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

Abstract

A series of small pure Au _m (2 ≥ m ≤ 7) and copper-doped Au _m-x Cu _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.