Ultrafast Optical Kerr Nonlinearity in LaPO4/g-C3N4 Heterojunctions for Optical Power Limiting Applications

Abstract

The present work reports the ultrafast photophysical carrier dynamics and third-order optoelectronic Kerr nonlinearity of a liquid-phase exfoliated LaPO₄/g-C₃N₄ heterojunction via a single-beam Z-scan technique using femtosecond (∼50 fs, 800 nm) and nanosecond (∼9 ns, 532 nm) pulses. A series of pure LaPO₄, bulk g-C₃N₄, exfoliated g-C₃N₄, and LaPO₄/g-C₃N₄ heterojunctions were successfully synthesized using a wet chemical method, and their structural, thermal, and phase stabilities were confirmed via preliminary material characterization such as X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Third-order nonlinear studies revealed enhanced three-photon- and two-photon-assisted nonlinear absorptions in an exfoliated 2D g-C₃N₄ heterojunction with 1D orthophosphate having nonlinear absorption coefficients on the order of 10^–4 (cm³/GW²) and 10^–10 m/W, respectively. Moreover, the nonlinear refractive index was estimated to be on the order of 10^–15 (cm²/W). Physical parameters such as the Kane energy and exciton reduced mass were calculated for composite samples. A type II heterojunction is formed and confirmed from band alignment using UV–visible absorption spectra and Fermi energy level calculations along with orbital contributions in LaPO₄ and g-C₃N₄ using density functional theory. The optical limiting properties of pure LaPO₄ and LaPO₄/g-C₃N₄ heterojunctions were examined, revealing that LaPO₄/g-C₃N₄ is a promising candidate for optical limiting applications from femtosecond and nanosecond lasers owing to its strong reverse saturable absorption.