M-lines spectroscopy (MLS) is an accurate, to within 10-3-10-4, nondestructive technique for measuring optogeometric
parameters (refractive index and thickness) of thin planar step-index waveguide films. Two exciting polarizations can be
used and information about the film anisotropy derived. Usually, MLS uses only one wavelength, which may be
disadvantageous in some cases. We have developed an MLS setup that includes a set of lasers emitting in the range of
405 to 1550 nm to conduct multi-wavelength MLS (MWMLS). MWMLS offers an opportunity to obtain more detailed
optical information, e.g., index profiles and dispersion curves, especially important for sol-gel prepared waveguide thin
films that are relatively porous and whose structure depends on the annealing treatment. The paper presents a detailed
description of the MWMLS setup. By using sol-gel prepared waveguide thin films of Y2O3, HfO2:Eu3+, and TiO2, optical
measurements are exemplified. Proceeding from the measurements, the advantages and limitations of the method are
discussed.
Sol-Gel optical film was widely used in high power laser system with the development of its preparation technique. Ultraviolet treating is an effective method to solidify the film and improve its performance. In this paper, optical films were deposited on K9, silica glass and silicon wafer substrates by Sol-Gel spin-coating method and ultraviolet light source was produced by 1000W high-pressure mercury lamp to irradiate these optical films. SEM, AFM, IR and ellipsometer were used to characterize the structure and optical properties of the films. Mechanical property of films was measured by pencil hardness-testing device. Laser damage threshold of films was measured by a Q-switched Nd:YAG high power laser whose wavelength was 1064nm and its pulse width 15ns. The results show that UV-irradiation can improve the mechanical property and increase the refractive index of the films. The nodules on the film surface can be changed into pits by UV-irradiation process so the laser damage threshold of Sol-Gel thin films increased. The laser damage threshold of ZrO2 single-layer film added with PVP reaches 50.6J/cm2 (1064 nm, 1 ns) after UV-irradiation process. It was found that UV-irradiation is an effective method to avoid infiltrating between the layers and improve the homogeneity of the multilayer films. The reflection of 5 layers of ZrO2/SiO2 multi-layer films is increased more than 10% after UV-irradiation treatment.
YAG optical planar waveguide was elaborated through sol-gel method. Acetate-alkoxide (YAG-I) and 2-alkoxides (YAG-II) methods were used for the sol preparations. The as-deposited layers were characterized by Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), m-lines, spectroscopy (MLS) Rutherford Backscattering Spectroscopy (RBS) and waveguide Raman spectroscopy (WRS). No intermediate phases were observed for both synthesis routes. In comparison, (YAG-II) layers presented a better adherence than YAG-I to SiO2 substrate, a higher refractive index and allowed to form a pure phase of YAG at lower temperature and short heat duration (900°C for 10min). However, the interdiffusion of Al and Si ions were detected to influence the refractive index improvement by heat treatments. The propagation loss of the fabricated YAG thin films in amorphous phase can be as low as 1.5±0.3 dB/cm.
The sol gel process is a rather simple route for the elaboration of pure or doped well known oxides such as TiO2, ZrO2, ... or less usual oxides such as Lu2O3, Gd2O3... in thin film form. A proof of the high optical quality of the layers is their ability to propagate light even if their application is not in the waveguide field. Moreover waveguiding properties of the layers allow their studies in waveguiding configuration. Among these methods, the most attractive for the sol gel films is the very low frequency waveguide Raman which provides the nanocrystals sizes and phases according to the annealing temperatures as illustrated by ZrO2 sol gel layers studies. For such measurements the thin films must be very carefully elaborated as described in this presentation. As example of application, Gd2O3 sol gel scintillating films are presented, when studied in waveguiding configuration and used in transmission configuration. The sol elaboration is described. The thickness and the refractive index of the films are analysed as a function of the annealing treatment. Results of the europium doped gadolinium oxide layer under X rays excitation are presented which underlined the attention paid on all the steps of the elaboration procedure.
Alumina thin films were elaborated by the sol-gel method, using aluminium butoxide as precursor and acetylacetone as chelating agent to stabilize the sol. The dip-coating parameters (sol concentration and withdrawal speed) were optimized in order to get waveguiding films. For a 15 successive layers deposition and a 700°C annealing treatment, waveguiding films of about 900nm thickness were obtained, which have a 1.582 refractive index at 543.5nm. The influence of the annealing temperature was studied in the 100°C to 1100°C range, in order to follow the elimination of the organic compounds and the existence of stresses. IR spectroscopy shows that an annealing temperature of 800°C is necessary for the complete elimination of the OH and C=O groups of the residual organic compounds, which are derived from the chelating agent. In a parallel way, the Al to O atomic ratio tends to the theoretical Al2O3 composition, according to Rutherford Backscattering Spectroscopy results. The stresses in the film were measured by an optical method. Their evolution with annealing temperature is characterized by two regimes, depending on the organic compounds elimination and the crystallization process.
The fabrication and the optical properties of sol-gel high quality DBRs and microcavities are described and the emission of the europium ions included in the cavity observed. The microcavities are constituted of an SiO2 half wave Eu3+ doped active layer inserted between two sol-gel Bragg reflectors. These reflectors are formed by a stack of alternated quarter wave films of SiO2 and TiO2. Films were deposited by a dip coating method. To fabricate high quality Bragg mirrors, a large number of layers has to be stacked, but sol gel thin layers develop internal stresses during the drying and firing processes, leading to defects and cracks into the stacked films. The study of the stresses in the layers shows that a short 900°C layer annealing solves this problem and the number of stacked layers can be greater than 60 without cracks. A microcavity with 7 doublets Bragg mirrors has been fabricated using this process. Eu3+ luminescence modification due to the cavity effect, intensity enhancement and modification of the lineshape, has been observed, showing a cavity quality factor of 1200. The reflectivity factor of the associated Bragg mirrors reaches 99.8% for seven alternated SiO2/TiO2 layers.
Sulfide glasses based on GeGaS are stabilized by the addition of a fourth element: antimony. They show very weak crystallization upon heating as compared to standard GeGaS glass. Because of the low phonon energy inherent in sulfide glasses, the 1.3 micrometers emission of Dy3+ ions can be observed. This transition is characterized in terms of emission cross-section and quantum efficiency. Refractive indices are measured for various concentrations of cesium halides in standard GeGaS glass.
Recent works in our laboratories investigated the microstructure of titania films prepared from a colloidal solution and used as planar waveguides. The transmission electron microscopy including high resolution observations together with waveguide Raman spectroscopy, especially in the very low frequency range, showed a strong influence of the heat-treatment procedure on the films' morphology. In addition, atomic force microscopy provided valuable information on their surface roughness. At lower temperatures (450 degree(s)C), the films' structure consists of a mixture of amorphous TiO2 and anatase nanocrystals (4.5 - 5 nm) and its surface is smooth. On the contrary, when increasing the annealing temperature, the nanocrystals grow (50 nm at 1000 degree(s)C), pores (5 - 20 nm) are clearly distinguished and the films' waveguiding properties disappear. Here, we discuss the optical behavior of these films and its relationship to microstructure. It is clearly shown that a high degree of crystallization induces an important porosity, a rough surface and thus dramatically affects the films' waveguiding properties. Finally, we report a relatively simple strategy to stabilize the amorphous phase and control the crystallization in order to improve the optical quality of the waveguides.
Due to the difficulties in geometrical arrangement, up to now the polarization arrangement for waveguide Raman spectroscopy (WRS) was limited to only a few simple configurations. In this report, we present the experiments in extending polarization configurations for WRS. The resulted polarized WRS spectra of thorium phosphate xerogel thin films will be presented. In the region of Raman shift from 850 to 1200 cm-1 ,two components with different polarization behavior exist. Based on the symmetry arguments, these components are then identified to the different vibration modes of P043 ions. From the spectra it is also concluded that the film is amorphous and isotropic and has chain-like structure.
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