The wavelength-dependent non-linear absorption and refraction of Au₂₅ and Au₃₈ monolayer-protected clusters

In the past decade, the structural and electronic properties of monolayer-protected metal clusters, which can be produced size-selected in macroscopic amounts, have received a lot of attention. Their great potential for optical applications has been identified. In the high intensity regime, monolayer-protected metal clusters show pronounced nonlinear absorption and refraction. Naturally, these phenomena are wavelength-dependent, however, such dependence is largely unexplored. Here, we quantify the wavelength-dependent non-linear optical absorption and refraction cross sections of atomically precise Au₂₅(DDT)₁₈ and Au₃₈(DDT)₂₄ clusters, using the z-scan technique in combination with a tunable nanosecond laser source. Qualitatively different non-linear optical phenomena were found to take place at different excitation wavelengths (two-photon and excited-state absorption, intensity saturation and non-linear refraction). Both clusters have high nonlinear absorption cross sections at 532 nm, and present a (local) maximum at 640 nm, together with a maximum in the absorption saturation. The nonlinear refraction is always negative for Au₂₅(DDT)₁₈, while it changes sign for Au₃₈(DDT)₂₄. Depending on the wavelength, the underlying mechanism of the nonlinear absorption effects is two-photon absorption or excited state absorption. The obtained very high nonlinear cross sections, on the order of 10⁷-10⁹ GM, demonstrate the great potential of those clusters as nonlinear absorption or refraction materials in optical applications.