Title: Laser Pulse Induced Second- and Third-Harmonic Generation of Gold Nanorods with Real-Time Time-Dependent Density Functional Tight Binding (RT-TDDFTB) Method

Abstract: In this study, we investigate second- and third-harmonic generation processes in Au nanorod systems using the real-time time-dependent density functional tight binding (RT-TDDFTB) method. Our study focuses on computation of nonlinear signals based on the time dependent dipole response induced by linearly polarized laser pulses interacting with nanoparticles. We systematically explore the influence of various laser parameters, including pump intensity, duration, frequency, and polarization directions, on the harmonic generation. We demonstrate all the results using Au nanorod dimer systems arranged in end-to-end configurations, and disrupting the spatial symmetry of regular single nanorod systems crucial for second harmonic generation processes. Furthermore, we study the impact of nanorod lengths, which lead to variable plasmon energies, on the harmonic generation, and estimates of polarizabilities and hyper-polarizabilities are provided.