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

Abstract

The third order optical susceptibility of metal-dielectric nanocomposite films (Cu:Al₂O₃) 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 Al₂O₃ matrix. They have metal volume fractions, p, ranging from 0.07 to 0.45, and morphologies that range from spherical particles (diameter, ⌀~2 nm) to a random network when close to the percolation threshold. In nanocomposites containing isolated oblate spheroids (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 (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 1.8X10⁻⁷ 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.