Strain-Mediated Second Harmonic Generation Enhancement of Bimetallic Metal–Organic Frameworks

Triggered by the demand of developing practical metal-organic framework (MOF) based nonlinear optical (NLO) devices, there is a surge of research interest in the regulation of NLO properties of MOFs. Strain engineering provides a new opportunity to regulate the optoelectronic properties of materials and device performance at the interface. Here we introduce the induction of the strain field in structures by constructing bimetallic Zn/Cu-MOFs. The NLO response of MOF1-9 with different metal element ratios was characterized in the near-infrared window. Surprisingly, MOF3 exhibited a giant second harmonic generation (SHG) enhancement compared with that of pristine Zn- and Cu-MOF. The determined second-order nonlinear susceptibility of MOF3 (∼19.86 p mV^-1) is one order of magnitude larger than that of pristine Zn-MOF (1.65 p mV^-1) and Cu-MOF (3.88 p mV^-1) and much higher than the commonly used NLO crystals. Moreover, the anisotropy ratios of MOF1-9 can be harnessed in a wide range (∼4.7-118.8). The anisotropy ratio of MOF4 (∼118.8) is much higher than that of the most excellent 2D material to date, to our best knowledge. A first-order perturbation of second-order polarization based on point group description was introduced to explore the regulation mechanism of the NLO response under the strain field of MOFs.