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Effect of Tm2O3 concentration and hydroxyl content on the emission properties of Tm doped silicate glasses
Journal of Luminescence 147 (2014) 341–345
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Effect of Tm2O3 concentration and hydroxyl content on the emission properties of Tm doped silicate glasses Xin Wang a,b,n, Kefeng Li a, Chunlei Yu a, Danping Chen a, Lili Hu a a b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China University of Chinese Academy of Sciences, Beijing 100039, China
art ic l e i nf o
a b s t r a c t
Article history: Received 13 June 2013 Received in revised form 16 September 2013 Accepted 4 November 2013 Available online 21 November 2013
Tm2O3-doped multi-component silicate glasses without alkali ions are prepared using a conventional melt quenching method. The Judd–Ofelt theory is applied to calculate the intensity parameters of the resultant glass. The intensity parameters of silicate glasses without alkali ions are larger than those of silicate glasses with alkali ions. The spontaneous radiation probability of the transition from 3F4 to 3H6 of glasses without alkali ions is 191.89 s 1, which is larger than that of glasses with alkali ions. Moreover, the measured lifetime of the 3F4 energy level of glasses is larger too. Emission cross section results show a maximum value 3.89 10 21 cm2 at 1882 nm. These findings indicate that the glass in this study can be used to realize 2 μm lasers. The effect of variations in Tm2O2 concentration on the emission properties is also studied. Cross-relaxation and emission intensity enhance with increasing Tm2O3 content. However, the lifetime of the 3F4 level decreases because of concentration quenching. The influence of OH groups on the emission properties is also investigated. & 2013 Elsevier B.V. All rights reserved.
Keywords: Silicate glass Thulium Laser
1. Introduction The emission and interaction features of rare earth ions, such as Er3 þ , Tm3 þ , Yb3 þ , and Ho3 þ have been intensively studied due to their extensive applications in infrared lasers [28–30]. Such studies have found, for example, that Tm3 þ emission from the first excitation level (3F4) lies in the 1.7 –1.9 μm region, thereby offering numerous applications in medicine, remote sensing, and atmospheric pollutant monitoring [1–3]. Other studies show that crossrelaxation (CR) allows the quantum efficiency of Tm3 þ to reach 200% when the rare earth ion is pumped by a commercial 800 nm laser diode. The high quantum efficiency of such a system compensates for the low Stokes efficiency of the infrared laser material resulting from the large energy gap between the pump and signal lasers. To develop Tm-doped materials for application in 2 μm lasers, the effects of OH groups on the system must be considered. Two processes influence the 2 μm emission of Tm3 þ as the rare earth concentration variation: favorable CR energy transfers and harmful concentration quenching. OH groups can quench 3F4-3H6 emissions and reduce emission efficiency. This study focuses on these effects on the 2 μm emission properties of Tm doped glass. Compared with other glasses, silicate glasses have many advantages, such as relatively low thermal expansion coefficient,
n
Corresponding author. E-mail address: [email protected] (X. Wang).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.11.025
excellent mechanical characteristics, low cost, and stable thermal properties. Tm-doped silicate glass for 2 μm emission has been widely investigated and fabricated into fibers to realize laser-like outputs [4,5]. However, most investigations thus far indicate that Tm-doped silicate glasses containing alkali metal ions are detrimental to 2 μm emissions compared with those containing alkaline–earth metal ions [6]. Thus, the aim of this study is to investigate silicate glass without alkali metal ions. 2. Experiment The silicate glasses studied were composed of (in mol%) SiO2 (40–60%), Al2O3 (5–10%), CaO/MgO (20–30%), and SrO/BaO (10– 20%). Tm2O3 was doped into silicate glasses at molar concentrations of 0.05, 0.25, 0.5, 0.75, 1, 1.25, and 1.5 and denoted as S1, S2, S3, S4, S5, S6, and S7, respectively. Well mixed 40 g batches were melted in a platinum crucible at 1450 1C for 1 h, cast onto a preheated steel plate, and annealed for about 4 h at 750 1C, which is near the glass transition temperature. During melting, oxygen was induced into the molten glass for 0.5 h to eliminate OH groups. The annealed glasses were fabricated and polished to dimensions of 10 mm 10 mm 1 mm to determine their optical properties. To investigate the effect of increasing OH concentration on the spectral properties of the glass, six samples doped with 0.5 mol% Tm2O3 were prepared under the following conditions: one sample was melted without gas bubbling (hereafter denoted as H1), two
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X. Wang et al. / Journal of Luminescence 147 (2014) 341–345
samples were melted with oxygen bubbling for 0.5 and 1 h (hereafter denoted as H2 and H3, respectively), two samples were melted with CCl4 bubbling for 0.5 and 1 h (hereafter denoted as H4 and H5, respectively), and one sample was melted in damp air (hereafter denoted as H6). The refractive index and density of the glasses were measured by the prism minimum deviation and Archimedes methods using distilled water as the immersion liquid. Absorption spectra were recorded using a UV/VIS/NIR spectrophotometer (Perkin Elmer Lambda 900). IR transmittance was measured using a Thermo Nicolet spectrophotometer. Emission spectra were obtained using an Edinburg FL920-type spectrophotometer. InAs detector (Hamamatsu Photonics, P7165) and InGaAs detector (Edinburg, NIR301/2) are used to measure emission spectrum and decay curve, respectively. All measurements were carried out at room temperature.
3. Results and discussion
Table 1 Judd–Ofelt parameters of Tm3 þ in various glasses. Host glass
Ω2 Ω4 Ω6 Reference (10 20 cm) (10 20 cm) (10 20 cm)
50SiO2–5AlO1.5– 24LiO0.5–12NaO0.5–9SrO 60SiO2–19CaO– 5Na2O–15K2O Silica S5
3.31
1.21
0.48
[12]
3.08
0.99
0.40
[14]
6.23 3.36
1.91 1.25
1.36 0.75
[13] This work
Table 2 Transition probability (A), radiative lifetime (τrad), and branch ratio (β) calculated using Judd–Ofelt theory. Transitions
A(s 1)
τrad (ms)
β (%)
3
3 243 92 1232 26 64 831 3 225 192
0.64
0 15 6 78 3 7 90 1 99 100
F3- H4 F3-3H5 3 F3-3F4 3 F3-3H6 3 H4-3H5 3 H4-3F4 3 H4-3H6 3 H5-3F4 3 H5-3H6 3 F4-3H6 3
3.1. Absorption spectra and Judd–Ofelt analysis The recorded absorption spectrum of S5 at room temperature is shown in Fig. 1. Six obvious peaks corresponding to transitions from the 3H6 ground state to the higher levels 1D2, 1G4, 3F2,3, 3H4, 3 H5, and 3F4 are labeled in the figure. The inset in Fig. 1 shows the energy level diagram of Tm3 þ . UV absorption arises from electronic transitions, which depend on the structure of glass and its chemical bonding [7]. To segregate the absorption bands of rare earth ions from the recorded absorption spectra, the UV absorption band and Fresnel losses are treated as a baseline, and the result is shown as “subtracted.” Judd–Ofelt theory [8,9] has been widely applied to determine important spectroscopic and laser parameters of rare earth-doped glasses. Based on the absorption bands of Tm3 þ in the silicate glass studied in this work, Judd–Ofelt parameters are calculated using Judd–Ofelt theory. The values of Ω2, Ω4, and Ω6 are 3.36 10–20, 1.25 10–20, and 0.75 10–20 cm2, respectively. Table 1 lists the Judd–Ofelt parameters of Tm3 þ in silica glass and silicate glass containing alkali ions. Clearly, all of the Judd– Ofelt parameters of Tm3 þ in the present glass samples are larger than those in silicate glass containing alkali ions and smaller than those in silica glass. This difference in Judd–Ofelt parameters stems from variations in the local environment of Tm3 þ in different glasses. Tm3 þ usually possesses six coordinated sites that act as glass modifier ions [10]. Different species of modifying elements tend to be distributed in modifier-rich regions, so
Fig. 1. Absorption spectrum of S5 glass at room temperature. Inset: Energy level of Tm3 þ .
3
1.09
4.37 5.21
modifier ions significantly influence Judd–Ofelt parameters. The Judd–Ofelt model suggests the increasing covalence and decreasing force constant of Tm–O bonds with decreasing Ω2, 4, 6 [11]. Judd–Ofelt parameters can be used to calculate the transition probability (A), radiative lifetimes (τrad), and branching ratios (β). The results of such calculations are listed in Table 2. The calculated transition probability of 3F4-3H6 in the present glass is 192 s 1, and the corresponding values for 50SiO2–5AlO1.5–24LiO0.5– 12NaO0.5–9SrO and 60SiO2–19CaO–5Na2O–15K2O glass are 161 s 1 [12] and 126 s 1 [9], respectively. As positive correlations are obtained between the Judd–Ofelt parameters and A, the calculated A in the present glass has a larger value. However, the transition probabilty (A) of the transition 3F4-3H6 in silica glass (219.36 s 1) is larger than the corresponding value in present glass [13].
3.2. Emission spectra and energy transfer process To investigate the emission properties of the prepared Tm2O3doped silicate glasses as a function of the doping content, the fluorescence spectra of samples with different doping concentration of rare earth ions pumped at 800 nm are recorded, as shown in Fig. 2. As shown in this figure, the 2 μm fluorescence intensity first increases and then decreases as the rare earth ion doping level increases. Glass with higher doping level has more effective rare earth ions to produce 2 μm emission, more opportunities to achieve CR (3H4 þ 3H6-3F4 þ 3F4), thus stronger 2 μm fluorescence intensity. However, too many rare earth ions in glass will induce harmful concentration quenching. When the silicate glass is doped with 1 mol% Tm2O3, the strongest peak intensity of the transition from 3F4 to 3H6 is obtained. As shown in Fig. 3, the intensity ratio of the transition 3F4→3H6 to the transition 3H4→3F4 increases as the Tm2O3 content increases, which indicates enhanced CR. The measured lifetime of the 3F4 energy level is displayed as a function of Tm2O3 concentration in Fig. 3. The 3F4 level clearly shows shorter lifetimes with increasing Tm2O3 doping levels.
X. Wang et al. / Journal of Luminescence 147 (2014) 341–345
Fig. 2. Emission spectra of glasses pumped at 800 nm.
343
Fig. 4. Reciprocals of 3F4 lifetime variations with changes in Tm2O3 content.
respectively proportional to the first and second powers of the rare earth ion concentration [19,20]. The total quenching rate can be calculated using the following equation: 1 1 Wq ¼ ¼
τ τw
Fig. 3. Intensity ratio and 3F4 lifetime variations with changes in Tm2O3 content.
Rate equation formalism is used to explain the variation trend of emission properties with changing rare earth ion contents. Based on the energy level diagram, the following rate equations can be written [6]: dn0 ¼ Rn0 þ n2 ðW 20 þ A20 Þ þ n1 ðW 10 þ A10 Þ W ET n0 n2 ; dt
ð1Þ
dn1 ¼ n2 ðW 21 þ A21 Þ n1 ðW 10 þ A10 Þ þ 2W ET n0 n2 ; dt
ð2Þ
dn2 ¼ Rno n2 ðW 21 þ A21 Þ−n2 ðW 20 þ A20 Þ W ET n0 n2 ; dt
ð3Þ
where R is the pumping rate and n0, n1, and n2 are the populations of the 3H6, 3F4, and 3H4 energy levels, respectively. Wij and Aij are the rates of multi-phonon relaxation and spontaneous transition, respectively. Under steady-state conditions, the following expression can be deduced based on Eq. (2): n1 W 21 þ A21 þ 2W ET n0 ¼ : n2 W 10 þ A10
ð4Þ
Because the fluorescence intensity is directly proportional to the population of its corresponding energy level, the population ratio above represents the ratio of emission intensity. Therefore, as the rare earth ion content increases, the fluorescence intensity ratio of the transition 3F4→3H6 to the transition 3H4→3F4 increases. The decrease in measured lifetime of the 3F4 level stems from concentration quenching. Two quenching centers must be considered: the impurity center (such as OH and transition metal ions) and the self-generated quenching center [15–18]. Quenching rates due to the first and second quenching centers are
9N 2 2π N 20 τw
þ CN;
ð5Þ
where τ is the measured lifetime of the 3F4 level, τw is the measured lifetime at weak concentrations, N is the Tm3 þ concentration, N0 is the critical concentration, N0 also corresponds to the concentration for which the self-generated quenching is as probable as photon emission [19] and C is a constant related to the quenching caused by impurity centers. The reciprocals of the measured lifetimes are displayed in Fig. 4 and simulated by Eq. (5). The obtained τw is 873 μs, which is slightly larger than the measured lifetime (847 μs) of the sample doped with 0.05 mol% Tm3 þ , and the N0 of the quenching process due to impurity centers is 2.36 mol%. The C obtained is 1144.85 (mol%) 2 μs 1. Given the parameters obtained from our simulations, quenching by impurity centers is considered to exert a major influence in this study because the concentration when the two quenching rates are equal is 2.37 mol%, which is much larger than the doping level considered in this study. When no saturation effects are present and no excited state absorption processes occur, the simulated emission cross-section, which is defined as the intensity gain of a laser beam per unit of population inversion, is extremely useful for determining the possibility of achieving laser effects. Based on the fluorescence spectra obtained, the emission cross section can be calculated using the Fuchtbauer–Ladenburg formula [21] sem ðλÞ ¼
λ4 Arad R λIðλÞ ; 8π cn2 λIðλÞdλ
ð6Þ
where λ is the wavelength, Arad is the transition probability calculated by Judd–Ofelt theory, I(λ) is the fluorescence intensity, n is the index of refraction, and c is the light speed. The calculation results are shown in Fig. 5. The largest emission cross-section of the transition from 3F4–3H6 at 1882 nm is 3.89 10–21 cm2. This value is larger than that in 60SiO2–19CaO–5Na2O–15K2O glass (3.62 10–20 cm2) because of the higher transition probability but smaller than that in 50SiO2–5AlO1.5–24LiO0.5–12NaO0.5–9SrO glass (6.2 10–20 cm2) because of the narrower spectral shape of the latter (159 nm). The maximum peak wavelength at 1882 nm is longer than that in glasses containing alkali ions (1861 nm in 60SiO2–19CaO–5Na2O–15K2O glass and 1835 nm in 50SiO2– 5AlO1.5–24LiO0.5–12NaO0.5–9SrO glass). Differences observed may
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X. Wang et al. / Journal of Luminescence 147 (2014) 341–345
Fig. 5. Simulated emission cross sections of the transition 3F4-3H6.
Fig. 6. Gain spectra of silicate glass doped with 0.75 mol% Tm2O3.
Table 3 Transfer constant of energy migration (EM) and cross-relaxation (CR) processes. EM (3H4 þ 3H6-3H6 þ 3H4) CR (3H4 þ 3H6-3F4 þ 3F4) M 0 ‰ Phonon assist 998 42 6 Cd–a (10 cm /s) 2040 Total Cd–a (10 42 cm6/s) 2044
1 2 4
0 209 86 411
1 762 313
2 29 12
3.3. Gain properties The gain coefficient G (λ) can be calculated based on the measured absorption cross section and deduced stimulated emission cross section if we assume Tm3 þ ions either in the ground state (3H6) or at the laser level (3F4). The wavelength dependence of the gain coefficient is a function of population inversion at the upper laser level [25] GðλÞ ¼ N½Psem ðλÞ ð1 PÞsabs ðλÞ;
stem from nephelauxetic effects because the glass in this study is more covalent than those in previous studies. The CR transfer process can be evaluated by calculating the absorption and emission cross sections. The extended integral method is widely used to analyze energy transfer processes. The energy transfer probability rate between the donor and acceptor can be estimated as [22] W d a ¼ C d a =R6 ;
ð7Þ
where R is the distance between the donor and the acceptor and Cd–a is the transfer constant, which can be calculated by [21] Z m 1 6cg dlow þ þ ð2n þ 1ÞS0 s0 ðn e þ1Þ sdems ðλm Þsaabs ðλm Þdλ; Cd a ¼ ∑ m! ð2π Þ4 n2 g dup m ¼ o ð8Þ where n ¼ 1=ðeℏω=kT 1Þ is the average occupancy of the phonon mode at temperature T, c is the light speed, n is the refractive index, g dup and g dlow are the degeneracies of the upper and lower þ levels of the donor, respectively, and λm ¼ 1=ð1=λ mℏωÞis the wavelength with m phonon creation. The calculated transfer constants of energy migration (EM) between 3H4 energy levels and CR processes are listed in Table 3. The transfer constant of EM between 3H4 energy levels is evidently larger than that of CR mainly because of resonant migration. Using the hopping model, the macroscopic CR probability WET can be expressed by the product of transfer constants [23]: 1=2 1=2 ðC CR Nd ; W ET ¼ 13ðC EM d aÞ d aÞ
ð9Þ
where Nd is the concentration of the donor, which is Tm3 þ in this case. Thus, the WET is directly proportional to the Tm3 þ content, which has been proven in a previous experiment [24]. Combining Eqs. (4) and (9), the fluorescence intensity ratio should increase proportionally with the square of the Tm2O3 concentration. However, the result in Fig. 3 does not show this variation tendency. Concentration quenching and simplification of the rate equation model may account for the variance observed.
ð10Þ
where P stands for the population at the upper laser level and N is the total rare earth ion concentration. The calculated gain coefficient of silicate glass doped with 0.75 mol% Tm2O3 with P ranging from 0 to 1 is shown in Fig. 6. Generally, the wavelength of the maximum gain coefficient of glass without alkali ions is longer than that of silicate glass with alkali ions [14], fluorophosphates glass [26], and fluoride glass [25]. With increasing population inversion, the gain peak shifts to shorter wavelengths and shows features typical of a quasi-threelevel laser system. 3.4. Effect of OH groups The infrared transmission spectra of samples obtained from different preparation process are shown in Fig. 7. A previous study on sodium silicate glasses showed that the absorption bands of OH groups in oxide glasses can be classified into three groups: (1) free OH groups at 3500 cm 1, (2) strongly bonded OH groups at 2650 cm 1, and (3) very strongly bonded OH groups at 2300 cm 1 [27]. As shown in Fig. 7, free OH groups play a major role in the IR absorption of the glass. Indeed, the preparation process greatly influences the OH group content. The absorption coefficient, αOH, for a sample of thickness L can be calculated using the following equation [31]: 1 L
αOH ¼ ln ðT=T 0 Þ
ð11Þ
where L is the thickness of the sample, T is the transmission at 3500 cm 1, and T0 is the transmission of the glass matrix. The calculated absorption coefficients of samples obtained from different preparation processes are listed in Table 4. The results show that bubbling of the molten glass with O2 or CCl4 can effectively remove OH groups. Glass bubbled with CCl4 has a lower OH content than that bubbled with O2 because CCl4 can chemically react with OH groups to generate CO2 and HCl. The presence of OH radicals greatly influences the emission properties of rare earth ions. Due to the large energy of OH
X. Wang et al. / Journal of Luminescence 147 (2014) 341–345
345
errors. The measured lifetimes of the 3F4 level are listed in Table 4. Clearly, the lifetime of the 3F4 level increases from 478.56 μs to 724.79 μs as OH groups are eliminated. These findings indicate the significant impact of OH groups on the emission properties.
4. Conclusion
Fig. 7. Transmission spectra of H series samples.
Table 4 Absorption coefficients of OH and measured 3F4 level lifetimes of samples obtained from different preparation processes. Glass
H1
H2
H3
H4
H5
H6
Process α (cm 1) 3 F4 lifetime (μs)
None 0.151 597
O2, 0.5 h 0.054 684
O2, 1 h 0.033 677
CCl4, 0.5 h 0.044 682
CCl4, 1 h 0.018 725
Wet air 0.459 479
1. The spectroscopic properties of silicate glasses without alkali ions are evaluated. Judd–Ofelt parameters, spontaneous transition probability, and 3F4 level lifetime values are larger in this type glass than in silicate glasses containing alkali ions. So silicate glasses without alkali ions provide a new choice of host glass for realizing 2 mm laser. 2. The effect of Tm2O3 concentration on the emission properties of the glass is investigated. The results show that both CR and the probability of concentration quenching increase as the Tm2O3 content increases. The glass doped with 1 mol% Tm2O3 shows maximum fluorescence intensity, so this doping level is the most suitable doping level for high gain. Our future work will focus on making fibers, using the glass investigated in present work doped with 1 mol% Tm2O3 as core, to realize laser. 3. The results show that OH groups can quench the 2 mm emission effectively. So OH groups must be removed. This work gives effective ways which are bubbling the molten glass with O2 or CCl4 for a long time. For future work, these ways can be used to prepare high quality glasses.
References
Fig. 8. Emission spectra of samples with different OH contents. Inset: Normalized emission spectra.
vibrations, rare earth ions at excited energy levels are likely to relax to lower levels when they interact with OH. The probability of this type of interaction can be determined by [20]: W OH ¼ kOH αOH N
ð12Þ
where kOH is a constant that determines the interaction force between Tm3 þ and OH radicals and N is the concentration of Tm3 þ . As the only difference in this series of samples is the OH content, the influence of OH groups on the emission properties can be evaluated. The fluorescence spectra are recorded and shown in Fig. 8, with the inset in the figure showing the fluorescence spectra normalized to the emission from 3H4 to 3F4. As the OH content increases, the fluorescence intensity ratio of 1.8 μm to 1.47 μm and the emission intensity generally decrease. The deviation of sample H3 from this variation tendency may stem from experimental
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Effect of Tm2O3 concentration and hydroxyl content on the emission properties of Tm doped silicate glasses Xin Wang a,b,n, Kefeng Li a, Chunlei Yu a, Danping Chen a, Lili Hu a a b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China University of Chinese Academy of Sciences, Beijing 100039, China
art ic l e i nf o
a b s t r a c t
Article history: Received 13 June 2013 Received in revised form 16 September 2013 Accepted 4 November 2013 Available online 21 November 2013
Tm2O3-doped multi-component silicate glasses without alkali ions are prepared using a conventional melt quenching method. The Judd–Ofelt theory is applied to calculate the intensity parameters of the resultant glass. The intensity parameters of silicate glasses without alkali ions are larger than those of silicate glasses with alkali ions. The spontaneous radiation probability of the transition from 3F4 to 3H6 of glasses without alkali ions is 191.89 s 1, which is larger than that of glasses with alkali ions. Moreover, the measured lifetime of the 3F4 energy level of glasses is larger too. Emission cross section results show a maximum value 3.89 10 21 cm2 at 1882 nm. These findings indicate that the glass in this study can be used to realize 2 μm lasers. The effect of variations in Tm2O2 concentration on the emission properties is also studied. Cross-relaxation and emission intensity enhance with increasing Tm2O3 content. However, the lifetime of the 3F4 level decreases because of concentration quenching. The influence of OH groups on the emission properties is also investigated. & 2013 Elsevier B.V. All rights reserved.
Keywords: Silicate glass Thulium Laser
1. Introduction The emission and interaction features of rare earth ions, such as Er3 þ , Tm3 þ , Yb3 þ , and Ho3 þ have been intensively studied due to their extensive applications in infrared lasers [28–30]. Such studies have found, for example, that Tm3 þ emission from the first excitation level (3F4) lies in the 1.7 –1.9 μm region, thereby offering numerous applications in medicine, remote sensing, and atmospheric pollutant monitoring [1–3]. Other studies show that crossrelaxation (CR) allows the quantum efficiency of Tm3 þ to reach 200% when the rare earth ion is pumped by a commercial 800 nm laser diode. The high quantum efficiency of such a system compensates for the low Stokes efficiency of the infrared laser material resulting from the large energy gap between the pump and signal lasers. To develop Tm-doped materials for application in 2 μm lasers, the effects of OH groups on the system must be considered. Two processes influence the 2 μm emission of Tm3 þ as the rare earth concentration variation: favorable CR energy transfers and harmful concentration quenching. OH groups can quench 3F4-3H6 emissions and reduce emission efficiency. This study focuses on these effects on the 2 μm emission properties of Tm doped glass. Compared with other glasses, silicate glasses have many advantages, such as relatively low thermal expansion coefficient,
n
Corresponding author. E-mail address: [email protected] (X. Wang).
0022-2313/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.11.025
excellent mechanical characteristics, low cost, and stable thermal properties. Tm-doped silicate glass for 2 μm emission has been widely investigated and fabricated into fibers to realize laser-like outputs [4,5]. However, most investigations thus far indicate that Tm-doped silicate glasses containing alkali metal ions are detrimental to 2 μm emissions compared with those containing alkaline–earth metal ions [6]. Thus, the aim of this study is to investigate silicate glass without alkali metal ions. 2. Experiment The silicate glasses studied were composed of (in mol%) SiO2 (40–60%), Al2O3 (5–10%), CaO/MgO (20–30%), and SrO/BaO (10– 20%). Tm2O3 was doped into silicate glasses at molar concentrations of 0.05, 0.25, 0.5, 0.75, 1, 1.25, and 1.5 and denoted as S1, S2, S3, S4, S5, S6, and S7, respectively. Well mixed 40 g batches were melted in a platinum crucible at 1450 1C for 1 h, cast onto a preheated steel plate, and annealed for about 4 h at 750 1C, which is near the glass transition temperature. During melting, oxygen was induced into the molten glass for 0.5 h to eliminate OH groups. The annealed glasses were fabricated and polished to dimensions of 10 mm 10 mm 1 mm to determine their optical properties. To investigate the effect of increasing OH concentration on the spectral properties of the glass, six samples doped with 0.5 mol% Tm2O3 were prepared under the following conditions: one sample was melted without gas bubbling (hereafter denoted as H1), two
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samples were melted with oxygen bubbling for 0.5 and 1 h (hereafter denoted as H2 and H3, respectively), two samples were melted with CCl4 bubbling for 0.5 and 1 h (hereafter denoted as H4 and H5, respectively), and one sample was melted in damp air (hereafter denoted as H6). The refractive index and density of the glasses were measured by the prism minimum deviation and Archimedes methods using distilled water as the immersion liquid. Absorption spectra were recorded using a UV/VIS/NIR spectrophotometer (Perkin Elmer Lambda 900). IR transmittance was measured using a Thermo Nicolet spectrophotometer. Emission spectra were obtained using an Edinburg FL920-type spectrophotometer. InAs detector (Hamamatsu Photonics, P7165) and InGaAs detector (Edinburg, NIR301/2) are used to measure emission spectrum and decay curve, respectively. All measurements were carried out at room temperature.
3. Results and discussion
Table 1 Judd–Ofelt parameters of Tm3 þ in various glasses. Host glass
Ω2 Ω4 Ω6 Reference (10 20 cm) (10 20 cm) (10 20 cm)
50SiO2–5AlO1.5– 24LiO0.5–12NaO0.5–9SrO 60SiO2–19CaO– 5Na2O–15K2O Silica S5
3.31
1.21
0.48
[12]
3.08
0.99
0.40
[14]
6.23 3.36
1.91 1.25
1.36 0.75
[13] This work
Table 2 Transition probability (A), radiative lifetime (τrad), and branch ratio (β) calculated using Judd–Ofelt theory. Transitions
A(s 1)
τrad (ms)
β (%)
3
3 243 92 1232 26 64 831 3 225 192
0.64
0 15 6 78 3 7 90 1 99 100
F3- H4 F3-3H5 3 F3-3F4 3 F3-3H6 3 H4-3H5 3 H4-3F4 3 H4-3H6 3 H5-3F4 3 H5-3H6 3 F4-3H6 3
3.1. Absorption spectra and Judd–Ofelt analysis The recorded absorption spectrum of S5 at room temperature is shown in Fig. 1. Six obvious peaks corresponding to transitions from the 3H6 ground state to the higher levels 1D2, 1G4, 3F2,3, 3H4, 3 H5, and 3F4 are labeled in the figure. The inset in Fig. 1 shows the energy level diagram of Tm3 þ . UV absorption arises from electronic transitions, which depend on the structure of glass and its chemical bonding [7]. To segregate the absorption bands of rare earth ions from the recorded absorption spectra, the UV absorption band and Fresnel losses are treated as a baseline, and the result is shown as “subtracted.” Judd–Ofelt theory [8,9] has been widely applied to determine important spectroscopic and laser parameters of rare earth-doped glasses. Based on the absorption bands of Tm3 þ in the silicate glass studied in this work, Judd–Ofelt parameters are calculated using Judd–Ofelt theory. The values of Ω2, Ω4, and Ω6 are 3.36 10–20, 1.25 10–20, and 0.75 10–20 cm2, respectively. Table 1 lists the Judd–Ofelt parameters of Tm3 þ in silica glass and silicate glass containing alkali ions. Clearly, all of the Judd– Ofelt parameters of Tm3 þ in the present glass samples are larger than those in silicate glass containing alkali ions and smaller than those in silica glass. This difference in Judd–Ofelt parameters stems from variations in the local environment of Tm3 þ in different glasses. Tm3 þ usually possesses six coordinated sites that act as glass modifier ions [10]. Different species of modifying elements tend to be distributed in modifier-rich regions, so
Fig. 1. Absorption spectrum of S5 glass at room temperature. Inset: Energy level of Tm3 þ .
3
1.09
4.37 5.21
modifier ions significantly influence Judd–Ofelt parameters. The Judd–Ofelt model suggests the increasing covalence and decreasing force constant of Tm–O bonds with decreasing Ω2, 4, 6 [11]. Judd–Ofelt parameters can be used to calculate the transition probability (A), radiative lifetimes (τrad), and branching ratios (β). The results of such calculations are listed in Table 2. The calculated transition probability of 3F4-3H6 in the present glass is 192 s 1, and the corresponding values for 50SiO2–5AlO1.5–24LiO0.5– 12NaO0.5–9SrO and 60SiO2–19CaO–5Na2O–15K2O glass are 161 s 1 [12] and 126 s 1 [9], respectively. As positive correlations are obtained between the Judd–Ofelt parameters and A, the calculated A in the present glass has a larger value. However, the transition probabilty (A) of the transition 3F4-3H6 in silica glass (219.36 s 1) is larger than the corresponding value in present glass [13].
3.2. Emission spectra and energy transfer process To investigate the emission properties of the prepared Tm2O3doped silicate glasses as a function of the doping content, the fluorescence spectra of samples with different doping concentration of rare earth ions pumped at 800 nm are recorded, as shown in Fig. 2. As shown in this figure, the 2 μm fluorescence intensity first increases and then decreases as the rare earth ion doping level increases. Glass with higher doping level has more effective rare earth ions to produce 2 μm emission, more opportunities to achieve CR (3H4 þ 3H6-3F4 þ 3F4), thus stronger 2 μm fluorescence intensity. However, too many rare earth ions in glass will induce harmful concentration quenching. When the silicate glass is doped with 1 mol% Tm2O3, the strongest peak intensity of the transition from 3F4 to 3H6 is obtained. As shown in Fig. 3, the intensity ratio of the transition 3F4→3H6 to the transition 3H4→3F4 increases as the Tm2O3 content increases, which indicates enhanced CR. The measured lifetime of the 3F4 energy level is displayed as a function of Tm2O3 concentration in Fig. 3. The 3F4 level clearly shows shorter lifetimes with increasing Tm2O3 doping levels.
X. Wang et al. / Journal of Luminescence 147 (2014) 341–345
Fig. 2. Emission spectra of glasses pumped at 800 nm.
343
Fig. 4. Reciprocals of 3F4 lifetime variations with changes in Tm2O3 content.
respectively proportional to the first and second powers of the rare earth ion concentration [19,20]. The total quenching rate can be calculated using the following equation: 1 1 Wq ¼ ¼
τ τw
Fig. 3. Intensity ratio and 3F4 lifetime variations with changes in Tm2O3 content.
Rate equation formalism is used to explain the variation trend of emission properties with changing rare earth ion contents. Based on the energy level diagram, the following rate equations can be written [6]: dn0 ¼ Rn0 þ n2 ðW 20 þ A20 Þ þ n1 ðW 10 þ A10 Þ W ET n0 n2 ; dt
ð1Þ
dn1 ¼ n2 ðW 21 þ A21 Þ n1 ðW 10 þ A10 Þ þ 2W ET n0 n2 ; dt
ð2Þ
dn2 ¼ Rno n2 ðW 21 þ A21 Þ−n2 ðW 20 þ A20 Þ W ET n0 n2 ; dt
ð3Þ
where R is the pumping rate and n0, n1, and n2 are the populations of the 3H6, 3F4, and 3H4 energy levels, respectively. Wij and Aij are the rates of multi-phonon relaxation and spontaneous transition, respectively. Under steady-state conditions, the following expression can be deduced based on Eq. (2): n1 W 21 þ A21 þ 2W ET n0 ¼ : n2 W 10 þ A10
ð4Þ
Because the fluorescence intensity is directly proportional to the population of its corresponding energy level, the population ratio above represents the ratio of emission intensity. Therefore, as the rare earth ion content increases, the fluorescence intensity ratio of the transition 3F4→3H6 to the transition 3H4→3F4 increases. The decrease in measured lifetime of the 3F4 level stems from concentration quenching. Two quenching centers must be considered: the impurity center (such as OH and transition metal ions) and the self-generated quenching center [15–18]. Quenching rates due to the first and second quenching centers are
9N 2 2π N 20 τw
þ CN;
ð5Þ
where τ is the measured lifetime of the 3F4 level, τw is the measured lifetime at weak concentrations, N is the Tm3 þ concentration, N0 is the critical concentration, N0 also corresponds to the concentration for which the self-generated quenching is as probable as photon emission [19] and C is a constant related to the quenching caused by impurity centers. The reciprocals of the measured lifetimes are displayed in Fig. 4 and simulated by Eq. (5). The obtained τw is 873 μs, which is slightly larger than the measured lifetime (847 μs) of the sample doped with 0.05 mol% Tm3 þ , and the N0 of the quenching process due to impurity centers is 2.36 mol%. The C obtained is 1144.85 (mol%) 2 μs 1. Given the parameters obtained from our simulations, quenching by impurity centers is considered to exert a major influence in this study because the concentration when the two quenching rates are equal is 2.37 mol%, which is much larger than the doping level considered in this study. When no saturation effects are present and no excited state absorption processes occur, the simulated emission cross-section, which is defined as the intensity gain of a laser beam per unit of population inversion, is extremely useful for determining the possibility of achieving laser effects. Based on the fluorescence spectra obtained, the emission cross section can be calculated using the Fuchtbauer–Ladenburg formula [21] sem ðλÞ ¼
λ4 Arad R λIðλÞ ; 8π cn2 λIðλÞdλ
ð6Þ
where λ is the wavelength, Arad is the transition probability calculated by Judd–Ofelt theory, I(λ) is the fluorescence intensity, n is the index of refraction, and c is the light speed. The calculation results are shown in Fig. 5. The largest emission cross-section of the transition from 3F4–3H6 at 1882 nm is 3.89 10–21 cm2. This value is larger than that in 60SiO2–19CaO–5Na2O–15K2O glass (3.62 10–20 cm2) because of the higher transition probability but smaller than that in 50SiO2–5AlO1.5–24LiO0.5–12NaO0.5–9SrO glass (6.2 10–20 cm2) because of the narrower spectral shape of the latter (159 nm). The maximum peak wavelength at 1882 nm is longer than that in glasses containing alkali ions (1861 nm in 60SiO2–19CaO–5Na2O–15K2O glass and 1835 nm in 50SiO2– 5AlO1.5–24LiO0.5–12NaO0.5–9SrO glass). Differences observed may
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Fig. 5. Simulated emission cross sections of the transition 3F4-3H6.
Fig. 6. Gain spectra of silicate glass doped with 0.75 mol% Tm2O3.
Table 3 Transfer constant of energy migration (EM) and cross-relaxation (CR) processes. EM (3H4 þ 3H6-3H6 þ 3H4) CR (3H4 þ 3H6-3F4 þ 3F4) M 0 ‰ Phonon assist 998 42 6 Cd–a (10 cm /s) 2040 Total Cd–a (10 42 cm6/s) 2044
1 2 4
0 209 86 411
1 762 313
2 29 12
3.3. Gain properties The gain coefficient G (λ) can be calculated based on the measured absorption cross section and deduced stimulated emission cross section if we assume Tm3 þ ions either in the ground state (3H6) or at the laser level (3F4). The wavelength dependence of the gain coefficient is a function of population inversion at the upper laser level [25] GðλÞ ¼ N½Psem ðλÞ ð1 PÞsabs ðλÞ;
stem from nephelauxetic effects because the glass in this study is more covalent than those in previous studies. The CR transfer process can be evaluated by calculating the absorption and emission cross sections. The extended integral method is widely used to analyze energy transfer processes. The energy transfer probability rate between the donor and acceptor can be estimated as [22] W d a ¼ C d a =R6 ;
ð7Þ
where R is the distance between the donor and the acceptor and Cd–a is the transfer constant, which can be calculated by [21] Z m 1 6cg dlow þ þ ð2n þ 1ÞS0 s0 ðn e þ1Þ sdems ðλm Þsaabs ðλm Þdλ; Cd a ¼ ∑ m! ð2π Þ4 n2 g dup m ¼ o ð8Þ where n ¼ 1=ðeℏω=kT 1Þ is the average occupancy of the phonon mode at temperature T, c is the light speed, n is the refractive index, g dup and g dlow are the degeneracies of the upper and lower þ levels of the donor, respectively, and λm ¼ 1=ð1=λ mℏωÞis the wavelength with m phonon creation. The calculated transfer constants of energy migration (EM) between 3H4 energy levels and CR processes are listed in Table 3. The transfer constant of EM between 3H4 energy levels is evidently larger than that of CR mainly because of resonant migration. Using the hopping model, the macroscopic CR probability WET can be expressed by the product of transfer constants [23]: 1=2 1=2 ðC CR Nd ; W ET ¼ 13ðC EM d aÞ d aÞ
ð9Þ
where Nd is the concentration of the donor, which is Tm3 þ in this case. Thus, the WET is directly proportional to the Tm3 þ content, which has been proven in a previous experiment [24]. Combining Eqs. (4) and (9), the fluorescence intensity ratio should increase proportionally with the square of the Tm2O3 concentration. However, the result in Fig. 3 does not show this variation tendency. Concentration quenching and simplification of the rate equation model may account for the variance observed.
ð10Þ
where P stands for the population at the upper laser level and N is the total rare earth ion concentration. The calculated gain coefficient of silicate glass doped with 0.75 mol% Tm2O3 with P ranging from 0 to 1 is shown in Fig. 6. Generally, the wavelength of the maximum gain coefficient of glass without alkali ions is longer than that of silicate glass with alkali ions [14], fluorophosphates glass [26], and fluoride glass [25]. With increasing population inversion, the gain peak shifts to shorter wavelengths and shows features typical of a quasi-threelevel laser system. 3.4. Effect of OH groups The infrared transmission spectra of samples obtained from different preparation process are shown in Fig. 7. A previous study on sodium silicate glasses showed that the absorption bands of OH groups in oxide glasses can be classified into three groups: (1) free OH groups at 3500 cm 1, (2) strongly bonded OH groups at 2650 cm 1, and (3) very strongly bonded OH groups at 2300 cm 1 [27]. As shown in Fig. 7, free OH groups play a major role in the IR absorption of the glass. Indeed, the preparation process greatly influences the OH group content. The absorption coefficient, αOH, for a sample of thickness L can be calculated using the following equation [31]: 1 L
αOH ¼ ln ðT=T 0 Þ
ð11Þ
where L is the thickness of the sample, T is the transmission at 3500 cm 1, and T0 is the transmission of the glass matrix. The calculated absorption coefficients of samples obtained from different preparation processes are listed in Table 4. The results show that bubbling of the molten glass with O2 or CCl4 can effectively remove OH groups. Glass bubbled with CCl4 has a lower OH content than that bubbled with O2 because CCl4 can chemically react with OH groups to generate CO2 and HCl. The presence of OH radicals greatly influences the emission properties of rare earth ions. Due to the large energy of OH
X. Wang et al. / Journal of Luminescence 147 (2014) 341–345
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errors. The measured lifetimes of the 3F4 level are listed in Table 4. Clearly, the lifetime of the 3F4 level increases from 478.56 μs to 724.79 μs as OH groups are eliminated. These findings indicate the significant impact of OH groups on the emission properties.
4. Conclusion
Fig. 7. Transmission spectra of H series samples.
Table 4 Absorption coefficients of OH and measured 3F4 level lifetimes of samples obtained from different preparation processes. Glass
H1
H2
H3
H4
H5
H6
Process α (cm 1) 3 F4 lifetime (μs)
None 0.151 597
O2, 0.5 h 0.054 684
O2, 1 h 0.033 677
CCl4, 0.5 h 0.044 682
CCl4, 1 h 0.018 725
Wet air 0.459 479
1. The spectroscopic properties of silicate glasses without alkali ions are evaluated. Judd–Ofelt parameters, spontaneous transition probability, and 3F4 level lifetime values are larger in this type glass than in silicate glasses containing alkali ions. So silicate glasses without alkali ions provide a new choice of host glass for realizing 2 mm laser. 2. The effect of Tm2O3 concentration on the emission properties of the glass is investigated. The results show that both CR and the probability of concentration quenching increase as the Tm2O3 content increases. The glass doped with 1 mol% Tm2O3 shows maximum fluorescence intensity, so this doping level is the most suitable doping level for high gain. Our future work will focus on making fibers, using the glass investigated in present work doped with 1 mol% Tm2O3 as core, to realize laser. 3. The results show that OH groups can quench the 2 mm emission effectively. So OH groups must be removed. This work gives effective ways which are bubbling the molten glass with O2 or CCl4 for a long time. For future work, these ways can be used to prepare high quality glasses.
References
Fig. 8. Emission spectra of samples with different OH contents. Inset: Normalized emission spectra.
vibrations, rare earth ions at excited energy levels are likely to relax to lower levels when they interact with OH. The probability of this type of interaction can be determined by [20]: W OH ¼ kOH αOH N
ð12Þ
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