Short-range order around Er3+ in silica waveguides containing aluminium, titanium and hafnium

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Abstract

Er3+-doped silica waveguides, co-doped with aluminium, titanium, and hafnium oxides, were prepared using the sol–gel method and dip-coating processing. Here, we present a characterisation of the local environment around Er3+ ions, as determined from the Er L3-edge extended X-ray absorption fine structure (EXAFS) measurements performed at ESRF (France). The first coordination shell is composed of 5–6 oxygen atoms at distances ∼2.32–2.35 Å, slightly varying as a function of the modifier oxide. Er3+ nearest neighbors distance does not show a significant compositional dependence. On the contrary, outer shells analysis shows significant differences: Al2O3 doping (less than 2% mol) induces an ordering around Er and increases the distance of the second shell, probably due to the substitution of Si by Al atoms; for TiO2 doping (7–15% mol), it is most probable that the second shell is composed from Si atoms as the case of pure SiO2; for HfO2 doping (from 10 to 50 mol%) there is a very clear evidence of Er–Hf coordination already at the lowest Hf content, but still in amorphous environment.

Introduction

Among the different technologies which are employed to develop materials suitable for photonics, sol–gel processing exhibits several advantages in terms of rare-earth solubility, composition, design, tailoring of optical properties as well as fabrication of films, waveguides, photonic crystals, and bulk glasses. The binary silica-based systems, such as silica–titania, silica–hafnia, and silica–alumina, are of particular interest allowing the tailoring of the optical and spectroscopic properties [1]. Information on the nearest and next-nearest coordination shells of the rare-earth ions in the glass host is of crucial importance to design and select the system with optimised spectral properties for fabricating active devices such as planar optical amplifiers.

For this purpose, we present here a comparison between the local structure around Er3+ ions in silica glass co-doped with different modifier oxides. In previous publications we presented the local structure around Er3+ ions in pure SiO2 and SiO2–Al2O3 networks [2], [3]. We showed that co-doping of SiO2 with few mol% of Al was sufficient to induce significant changes beyond the first coordination shell. Here we report on new EXAFS experiments on SiO2–TiO2 and SiO2–HfO2 waveguides. In the last system, in particular, spectroscopic studies have shown a strong modification of the crystal field strength around the Er3+ ion, leading to an increase of the electric dipole component of the 4I13/24I15/2 transition probability [4], [5].

Section snippets

Experimental and data analysis method

Er3+-doped SiO2–TiO2 and SiO2–HfO2 solutions with different modifier contents (7, 12, and 15 mol% for TiO2; 10, 20, 30, 40, and 50 mol% for HfO2) were prepared using the sol–gel method and deposited on v-SiO2 substrates using the dip-coating processing [4]. Composition and thermal history of different samples are compiled in Table 1. X-ray absorption measurements were performed at room temperature (RT) in fluorescence mode at the Er L3-edge using synchrotron radiation at the BM08 GILDA CRG

Results and discussion

Let us first discuss the results relative to the first coordination shell of Er3+ in the different oxide matrices. From the experimental and calculated Fourier transforms reported in Fig. 1, and the structural parameters compiled in Table 1, the first coordination shell in all the investigated samples is composed of 5–6 oxygen atoms at distances ∼2.32–2.35 Å. The first coordination shell is not influenced significantly neither by varying the network modifier nor its concentration. This can be

Conclusions

The first coordination shell around erbium in Er3+-doped SiO2 co-doped with different modifier oxides is composed from 5 to 6 oxygen atoms at distance ∼2.32–2.35 Å. This shell is not influenced, significantly, neither by varying the type nor the content of the network modifier.

The situation for the outer coordination shell is very different. The presence of titanium is not detectable in the local environment of erbium. The main effect of co-doping with a low molar content of aluminium is to

Acknowledgement

This work was partially supported by MIUR, Italy, through the FIRB project “Sistemi Miniaturizzati per Elettronica e Fotonica”. We are grateful to Luca Zampedri for significant technological and scientific assistance in waveguide fabrication. The financial support by ESRF (F) and INFM (I) is acknowledged. Authors are grateful to the staff of the BM08-GILDA beamline for assistance during the measurements.

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