SiO₂-SnO₂:Er³⁺ Glass-Ceramic Monoliths

Featured Application

The goal of this work is to demonstrate: (1) a reliable fabrication protocol of monolithic SiO₂-SnO₂:Er³⁺ glass-ceramics and (2) the luminescence efficiency of this system. Based on these fundamental results, we are working on developing a proof of concept of a solid-state laser with lateral pumping as drawn below.

Abstract

The development of efficient luminescent systems, such as microcavities, solid-state lasers, integrated optical amplifiers, and optical sensors is the main topic in glass photonics. The building blocks of these systems are glass-ceramics activated by rare-earth ions because they exhibit specific morphologic, structural, and spectroscopic properties. Among various materials that could be used as nanocrystals to be imbedded in a silica matrix, tin dioxide presents some interesting peculiarities, e.g., the presence of tin dioxide nanocrystals allows an increase in both solubility and emission of rare-earth ions. Here, we focus our attention on Er³⁺—doped silica—tin dioxide photonic glass-ceramics fabricated by a sol-gel route. Although the SiO₂-SnO₂:Er³⁺ could be fabricated in different forms, such as thin films, monoliths, and planar waveguides, we herein limit ourselves to the monoliths. The effective role of tin dioxide as a luminescence sensitizer for Er³⁺ ions is confirmed by spectroscopic measurements and detailed fabrication protocols are discussed.

1. Introduction

Looking at the literature from the last few years, it is evident that glass-based rare-earth-activated optical structures represent the technological pillar of a huge of photonic applications covering health and biology, structural engineering, environment monitoring systems, and quantum technologies. Among different glass-based systems, a strategic place is assigned to transparent glass-ceramics and nanocomposite materials, which offer specific characteristics of capital importance in photonics [1,2,3]. These two-phase materials are constituted by nanocrystals or nanoparticles dispersed in a glassy matrix. The respective composition and volume fractions of crystalline and amorphous phase determine the properties of the glass-ceramics. The key to making the spectroscopic properties of the glass-ceramics very attractive for photonic applications is to activate the nanocrystals by using rare-earth ions as luminescent species [4]. From a spectroscopic point of view, the more appealing feature of glass-ceramic systems is that the presence of the crystalline environment for the rare-earth ions allows high absorption and emission cross sections, reduction of the non-radiative relaxation thanks to the lower phonon cut-off energy, and tailoring of the ion–ion interaction by the control of the rare-earth ion partition [5]. Here we focus on glass-ceramic photonic systems based on rare-earth activated SiO₂-SnO₂ monoliths produced by sol-gel route. Although the system has been investigated for several years, the research activity is still undergoing because of the need to develop reliable fabrication protocols and to control the ion–ion interaction [4,5,6]. Both these problems are highly detrimental for the efficiency of active devices [2,7,8,9]. Among the different materials that are successfully used as nanocrystals to be embedded in the silica matrix, tin dioxide presents specific interesting characteristics. Rare-earth-activated SnO₂-based bulk glass ceramics have been extensively studied for improving luminescence efficiencies of several rare-earth ions by exciton mediated energy transfer from SnO₂ nanocrystals to the rare-earth ion [6,10,11]. SnO₂ is a wide-band gap semiconductor (E_g = 3.6 eV at 300 K) with a maximum phonon energy of 630 cm⁻¹, exhibiting a broad window of transparency from visible to infrared covering a significant emission range of rare-earth ions [12].

Here we look for two significant outcomes: (i) a fabrication protocol of SiO₂-SnO₂:Er³⁺ glass-ceramic monoliths, and (ii) efficient Er³⁺ sensitizing by SnO₂ nanocrystals pumping. We will present recent results concerning sol-gel fabrication of SiO₂-SnO₂:Er³⁺ glass-ceramic monoliths and their spectroscopic assessment for the development of luminescent systems such as solid-state lasers and active fibers.

2. Materials and Methods

2.1. Sample Preparation: Sol-Gel Derived Route

In this work, a sol-gel derived route was employed to synthesize the tin dioxide-based glass-ceramic monoliths. The monoliths were prepared following five consecutive stages: sol formation, gelation, aging, drying, and heat treatment. Since the final monoliths were obtained based on the phase transformation from gels to glasses, the first four stages played critical roles in assembling the gel skeleton, and it in turn defined a specific strategy for the heat treatment to obtain the glass-ceramics.

Table 1. Table of the detailed composition of (100 − x)SiO₂-xSnO₂:yEr³⁺ monoliths.

SnO2 Content x (mol%)	Er3+ Concentration $\mathbf{y}=\frac{{\mathbf{n}}_{\mathbf{E}{\mathbf{r}}^{3+}}}{{\mathbf{n}}_{\mathbf{S}\mathbf{i}{\mathbf{O}}_2}+{\mathbf{n}}_{\mathbf{S}\mathbf{n}{\mathbf{O}}_2}}$ (mol%)	H2O/TEOS	EtOH/TEOS	HCl/TEOS
10	0.5	10	4	0.009

describes the synthesis recipe used for sol formation which is similar to the one reported elsewhere [13]. Briefly, the syntheses started by dissolving TEOS, SnCl₂∙2H₂O, and Er(NO₃)₃∙5H₂O in ethanol separately, and then the solutions were mixed together. The solution of water and hydrochloric acid was poured drop by drop into the mixture. After that, the mixture was stirred for 1 h to form the resulting solution. This solution was transferred into the containers and sealed before being applied to any further treatment.

However, since our target was to increase the SnO₂ content higher than 5 mol %, as in Reference [13], it was necessary to modify the condition of the next stages, i.e., gelation, aging, drying, and heat treatment. This change helped avoid any phase separation when the content of SnO₂ was increased up to 10 mol %. The schematic synthesis procedure of 90SiO₂-10SnO₂:0.5Er³⁺ monoliths is shown in Figure 1.

Figure 2 below shows the photos of two examples of the crack-free and transparent 90SiO₂-10SnO₂:0.5Er³⁺ monolithic square and cylinder after the heat treatment at 900 °C for 40 h.

2.2. Charaterization Methods

To check the effective role of tin dioxide as a luminescence sensitizer for Er³⁺ ions, the spectroscopic measurements based on different excitation sources were carried out on the 90SiO₂-10SnO₂:0.5Er³⁺ monolith that was heat treated at 900 °C for 40 h. By the use of Xenon lamp 450 W (Edison, NJ, USA) coupled to monochromator Horiba mod. microHR (Edison, NJ, USA), the 1500 nm emission spectra excited at different wavelengths and the excitation spectrum were performed. The excitation range was from 300nm to 750 nm with a 1 nm scanning step and a spectral resolution of 10 nm. The results proved the energy transfer from SnO₂ to Er³⁺ and its effective role in this indirect excitation scheme in comparison with other direct ones. For the lifetime acquisition of the ⁴I_13/2-⁴I_15/2 Er³⁺ transition, the 514.5 nm coherent laser beam from the Ar⁺ laser Coherent mod. Innova-Sabre TSM 15 (Santa Clara, CA, USA) was then employed to perform the time-resolved 1500 nm florescence spectroscopy of the monolith. All of the luminescence signal was dispersed by a 320 mm single-grating monochromator with a resolution of 0.5 nm and 2 nm for the emission and excitation spectra, respectively, and was detected using a Hamamatsu photomultiplier tube (Shizuoka, Japan) and standard lock-in technique. Measurements were performed at room temperature.

3. Results

3.1. Emission Spectra

Figure 3 shows the photoluminescence spectra of the 90SiO₂-10SnO₂: 0.5Er³⁺ monolith acquired at 1500 nm using a Xenon lamp as an excitation source. Two different excitation schemes are presented in this figure. One is the indirect excitation, when the sample was excited at 330 nm, corresponding to the maximum of the absorption band of SnO₂. The other at 514 nm is the direct excitation of Er³⁺ to the ²H_11/2 excited state. The Stark splitting and the enhancement of the ⁴I_13/2 → ⁴I_15/2 emission of Er³⁺ ions upon 330 nm indirect excitation are clearly shown. On the contrary, the 514 nm excitation led to a broad and weaker emission band at 1500 nm typical of an Er³⁺ ion embedded in glass [14,15].

In Figure 4, the 1500 nm emission characteristics of the direct excitation was more evident under the coherent 514.5 nm laser beam excitation. The spectrum also revealed the Stark splitting, but it was less pronounced in comparison with the emission spectrum obtained upon 330 nm excitation (Figure 3).

3.2. Excitation Spectra

Figure 5 shows the excitation spectra obtained by recording the luminescence signal at 1553.5 nm and 1535.5 nm, respectively. The emission peak at 1535.5 nm is the fingerprint of the ⁴I_13/2 → ⁴I_15/2 transition in silica, i.e., in an amorphous environment, as shown in Figure 3 (black line) and Figure 4 [15]. The detection at 1553.5 nm mainly concerns the Er³⁺ ion in a crystalline environment, as shown by the red curve in Figure 3 [13,16]. From Figure 5, it is evident that for both the detection wavelengths the more intense emission from the Er³⁺ metastable state ⁴I_13/2 was achieved by indirect pumping, i.e., by excitation at 330 nm in the SnO₂ band gap. The direct excitation in the Er³⁺ electronic states at 489 nm, 520 nm, and 655 nm resulted in an extremely lower emission intensity, confirming the results presented in Figure 3.

3.3. Lifetime

Figure 6 shows the decay curves of the luminescence of the ⁴I_13/2 metastable state of Er³⁺ recorded at both wavelengths 1535.5 nm and 1553.5 nm, acquired using the 514.5 nm Ar⁺ laser beam. Considering the 1/e decay time, the two curves exhibit the same value: τ_1/e = 1.2 ms. This is not surprising because the direct excitation of the ²H_11/2 level of the Er³⁺ ions involved both the ions embedded in SnO₂ suffering different local crystalline fields and those in the silica matrix, also discussed by Joaquín Fernández et al. [17].

The decay curves do not exhibit a single exponential profile but instead they can be described as a sum of two exponentials:

$$\mathsf{\varphi }\left(\mathrm{t}\right)={\mathrm{A}}_1\exp \left[-\frac{\mathrm{t}}{{\mathsf{\tau }}_1}\right]+{\mathrm{A}}_2\exp \left[-\frac{\mathrm{t}}{{\mathsf{\tau }}_2}\right]$$

(1)

Table 2. Table of the obtained values of A₁, τ₁, A₂, τ₂, N₁, and N₂.

A1	τ1 (ms)	A2	τ2 (ms)	$\frac{{\mathbf{N}}_1}{{\mathbf{N}}_1+{\mathbf{N}}_2}=\frac{{\mathbf{A}}_1{\mathsf{\tau }}_1}{{\mathbf{A}}_1{\mathsf{\tau }}_1+{\mathbf{A}}_2{\mathsf{\tau }}_2}$
0.32	4.9	1.03	0.5	75%

summarizes the obtained values of A₁, τ₁, A₂, and τ₂. In addition, in this table, the ratio of the numbers N₁ and N₂ of the ions that decay with the lifetime τ₁ and τ₂ respectively are also listed following the approximation of the number of the total ions:

$$\mathrm{N}={\mathrm{N}}_1+{\mathrm{N}}_2={\mathrm{A}}_1{\mathsf{\tau }}_1+{\mathrm{A}}_2{\mathsf{\tau }}_2$$

(2)

4. Discussion

Under the indirect 330 nm excitation, which is associated to the SnO₂ band-gap, the ⁴I_13/2 → ⁴I_15/2 emission spectrum exhibits Stark splitting and narrow peaks (see Figure 3). This aspect revealed two important points: (i) the location of Er³⁺ in the crystalline environment, i.e., SnO₂ nanocrystals [13,16,18,19], and (ii) the energy transfer from SnO₂ to the rare earth ions. The 1500 nm broad band emission acquired by directly exciting Er³⁺ ions using the 514 nm emission of the Xenon lamp disclosed the location of Er³⁺ in a disordered environment. Although the intensity-based analysis can suffer variations from the experimental factors, e.g. light sources, detectors, and refractive indices, the difference in the integrated intensity of Er³⁺ emission band centered at 1500 nm in the case of the two excitation schemes was evident. The emission intensity was higher for the 330 nm excitation with respect to that recorded in the case of the 514 nm excitation. The more intense emission of Er³⁺ upon excitation at 330 nm proved the efficient role of SnO₂ as a luminescence sensitizer for the rare earth ions. A similar effect was demonstrated in the case of silica-tin oxides waveguides activated by Eu³⁺ ions [4] and Er³⁺ ions [10].

⁴I_13/2 → ⁴I_15/2 photoluminescence spectrum obtained upon direct excitation of the ²H_11/2 level of the Er³⁺ ions is shown in Figure 4. The large emission bandwidth, together with less pronounced Stark splittings, are observed. This shows the presence of Er³⁺ ions in an amorphous environment.

The excitation spectra in Figure 5 clearly show that the dominant contribution to both 1553.5 nm and 1535.5 nm emission was due to energy transfer from the SnO₂ nanocrystals to the imbedded Er³⁺ ions, i.e., the indirect excitation scheme. The weak bands observed at 489 nm, 520 nm, and 655 nm are due to direct excitation of Er³⁺ electronic states. These results again confirm that SnO₂ were efficient sensitizers of Er³⁺ luminescence.

To assess some parameters that will be useful for the modelling of a possible laser, the lifetimes and the corresponding fractions of the ions in the ⁴I_13/2 metastable state were determined. The decay curve of Figure 6 is similar to the ones already observed in the (100 − x)SiO₂-xTiO₂-1Er₂O₃ glass-ceramic system in Reference [20]. The results listed in Table 2 show that about 75% of the Er³⁺ ions in the ⁴I_13/2 state had an exponential decay of about 4.9 ms. Considering that the lifetime of the metastable state of Er³⁺ in SnO₂ crystals is in the order of 6 ms [21], is reasonable to assume that the majority of the Er³⁺ ions were imbedded in the SnO₂ crystals [22]. The short decay component of 0.5 ms could be assigned to the ion–ion interaction energy transfer or Er-OH centers. As recently discussed by Joaquín Fernández et al. [17], the spectral response of the Er³⁺ in SnO₂ is highly complex and more site-selective spectroscopic measurements are mandatory to accurately define the more suitable pumping schema for a solid-state laser based on the system presented here. It could be that, following the paper already mentioned, besides well-defined narrow band crystalline-like emission, corresponding to substitutional sites, broader band emission is also present, which suggests the presence of a wide variety of crystal fields at the Er³⁺ sites of SnO₂ [17]. In any case, this could be useful for the laser system increasing the number of ions available for the population inversion.

5. Conclusions

A viable sol-gel based fabrication protocol for the SiO₂-SnO₂:Er³⁺ glass-ceramic monoliths has been demonstrated. Based on different spectroscopic characterizations, the effective luminescence sensitizer role of SnO₂ for Er³⁺ has been assessed. The emission and excitation spectra showed the luminescence effectiveness of the energy transfer from SnO₂ to Er³⁺ in comparison with the direct excitation of Er³⁺ ions. About 75% of the Er³⁺ ions were imbedded in the SnO₂ nanocrystals.

Finally, a SiO₂-SnO₂:Er³⁺ glass-ceramic is surely a fantastic host for rare earth ions and it appears that a pumping schema resonant with the SnO₂ energy gap absorption band could be of some interest in developing solid-state lasers.