M. Mattarelli

Pulsed laser deposition of active waveguides

We report on the pulsed laser deposition (PLD) of thin films of chalcogenide and tellurite glasses doped with RE ions. The depositions were performed in vacuum, for chalcogenide films, and in a low oxygen pressure for the tellurite films by using an excimer laser. After the deposition, the morphological, structural, optical and spectroscopical characteristics are analysed by different techniques: Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), Rutherford Backscattering Spectrometry (RBS), optical profilometry, m-lines spectroscopy, photoluminescence etc. The experimental results demonstrated that PLD is a suitable technique for the realisation of films of complex materials for optoelectronic and photonic applications.

Femtosecond laser direct writing of gratings and waveguides in high quantum efficiency erbium-doped Baccarat glass

The femtosecond laser direct writing technique was employed to inscribe gratings and waveguides in high quantum efficiency erbium-doped Baccarat glass. Using the butt coupling technique, a systematic study of waveguide loss with respect to input pulse energy and writing speed was performed to achieve the best waveguide with low propagation loss (PL). By pumping at 980 nm, we observed signal enhancement in these active waveguides in the telecom spectral region. The refractive index change was smooth and we estimated it to be ~10−3. The high quantum efficiency (~80%) and a best PL of ~0.9 dB cm−1 combined with signal enhancement makes Baccarat glass a potential candidate for application in photonics.

Er3+ activated silica-hafnia glass-ceramics planar waveguides

Silica-hafnia glass-ceramics waveguides activated by Er3+ ions were fabricated by sol-gel route. X ray diffraction and optical spectroscopy showed that after an adapted heat treatment, the resulting materials showed a crystalline environment. Analysis of the luminescence properties has demonstrated that erbium ions are, at least partially, trapped in a crystalline phase. Losses measurements at different wavelength highlight a very low attenuation coefficient indicating that this nanostructured material is suitable for a single band waveguide amplifier in the C band of telecommunication.

Spectroscopic assessment of silica–titania and silica–hafnia planar waveguides

Silicate glasses remain the most investigated systems for optical planar waveguides, since they offer a reasonable solubility for rare-earth ions, they are transparent in the near-infrared–visible region and they are compatible with integrated optics (IO) technology. In the last decade, various technologies have been employed for the fabrication of silica (SiO2)-based IO components and a broad variety of silicate glass systems have been investigated. Besides the SiO2–titania (TiO2) system, which has been widely studied, it has recently been shown that SiO2–hafnia (HfO2) could be a further viable system for 1.5 µm applications. This paper compares spectroscopic results, in particular infrared and Raman spectra, in order to assess the structural and optical properties of erbium-activated SiO2–TiO2 and SiO2–HfO2 planar waveguides, prepared by two different techniques: rf sputtering and the sol–gel method. Particular attention is devoted to the homogeneity of the material structures obtained in each case.

Vibration spectroscopy of weakly interacting mesoscopic colloids

Brillouin spectroscopy has been used to study the vibrational dynamics of clusters of spherical polystyrene (PS) particles with different size. In a first approximation, the spectra can be described by a single particle model within the continuum approximation of the Lamb theory. The model yields excellent results for particles with diameter d ≳ 500 nm, but fails in accounting for the lineshapes of the observed lowest frequency signals in the vibration spectrum of smaller particles due to a broadening and shift of the lines of confined vibrations. The model also fails to predict an additional very low frequency broad band in the case of small particles (d ≲ 400nm). This band is attributed to phonon propagation in multiple-particle clusters governed by the interactions among particles. These interactions also produce extended modes in close relation to the Lamb modes of the single sphere. A simple model for the particle interaction allows to represent the new spectral features and estimate the strength of the interactions and the long wavelength longitudinal velocity in the colloidal clusters. Their thermal annealing near the glass transition of PS enhances the interactions which are manifested in the low frequency particle vibration spectrum and the phonon bands associated with the vibrations of the individual spheres. The emerging particle vibration spectroscopy becomes a sensitive tool of the colloids’ thermo-mechanical properties as well their interactions.

Ultratransparent glass ceramics: The structure factor and the quenching of the Rayleigh scattering

Glass ceramics with nanocrystals present a transparency higher than that expected from the theory of Rayleigh scattering. This ultratransparency is attributed to the spatial correlation of the nanoparticles. The structure factor is calculated for a simple model system, the random sequential addition of equal spheres, at different volume filling factor. The spatial correlation given by the constraint that particles cannot superimpose produces a diffraction peak with a low S(q) in its low-q tail, which is relevant for light scattering. The physical mechanism producing high transparency in glass ceramics is demonstrated to be the low density fluctuation in the number of scatterers.

Nanocomposite Er-Ag silicate glasses

Particular attention has being given to metal–dielectric nanostructured materials, due to the well known surface plasmon resonance, described as the oscillation of the free electrons with respect to the ionic background of the nanoparticle when they are collectively excited by laser irradiation. It is claimed that metal nanoparticles can be used for increasing the intensity of the luminescence emitted by rare earth ions. This effect is attributed to the strong absorption cross section related to the surface plasmon excitation in noble-metal nanoparticles and/or to the large local field enhancement generated around the excited nanoparticles. In spite of the large amount of work published on this topic, the mechanism of optical amplification remains controversial. Here we present x-ray photoelectron spectra and transmission electron images together with photoluminescence absorption and emission measurements, with the aim of providing a better understanding of the effective role of silver as a sensitizer for erbium.

Sol-Gel Erbium-Doped Silica-Hafnia Planar and Channel Waveguides

Erbium activated SiO2-HfO2 planar waveguides, doped with Er3+ concentrations ranging from 0.01 to 4 mol%, were prepared by sol-gel method. The films were deposited on v-SiO2 and silica-on-silicon substrates using dip-coating technique. The waveguides show high densification degree, effective intermingling of the two film components, and uniform surface morphology. The waveguide deposited on silica-on-silicon substrates shows one single propagation mode at 1.5μm, with a confinement coefficient of 0.81 and an attenuation coefficient of 0.8 dB/cm at 632.8nm. Emission in the C-telecommunication band was observed at room temperature for all the samples upon continuous-wave excitation at 980 nm or 514.5 nm. The shape of the emission band corresponding to the 4I13/2→4I15/2 transition is found to be almost independent both on erbium content and excitation wavelength, with a FWHM between 44 and 48 nm. The 4I13/2 level decay curves presented a single-exponential profile, with a lifetime ranging between 1.1-6.6 ms, depending on the erbium concentration. Infrared to visible upconversion luminescence upon continuous-wave excitation at 980 nm was observed for all the samples. Channel waveguide in rib configuration was obtained by etching the active film in order to have a well confined mode at 1.5 μm.

Pulsed Laser Deposition of Er doped tellurite films on large area

Thin films of Er3+-doped tungsten tellurite glass have been prepared by the pulsed laser deposition technique using an ArF excimer laser. The depositions were performed at different O2 pressure (5, 10 Pa) and at different substrate temperatures (RT, 100°C and 200°C). The composition and the optical properties of the deposited films, such as transmission, dispersion curves of refraction index and extinction coefficient, and film thickness were studied for the different deposition parameters. Transparent films at the highest substrate temperature were obtained only for a higher oxygen pressure with respect to the RT conditions.

Fabrication and optical assessment of sol-gel-derived photonic bandgap dielectric structures

We present the details of the Sol-gel processing used to synthesize silica spheres, with particular attention to the conditions that permit to tailor their dimension. We have elaborated a protocol in order to obtain silica micro spheres with low polydispersivity and we have demonstrated that large well-ordered crystals of synthetic opal, that exhibit a photonic stopband, can be produced in few days by vertical deposition and evaporation-assisted sedimentation deposition methods. Scanning electron microscope was employed to characterize the samples reflectance and transmission measurements were used to put in evidence the high quality of the realized opals. Starting from the silica spheres, core-shell-like Er3+-activated silica spheres were also prepared, where the core is the silica sphere and the shell is an Er2O3-SiO2 coating. Morphologic, structural and spectroscopic properties were investigated by scanning electron microscope and luminescence spectroscopy. The emission of 4I13/2→4I15/2 of Er3+ ion transition with a 27 nm bandwidth was observed upon excitation at 514.5 nm. The 4I13/2 level decay curves presented a single-exponential profile, with a lifetime of 12.8 ms.