Nonlinear optical spectroscopy of isotropic and anisotropic metallic nanocomposites

On the physical contributions to the third-order nonlinear optical response in plasmonic nanocomposites

“…(3) m for both metals, at different wavelengths, as shown in figure 11. The figure shows, for Au and Ag, a sign reversal between metal nanoparticle and composite nonlinear susceptibilities, given by the local field factors between them; this is the first time that this has been observed in silver, although only for long wavelengths (600 and 750 nm) [20]. In the case of gold, there is no sign reversal at 600 nm.…”

“…Otherwise, if one considers both types of NLA, the fitting improves radically in both cases, as shown in figure 4 for low (1.0 × 10 15 W m −2 ) and high (6.7 × 10 15 W m −2 ) irradiances, respectively. Therefore, from here on, we considered this fact in the estimation of the nonlinear coefficients from the theoretical fitting of the experimental data, for all wavelengths [20].…”

“…the intra-band transitions and the hot-electrons, but, because of the low energy of the photons in this region, the hot-electron contribution is not as strong as near the plasmon resonance and, consequently, a positive NLA is always dominant. However, we must consider that the shorter the wavelength, the higher the energy of the photon, consequently, the production of hot-electrons is higher for shorter wavelengths, and the positive NLA could cease to be dominant for a given irradiance and different wavelengths [28]. For the nonlinear refractive index n 2 , there is a simple calculation of n 2 following Boyd [29], which is given by…”

“…In a certain way, these results coincide with the ones obtained previously using femtosecond pulses, where anisotropy in the NLA of elongated Ag NPs has been observed, although in that work only saturable absorption was observed for both axes [33]. On the other hand, as previously observed for the isotropic case [4,24,25,34], and calculated for the anisotropic one [35], using Maxwell-Garnett theory [26], we can extract the nanoparticle susceptibility χ where f 1 is the local field correction factor, f 1 = 3ε d /(ε m (ω)+ 2ε d ), with ε d the dielectric function of the dielectric host, and ε m the complex dielectric function of the metal inclusion; and p is the volume ratio of the metal implanted in the hosted matrix (filling factor), calculated to be 0.014 for Au and 0.011 for Ag, as said before in section 3 [20]. Using the tabulated data for ε d of silica, and ε m for Au, or Ag [36], we were able to extract χ…”