From Molecular Structure to Optical Functionality: Tailoring Nonlinear Optical Behavior in Phosphorus-Doped g-C3N4

Abstract

Phosphorus (P)–doped graphitic carbon nitride (g-C₃N₄) represents a significant advancement in nonlinear optical (NLO) materials, where subtle atomic modifications induce notable changes in electronic behavior. In this study, we conduct a comprehensive quantum-chemical and wavefunction-based investigation into the electronic and NLO properties of both pristine and P-doped g-C₃N₄, under static and dynamic regimes. Using advanced real-space function analyses, we reveal complex patterns of electronic excitation and charge redistribution, highlighting P-doping's pivotal role in enhancing van der Waals attractions and exchange-repulsion forces, which are essential for the material's binding interactions. The modulation of molecular (hyper)polarizabilities is systematically explored through sophisticated tools such as unit sphere representation and second-order NLO spectra. Our findings show that P-doping significantly enhances polarization anisotropy. Notably, the second-order hyperpolarizability exhibits pronounced optical anisotropy, particularly in the xy-plane, with a remarkable dispersion effect at 589 nm (γ_yyyy = 3.55 × 10¹⁰ a.u.), demonstrating unprecedented control over optical properties. The selected S2 and S4 structures exhibit strong octupolar behavior, with the octupolar molecular tensor contributing up to 80% of the NLO response. This study not only positions P-doped g-C₃N₄ at the forefront of high-performance NLO material design but also paves the way for further exploration of heteroatom-engineered carbon-based nanostructures. These materials offer versatile platforms for advanced photonic and optoelectronic devices. Future directions could delve into the synergistic integration of P-doped g-C₃N₄ within hybrid architectures, broadening its potential for tunable NLO systems in fields such as quantum optics, telecommunications, and molecular sensing.