Ultrathin Metal-Organic Framework: An Emerging Broadband Nonlinear Optical Material for Ultrafast Photonics

Crystalline porous metal-organic frameworks (MOFs) with nanometer-sized void spaces, large surface areas and ordered reticular motifs have offered a platform for achieving disruptive successes in divisional fields. Great progress in exploring the linear and nonlinear optical features of MOFs has been achieved, yet third-order optical nonlinearities in two-dimensional (2D) MOFs have rarely been studied. Here, a broadband nonlinear optical amplitude modification and phase shift are demonstrated in a few-layer nickel-p-benzenedicarboxylic acid MOF (Ni-MOF). The calculated bandgap of Ni-MOF decreases from 3.12 eV to 0.85 eV as the doping of Ni ions increases, indicating the ability of this material to be used for optical amplitude modulation from the visible to the near-infrared region, which is experimentally confirmed via a Z-scan technique. The determined third-order optical nonlinearities resemble those of other low-dimensional nonlinear optical materials, suggesting the wide potential of Ni-MOF for application in optoelectronics. As an example, a Ni-MOF-based saturable absorber was implemented into fiber resonators to demonstrate its broadband mode-locking operations. A femtosecond laser pulse was readily obtained in the telecommunication wavelength window in an integrated all-fiber resonator. Considering the chemical compatibility and rich variability, these primary investigations pave the way towards advanced photonics based on multifeature MOF materials.