Fifth-order nonlinear optical properties of through space charge transfer pyridinium salt

The fifth-order nonlinear optical (NLO) properties of a through space charge transfer (TSCT) pyridinium organic salt were investigated using the Z-scan technique with a femtosecond laser. Unlike previous studies, focusing on through bond charge transfer molecules and their third-order nonlinear effects, this work presents the first evidence that TSCT designed organic molecules show significant fifth-order nonlinear refraction (n ₄) with a negligible third-order nonlinear refraction (n ₂) component. The TSCT designed molecule demonstrated weak two-photon absorption (2PA) and a strong fifth-order nonlinear refractive index response, with peak values occurring near 700 nm. The spectral dispersion of the n ₄ coefficient correlated with the 2PA spectrum, suggesting a linked electronic mechanism. The measured n ₄ values are on the order of 10^-24 to 10^-25 cm⁴ W^-2, comparable to those observed in other high-performance organic NLO materials, but with lower nonlinear optical losses. These findings highlight the potential of TSCT molecules for high-intensity, low-loss photonic and optoelectronic applications where higher-order nonlinearity is advantageous.