Nonlinear optical response of ZnO/HfO₂ core/shell nanorod arrays under continuous wave laser irradiation

ZnO nanorods (NRs) were synthesized by the hydrothermal method and coated by HfO₂ sell layers by using the atomic layer deposition to form ZnO/HfO₂ core/shell NRs arrays. XRD showed the hexagonal wurtzite structure of ZnO NRs and tetragonal phase of HfO₂. Scanning electron microscope and atomic force microscopy analysis illustrated vertically well-aligned ZnO/HfO₂ core/shell NRs structure. Optical property was studied by PL which revealed that the near band-edge emission of the ZnO NRs improved after coating by HfO₂ shells. The band gap energy was estimated about 3.51 eV by UV–Vis. For the first time, the measurements of nonlinear optical (NLO) properties of ZnO/HfO₂ core/shell NRs structures have been performed by using a continuous-wave (cw) Nd:YAG laser at 532 nm by the Z-scan technique. A negative nonlinear refractive index and a positive absorption coefficient were calculated equal to 8.6 × 10⁻⁶ cm²/W and 1.75 cm/W for ZnO/HfO₂ core/shell NRs, respectively. A third-order NLO susceptibility of ZnO/HfO₂ core/shell NRs was measured of the order of 10⁻² esu which is a large value. The results suggest that ZnO/HfO₂ core/shell NRs are promising candidates for NLO applications.