Third-order nonlinear optical properties of epsilon-near-zero thin films constructed by amine- and thione-substituted triazole

Abstract

The new class of materials with a vanishing permittivity, known as epsilon-near-zero (ENZ) materials, has been reported to exhibit unprecedented ultrafast third-order nonlinear efficiencies within sub-wavelength propagation lengths. Organic ENZ thin films have arisen as promising materials to new nanophotonic architectures in integrated optics and metasurface devices. As a kind of donor-π-acceptor (D-π-A) molecules, the study of third-order nonlinear optical (NLO) properties of organic ENZ thin films constructed by amine- and thione-substituted triazole molecules is necessary to get success in nanophotonic devices, which is practically absent in the literature. In this work, amine- and thione-substituted triazole molecules: 4-furfuralideneamino-3-methyl-4,5-dihydro-1H-1,2,4-triazole-5-thione (C₈H₈N₄OS, FMTT) and 4-benzylideneamino-3-methyl-4,5-dihydro-1H-1,2,4-triazole-5-thione (C₁₀H₁₀N₄OS, BMTT) have been synthesized and characterized by X-ray single crystal diffraction. The liquid phase epitaxy (LPE) method were used to fabricate their ENZ thin films on quartz substrates. Z-scan technique were used to analyze the third-order NLO properties of the ENZ thin films in picosecond regime at wavelength 532 nm and found their nonlinear refraction coefficient $n_2^I$ value reaching the order of 10⁻¹² m²/W. The molecular orbital, total density-of-state and molecular second hyperpolarizability γ value were calculated by time-dependent density functional theory (TDDFT). These findings open the path toward search for new ENZ enhancement organic NLO materials.