Optical properties of gold nanoparticles in vitreous sol-gel-derived silica preforms and fibers
Résumé
Beside the continuous improvement of active ions-doped optical fibers, there is a growing interest in the development of optical fibers doped with nanostructures like metallic gold nanoparticles (GNPs). Such systems exhibit unique optical properties, dependent on their size and shape as well as on their dielectric surrounding. Hence, dense silica glasses containing GNPs are of significant interest in various fields including nonlinear optics[1]. However these potential technological applications require a controlled synthesis of metal-dielectric nanocomposites with well-defined and thermally stable NPs, compatible with fiber manufacturing. To obtain those NPs in pure silica glass, the sol-gel process has been successfully used for several years but essentially in thin films[2] or at least in porous structures. This lack of experimental achievements is particularly true for gold nanocrystals in vitreous silica matrix. In this work we report on a novel method based on the embedment of zirconia-coated GNPs in a silica sol, so as to prepare vitrified silica glasses doped with GNPs for photonic applications. The typical size of the GNPs synthesized in solution is about 5 nm at room temperature. Then the GNPs/ZrO2 solution is incorporated in the SiO2 sol to obtain a similar concentration of dopant in each part of the matrix. The obtained sample is dried at 120°C and then annealed in air at 850 °C to eliminate the organic residues. In order to check an eventual influence of the GNPs insertion on the structure of the silica matrix and to follow the evolution of this structure under heat-treatment, Raman spectra of the doped xerogel are recorded. Such nanocomposite xerogels are then densified under air at 1100 °C. After this sintering step the low-wavenumber peak (Boson peak) appears around 50 cm−1 which is commonly interpreted as a proof for the gel-to-glass transition, namely a complete densification of the silica network. The role of the zirconia is here to protect GNPs during this high temperature step thanks to its refractory properties. In this way pink and monolithic glass rods suitable for serving as optical fiber preforms are obtained. The persistent pink color suggests the conservation of GNPs inside the silica matrix during the xerogel-to-glass transition. TEM analysis confirms the presence of GNPs with diameters between 10 nm in the xerogel stabilized at 850°C and around 30 nm after sintering at 1100 °C. The slight increase in the GNPs size between the two annealing temperatures is a normal behavior for nano-objects explained by the Ostwald ripening phenomenon in solid solutions. During the conference, we will detail the structure and morphology of the GNPs with inter reticular spaces in good agreement with the distance between the (111) planes of “cubic” gold. Presence of Zirconia in the surrouding of GNPs could not be clearly detected with TEM and EDX analysis. We will also detail the optical properties of the doped silica glass. Linear optical properties first with the report of a well-defined absorption band around 520 nm, doubtlessly attributed to the surface plasmon resonance (SPR) of GNPs. In fact, the observed SPR band positions remain compatible with small spherical GNPs in a silica matrix. Even if the presence and the role of zirconia around GNPs is not clearly established, the present synthesis method seems to favor the conservation of smaller nanoparticles inside the glass with a better control of the size distribution, as compared with the GNPs synthesis in pure silica glass proposed in our previous work [3]. As regards the third-order optical nonlinear properties of sample, behavior of this GNP/ZrO2-doped silica glass is analyzed by Z-scan technique based on the conventional one. Closed-aperture Z-scan transmittance are numerically processed from the acquired images by a CCD camera [4]. The sample is moved in the focal region along the beam propagation direction (Z axis) and the incident intensity at 532 nm adjusted by a polarizing system at the entry of the setup. The experimental profile of the transmittance spectra recorded is characteristic of saturable absorption phenomena and the numerical fit gives a negative nonlinear absorption coefficient, β, of about -13.7 cm/GW close to the SPR wavelength. Then a linear optical absorption spectrum is recorded in the irradiated zone after the nonlinear measurements to check a possible photobleaching of the GNPs. Furthermore, a careful examination of the sample surface under microscope showed that it remained intact at the point the laser beam was focused on. It is a difficult to compare the obtained nonlinear absorption coefficient β with other values taken in the literature because of the strong dependence of this coefficient on the size of the GNPs, on their concentration and on the surrounding of GNPs, as well as on the experimental conditions. One can also note that only few works concern nonlinearities of GNPs inside a densified glass matrix. For example, a saturable absorption behavior with a dynamics in the picosecond scale has been reported about GNPs in a borosilicate glass[5] but the magnitude of the corresponding resonant nonlinear coefficient was not estimated. The case of highly doped Au:SiO2 cermet films can also be mentioned, where giant negative β coefficients could be measured (up to -2.2.107 cm/GW), but in nanosecond regime[6]. Thermal effects are known to artificially increase the nonlinear coefficients when nanosecond pulses are used for their measurement[7]. Our value of β, measured using ps pulses, normally includes only electronic effects. However, it is of the same order of magnitude as the value obtained in lanthanum borate glasses doped with very low concentration (10 ppm) of gold, but measured in nanosecond regime and in non-resonant condition[8]. In a conclusion, we have presented a new way for the synthesis of a GNP-doped dense silica glass via a sol-gel route. Comparison of the SPR absorption bands and TEM measurements showed the presence of GNPs in the bulk silica matrix, with a diameter increasing from 8-18 nm in the porous xerogel to 10-30 nm in the glass. Despite this moderate particle growth, zirconia seems to be useful to embed GNPs into silica and protect them during the densification high temperature treatment. Resonant nonlinear characterization using the Z-scan technique with ultra-short laser pulses allowed us to demonstrate a reversible NL saturable absorber behavior of the densified doped silica glass. The nonlinear absorption coefficient β of about -14 cm/GW could not be strictly compared with other results from the literature because of the originality of our measurement and of our material. The saturable absorber may be exploited to induce Q-switching or mode-locking in a pulsed laser cavity. Fabrication of a nonlinear optical fiber with insertion of the obtained GNP-doped rod in a microstructure perform is being implemented.
This work was partly supported by the ”Fonds Européen de Développement Economique Régional” and the Labex CEMPI and Equipex FLUX through the ”Programme Investissements d’Avenir”. This study was also partly supported by an ANR (French National Research Agency) grant (JCJC SIMI 9- AMPEROR, 2014-2017)
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