Non-linear optical properties of gold quantum clusters. The smaller the better

Abstract

By developing a new method for synthesizing atomically monodisperse Au₁₅ nanoclusters stabilized with glutathione molecules and using the current state-of-the-art methods for synthesizing monodisperse protected Au₂₅ nanoclusters, we investigated their nonlinear optical (NLO) properties after two-photon absorption. The two-photon emission spectra and the first hyperpolarizabilities of these particles were obtained using, in particular, a hyper-Rayleigh scattering technique. The influence on NLO of the excitation wavelength, the size as well as the nature of the ligands is also explored and discussed. Au₁₅, the smallest stable thiolated gold nanocluster, presents remarkable nonlinear properties with respect to two-photon processes. The two-photon absorption cross-section at 780 nm for Au₁₅ is ∼65 700 GM. This experimental cross-section value points to a quantum yield for two-photon emission of about 3 × 10⁻⁷ at 475 nm for Au₁₅. The first hyperpolarizability β for Au₁₅ clusters (509 × 10⁻³⁰ esu), as compared to Au₂₅ clusters (128 × 10⁻³⁰ esu), is larger considering the difference in the number of gold atoms. Also, 10³⁰ β per atom values reported for Au₁₅ and Au₂₅ clusters are more than two orders of magnitude larger than the values reported for Au NPs in the size range 10–50 nm, outlining the quantum cluster regime.