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

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 Al m N n (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 Al m N n 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.