Elsevier

Optical Materials

Volume 27, Issue 11, October 2005, Pages 1743-1747
Optical Materials

Assessment of spectroscopic properties of erbium ions in a soda-lime silicate glass after silver–sodium exchange

https://doi.org/10.1016/j.optmat.2004.11.044Get rights and content

Abstract

Spectroscopic properties of Ag/Er co-doped thin plates of silicate glass were investigated with the aim of assessing the effective role of silver as a sensitizer for erbium. Additive heat treatments in air at different temperatures were performed on both a silver-exchanged and a silver-free plate in order to promote the formation of silver nanoparticles in the former and to refer to the later in the spectroscopic characterization. Absorption as well as photoluminescence and lifetime measurements in the region of the 4I13/2  4I15/2 transition of the Er3+ ion were performed; excitation wavelengths in the range from 360 to 750 nm were used. Enhancement of the Er3+ luminescence at 1.53 μm was observed when the excitation wavelength was in the blue region. This spectral range typically coincides with the excitation energy of the surface plasmon resonance of nanometer-sized spherical silver particles.

Introduction

Nanostructured materials have been attracting a very large interest in both academic and industrial communities over the past decade, due to the remarkable variations in fundamental electrical, optical and magnetic properties that occur as one moves from a bulk “homogeneous” material to a particle or a cluster of dimensions in the 1 to 100 nm range [1]. Understanding of the optical properties of metal nanocrystals and nanocomposites has been the subject of numerous studies; besides significant fundamental interest, more and more photonic applications are emerging, where—for instance—colorimetric, surface-enhanced Raman scattering (SERS), and surface plasmon resonance (SPR) effects can be adequately exploited. Particular attention is being devoted to metal-dielectric nanostructured materials [2], due to the well-known surface plasmon resonance, which originates from a confinement effect on the electronic properties in metal systems of finite size [3]. Glasses containing metal nanoparticles have been exploited since the Medieval Ages, for the fabrication of colored stained glasses in cathedrals, but they are now attracting a novel interest for photonic applications [4].

The phenomenon of plasmon resonance is classically 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. The strong absorption cross-section related to the surface plasmon excitation in noble-metal nanoparticles and/or the large local field enhancement that is generated around the excited nanoparticle make it possible to use such metal nanoparticles for the enhancement of the luminescence intensity emitted by rare-earth ions.

The works of Malta et al. [5] and Hayakawa et al. [6] have already proposed to intensify the luminescence of rare-earth ions by inserting very small silver particles in the glass host. These authors have noticed that the luminescence of Eu3+ ions in melted calcium boron oxyfluoride glass [5] and bulk sol–gel-derived silica glass [6] was enhanced by doping those glasses with Ag(NO3)3 and precipitating the incorporated silver ions in the form of metal Ag particles. The size of these particles was estimated not to exceed a few nanometers.

More recently, a study on the photoluminescence (PL) of erbium in borosilicate glass [7], with or without the presence of silver, indicated that, concerning the rare-earth ion excitation, the dominant process would not involve silver nanoparticles, neither via local field enhancement effects due to their surface plasmon resonance, nor via absorption and subsequent energy transfer to Er3+ ions. Furthermore, no clear influence of the presence of Ag0 particles was detected on the spectroscopic properties of erbium in sol–gel-derived silica–titania nanocomposite planar waveguides [8].

In such a controversial scientific context, we decided it was worthwhile to investigate the optical and spectroscopic properties of Er/Ag co-doped thin plates of silicate glass, with the aim of better understanding the effective role of silver as a sensitizer for erbium.

Section snippets

Experimental

The composition of the rare-earth doped silicate glass we decided to use for our tests is (mol%): 71.5 SiO2, 15 Na2O, 10.4 CaO, 1.2 Al2O3, 0.4 P2O5, 0.6 K2O, 0.3 Er2O3 and 0.6 Yb2O3; it was one of a set of glasses we had developed for the fabrication of ion-exchanged integrated optical amplifiers [9]. From this glass, two plates were cut, thinned down to 200 μm, and optically polished. One of these two plates was kept as reference (labeled as Ref) while the other was fully ion exchanged, by

Results

Fig. 2 shows the luminescence spectra of the reference and as-prepared Ag-exchanged plates of silicate glass. No effect of the Ag exchange on the profile of the Er3+ PL band at 1.5 μm was observed. The PL measurements of the Ref and AgEr0 samples showed some difference only for what concerned the lifetimes of the Er3+4I13/2 metastable level. Previous results reported so far [7], [10] suggested a decrease of the lifetime due to defects related to silver exchange in the glass matrix. Here, on the

Discussion

From the present experimental results, some enlightenment is brought in the understanding of the sensitization of erbium by silver. The excitation spectrum of Fig. 5 clearly shows the tail of a band in the blue region, extending to about 500 nm. Such an excitation band seems to be associated with the absorption shoulder observed in Fig. 3(b), and is not correlated with the broadband at 480 nm revealed in Fig. 3(a). This last point is supported by the following arguments: (i) a similar broadband

Conclusions

Erbium-doped soda-lime silicate thin plates were co-doped with Ag by using silver–sodium ion exchange and later submitted to various heat treatments in air. After two annealing processes of 30 min each at 500 °C, the Ag-exchanged silicate glass exhibited a shoulder in its absorption spectrum, between 400 and 500 nm. A stronger absorption band centered in the blue region was observed after the heat treatment was performed at higher temperature (600 °C).

Due to their spectral characteristics and

Acknowledgments

The authors wish to acknowledge Roberto Calzolai (IFAC CNR) and Enrico Moser (Trento University) for their technical assistance. This research has been supported by MIUR-FIRB “Miniaturized systems for electronics and photonics”, and PAT 2004–2006 FAPVU “Fabrication of ultra transparent glass–ceramics based planar optical amplifiers” Projects.

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