We report experiments on gold clusters in the size range 2–4 nm, embedded in an alumina matrix. The metallic particles are produced with a laser vaporization source and codeposited with a dielectric vapor as a thin film on a substrate. Our technique allows varying the cluster size at a given metal concentration. These composite materials are studied through optical absorption and ellipsometric measurements, allowing determination of their complex index of refraction. Various complementary techniques provide information about their morphology, their chemical composition, the thickness of the films, and the size distribution of the clusters. The surface plasmon resonance in the absorption spectra is shown to be damped and blueshifted with decreasing cluster size. Theoretical calculations in the framework of the time-dependent local-density approximation allow a clear understanding of these experimental results.

The surface plasmon energy of silver clusters is investigated in the framework of the time-dependent local density approximation. The results are in good agreement with experimental data on free ${\mathrm{Ag}}_N^{+}$ clusters and on ${\mathrm{Ag}}_N$ clusters embedded in solid argon and alumina. The size effects are weak and very sensitive to the matrix, the porosity at the interface, and the cluster charge. It is shown that the core-electron contribution to the metal dielectric function is mainly responsible for the quenching of the size effects in the optical response.

The electron dynamics is studied in silver nanoparticles of different sizes embedded either in a silicate glass or in a porous alumina matrix, using time-resolved optical spectroscopy. The femtosecond pump–probe measurements, made under various conditions of excitation, reveal that the energy relaxation is faster as the particle size decreases. For particles with identical size and excitation conditions, the relaxation time is found to be faster for clusters in the alumina matrix. The influence of the surface acoustic modes and of the heat conductivity of the matrix on the energy relaxation is discussed.

Absorption and low-frequency Raman-scattering experiments have been performed on thin films consisting of small silver clusters embedded in a porous alumina matrix. When the Raman excitation wavelength is close to the maximum (≈420 nm) of the Mie band (dipolar surface plasmon resonance) the Raman spectra exhibit a strong band located around 10 ${\mathrm{cm}}^{\mathrm{-}1}$, the maximum of which depends on the cluster diameter D at the maximum of the cluster-size distribution in the sample according to the approximate law ${\omega }_{\mathrm{vib}}\propto D^{-1}.$ The Raman band corresponds to the excitation of the quadrupolar vibration mode of the clusters, via the plasmon-phonon interaction. Moreover, the maximum of the Raman band shifts towards lower frequencies when the excitation light is shifted to the red. This feature, as well as the rather large Mie-band width, is shown to reflect the ellipsoidal shape distribution of part of the embedded clusters.

The surface phonon-polaritons contribution to the thermal conductivity of a nano thin film of silicon dioxide is investigated based on the Maxwell equations and the Boltzmann transport equation. It is shown that: (1) a small difference between the permittivities of the substrate and superstrate of the film can generate giant propagation lengths and therefore remarkably enhances its thermal conductivity with respect to values obtained for a freestanding one. (2) The propagation of surface phonon-polaritons is present in a broad band of frequencies and exhibits its largest propagation lengths at the frequency where the absorption of energy is minimal. (3) The increase of the thermal conductivity of the film as its thickness decreases is higher when it is deposited on potassium bromide instead of being suspended in air. The difference in the thermal conductivity for these two systems increases with increasing temperature and reducing the film thickness. A thermal conductivity as high as 2.5 W/m K is obtained for a 30 nm-thick thin film at room temperature, which is about 1.8 times larger than its bulk phonon value. The obtained results show that the propagation of surface phonon-polaritons has the potential not only to offset the reduction of the phonon thermal conductivity of a nano thin film, when its sizes are scaled down, but also to enhance it, by choosing properly the permittivity of its substrate.