Mid-Infrared Nonlinear Optical Responses in Strong Charge-Transfer Dominated Superalkali-Superhalogen Hybrid Clusters

Abstract Optoelectronic devices for national defense and security, environmental monitoring, and biomedical diagnostics demand greater mid-infrared nonlinear optical (MIR NLO) performance. However, conventional MIR NLO crystals suffer from bottlenecks like narrow transmittance range, low laser damage threshold, or stringent phase matching, failing to meet high-end device requirements. Here we propose a cluster-assembly strategy that delivers record-level NLO performance by pairing superalkali (NM 4 , M = Li, Na, and K) with superhalogen (MgF 3 ) clusters. Density functional theory (DFT) calculations reveal that the resulting dimers are thermodynamically robust and exhibit exceptional NLO properties. The first hyperpolarizability ( β 0 ) of NK4-MgF3 reaches 35127 a.u. (approximately 1.13×10 -50 C 3 m 3 J -2 ), far exceeding that of most donor-acceptor (D-A) systems. The excellent nonlinear optical performance is attributed to a strong electron transfer process between the building blocks and the low transition energy of the crucial excited states. The transparent window is determined by the combination of infrared and electronic absorption spectroscopy. Notably, the NK 4 -MgF 3 dimer can stably anchor on single-layer hexagonal boron nitride (h-BN), and its second-order NLO coefficient is an order of magnitude higher than that of single-layer h-BN and most traditional MIR NLO crystals. This underscores the great potential of the cluster assembly strategy for preparing high-performance two-dimensional NLO materials.