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

摘要

By developing a new method for synthesizing atomically monodisperse Au_(15) nanociusters stabilized with glutathione molecules and using the current state-of-the-art methods for synthesizing monodisperse protected Au_(25) nanociusters, 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. Au_(15), 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 Au_(15) is ~65 700 GM. This experimental cross-section value points to a quantum yield for two-photon emission of about 3 x 10~(-7) at 475 nm for Au_(15). The first hyperpolarizability p for Au_(15) clusters (509 x 10~(-30) esu), as compared to Au_(25) clusters (128 x 10~(-30) esu), is larger considering the difference in the number of gold atoms. Also, 10~(30) β per atom values reported for Au_(15) and Au_(25) 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.