Tailoring Nonlinear Optical Response: Impact of Substituents in Thiazole-Azo Polymers

This study explores how different chemical substituents impact the nonlinear optical (NLO) behavior of specialized thiazole-azo polymer thin films. Using the well-known Maker fringe technique, we dive into their second- and third-harmonic generation capabilities (SHG and THG, respectively), revealing key NLO parameters such as second- and third-order susceptibility. The findings show that these thiazole-azo polymers exhibit impressive NLO responses, positioning them as strong candidates for advanced optoelectronic applications driven by NLO phenomena. These materials, with their remarkable NLO enhancements, hold promise for the future optical devices. To investigate the microscopic second- and third-order NLO phenomena of title structures, we compute the electric dipole moments (μ), static and dynamic dipole polarizabilities (α), second-order (β) and third-order (γ) hyperpolarizabilities using density functional theory (DFT). We calculate the frequency-dependent second- (χ⁽²⁾) and third-order (χ⁽³⁾) susceptibilities through the time-dependent Hartree-Fock (TDHF) technique. The one-photon absorption (OPA) properties of the examined thin films are also computed, and both theoretical and experimental UV-Vis spectra are presented. Furthermore, the outcomes of the analyzed and simulated OPA spectra, (χ⁽²⁾) and (χ⁽³⁾) susceptibilities are compared to highlight their compatibility. The investigated materials demonstrate significant quadratic and cubic hyperpolarizabilities and susceptibilities with non-zero values, showcasing substantial second- and third-order optical nonlinearity. To enhance understanding of the molecular orbital (MO) structures, we interpret the highest occupied molecular orbitals (HOMOs), lowest unoccupied molecular orbitals (LUMOs), and HOMO-LUMO energy gaps from DFT.