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Optoelectronics & Communications

Rare-earth-activated glass-ceramic waveguides: ideal systems for photonics

Rare-earth-activated glass ceramics consisting of nanocrystals embedded in an amorphous matrix combine the advantages of optical glasses with crystal-like spectroscopic characteristics.
22 November 2011, SPIE Newsroom. DOI: 10.1117/2.1201111.003940

Amorphous waveguide structures activated with rare earth metal ions are becoming widely adopted in photonic applications such as integrated optical amplifiers, laser systems, and solar energy converters. These waveguides offer efficient luminescence quantum yield (i.e., the ratio of emitted to absorbed photons), effective broad bandwidth, and low attenuation coefficients. However, previous investigations on the enhancement of the luminescence quantum yield have identified phenomena detrimental to the efficiency of the active waveguide,1 such as ion-ion interactions and non-radiative relaxation processes. The search for more efficient glass compositions and guiding structures is thus ongoing.

Glass-ceramic waveguides overcome some of the efficiency problems experienced with conventional waveguides. These two-phase materials are composed of nanocrystals embedded in an amorphous matrix. The respective volume fractions of the crystalline and amorphous phases determine the properties of the glass ceramic. They also represent a valid alternative to widely used glass hosts such as silica as an effective optical medium for light propagation and luminescence enhancement.2 The crystalline environment of rare earth ions creates a waveguide with high absorbance and emission cross sections. It also reduces non-radiative relaxation by lowering the phonon cut-off energy of the waveguide2 and by increasing the spacing between particles, diminishing unfavorable ion-ion interactions. For photonic applications, transparency of the waveguide is of paramount importance. Fabrication protocols and materials must be tailored to optimize the spectroscopic features of active ions and to minimize attenuation coefficients.1

Figure 1. High-resolution transmission electron micrographs of a 1mol% erbium ion (Er3+)-activated silica-hafnia (SiO2-HfO2) glass-ceramic waveguide, showing HfO2 single nanocrystals homogeneously dispersed in an amorphous matrix.

Figure 2. Room-temperature photoluminescence spectra of Er3+in zirconium erbium lanthanum aluminum [gallium] fluoride planar waveguide glass ceramics. WGF indicates fluoride waveguide. WGF6 is obtained after heat treatment and comprises lanthanum zirconium fluoride (LaZr3F15) crystals. WGF7 comprises LaF3 crystals grown during the deposition step with a substrate temperature higher than for WGF6. Luminescence decay curves from the 4I13/24I15/2 metastable state of Er3+ are reported in the inset. The increased bandwidth of WGF7 as the waveguide material is changed from glass to glass ceramic, caused by the presence of LaF3 nanocrystals, is clearly evident as a wider waveform. Er3+ in both amorphous glass and crystalline environments contributes to the spectrum.

We previously showed silica-hafnia (SiO2-HfO2) thin-film systems to be suitable for fabricating amorphous planar waveguides, glass-ceramic waveguides, spherical microresonators, and tapered rib waveguide lasers.1 Using appropriate top-down and bottom-up techniques, we can make erbium (Er3+)-activated glass-ceramic planar waveguides. Top-down approaches seek to create nanocomposite devices by using larger externally controlled zones to construct a new system, while bottom-up methods focus on smaller components and arrange them into a more complex system. These materials exhibit attenuation coefficients as low as 0.3dB/cm at 1542nm, and the HfO2 crystalline phase greatly enhances the spectroscopic properties of embedded erbium ions. X-ray diffraction and high-resolution transmission electron microscopy analyses have shown the formation of tetragonal HfO2 nanocrystals with dimensions of about 3–5nm (see Figure 1), depending on the HfO2 content.1

While silica remains one of the best low-cost materials, fluoride glasses are attractive because of their ability to solubilize rare earth ions (> 5×1021 ions cm−3) and their inherently low phonon cut-off energy. Mortier et al. showed that transparent glass ceramics can be obtained by heating zirconium fluoride-based glass with high erbium content (8mol%) at 70°C above the glass-transition temperature (390°C) for 40min.3 This causes a so-called spinodal decomposition, whereby the chemical composition of the glass fluctuates continuously until it decomposes into two separate and distinguishable phases, producing a glass ceramic with absorbance cross section increased by 20%. The morphology of the crystallites is dendritic with high connectivity. We have obtained fluoride glass waveguides with composition close to bulk glass by physical vapor deposition,2 which opens up the possibility of using fluoride glass-ceramic materials for photonic applications.

Fluoride glasses provide an interesting system for fabricating both amorphous and glass-ceramic waveguides activated by rare earth ions. They take advantage of high Er3+ solubility and low phonon energy compared with oxide glasses. Moreover, they enable incorporation of rare earth ions into the crystal phase after thermal annealing (see Figure 2). Bulk fluoride glasses and glass ceramics activated by Er3+ and ytterbium ions (Yb3+) were investigated with the aim of quantifying the influence of Yb3+ on the spectral characteristics of Er3+ in these systems. Glassy samples co-doped with Yb3+ with a ratio Yb:Er of 5:1 present an absorption coefficient and emission intensity four times higher at 1532nm than samples activated only with Er3+.2

In summary, glass-ceramic waveguides activated by rare earth ions are nanocomposite systems that exhibit specific morphological and spectroscopic properties. They allow exploration of interesting new physical concepts and enable us to construct novel photonic devices based on luminescence enhancement. Fabrication techniques based on both bottom-up and top-down approaches have been shown to be viable, though it remains true that precise adherence to protocol is required to achieve the reliability and reproducibility necessary for such devices. Our work now focuses on fabrication protocols that preferentially embed rare earth ions inside the crystal phase, and on patterning techniques that minimize degradation of optical properties.

Alessandro Chiasera, Maurizio Ferrari
Institute of Photonics and Nanotechnology, CNR
Trento, Italy

Alessandro Chiasera is currently a member of the Characterization and Development of Materials for Photonics and Optoelectronics group. He works on the development and investigation of photonic devices based on glass matrix materials fabricated by techniques such as sol-gel and RF sputtering.

Guillaume Alombert-Goget
University of Versailles
Vélizy Villacoublay, France
Simone Berneschi
Enrico Fermi Center for Research and History of Physics
Rome, Italy
Nello Carrara Institute of Applied Physics
Sesto Fiorentino, Italy
Stefano Pelli
Nello Carrara Institute of Applied Physics
Sesto Fiorentino, Italy
Claire Duverger Arfuso, Brigitte Boulard
Laboratory of Oxides and Fluorides
University of Maine
Le Mans, France

1. A. Chiasera, G. Alombert-Goget, M. Ferrari, S. Berneschi, S. Pelli, B. Boulard, C. Duverger Arfuso, Rare earth–activated glass-ceramic in planar format, Opt. Eng. 50, pp. 071105, 2011. doi:10.1117/1.3559211
2. B. Boulard, O. Péron, Y. Jestin, M. Ferrari, C. Duverger-Arfuso, Characterization of Er3+-doped fluoride glass ceramics waveguides containing LaF3 nanocrystals, J. Lumin. 129, pp. 1637-1640, 2009. doi:10.1016/j.jlumin.2009.01.031
3. M. Mortier, A. Monteville, G. Patriarche, G. Mazé, F. Auzel, New progresses in transparent rare-earth doped glass-ceramics, Opt. Mater. 16, no. 1–2. pp. 255-267, 2001. doi:10.1016/S0925-3467(00)00086-0