Enhanced nonlinear optical response and ultrafast carrier dynamics in amorphous Fe-doped ZIF-67

Abstract

Developing novel materials with excellent nonlinear optical response and ultrafast carrier dynamics is of great significance for the development of optoelectronic devices. In this contribution, amorphous Fe-doped ZIF-67 (Fe-ZIF-67) was successfully synthesized and characterized in detail. Femtosecond-resolved transient absorption spectroscopy (TAS) clearly revealed the ultrafast carrier dynamics of Fe-ZIF-67 and ZIF-67. The results showed that amorphous Fe-ZIF-67 exhibited a deeper photoinduced bleaching valley and a higher photoinduced absorption peak when compared with the crystalline ZIF-67. Meanwhile, the introduction of Fe ions promoted the charge transfer in ZIF-67 and significantly inhibited the photogenerated carrier recombination, thus greatly enhancing the nonlinear optical response of Fe-ZIF-67. This indicates its great potential in nonlinear optical applications. In addition, amorphous Fe-ZIF-67 displayed excellent nonlinear saturation absorption features at 1.5 μm, with a modulation depth of 3.1% and a saturation light intensity of 1.84 MW cm⁻². Subsequently, Fe-ZIF-67 was inserted into an erbium-doped fiber ring cavity to generate conventional soliton mode-locking laser pulses with a pulse width of 1.23 ps and a wavelength of 1560.8 nm. Surprisingly, the harmonic order of mode locking could be tuned by adjusting the polarization controller (PC), which is different from the previously known method of adjusting the pump power. Finally, a 95th-order harmonic mode-locking pulse output was obtained with a repetition rate of 694.8 MHz. Compared to the crystalline ZIF-67 saturable absorber (SA), amorphous Fe-ZIF-67 SA increased the mode-locking frequency to the order of several hundred megahertz. Therefore, it is of great significance to synthesize amorphous advanced MOF functional materials by changing the crystal structure through doping and to further investigate their feasibility for application in the field of optoelectronics.