Third order nonlinear optical susceptibility of Cu:Al2O3 nanocomposites: From spherical nanoparticles to the percolation threshold

The third order optical susceptibility of metal-dielectric nanocomposite films $({\mathrm{Cu:Al}}_2{\mathrm{O}}_3)$ has been determined by degenerate four wave mixing. The films have been synthesized by alternate pulsed laser deposition and consisted of Cu nanoparticles in an amorphous ${\mathrm{Al}}_2{\mathrm{O}}_3$ matrix. They have metal volume fractions, p, ranging from 0.07 to 0.45, and morphologies that range from spherical particles (diameter, φ∼2 nm) to a random network when close to the percolation threshold. In nanocomposites containing isolated oblate spheroids $\text{(p⩽0.17),}$ the optical response at wavelengths close to that of the surface plasmon resonance (SPR) can be described in the frame of the Maxwell-Garnett effective medium theory. Above the particle coalescence threshold, in nanocomposites with higher Cu content $\text{(p⩾0.2),}$ both the linear absorption in the near-infrared and the third order nonlinear optical susceptibility at the SPR are greatly enhanced, the latter achieving values as high as ${\text{1.8×10}}^{\text{−7}} \mathrm{esu}.$ These results are discussed in terms of multipolar interactions among particles and giant local resonance effects that appear as a consequence of the particle coalescence and the increase in size of the nanocrystals.