Control of silver clustering for broadband Er3+ luminescence sensitization in Er and Ag co-implanted silica
Introduction
The field of optical communications continuously requires the development of new materials for the transmission and the amplification of light signals. In this context, insulating matrices doped with rare-earth species have represented an active research field during the last decades [1], [2], [3], [4], [5], [6], [7], [8]. Particularly, the superior chemical resistance and the compatibility to optical fiber technology make silica-based matrices one of the elective hosts for the realization of Er-based devices, since the ion can be naturally incorporated in an optically active site. Furthermore, silica glass can accommodate a wide variety of doping elements, whose structural and optical properties can be controlled and even tuned by suitable post-doping treatments. Depending on the functionality required for a specific device, a crucial aspect is the possibility to improve the optical response of the rare-earth, being this species intrinsically characterized by small excitation cross-section values.
With the aim at realizing Er-based waveguide systems with enhanced optical performances, several synthesis techniques were explored [3], [9], [10], [11], [12] to achieve a real improvement in terms of rare-earth distribution homogeneity, optimized doped layer geometry, limited structural defectivity. Furthermore, to enhance the Er luminescence, it is well established that the incorporation in the host of suitable co-doping species can lead to an overall intensity increase of the rare-earth emission features, including those falling within the telecommunication wavelength window (usually around 1.5 µm).
As concerns the use of metallic species as rare-earth luminescence sensitizers, in the 80's a few papers reported about the possibility to increase of Eu3+ ion emissions in calcium boron oxyfluoride [13] and sol-gel derived silica glasses [14] after the incorporation of Ag precursor and following precipitation of metallic nanoparticles (NPs). In the latter case, the authors proposed that the increased fluorescence stemmed from local-field enhancement around Eu ions owing to surface plasmon resonance (SPR) features occurring in the Ag NPs. At the turn of 2000's, it was demonstrated that for Eu [15], [16] and Er [17], [18] the presence of a silver surface near the region doped with optically active ions can relax the momentum mismatch between photon and the propagating surface plasmon waves. This process thus contributes to the far-field fluorescence of the rare-earth emitter, effectively increasing its radiative emission rate. The emission enhancement is attributed to near-field coupling of Er3+ to Ag surface plasmon polaritons, that subsequently re-radiate at well-defined resonance conditions. In the following years, the advent of plasmonics has determined a new paradigm for the realization of nanoscale devices operated at the optical and telecom wavelengths, thanks to novel functionalization in terms of light generation, manipulation and guiding, including the possibility to improve the emission properties of luminescent materials.
In addition to this research line, involving Plasmon-enhanced luminescence phenomena driven by metal NPs, it must be considered the pioneering work by Strohhöfer and Polman [19], that dealt with the luminescence sensitization effect occurring in Er-implanted borosilicate glasses after Ag ion-exchange procedure. Supported by following studies, it was evidenced that the presence of ultra-small, non-plasmonic Ag clusters promotes the activation of a non-resonant energy transfer process to the Er3+ ions, then determining the broadening and the enhancement of the rare-earth absorption spectrum, with consequent improvement of the luminescence activity [20], [21], [22], [23], [24]. Furthermore, these sensitizing Ag-related species, whose formation characterizes the early stage of the overall clustering process, exhibit peculiar luminescence activity characterized by spectral band across the UV–visible range [25], [26], [27].
Later, several studies focused the occurrence of luminescence enhancement effects driven by different metal sensitizing agents, like Au [28], [29], [30], [31], [32], Cu [33], [34], [35], Sn [36], Bi [37], for Er ions and, more in general, for other rare-earth species [38], [39], [40], [41], [42]. It is worth pointing out that the photophysical mechanism at the basis of the interaction between rare-earth and Ag (or metal) sensitizing species strongly resembles the well-known Er broadband sensitization process promoted by the coupling with Si nanocluster-based energy transfer mediators [43], [44], [45], [46], [47].
In this work, we investigated the optical properties of Er and Ag co-doped silica thin films, focusing on the occurrence of a rare-earth luminescence improvement through the coupling with metal sensitizers. The samples were prepared by a full ion implantation route, since this technique represents an efficient way to generate a homogeneous, step-like active layer with controlled dopant concentration, even beyond the solubility limit allowed by the specific host. Then, metal clustering process was promoted and carefully controlled by following post-implantation heating treatment, for an effective size tuning of the precipitated nanostructures. Spectroscopic and time-resolved photoluminescence studies were carried out with the aim at determining the best conditions for material synthesis and processing, in order to maximize the Ag-driven Er sensitization process and thus the yield of the rare-earth luminescence emission at 1.54 µm.
Section snippets
Sample preparation
Sequential ion implantation procedures were carried out on silica substrates (Herasil 102 by Haereus; slide area of 7.5×2.5 cm2) at room temperature for preparing a series of Er and Ag co-doped samples. To obtain step-like concentration profile and to extend the doped region, a sequence of three consecutive implantations at specific energies were used for both the doping elements. Fluence and implant energy values were chosen for optimizing the overlap of the respective concentration profiles.
Enhancement of 1.54 µm Er3+ emission
The occurrence of a photosensitization process after the incorporation of Ag ions in Er implanted silica has been evidenced by a luminescence study of the rare-earth emission around 1540 nm, both in resonant and non-resonant pumping conditions. To this aim, we used the 488 and the 476.5 nm emission lines of an Ar laser operated at 6.5 mW and mechanically chopped at 9 Hz.
Fig. 1 presents a comparison between the PL spectra in the 1450–1650 nm range of the Er reference sample and the co-implanted
Conclusions
We investigated the broadband enhancement of the 1.54 µm emission in Er and Ag co-doped silica slabs, realized by a full ion implantation approach. The increase of Er3+ PL emission at 1.54 µm is due to a photosensitization mechanism originating from light absorption by few-atom Ag clusters or multimeric structures in a wide wavelength range, and subsequent energy transfer towards the rare-earth ions, while an improvement induced by a local field enhancement due to surface plasmon resonance at
Acknowledgments
The NanoStructures Group gratefully acknowledges the Italy-Mexico Project of Major Importance (MX14MO09) of the Italian Ministry of Foreigner Affairs and International Cooperation (MAECI).
Francesco Enrichi gratefully acknowledges VINNOVA for support, under the Vinnmer Marie Curie Incoming - Mobility for Growth Programme (project “Nano2solar” Ref. No. 2016-02011) and the PLESC Project between South Africa and Italy (contributo del Ministero degli Affari Esteri e della Cooperazione
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