Theoretical Insight into the Optical Properties of Al m N n ( m = 1–8, n = 1–2) Clusters under External Electric Fields

Abstract Nonlinear optical (NLO) materials are indispensable functional components for optical communication, photonic data storage, and solid-state laser devices. Current strategies for regulating the NLO performances of aluminum-based nanoclusters mainly rely on compositional modulation, whereas modulating NLO responses without perturbing cluster stoichiometry remains a formidable experimental challenge. Herein, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations are performed to systematically investigate the NLO characteristics of AlmNn (m = 1–8, n = 1–2) clusters under external electric field (EEF) modulation. Structural scrutiny reveals that incremental Al stoichiometry elicits conspicuous geometric reconstruction, unveiling an explicit size-correlated structural evolution. Notably, appropriate EEF polarization enables the conversion of various non-superhalogen units into superhalogen species, which significantly improves the NLO performance of the clusters. Numerical results demonstrate that EEF regulation delivers extraordinary enhancement in the first hyperpolarizability of typical AlmNn clusters. The Al8N2 cluster presents a 669-fold improvement, the Al2N cluster exhibits a roughly 360-fold elevation, and the Al3N cluster obtains a 44-fold increment. Mechanistic studies demonstrate that EEF stimulation triggers intramolecular charge density redistribution, reduces the HOMO–LUMO energy gap, and consequently enhances the intrinsic NLO susceptibility. Furthermore, EEF modulation increases the dipole moment of clusters by 1–3 orders of magnitude and remarkably promotes molecular polarizability, hyperpolarizability, and electron spatial delocalization, which fundamentally accounts for the enhanced NLO activity. This work affords a robust and reliable paradigm for the rational design and precise optimization of high-efficiency AlN-based NLO functional nanomaterials.