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.