Er and Cu codoped SiO2 films obtained by sputtering deposition: Enhancement of the rare earth emission at 1.54 μm mediated by metal sensitizers

doi:10.1016/j.optmat.2012.09.036

Optical Materials

Volume 35, Issue 11, September 2013, Pages 2018-2022

https://doi.org/10.1016/j.optmat.2012.09.036 Get rights and content

Abstract

The modification of the Er³⁺ emission properties by means of the interaction with metallic species is the object of this paper. In particular, this research has evidenced the enhancement of the characteristic Er ion emission at 1.54 μm after copper incorporation in Er-doped SiO₂ films deposited by RF magnetron sputtering. Metal sensitizer nature was studied by evaluating the impact of the sputtering deposition conditions, together with during- and post-deposition energetic treatments, on the structural and optical properties of the synthesized films. Since the rare earth sensitization occurrence is strictly dependent on the metal clustering state, the reported results suggest that 1.54 μm PL enhancement is achieved after copper incorporation, but preventing the formation of crystalline clusters with size larger than 1 nm. Concerning the mechanism involved in the sensitization process, it is argued that this phenomenon is due to an energy transfer process mediated by Cu-related sensitizers, that can be photostimulated even at frequencies non-resonant with the Er absorption lines in the visible range. The comparison of the estimated Er ion excitation cross section between the Er-doped and the Er and Cu codoped systems has revealed a real enhancement of this parameter when the Cu-mediated sensitization occurs.

Highlights

► PL properties of Er ion doped silica films are improved after Cu ion incorporation. ► 1.54 μm Er PL emission increase is due to a Cu-mediated sensitization mechanism. ► Er ions can be excited via Cu sensitizers with enhanced excitation cross section.

Introduction

The realization of highly performing light sources and optical amplifiers in planar waveguide is a crucial point for the development of the integrated optoelectronic and lightwave communication technology. In this field, rare earth based dielectrics find large application and, in particular, Er-doped glasses are one of the reference materials for the realization of optical amplifiers operating at the standard telecommunication wavelength in the near-IR window [1], [2], [3]. Anyway the possible use in planar photonic devices requires amplifiers providing an adequate optical gain, even considering the reduced size of the system [4].

One of the major limitations for the Er-doped glasses is the small absorption cross section for direct excitation of the rare earth ion. In the case of devices based on external optical pumping, a possible way to enhance the Er optical response is given by the so-called sensitization effect [5]. This consists in coupling the rare earth ion network to a suitable sensitizing agent, which act as light antenna being photostimulated even at frequencies non-resonant with the characteristic Er ion absorption lines. Subsequent energy transfer process to the rare earth brings it to relax part of this indirect excitation through radiative emission at 1.54 μm. The result is an overall enhancement in the photon capture efficiency of the system mediated by the sensitizer. This phenomenon has been observed in Er–based glasses codoped with other elements, such as rare earth ions [6], [7], Si nanostructures [8], [9], [10] or clusters of noble metals, like Ag [11], [12], [13], [14] and Au [15], [16], [17].

Following the research that our group has developed on both the themes of Er-doped dielectric waveguides [18], [19], [20], [21] and metal nanocluster composite glasses (MNCGs) [22], [23], [24], we have checked the possibility to modify and eventually to improve the luminescence response of Er-doped silica film through copper incorporation [25]. As also shown for other noble metal elements [26], [27], [28], [29], copper exhibits optical activity in the visible range when in ionic or aggregate form [30], [31], [32], [33]. This might point out the occurrence of a possible interaction with the rare earth ions, like energy transfer mechanism after suitable Cu-related antenna excitation.

A simple way to prepare Er-doped silica film consists in using sputtering deposition starting from solid targets [18], [19], [20]. This technique allows both the synthesis of glassy layers with well-controlled compositional and structural properties, and the incorporation of a huge variety of possible doping elements. During and post-synthesis treatments play a crucial role for the resulting film microstructure and the optical response activation for the doping elements. These treatments include substrate heating and low-energy ion bombardment together with post-synthesis annealing in conventional oven.

Our study has evidenced that, choosing appropriate synthesis conditions, it is possible to achieve an enhancement of the Er luminescence response when codoping the silica host with copper. As a manifest fingerprint for the occurrence of an energy transfer process, the sensitization effect is associated to the opening of an indirect excitation path mediated by the sensitizer agent with possible Er ion photostimulation even at wavelengths not directly absorbed by the rare earth itself [25]. Since this implies a modification of the photophysics involved in the overall Er luminescence process, we paid particular attention on the analysis and the determination of the parameters characterizing the optical properties of our doped silica films. In this regard, here we discuss the results concerning the evaluation of the Er ion excitation cross section, based on a simplified model for treating the experimental data. The values obtained for the Er and Cu codoped system show a manifest increase of the cross section, as a consequence of the sensitization mechanism.

Section snippets

Experimental

For this study, doped silica films of about 0.5 μm were deposited by multisource RF magnetron sputtering on fused silica slabs. The 13.56 MHz radiofrequency sources worked in a neutral atmosphere of pure Ar at a pressure of 0.40 Pa, and the power was set at 4, 12 and 150 W for copper, erbia and silica targets, respectively. During the 75 min deposition, the sample holder was rotated at 5 rpm to have a good homogeneity of film composition and thickness. Afterwards, a 15 min single silica deposition was

Impact of Cu codoping on Er ion PL properties

Fig. 1 reports the erbium near-IR PL spectra associated to the ⁴I_13/2 → ⁴I_15/2 Er ion transition for some of the synthesized films. The spectra in the left panel of Fig. 1 result from pumping in–resonance condition, whereas the ones in the right panel are obtained non-resonantly.

For the Er-doped system, the as-deposited film exhibits poor luminescence properties and a high temperature annealing is always needed to fully activate the 1.54 μm emission (only by resonant excitation). By increasing

Conclusions

In this work we discussed the possibility to enhance the characteristic Er ion emission at 1.54 μm by the interaction with Cu related sensitizers. The system object of this study was constituted by silica film obtained by sputtering deposition, where Er and Cu ions are incorporated in the glassy host as doping elements. Additional during and post-synthesis energetic treatments play a crucial role giving the possibility to optimize the Er³⁺ emission yield and to control the Cu clustering

Acknowledgements

The authors thank dr. Jasper Rikkert Plaisier of the MCX beamline (Elettra synchrotron light source, Basovizza (Trieste), Italy) for his help during grazing incidence XRD measurements.

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We present the thermal stability and the photoluminescence properties of RE-doped (RE = Ho, Er, Tm) alumina nanoparticles in the phase system Al₂O₃-SiO₂ with respect to the chemical composition and the thermal processing conditions applied in the fiber-optic technology. The alumina and silica soot reacted together to form mullite when the Al₂O₃ concentration was higher than 5 mol. %. We have demonstrated that the solubility limits of RE ions in the mullite nanocrystals are strongly limited. The RE ions preferentially occupy highly disordered positions on the nanoparticle surface or in the amorphous Al³⁺-enriched shell around the nanoparticles, exhibiting maximal lifetime of approx. 1.2 ms, 10.0 ms and 0.6 ms in the Ho-, Er- and Tm-doped samples. Rapid cooling of the samples with stoichiometric composition 3Al₂O₃·2SiO₂ managed to prepare highly defective mullite nanocrystals with embedded RE ions, exhibiting promising photoluminescence lifetimes of 5.6 ms and 2.4 ms in the case of Ho³⁺ and Tm³⁺ ions, respectively. In optical fibers with 5 mol. % Al₂O₃, the formation of amorphous Al³⁺-enriched nanoparticles was observed and the photoluminescence lifetime was in a good agreement with corresponding bulk samples. Exploitation of the RE-doped stoichiometric mullite in the fiber-optic technology may be a perspective way to improve the photoluminescence efficiency of active optical fibers for high-power applications.
Excitation-dependent enhancement and quenching of the 1.54 μm emission from Er3+ ions in dichroic Cu nanocomposite glass
2020, Solid State Communications
Citation Excerpt :
For instance, Jiménez et al. [10,11], reported on the near-UV sensitizing effect of Cu+ ions on the NIR emission of Er3+ ions in phosphate-based glasses. On the other hand, Cattaruzza et al. [5] and Trave et al. [6] found that copper aggregates could serve as sensitizers for Er3+ in sputtered glass silica films, as long as the Cu particles do not exceed ~1–2 nm so these remain non-plasmonic. More recently, Machado et al. [8] reported on the 1.55 μm photoluminescence (PL) enhancement of Er3+ in tellurite glasses, and claimed it was due to the plasmonic activity of Cu NPs.
The 1.54 μm emission of Er³⁺ ions embedded in a dichroic Cu nanocomposite glass is shown to be either quenched or enhanced depending on the excitation wavelength. The synthesized material consisted of an aluminphosphate matrix co-doped with Cu and Er as prepared by thermal processing, and characterized by optical absorption and photoluminescence (PL) spectroscopy including an assessment of decay dynamics. The extinction spectrum presented a broad surface plasmon resonance (SPR) band for Cu nanoparticles (NPs) around 588 nm, while the glass appeared to scatter red light but transmit blue, i.e. exhibiting dichroism. Interestingly, the PL obtained for the ⁴I_13/2 → ⁴I_15/2 emission around 1.54 μm was enhanced under the excitation of the ⁴I_15/2 → ⁴F_9/2 transition at 650 nm, which turned out to be located toward the low energy wing of the SPR of Cu NPs. About a 12% PL boost was estimated relative to a merely Er³⁺-doped reference. Conversely, a PL quenching effect was manifested for resonant Er³⁺ excitations (e.g. ⁴I_15/2 → ²H_11/2 transition at 520 nm) overlaid with the 3d → 4sp interband transitions absorption in copper. It is suggested that favorable conditions for the PL enhancement are achieved for when the excitation of Er³⁺ ions can benefit from light scattering by plasmonic Cu particles, while an excitation energy transfer from Er³⁺ to Cu NPs via interband transitions leads to the quenching.
In situ-monitored enhancement and quenching effect of Cu nanoclusters on Sm3+ photoluminescence in glass
2020, Physics Letters, Section A: General, Atomic and Solid State Physics
An assessment of the influence of heat treatment temperature and retention time on the optical properties of a Cu⁺ and Sm³⁺ co-doped phosphate glass of interest to photonic applications is reported. Particularly, the enhancement of the orange-red photoluminescence (PL) from Sm³⁺ ions has been realized in the glass through the development of non-plasmonic Cu clusters. It was exposed during an in situ isothermal treatment wherein optical absorption and PL were monitored together in real time. Following the PL increase, a drop in the Sm³⁺ emission intensity was produced concurring with the appearance of Cu nanoparticles (NPs) exhibiting the surface plasmon resonance. Hence, the connection between energy donor (sensitizing) effects of small Cu clusters and the energy acceptor (quenching) character of Cu NPs is supported. It is the first time to the author's knowledge that the enhancement of Sm³⁺ luminescence via Cu clusters embedded in glass is indicated.
Thermal and spectroscopic characterization of copper and erbium containing aluminophosphate glass
2020, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy
Thermal, Raman scattering, optical absorption, and photoluminescence characterizations were carried out on aluminophosphate glass containing Cu⁺/Cu²⁺ along with near-infrared (IR) emitting Er³⁺ ions of interest to photonic applications. Material synthesis was carried out by the melt-quench technique wherein Cu⁺ ions were incorporated at a high concentration by addition of 10 mol% of Cu₂O together with SnO. The copper oxide doping was recognized to result in a decrease of the glass transition temperature of the matrix, however Er³⁺ doping displayed opposite propensity. Raman measurements under 785 nm excitation were consistent with calorimetry data indicating that copper ions modify glass structural features. The degree of copper oxidation during material preparation was assessed quantitatively through the Cu²⁺ absorption feature around 850 nm. The presence of substantial Cu⁺ concurred with the significant red shift in the near-ultraviolet glass absorption edge, and was analyzed in the context of optical band gap determinations. An evaluation of the luminescence decay kinetics of Cu⁺ ions in the presence of Er³⁺ agreed with a non-radiative energy transfer which appeared more effective for excitation of Cu⁺ near the glass absorption edge at 400 nm. Such excitation was confirmed to result in the sensitized near-IR emission from Er³⁺ ions around 1.53 μm of interest to lasers, the telecommunications, and spectral conversion in photovoltaic cells.
The influence of copper and silver in various oxidation states on the photoluminescence of Ho3+/Yb3+ doped zinc-silicate glasses
2019, Optical Materials
Citation Excerpt :
Silver has been extensively studied mainly for its effect on the photoluminescence of erbium, which it enhances by up to 300% [16–20]. Copper is of relatively new interest; several promising studies have been conducted on its effect on the photoluminescence of erbium [21,22]. However, its effect on holmium has yet to be studied.
Transition metals, such as silver or copper, can be used to enhance the photoluminescence of rare-earth ions. In this work, a novel zinc-silicate glass containing holmium and ytterbium was prepared and subsequently doped with silver or copper by means of ion exchange. The concentration of transition metals was evaluated by electron microprobe analysis (EMPA), and the oxidation state was analysed by absorption spectroscopy and transmission electron microscopy (TEM). The waveguiding properties of the prepared samples were evaluated by m-line spectroscopy. Most importantly, the effect of transition metals on the photoluminescence of Ho³⁺ ions was evaluated by photoluminescence spectroscopy. The results show that copper has no positive effect on the photoluminescence of Ho³⁺ ions. On the other hand, silver shows significant promise because the photoluminescence of the glass increased by as much as 20%. It has been determined that the main mechanism behind this increase is energy transfer from small clusters rather than the surface plasmon resonance of silver nanoparticles.
Control of silver clustering for broadband Er3+ luminescence sensitization in Er and Ag co-implanted silica
2018, Journal of Luminescence
In this work, the optical properties of Er and Ag co-implanted silica slabs were investigated in order to shed light on the observed improvement of the rare-earth emission properties through a sensitization process activated by Ag implantation. A full ion implantation approach was adopted since it represents an effective way to create a thin doped layer, where luminescent Er ions can interact with Ag-related sensitizing species. The results evidenced that the sensitization process is effectively promoted in presence of Ag ultra-small structures, like few-atom aggregates or multimers, which can be already formed at the early stages of the metal clustering process. On the other hand, the precipitation of large, plasmonic clusters, occurring at high temperature post-Ag implantation annealing, produces a decrease of the fluorescence enhancement effect. Furthermore, it is suggested that the overall sensitization mechanism originates from an Ag-Er energy transfer that determines the possibility of a broadband photostimulation of the rare-earth ions, even by pumping in non-resonant excitation condition. Thanks to these features, the investigated Er and Ag co-implanted system can be considered for the realization of high-performing optical amplifiers in waveguide.

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Optical Materials

Abstract

Highlights

Introduction

Section snippets

Experimental

Impact of Cu codoping on Er ion PL properties

Conclusions

Acknowledgements

References (36)

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Cited by (14)

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In situ-monitored enhancement and quenching effect of Cu nanoclusters on Sm<sup>3+</sup> photoluminescence in glass

Thermal and spectroscopic characterization of copper and erbium containing aluminophosphate glass

The influence of copper and silver in various oxidation states on the photoluminescence of Ho<sup>3+</sup>/Yb<sup>3+</sup> doped zinc-silicate glasses

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