J. Mugnier

Elaboration and scintillation properties of Eu3+-doped Gd2O3 and Lu2O3 sol–gel films

Abstract High-resolution X-rays imaging requires thin films of dense materials with high light yield under X-ray excitation. Polycrystalline films of europium-doped gadolinium and lutetium oxide have been prepared by sol–gel method. The scintillation performances of these compounds for X-rays imaging are presented and discussed.

Optical properties of europium-doped Gd2O3 waveguiding thin films prepared by the sol–gel method

Abstract Recently, there has been a growth of interest in the preparation of new phosphor films for high-resolution X-ray imaging systems. Europium activated gadolinium oxide is very interesting because of its scintillation properties especially as a red component. The sol–gel method has been used to synthesize europium-doped gadolinium oxide films. The films present waveguiding properties and this special feature is used to study their microstructure by Raman spectroscopy in waveguiding configuration. Structural results, confirmed by X-ray diffraction (XRD) and transmission electron microscopy (TEM) observations, show that the crystallization in the cubic phase occurs at 700 °C. Opto-geometrical parameters were determined with respect to the annealing temperature. After annealing at 1000 °C, very dense europium-doped gadolinium oxide films are obtained with a thickness of 390 nm and a refractive index of 1.88 at 632.8 nm. Spectroscopic results constituted by emission spectra (UV and X-ray excitation) and decay measurements are presented.

XPS study of amorphous zirconium oxide films prepared by sol–gel

Abstract Zirconium oxide gel films deposited by dip coating on Si substrates have been studied by XPS as a function of the annealing temperature between 150 to 400°C and Ar+ ion irradiation time. The use of both ultra-thin (≤4 nm) and thick (∼100 nm) films allowed separation of chemical from charging shifts in the O1S and Zr3d binding energies. The preferential sputtering of C and O with respect to Zr under Ar+ bombardment was also shown together with the radiation enhanced electrical conductivity of the damaged depth in films annealed at temperature lower than 300°C containing organic species. The irradiation defects in the gel were also shown to be very chemically reactive for atmospheric OH− groups.

Eu3+-doped microcavities fabricated by sol–gel process

The fabrication of microcavities by a sol–gel process and their optical properties are described. The cavities are constituted of an Eu3+-doped SiO2 active layer inserted between two Bragg mirrors, fabricated by stacking alternatiely undoped TiO2 and SiO2 sol–gel thin films. Eu3+ luminescence modification due to the cavity effect, intensity enhancement and modification of the line shape has been observed, and shows a cavity quality factor of 1200. The reflectivity factor of the associated Bragg mirrors reaches 99.8% for seven alternate SiO2/TiO2 layers.

Preparation and characterization of sol–gel Y2O3 planar waveguides

Abstract Y2O3 thin film waveguides were prepared using a sol–gel method and a dip-coating procedure. Multicoating operations were necessary to produce multimode and low attenuation coefficient waveguides. Thickness and refractive index were determined by m-lines spectroscopy. X-ray diffraction (XRD) and conventional transmission electron microscopy (CTEM) were used for structural investigations and nanocrystals size determination as a function of the annealing temperature. The Y2O3 amorphous phase was observed at low heat-treatment temperature (300°C) and the layers turn to cubic nanocrystalline form when the annealing temperature reached 400°C. An annealing treatment at 600°C was necessary to provide more densified and good optical planar waveguides exhibiting an attenuation coefficient lower than 2 dB/cm.

Designable buried waveguides in sapphire by proton implantation

Buried and stacked planar as well as buried single and parallel channel waveguides are fabricated in sapphire by proton implantation. Good control of the implantation parameters provides excellent confinement of the guided light in each structure. Low propagation losses are obtained in fundamental-mode, buried channel waveguides without postimplantation annealing. Choice of the implantation parameters allows one to design mode shapes with different ellipticity and/or mode asymmetry in each orthogonal direction, thus demonstrating the versatility of the fabrication method. Horizontal and vertical parallelization is demonstrated for the design of one- or two-dimensional waveguide arrays in hard crystalline materials.

Optical and structural analysis of Eu3+-doped alumina planar waveguides elaborated by the sol–gel process

Abstract Eu 3+ ion-doped Al 2 O 3 waveguiding thin films were dip-coated by the sol–gel route. Crack-free films were obtained, about 880 nm thick for 15 layers. According to the results of grazing incidence X-ray diffraction analysis and transmission electron microscopy (TEM), the film contains γ-Al 2 O 3 nanocrystals in an amorphous surrounding. The refractive index determined by m-lines spectroscopy is 1.582 at 543.7 nm. A good agreement between film thicknesses determined by m-lines spectroscopy measurements and TEM observation is obtained. The propagation is good (propagation losses less than 2 dB/cm) and waveguide fluorescence spectroscopy (WFS) could be used to register the Eu 3+ fluorescence spectrum, which shows disordered environment for Eu 3+ ions.

Study of erbium doped ZrO2 waveguides elaborated by a sol–gel process

Er3+ doped ZrO2 thin films (1 mol%) are prepared by sol–gel process and dip-coating technique. After annealing the films exhibit good optical quality and waveguiding properties. The observed fluorescence of Er3+ ions, especially at 1.53 μm, is used by Waveguide Fluorescence Spectroscopy (WFS). It is shown that the band shape of the 4I13/2 → 4I15/2 transition depends on the annealing temperature. Waveguide Raman Spectroscopy (WRS) is used at room temperature in order to analyse the films structure. Such study was conducted for different annealing temperatures at an excitation wavelength where erbium absorption is low (i.e. 676.4 nm). An amorphous waveguide is obtained below 400°C. Up to 800°C, the tetragonal phase mainly appears, although both monoclinic and tetragonal phases are simultaneously detected in undoped waveguides at this temperature. High Resolution Transmission Electron Microscopy (HRTEM) observations provide the mean diameter of ZrO2 crystallites which increases up to 15 nm after a 800°C heat-treatment. This large size explains in part the high attenuation measured on this film at 676.4 nm (around 20 dB/cm).

Sol–gel fabrication of thick multilayers applied to Bragg reflectors and microcavities

Abstract We discuss attractive potentialities of the sol–gel process applied to planar microcavities with distributed Bragg reflectors (DBRs). This method is well known for its good flexibility and the optical quality of the thin films obtained. The DBRs are composed of alternated TiO2 and SiO2 thin films. One of the main problems of the sol–gel process is the stresses induced during the layers heat treatment leading to defects and cracks in the films. The study of these stresses shows that with the appropriate annealing temperature and duration of the firing process, the stress in the SiO2 layers partially compensates the stress in the TiO2 layers. DBRs and microcavities formed by 60 stacked layers have been elaborated in these conditions. The reflectivity of such sol–gel DBRs reaches 99.7%. The sol–gel DBRs are used to fabricate microcavities containing Eu3+ rare earth ions with a quality factor of approximately 1000.

Planar ZrO2 waveguides prepared by the sol-gel process: Structural and optical properties

ZrO2 waveguides are prepared by the sol-gel process from a solution containing zirconiumn-propoxide and acetylacetone in propanol-2. Structural characterizations are investigated for different annealing temperatures using suitable techniques including Waveguide Raman Spectroscopy, Electron Microscopy and Atomic Force Microscopy. Films are amorphous at 300°C and the pure ZrO2 tetragonal crystalline phase appears beyond 400°C. Crystallized films present a dense, uniform and polycrystalline structure made up by randomly oriented nanocrystallites, the diameter of which increases from 38 Å at 400°C to 53 Å at 600°C. Waveguides are at least monomode TE0 at 632.8 nm. At this wavelength, optical losses are about, 0.8±0.2dB/cm for amorphous layers and increase up to 2.5±0.4 dB/cm for 600°C heat-treated waveguides.