Evidence for electron thermal effect in the third-order nonlinear optical response of matrix-embedded gold nanoparticles

The third-order nonlinear optical response of materials composed of noble metal nanoparticles embedded in a dielectric matrix is large around the surface plasmon resonance frequency, due to local electric field enhancement in the particles. This response can be described by both nonlinear refraction and absorption, related to the complex third-order susceptibility, χ (3) , of the composite material. χ (3) is linked with the intrinsic metal particle susceptibility, χ (3) m , whose value is ruled by interband and intraband transitions. Depending on the incident pulse power and duration, very high conduction electron temperatures can be reached subsequent to the pulse absorption, and can result in a modification of the nonlinear response ("hot electron" effect). The χ (3) real and imaginary parts of Au:SiO 2 thin films, synthesized by radio-frequency sputtering, are measured simultaneously by the z-scan technique, with both nanosecond and femtosecond laser pulses at 560 nm. Comparing the results obtained in both regimes, we show, by using a simple thermal model, that the "hot electron" phenomenon which is significant when exciting with ultrashort pulses, not only reduces the modulus of χ (3) by three orders of magnitude, but also greatly affects its phase.