan technique. The XRD spectra confirmed the formation of a highly pure covellite CuS phase with a hexagonal crystal structure in the P6₃/mmc (194) space group. The EDS analysis validated the purity of the synthesized samples. HR-TEM micrographs of 1 wt% Zn-doped CuS QDs clearly showed the formation of very narrow-sized Zn-doped CuS QDs with average sizes of 2-3 nm. A strong blue-shift in the maximum absorption wavelength is observed for the 3 wt% Zn-doped sample with a band gap of 2.24 eV. Considerably enhanced third-order nonlinear refraction and absorption coefficients were obtained for the 3 wt% Zn-CuS-PMMA nanocomposite thin films, indicating their potential use in optical limiters and photonic devices.