applied sciences Article
SiO2-SnO2:Er3+ Glass-Ceramic Monoliths Lam Thi Ngoc Tran 1,2,3, *, Damiano Massella 1,4 , Lidia Zur 1,5 , Alessandro Chiasera 1 , Stefano Varas 1 , Cristina Armellini 1 , Giancarlo C. Righini 5,6 , Anna Lukowiak 7 , Daniele Zonta 1,2,8 and Maurizio Ferrari 1,5, * ID 1
2 3 4 5 6 7 8
*
FBK Photonics Unit, IFN-CNR CSMFO Lab, Povo, 38123 Trento, Italy;
[email protected] (D.M.);
[email protected] (L.Z.);
[email protected] (A.C.);
[email protected] (S.V.);
[email protected] (C.A.);
[email protected] (D.Z.) Department of Civil, Environmental and Mechanical Engineering, University of Trento, Mesiano, 38123 Trento, Italy Department of Applied Sciences, Ho Chi Minh City University of Technology and Education, Linh Chieu, Thu Duc, Ho Chi Minh City 720214, Vietnam Department of Physics, University of Trento, Povo, 38123 Trento, Italy Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Piazza del Viminale 1, 00184 Roma, Italy;
[email protected] MiPLab, IFAC-CNR, 50019 Sesto Fiorentino, Italy Institute of Low Temperature and Structure Research, PAS, 50422 Wroclaw, Poland;
[email protected] Department of Civil and Environmental Engineering, University of Strathclyde, Glasgow G1 1XJ, UK Correspondence:
[email protected] (L.T.N.T.);
[email protected] (M.F.); Tel.: +39-0461-314924 (L.T.N.T.); +39-0461-314918 (M.F.)
Received: 4 July 2018; Accepted: 6 August 2018; Published: 10 August 2018
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. Keywords: transparent glass-ceramics; luminescence sensitizer; SiO2 -SnO2 ; erbium; sol-gel; time-resolved spectroscopy
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 Appl. Sci. 2018, 8, 1335; doi:10.3390/app8081335
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nanocomposite materials, which offer specific characteristics of capital importance in photonics [1–3]. 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 [4–6]. Both these problems are highly detrimental for the efficiency of active devices [2,7–9]. 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 (Eg = 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 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 ·2H2 O, and Er(NO3 )3 ·5H2 O 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. Table 1. Table of the detailed composition of (100 − x)SiO2 -xSnO2 :yEr3+ monoliths. SnO2 Content x (mol%)
Er3+ Concentration n y = nSiO Er+3n+SnO (mol%)
H2 O/TEOS
EtOH/TEOS
HCl/TEOS
10
0.5
10
4
0.009
2
2
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
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3+3+ uptoto1010mol mol%.%.The Theschematic schematicsynthesis synthesisprocedure procedureofof90SiO 90SiO 2-10SnO 2:0.5Er monolithsisisshown showninin up 2-10SnO 2:0.5Er monoliths 3+ monoliths was increased up to 10 mol %. The schematic synthesis procedure of 90SiO -10SnO :0.5Er Figure 1. 2 2 Figure 1. is shown in Figure 1.
3+ 3+ 3+ Figure The flow-chart illustrating the synthesis procedure ofof90SiO 90SiO -10SnO Figure1.1.1.The Theflow-chart flow-chartillustrating illustratingthe thesynthesis synthesisprocedure procedureof 90SiO 2-10SnO 2:0.5Er :0.5Er monoliths. Figure 22-10SnO 22 :0.5Er monoliths.
Figure2 22below below shows thethe photos two examples theof crack-free andtransparent transparent 90SiO Figure shows the photos ofoftwo ofofthe crack-free and 90SiO 2-2Figure below shows photos of examples two examples the crack-free and transparent 3+ monolithic 3+ ◦ 3+ 10SnO 2 :0.5Er square and cylinder after the heat treatment at 900 °C for 40 h. 10SnO square and cylinder the heat at 900 °C at for900 40 h.C for 40 h. 90SiO22:0.5Er -10SnOmonolithic monolithic square andafter cylinder aftertreatment the heat treatment 2 :0.5Er
2 and 3+ 3+ 2 and Figure2.2. 2.Photo Photoofof ofthe the as-prepared 90SiO 2-10SnO 2:0.5Er monolithic square with size cm 3+ 2 Figure 90SiO 2-10SnO 2:0.5Er monolithic square with size ofof1 1of × ×111cm Figure Photo theas-prepared as-prepared 90SiO monolithic square with size × 1 cm 2 -10SnO2 :0.5Er thickness of ≈0.3 cm and the cylinder with diameter of 0.5 cm and length of 1.5 cm obtained after the thickness of ≈0.3 cm and the cylinder with diameter of 0.5 cm and length of 1.5 cm obtained after the and thickness of ≈0.3 cm and the cylinder with diameter of 0.5 cm and length of 1.5 cm obtained after heatheat treatment 900 °Cfor for 40h.h. ◦ C40for heat treatment atat900 the treatment at°C 900 40 h.
2.2.Charaterization CharaterizationMethods Methods 2.2. 2.2. Charaterization Methods 3+3+ions, Tocheck check the effective role tindioxide dioxide luminescence sensitizer for ions,the the To check the effective tin asasa aluminescence for ErEr To the effective rolerole of tinofof dioxide as a luminescence sensitizer sensitizer for Er3+ ions, the spectroscopic spectroscopic measurements based on different excitation sources were carried out on the 90SiO 2spectroscopic measurements basedexcitation on different excitation were out2 -10SnO on the 290SiO measurements based on different sources were sources carried out oncarried the 90SiO :0.5Er2-3+ 3+ 3+ 10SnO 2:0.5Er monolith thatwas was heat at900 900the °Cfor for40 40h. h.By Bythe theuse use Xenon lamp 450 ◦treated 10SnO 2:0.5Er that °C ofofW Xenon lamp 450 WW monolith that monolith was heat treated at heat 900 Ctreated for 40ath. By use of Xenon lamp 450 (Edison, NJ, USA) (Edison, NJ, USA) coupled to monochromator Horiba mod. microHR (Edison, NJ, USA), the 1500 nm (Edison, USA) coupled to Horiba monochromator Horiba(Edison, mod. microHR (Edison, NJ,nm USA), the 1500 nm coupledNJ, to monochromator mod. microHR NJ, USA), the 1500 emission spectra emission spectra excited different wavelengths and theexcitation excitation spectrumwere were performed. The emission spectra excited atatdifferent and the spectrum The excited at different wavelengths andwavelengths the excitation spectrum were performed. The performed. excitation range excitation range was from 300nm to 750 nm with a 1 nm scanning step and a spectral resolution of excitation range was from 300nm to 750 nm with a 1 nm scanning step and a spectral resolution of 1010 was from 300nm to 750 nm with a 1 nm scanning step and a spectral resolution of 10 nm. The results 3+ and its effective role in this indirect 3+ nm. The results proved the energy transfer from SnO 2 to Er 3+ nm. The the results proved the from energy transfer from SnO2 to Er and its effective role in this indirect proved energy transfer SnO 2 to Er and its effective role in this indirect excitation scheme 4I413/2 excitation scheme in comparison with other direct ones. Forthe thelifetime lifetime ofthe the I13/2 I15/2 4acquisition 3+ excitation scheme in comparison with other direct ones. For -4-I415/2 in comparison with other direct ones. For the lifetime acquisition of the acquisition I13/2 -4 I15/2ofEr transition, 3+ + 3+ transition, + laser Er transition, the 514.5 nm coherent laser beam from the Ar laser Coherent mod. Innova-Sabre TSM + Er the 514.5 nm coherent laser beam from the Ar Coherent mod. Innova-Sabre TSM the 514.5 nm coherent laser beam from the Ar laser Coherent mod. Innova-Sabre TSM 15 (Santa Clara, -resolved1500 1500 nmflorescence florescence 15(Santa (Santa Clara, CA, USA)was was then employed performthe thetime time nm 15 CA, USA) employed totoperform CA, USA)Clara, was then employed tothen perform the time-resolved 1500 nm-resolved florescence spectroscopy of the spectroscopy of the monolith. All of the luminescence signal was dispersed by a 320 mm singlespectroscopy All of the was luminescence was dispersed by a 320 mm singlemonolith. Allofofthe themonolith. luminescence signal dispersed signal by a 320 mm single-grating monochromator gratingmonochromator monochromatorwith witha aresolution resolutionofof0.5 0.5nm nmand and2 2nm nmfor forthe theemission emissionand andexcitation excitationspectra, spectra, grating
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with a resolution of 0.5 nm and 2 nm for the emission and excitation spectra, respectively, and was respectively, respectively,and andwas wasdetected detectedusing usinga aHamamatsu Hamamatsuphotomultiplier photomultipliertube tube(Shizuoka, (Shizuoka,Japan)and Japan)and detected using a Hamamatsu photomultiplier tube (Shizuoka, Japan) and standard lock-in technique. standard lock-in room standard lock-intechnique. technique.Measurements Measurementswere wereperformed performedatat roomtemperature. temperature. Measurements were performed at room temperature. 3.3.Results Results
3. Results
3.1. Emission Spectra 3.1. Emission Spectra
3.1. Emission Spectra
3+ 3+ Figure 2-10SnO 2: 20.5Er monolith Figure3 3shows showsthe thephotoluminescence photoluminescencespectra spectraofofthe the90SiO 90SiO 2-10SnO : 0.5Er monolithacquired acquired 3+ monolith acquired Figure 3 shows the photoluminescence spectra of the 90SiO -10SnO : 0.5Er at 1500 nm using a Xenon lamp as an excitation source. Two different excitation 2 different 2 excitationschemes at 1500 nm using a Xenon lamp as an excitation source. Two schemesare are presented ininthis figure. One indirect when the was 330 nm, at 1500 nm using a this Xenon lamp asisan excitation source. Two different excitation schemes presented presented figure. One isthe the indirectexcitation, excitation, when thesample sample wasexcited excitedatatare 330 nm, corresponding the maximum ofofthe band ofofSnO 2. 2The nm is isthe corresponding theindirect maximum theabsorption absorption band SnO .excited Theother other at514 514 nm thedirect direct to in this figure. One to istothe excitation, when the sample was atat330 nm, corresponding 3+ 3+ 2H 4I13/2 4I15/2 2H 4I13/2 4I15/23+ excitation ofof Erthe toabsorption the 11/2 excited The Stark splitting and the enhancement ofexcitation the →→ excitation of Er to the 11/2 excited state. The Stark splitting and the enhancement of the the maximum bandstate. of SnO . The other at 514 nm is the direct of Er to 2 3+ 3+ emission of Er ions upon 330 nm indirect excitation are clearly shown. On the contrary, the 514 nm 2 4 4 emission of Er ions upon 330 nm indirect excitation are clearly shown. On the contrary, the 514 nm the H11/2 excited state. The Stark splitting and the enhancement of the I13/2 → I15/2 emission of 3+ 3+ excitation led a broad weaker band atat1500 nm typical ofofananErEr ion inin excitation ledtoto broadand and weakeremission emission band 1500 nm typical ionembedded embedded Er3+ ions upon 330 nma indirect excitation are clearly shown. On the contrary, the 514 nm excitation led glass glass[14,15]. [14,15]. 3+
to a broad and weaker emission band at 1500 nm typical of an Er
ion embedded in glass [14,15].
5 2x10 2x105
λexλ= 330 nm = 330 nm ex
Intensity (a.u.) Intensity (a.u.)
10% SnO 10% SnO 2 2 5 1x10 1x105
λexλ= 514 nm = 514 nm ex
4 5x10 5x104
x10 x10
1400 1400
1450 1450
1500 1500 1550 1550 1600 1600 Wavelength (nm) Wavelength (nm)
1650 1650
1700 1700
3+ monolith heat treated at 900 ◦ C for 40 h excited at Figure 3. Emission spectra of of 90SiO 3+ monolith 2 -10SnO 22:0.5Er Figure 3. 3. Emission spectra 90SiO 2-10SnO :0.5Er heat treated atat 900 °C°C forfor 4040 h excited atat Figure Emission spectra of 90SiO 2-10SnO 2:0.5Er3+ monolith heat treated 900 h excited 330 nm and 514 nm by using a Xenon lamp as an excitation source. 330 nm and 514 nm by using a Xenon lamp asas anan excitation source. 330 nm and 514 nm by using a Xenon lamp excitation source.
InIn Figure 4,4, the 1500 nm emission characteristics ofof direct excitation was more evident under Figure the 1500 nm emission characteristics the direct excitation was more evident under In Figure 4, the 1500 nm emission characteristics ofthe 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 the coherent 514.5 nm laser beam excitation. The spectrum also revealed the Stark splitting, but it was less the coherent 514.5 nm laser beam excitation. The spectrum also revealed the Stark splitting, but it was pronounced inin comparison with the emission spectrum obtained upon 330 nm excitation (Figure 3).3). pronounced comparison with the emission spectrum obtained upon 330 nm excitation (Figure
less pronounced in comparison with the emission spectrum obtained upon 330 nm excitation (Figure 3). 7575 10% SnO 10% SnO 2 2
nm λexλ= 514.5 = 514.5 nm ex
Intensity (a.u.) Intensity (a.u.)
6565 5555 4545 3535 2525 1515 55
1425 1425
1475 1475
1525 1525 1575 1575 1625 1625 Wavelength (nm) Wavelength (nm)
1675 1675
Figure 4. Emission spectrum of 90SiO2 -10SnO2 :0.5Er3+ monolith heat-treated at 900 ◦ C for 40 h excited at 514.5 nm by using an Ar+ laser as an excitation source.
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Figure 4. Emission spectrum of 90SiO2-10SnO2:0.5Er3+ monolith heat-treated at 900 °C for 40 h excited 5 of 8 at 514.5 nm by using an Ar+ laser as an excitation source.
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3.2.Excitation ExcitationSpectra Spectra 3.2. Figure 4. Emission spectrum of 90SiO2-10SnO2:0.5Er3+ monolith heat-treated at 900 °C for 40 h excited at 514.5 nm by using an Ar+ laser as an excitation source. Figure shows theexcitation excitation spectra obtained recording luminescence signal at 1553.5 Figure 55shows the spectra obtained by by recording the the luminescence signal at 1553.5 nm 4 4 I 4I15/2 4 nm 1535.5 and 1535.5 nm, respectively. The emission peak at 1535.5 nm is fingerprint the fingerprint of the I 13/2 → → and nm, respectively. The emission peak at 1535.5 nm is the of the I 13/2 15/2 3.2. Excitation Spectra transitionininsilica, silica,i.e., i.e., amorphous environment, as shown in Figures 3 (black line) and 44 [15]. [15]. transition inin anan amorphous environment, as shown in Figure 3 (black line) and Figure 3+3+ Figure at 5 shows the excitation spectra obtained by recording the luminescence signalasatshown 1553.5 by the The detection 1553.5 nm mainly concerns the Er ion in a crystalline environment, The detection at 1553.5 nm mainly concerns the Er ion in a crystalline environment, as shown by nm andin1535.5 nm,3 respectively. The emission peak at 1535.5 nmfor is the fingerprint of the 4wavelengths I13/2 → 4I15/2 redred curve Figure [13,16]. From Figure 5, it5, isitevident that both thethe detection the the curve in Figure 3 [13,16]. From Figure is evident that for both detection wavelengths transition in silica, i.e., in an amorphous environment, 4as shown in Figures 3 (black line) and 4 [15]. 3+ metastable 3+ 4 more intense emission from the Er state I 13/2 was achieved by indirect pumping, i.e., by the more intense emission from the Er metastable state I13/2 wasenvironment, achieved by indirect pumping, The detection at 1553.5 nm mainly concerns the Er3+ ion in a crystalline as shown by the 3+ electronic 3+ excitation at 330 nm in the SnO 2 band gap. The direct excitation in the Er states at 489 nm, i.e., by at 3303 nm in the SnO The direct excitation in the Er wavelengths electronic the states at 2 band redexcitation curve in Figure [13,16]. From Figure 5, itgap. is evident that for both the detection 520nm, nm, and 655 nm resulted in an extremely lower emission intensity, confirming the results 3+ 4 489 520 nm, and 655 nm resulted in an extremely lower emission intensity, confirming the more intense emission from the Er metastable state I13/2 was achieved by indirect pumping, i.e., byresults presented in Figure 3. 3+ presented in Figure 3. in the SnO2 band gap. The direct excitation in the Er electronic states at 489 nm, excitation at 330 nm
520 nm, and 655 nm resulted in an extremely lower emission intensity, confirming the results 7000 presented in Figure 3. 6000 Emission intensity (nm)
7000
10% SnO2
5000
6000
Emission intensity (nm)
10% SnO2
4000 5000 3000 4000 3000 2000
λemission = 1553.5nm λ
2000 1000
= 1535.5nm
emission λemission = 1553.5nm
λemission = 1535.5nm
1000
0 300
0 300
400 400
500 600 Excitation wavelength (nm)
500 600 Excitation wavelength (nm)
700 700
3+ Figure 90SiO 2 -10SnO 2 :0.5Er Figure5.5.Excitation Excitationspectra spectradetected detectedatat1553.5 1553.5nm nmand and1535.5 1535.5nm nmofof 90SiO 2-10SnO 2:0.5Er3+monoliths monoliths 3+ ◦ C for Figure 5.at Excitation spectra detected at 1553.5 nm and 1535.5 nm of 90SiO2-10SnO2:0.5Er monoliths heat heattreated treated at900 900 °C for40 40h.h.
heat treated at 900 °C for 40 h.
3.3. 3.3.Lifetime Lifetime 3.3. Lifetime 4I 4I13/2 metastable Figure 66 shows shows the decay curves of luminescence state of Er3+ Figure the of the thethe luminescence Er3+ recorded 4of 13/2 metastable Figure 6 shows thedecay decaycurves curves of luminescence of of thethe I13/2the metastable state state of Er3+ofrecorded + laser recorded at wavelengths both wavelengths 1535.5 andnm, 1553.5 nm, using acquired using the Ar 514.5 Ar+ beam. laser + laser at both wavelengths 1535.5 andnm 1553.5 nm, acquired using nm Arnm at both 1535.5 nm nm 1553.5 acquired the the 514.5514.5 nm beam. beam. Considering the 1/e decay time, the two curves exhibit the same value: τ = 1.2 ms. This is Considering decaytime, time, the the two thethe same value: τ1/e =τ1/e 1.2=1/e ms. not is not Considering thethe1/e1/edecay two curves curvesexhibit exhibit same value: 1.2 This ms. isThis 2 3+ 2 3+ 2H 3+ Er not surprising because the direct excitation of the H of Er the ions involved both the surprising because the direct excitation of H 11/2 level oflevel thethe Er ions involved both the surprising because the direct excitation of the the 11/2 level of ions involved bothions the ions 11/2 embedded in SnO 2 suffering different local crystalline fields and those in the silica matrix, also ions embedded in SnO suffering different local crystalline fields and those in the silica matrix, also embedded in SnO2 suffering different local crystalline fields and those in the matrix, also 2 discussed by Joaquín Fernándezetet etal. al.[17]. [17]. discussed by Fernández discussed byJoaquín Joaquín Fernández al. [17].
Intensity (a.u.)
Intensity (a.u.)
1
1
λemission = 1535.5 nm
10% SnO2
λemission = 1535.5 λemission = 1553.5 nm nm
10% SnO2
λemission = 1553.5 nm fitting Eq.(1) fitting Eq.(1)
λexcitation = 514.5nm
0.1
λexcitation = 514.5nm
0.1
0.01 0
0.01 0
10 time (ms)
10
20
20
time4 I(ms) Figure 6. Decay curves of the luminescence from the 13/2 metastable state recorded at 1535.5 nm (black dots) and 1553.5 nm (green dots), acquired using the 514.5 nm Ar+ laser beam, of Er3+ in 90SiO2 -10SnO2 :0.5Er3+ monolith heat treated at 900 ◦ C for 40 h. The fitting curve is acquired based on Equation (1).
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The decay curves do not exhibit a single exponential profile but instead they can be described as a sum of two exponentials: t t (1) + A2 exp − φ(t) = A1 exp − τ1 τ2 Table 2 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: N = N1 + N2 = A1 τ1 + A2 τ2
(2)
Table 2. Table of the obtained values of A1 , τ1 , A2 , τ2 , N1 , and N2 . A1
τ1 (ms)
A2
τ2 (ms)
0.32
4.9
1.03
0.5
N1 N1 +N2
=
A1 τ1 A1 τ1 +A2 τ2
75%
4. Discussion Under the indirect 330 nm excitation, which is associated to the SnO2 band-gap, the 4 I13/2 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 4 2 13/2 → I15/2 photoluminescence spectrum obtained upon direct excitation of the H11/2 level 3+ of the Er 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 4 I13/2 metastable state were determined. The decay curve of Figure 6 is similar to the ones already observed in the (100 − x)SiO2 -xTiO2 -1Er2 O3 glass-ceramic system in Reference [20]. The results listed in Table 2 show that about 75% of the Er3+ ions in the 4 I13/2 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 4I 15/2
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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. Author Contributions: L.T.N.T., M.F., D.Z., and A.L. conceived and designed the experiments; L.T.N.T., D.M., A.C., S.V., and C.A. performed the experiments; D.M., L.T.N.T., and L.Z. analyzed the data; L.T.N.T., M.F., D.Z., A.L., D.M., L.Z., and G.C.R. wrote and revised the paper. Funding: The research activity is performed in the framework of COST Action MP1401 Advanced fiber laser and coherent source as tools for society, manufacturing, and lifescience (2014–2018) and Centro Fermi MiFo (2017–2020) project. L.T.N. Tran acknowledges the Vietnamese Ministry of Education and Training for her PhD scholarship. Conflicts of Interest: The authors declare no conflicts of interest.
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