Oxygen vacancy engineering of MOF-derived ZnO/Co3O4 nanocomposites for high harmonic mode-locking operation

Abstract

In this work, we successfully prepared ZnO/Co₃O₄ nanocomposites with different oxygen vacancy concentrations by calcining core–shell ZIF-8@ZIF-67. ZnO/Co₃O₄ nanocomposites possessing remarkable nonlinear optical (NLO) properties in the near-infrared region. ZnO/Co₃O₄ nanocomposites as saturable absorbers were successfully applied in an Er-doped mode-locking fiber laser for the first time. At a pump power of 30 mW, conventional soliton mode-locking pulses with a central wavelength of 1558.3 nm, a pulse duration of 1.71 ps, a spectral width of 1.55 nm, and a fundamental frequency of 6.06 MHz were generated. High harmonic mode-locking pulses of 242nd and 181st harmonics (corresponding to the repetition rates of 1.465 and 1.114 GHz) were also separately obtained, and the harmonic mode-locking pulse duration was ∼1.5 ps. These experimental results well indicate that oxygen vacancies in ZnO/Co₃O₄ can improve its photogenerated electron–hole separation, thus enhancing the effect of light–matter interactions and the nonlinear optical response. Besides, its lower saturation intensity and larger modulation depth could benefit harmonic mode-locking pulse generation.