In recent years, nanometer sized particles have attracted much attention, as they present an interest both for fundamental studies and potential applications due to their magnetic or optical properties. These properties, below a critical size, depending on the material and considered property, are size dependent. As a consequence, it is necessary to control the size of the particles and, in numerous cases, to protect them against degradation especially against oxidation. Embedding the particles in an inert matrix can be a suitable way to achieve these goals. Among the possible materials which can be used, silica based ones offer the advantages of versatility, chemical inertness and optical transparency. One of the most ancient examples of such composite materials is the use of noble metals (Cu, Ag, Au) colloidal dispersions to get coloured silicate glasses or enamels. More recently, numerous methods have been used to produce these embedded particles. They can be roughly classified as chemical and physical methods. The most often used are, among the first group, ion exchange processes and the sol-gel derived methods and among the second one, evaporation, sputtering and ion implantation. Our purpose is to illustrate the potentiality of some of these methods giving some examples. Two of them are concerned with metal particles and two others with organic crystals. Metal particles

Porous sol-gel glasses, either impregnated with pure C60 or doped with a methanofullerene derivative, have been studied and induced absorption or “reverse saturable absorption” (RSA) has been observed in both types of solid materials. The samples impregnated by pure C60 mainly contain well-dispersed fullerene molecules. Unlike crystalline films of C60, their absorption dynamics can be well described by a 5-level model, developed for non-interacting C60-molecules in solutions. Methanofullerene samples, on the other hand, show signs of micellar aggregation and therefore RSA dynamics that are influenced by solid state effects. We observe an important decrease of transmission at high fluences for both kinds of samples, a shortened singlet-state lifetime to that observed in solution, but nonetheless, a triplet yield, that cannot be considered as negligible. In the case of pure C60 in a sol-gel matrix, we can explain the faster de-excitation dynamics, relative to behavior in solution, mainly by the absence of stabilizing aromatic solvents and also by the interaction of the amorphous environment with the molecules. Concerning the methanofullerene samples, the acceleration of the de-excitation dynamics can be principally attributed to solid-state effects due to the micellar aggregation.

Thin silica-titania films doped with CdS and PbS nanocrystals have been prepared by the sol-gel route. Their nonlinear properties have been studied using the techniques of degenerated four-wave mixing and m-lines with picosecond (ps) and nanosecond laser pulses. Depending on wavelength, doping level, and laser pulse duration, high negative nonlinearity was found for CdS-doped (n2 = -2 x 10-8 cm2 per kW) and PbS-doped films (-10-10 to -2 x 10-7 cm2 per kW). The response time of the nonlinearity was below 35 ps. Saturation of the nonlinearity was observed. Straight, monomode channel waveguides have been fabricated on these films. The influence of MIE-scattering due to the nanoparticles is investigated.

Amphiphilic methanofullerene derivative 1 is water soluble and micellar aggregation has been evidenced by small-angle X-ray scattering measurements and UV/Vis spectroscopy. Thanks to the high solubility of 1 in polar solvents widely used in sol–gel processing, successful inclusion of 1 in the sol–gel could be easily achieved. The optical limiting properties of the doped sol–gel samples have been evaluated and a fast S1–S0 relaxation has been observed for these samples. This observation appears to be consistent with the presence of micellar aggregates of 1 in the sol–gel. Indeed, the interactions between the fullerene spheres of neighbouring molecules in the clusters may be at the origin of this fast S1–S0 relaxation as already shown in solid C60-films.

Embedding nano-aggregates in silica based supports may be a usefull method to control their size and protect them against degradation. Some Some examples are given to illustrate the potentiality of this method. Metal aggregates can be obtained on reducing an oxide or salt dispersed in a silica gel or by ion implantation in dense silica layers. In both cases the particles size can be controlled by the conditions of elaboration. Organic nano-crystals such as fullerenes and polydiacetylenes can be also incorporated in mesoporous silica. In this case, the size of the crystals is controled by the site of the pores.

Thin silica–titania films doped with different concentration of PbS quantum dots (PbS/oxide molar ratios ranging from 5% to 25%) were fabricated via a sol-gel route. The structural properties were studied by x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering spectrometry. The PbS crystals were found to have a mean diameter in the range 2.3–3.5 nm with narrow size distribution. The resonant nonlinear optical properties were studied by the nonlinear m-line technique and degenerate four-wave mixing. High negative nonlinear refractive indices (n2) were measured at 1.064 μm. Different n2 values were obtained for nanosecond excitation (n2=10−7–10−8 cm2/kW) and for picosecond excitation (n2=10−9–10−10 cm2/kW). The differences can be explained by saturation effects. Measurements at 532 nm showed n2 values ten times higher than at 1.064 μm. The response time of the nonlinearity for both wavelengths was below 35 ps.

Thin silica-titania planar waveguides doped with different concentrations of lead sulfide (10-25 mol%) have been prepared by a sol-gel process. It consists of three steps: a) preparation of a colloidal sol of semiconductor particles; b) preparation of an alkoxide solution, precursor of the glass-like matrix; c) mixing of the colloidal sol and the alkoxide solution. Films were deposited on fused silica by dipping and densified for 1h at 300 degrees C in nitrogen. The mean particle size is 3 nm and the optical absorption edge is situated around 1100 nm.The nonlinear properties have been investigated using degenerated four wave mixing (DFWM) and a nonlinear m-line technique. Different Nd:YAG lasers with pico- and nanosecond pulses at 1064 nm have been used. Depending on the PbS concentration we measured a high negative nonlinear refractive index of n(2) = -3 to -9 10(-8) cm(2)/kW for nanosecond pulses (m-lines) and -1 to -10 10(-10) cm(2)/kW for picosecond pulses (m-lines and DFWM). The response time of the nonlinearity is below 30 ps. All observed nonlinear effects are fully reversible and we did not observe any photodarkening. Straight, monomode channel waveguides have been fabricated on theses films.

We investigate the nonlinear absorption of C60-doped porous sol-gel glasses by single- and double-pulse pump–probe experiments. We find that the reverse saturable absorption (RSA) of these samples can be explained in the frame of a five level system as it is commonly used for C60 solutions. We observe a strong saturation of the RSA at high fluences, especially if the molecules are prepared in the triplet state. In a double-pulse pump–probe experiment we measure the triplet quantum yield for C60 solutions (0.8) and doped glasses (0.25) and the singlet and triplet absorption cross sections. In the first excited singlet state (S1) lifetimes of 1 ns and 65 ps are determined in solutions and glasses, respectively. We find that the dynamics of both depopulation processes, direct S1–S0 relaxation, and intersystem crossing is faster in glasses than in the case where C60 is in solution in a liquid aromatic solvent. We tentatively explain these findings by the absence of a stabilizing solvent and a perturbation of the molecular energy levels due to interaction with the solid glass matrix. We finally determine the lifetime of the triplet states in our glass samples to 2 μs.

In this paper measurements of induced absorption and its dynamics in C60-doped solid xerogel matrices are presented. The measurements are performed in a pump-test geometry using picosecond pulses generated from a frequency doubled modelocked Nd:YAG laser. An induced absorption leading to optical limiting was observed. Outstanding optical limiting qualities for single shot excitation were found. Furthermore, limiting dynamics are studied. A decrease of the initial laser-induced absorption with a time constant of 150 picoseconds was observed. This is attributed to a transition from excited singlet to triplet electronic states in the C60 molecule. A long-living component due to the resulting triplet state population was also observed.

We present measurements of induced absorption and its dynamics in C60-doped solid xerogel matrices. The measurements are performed in a pump-test geometry using picosecond pulses generated from a frequency doubled mode locked Nd:YAG laser at 532 nm. We observe an induced absorption leading to optical limiting. We find outstanding optical limiting qualities for single shot excitation but some degradation above a certain threshold intensity for repetitive pulses. Below this degradation threshold, the material is stable and its limiting dynamics is studied. A decrease of the initial laser-induced absorption with a time constant of 150 ps is observed, down to a very long-lived constant level. We attribute this dynamics of induced absorption to a transition from excited singlet to triplet electronic states in the C60 molecule.

Using the sol-gel technique we have investigated PbS-doped thin films. The resonant nonlinear refractive index of these samples was measured, using two different techniques: nonlinear m-lines and degenerate four-wave mixing (DFWM). The measured values of the nonlinear refractive index are and for the m-lines and DFWM, respectively. We explain this difference as being due to the different durations of the laser pulses used (7 ns for m-lines and 30 ps for DFWM).