The optical response of materials based on gold nanoparticle assemblies depends on many parameters connected to both material morphology and light excitation characteristics. The optical energy absorbed is then converted into heat through different nanoscale energy exchange mechanisms. This heating subsequently modifies itself the optical properties. We investigate the interplay between the optical and thermal responses of nanocomposite media under its theoretical aspect. In this first paper, the thermal response of gold nanoparticle assemblies under pulsed optical excitation is considered. Both conventional and original modelling approaches are presented. We first underline the role of electromagnetic interactions between particles in a dense assembly in its linear optical response. We then show how the interaction of light with matrix-embedded gold nanoparticles can result in the generation of thermal excitations through different energy exchange mechanisms. Finally, we demonstrate the possible significant influence of the heat carrier ballistic regime and phonon rarefaction in the cooling dynamics of an embedded gold nanoparticle subsequent to ultrafast pulsed laser excitation.

Gold nanoparticle assemblies present an optical response which is very sensitive to different phenomena of thermal origin. In a recent paper, we have described how the interaction of light with matrix-embedded gold nanoparticles can result in the generation of thermal excitations. Here we show how thermal processes can affect the optical response of a nanoparticle assembly. We then connect both aspects and finally point out their involvement in the nonlinear optical response of nanocomposite media. This allows us to tackle two key issues in the field of third-order nonlinear properties of gold nanoparticles: The influence of the generalized thermal lens in the long laser pulse regime and the hot electron contribution to the gold particle intrinsic third-order susceptibility, including its spectral dispersion and intensity-dependence.

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:SiO2 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.

We report a study on the third-order nonlinear optical properties of nanocomposite thin films composed of gold particles embedded in a silica host matrix. Samples of various metal volume fractions, ranging from 8 to 35%, are synthesized by the sputtering technique. Some of them are annealed. Nonlinear optical measurements, which are performed by using the z-scan technique, reveal both a very large nonlinear absorption and a weak nonlinear refraction close to the surface plasmon resonance frequency of the particles. We especially study the effect of the metal concentration and the influence of thermal treatment on the real and imaginary components of the third-order nonlinear susceptibility. Our results reveal that, as the metal concentration reaches a few percent, the mutual electromagnetic interactions between particles greatly enlarge the nonlinear optical response of the material and can not be neglected in the theoretical analysis. Moreover, the thermal treatment leads, for a given concentration, to a significant increase of the nonlinear response, which is ascribed to a modification of the material morphology. We finally point out that the material nonlinear properties are very sensitive to the incident wavelength through the local field enhancement phenomenon.

We report an experimental and theoretical study on the third-order nonlinear optical properties of gold particles embedded in a dielectric host. Thin films of various metal concentrations are elaborated by the sputtering technique. Several complementary techniques provide information about the metal concentration, the film thickness and the size of the particles. The effective complex optical index of the film is determined by spectrophotometry and ellipsometry. The third-order nonlinear susceptibility, χ(3), is measured using the z-scan technique in the nanosecond regime. Experimental results reveal very large nonlinearities, increasing with the metal concentration. Theoretical calculations of the amplification factor, based on a recursive transfer method, allow a better understanding of these results.

We illustrate that a recently formulated recursive transfer matrix method can be used to reliably calculate the electromagnetic fields throughout three-dimensional systems of strongly scattering spheres, and/or coated spheres. The exceptional features of our technique are its particularly stable and reliable numeric implementations. In this work, we present new self-consistent formulae which permit the verification of the numerical stability at any stage of the calculations, and which ensure the satisfaction of the underlying multiple scattering equations for an arbitrary incident wave.

We report local electric field calculations in nanocomposite materials. These calculations are performed by means of a recursive transfer matrix method initially developed for calculating the electromagnetic field response for three-dimensional systems of scattering spheres. Illustrative calculations are presented for various morphologies of nanocomposites composed of gold particles in a silica matrix. We particularly point out that mutual interactions between particles are responsible for large local field enhancements as compared with fields inside isolated particles. Nonlinear optical measurements, performed by using the z-scan technique, are presented afterwards. We especially study the variations of the imaginary part of the third-order nonlinear susceptibility as a function of the metal concentration and we show that these results are in agreement with the previous field calculations.

Third-order nonlinear optical properties of Au:SiO2 thin films were studied at the surface plasmon resonance wavelength by the z-scan technique using a nanosecond laser. Films were prepared by a multilayer deposition sputtering technique. They were composed of 2 nm mean diam gold particles, with a metal volume fraction of 20%. Z-scan measurements performed both with and without aperture showed a very large nonlinear absorption masking the nonlinear refraction. The nonlinear absorption coefficient β was found to be negative and equal to −1.1×10−2 cm/W. The different mechanisms contributing to this absorption are discussed and the large value of β is correlated to the duration of the laser pulses. Moreover, it is shown that a mean field theory is not appropriate to evaluate the effective susceptibility at high metal concentrations.

Oxygen self-diffusion coefficients were determined in polycrystals of a-alumina either 'undoped' or doped with 500 wt.ppm (225 molppm) Y2O3 in the temperature range 1460-1720 degrees C, with O-18(2) and secondary-ion mass spectrometry. Oxygen grain-boundary diffusion coefficients obey the relationsD'O-->Al2O3(cm(2)s(-1)) = 1.6 x 10(16)exp(- 921(kJ mol(-1))/ RT)for 'undoped' alumina andD'(O)--Al2O3:Y2O3(cm(2)s(-1)) = 7.0 x 10(10)exp(- 800(kJ mol(-1))/RT)for 'yttrium-doped' alumina.The comparison between the two materials indicates that yttrium addition decreases the oxygen grain-boundary diffusion. For both materials, it was observed that the activation energy of grain-boundary diffusion is greater than the activation energy of the bulk diffusion. Similar results were obtained in single crystals for diffusion in the bulk and in subboundaries. These results were interpreted as being due to impurity segregation along boundaries and led to a model.

Oxygen self-diffusion in 'undoped' (i.e. unintentionally doped) alumina (Al2O3) single crystals was investigated using the gas-solid isotope exchange technique. After diffusion annealing, profiles of O-18 were determined by secondary ion mass spectrometry. These showed two parts: close to the initial surface, the first part was attributed to bulk self-diffusion, while the diffusion tails were attributed to diffusion in dislocation walls.In the temperature range 1500-1720 degrees C the bulk self-diffusion coefficients of the oxygen in Al2O3 are represented by: D-O (cm(2) s(-1)) = 206 exp [-(636 kJ mol(-1))/RT]. Possible diffusion mechanisms are proposed considering an extrinsic behaviour associated with silicon contamination. In the same temperature range, the oxygen diffusion coefficients in the Al2O3, subboundaries are described by: D-O('')(cm(2) s(-1)) = 3.1 x 10(14) exp[-(896 kJ mol(-1))/RT]. The high activation enthalpy observed is attributed to segregation effects on subboundaries.

Oxygen self-diffusion has been studied in alumina single crystals in the temperature range 1520-1750°C by means of the gas-solid isotope exchange technique. After diffusion annealing, profiles of oxygen-18 were determined by Secondary Ion Mass Spectrometry. Results indicate that two diffusion mechanisms are involved : a bulk diffusion mechanism and a subboundary one. The bulk diffusion mechanism can be described by: D(cm2/s) = 99.6 exp (- 626(kJ/mol)/RT) while subboundary coefficients obey: D' (cm2/s) = 3 x 1013 exp (-877(kJ/mol)/RT). The large value of the activation energy obtained for the subboundary diffusivity (larger than those related to the volume one) is attributed to the segregation of an impurity along the subboundaries.