Resonance Third-Order Nonlinear Optical Response from Molecular-Like Noble Metal Emitting Nanoclusters Embedded in a Silica Matrix

Metal nanoclusters (MNCs) have emerged as a novel class of highly efficient nano-emitters, characterized by their extremely small size (under 2 nm) and high photostability. In this work, we have studied the third-order nonlinear optical response of MNCs made of Au and Cu by using a two-wave mixing configuration at 355 nm with 29 ps pulses at 1 Hz to induce the optical Kerr effect. This excitation wavelength is in resonance with electronic transitions within Cu and Au clusters. We observe an enhanced Kerr signal from Cu and Au NCs compared to that exhibited for larger Au and Cu plasmonic nanoparticles (NPs). The synthesized Au and Cu NCs, averaging about 2-3 nm in size, do not exhibit plasmon resonances. We measure a two-fold and four-fold increase in the third-order susceptibility, X⁽³⁾, for Cu and Au NCs, respectively, as compared to larger metal NPs. The enhanced Kerr nonlinearities can be attributed to the molecular-like behavior of these ultrasmall size the Cu and Au NCs. Our experimental results can be analyzed considering the metal clusters as a closed two-level atomic system interacting with light under resonance conditions. These findings open potential applications for ultraminiaturized photonics and optoelectronic devices operating at ultraviolet wavelengths.