Ultrafast Optical Kerr Nonlinearity in LaPO 4 /g-C 3 N 4 Heterojunctions for Optical Power Limiting Applications

The present work reports the ultrafast photophysical carrier dynamics and third-order optoelectronic Kerr nonlinearity of a liquid-phase exfoliated LaPO 4 /g-C 3 N 4 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 LaPO 4, bulk g-C 3 N 4, exfoliated g-C 3 N 4, and LaPO 4 /g-C 3 N 4 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-C 3 N 4 heterojunction with 1D orthophosphate having nonlinear absorption coefficients on the order of 10 –4 (cm 3 /GW 2 ) and 10 –10 m/W, respectively. Moreover, the nonlinear refractive index was estimated to be on the order of 10 –15 (cm 2 /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 LaPO 4 and g-C 3 N 4 using density functional theory. The optical limiting properties of pure LaPO 4 and LaPO 4 /g-C 3 N 4 heterojunctions were examined, revealing that LaPO 4 /g-C 3 N 4 is a promising candidate for optical limiting applications from femtosecond and nanosecond lasers owing to its strong reverse saturable absorption.