Insights into the nonlinear optical (NLO) response of pure Aum (2 ≥ m ≤ 7) and copper-doped Aum–xCux clusters

Abstract

A series of small pure Au_m (2 ≥ m ≤ 7) and copper-doped Au_m−xCu_x clusters was evaluated by density functional theory (DFT) at the CAM-B3LYP/LANL2DZ level for their geometric, electronic, and nonlinear optical (NLO) properties. The charge transfer for the Au cluster significantly improved by reducing the HOMO–LUMO energy gap from 3.67 eV to 0.91 eV after doping with Cu atoms. The doping of Cu also showed noteworthy impacts on other optical and NLO properties, including a decrease in the excitation energy and increase in the dipole moment and oscillator strength. Furthermore, changes in the linear isotropic and anisotropic polarizabilities (α_iso and α_aniso) and first and second NLO hyperpolarizabilities (β_static, γ_static) were also observed in the pure and Cu-doped clusters, which enhanced the NLO response. The nonlinear optical properties of the clusters were evaluated by calculating the static and frequency dependent second- and third-order NLO polarizabilities at 1064 nm wavelength. Among all the doped structures, the Au₃Cu₁ cluster showed the largest static first hyperpolarizability of β_(total) = 4.73 × 10³ au, while the Au₁Cu₆ cluster showed frequency dependent first hyperpolarizability of β_(−2w;w,w) = 1.26 × 10⁶ au. Besides this, large static and frequency-dependent second hyperpolarizability values of 6.30 × 10⁵ au and 1.05 × 10 au were exhibited by Cu₇ and Au₁Cu₆, respectively. This study offers an effective approach to design high-performance NLO materials utilizing mixed metal clusters which might have broad applications in the fields of optoelectronics and electronics.