Excitation-dependent enhancement and quenching of the 1.54 μm emission from Er³⁺ ions in dichroic Cu nanocomposite glass

Highlights

• Dichroic phosphate glass containing plasmonic Cu and Er³⁺ ions prepared.

• Absorption and photoluminescence spectroscopy performed including decay dynamics.

• Excitation dependence of Er³⁺ near-infrared photoluminescence evaluated.

• Underlying interactions between Er³⁺ ions and Cu in dichroic nanocomposite discussed.

Abstract

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.

Introduction

Investigating the influence of noble metal aggregates such as nanoparticles (NPs) on Er³⁺ ions emission in a dielectric matrix such as inorganic glass has been an active area of research in the context of photonic/optoelectronic applications [[1], [2], [3], [4], [5], [6], [7], [8]]. Amongst the different noble metals, copper is relatively abundant and can sustain a strong surface plasmon resonance (SPR) absorption in the nanoscale, so it appears of interest [9]. However, thus far most research has focused on either the presence of copper ions or non-plasmonic copper aggregates for improving the near-infrared (NIR) emission of Er³⁺ ions around 1.54 μm. 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. Nevertheless, the plasmon absorption band characteristic of Cu NPs was not clearly distinguished [8]. Thus, it is still unclear under what conditions the PL of dielectric-embedded Er³⁺ ions can benefit from the presence of plasmonic copper.

In view of aforementioned setting, the present investigation addresses the excitation wavelength dependence of the NIR emission of Er³⁺ ions in glass containing plasmonic Cu particles. The selected glass is a durable aluminophosphate system which has been previously studied with regard to the influence of monovalent copper on Er³⁺ NIR emission [11]. Since the melt-quenched precursor glass contains tin(II) as reductant, a subsequent heat treatment (HT) was herein performed to induce the vigorous precipitation of Cu NPs resulting in the glass showing dichroism by transmitting blue light and scattering red. A similar dichroic Cu nanocomposite in the aluminophosphate glass system containing Sm³⁺ has been reported to exhibit an interesting effect, displaying an enhanced PL for the orange-red Sm³⁺ emission depending on the excitation and emission wavelength [12]. This prompted the question of whether the dichroic effect accompanying the expedient precipitation of Cu NPs could result in a beneficial effect on the NIR emission from Er³⁺. Following glass synthesis then, an optical investigation was carried out focusing on understanding the influence of different excitation wavelengths on the NIR emission of Er³⁺ ions relevant to the telecommunications, IR lasers, and solar energy conversion in photovoltaic cells.