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Yb3+ co-doped P2O5–CaO–Na2O–Al2O3–AgO phosphate glass
Optics Communications 284 (2011) 1868–1871
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Optical character of Er3+/Yb3+ co-doped P2O5–CaO–Na2O–Al2O3–AgO phosphate glass Chengguo Ming, Feng Song ⁎, Yin Yu, Gong Zhang, Qingru Wang, Hua Yu, Tongqing Sun, Jianguo Tian Photonics Center, College of Physical Science, Nankai University, Tianjin 300071, China The Key Laboratory of Weak Light Nonlinear Photonics , Ministry of Education, Nankai University, Tianjin 300457, China
a r t i c l e
i n f o
Article history: Received 3 July 2010 Received in revised form 1 December 2010 Accepted 3 December 2010 Available online 24 December 2010 Keywords: Er3+/Yb3+ co-doped Rate equation Luminescence
a b s t r a c t The up-conversion (UC) and near infrared (NIR) luminescence of Er3+/Yb3+ co-doped phosphate glass are investigated. In the UC emission range, the 523 nm, 546 nm green emissions and the 659 nm red emission are observed. With the increasing pump power, the intensity ratios of I523/I659, I546/I659 and I523/I546 increase gradually. The phenomenon is reasonably interpreted by theoretical analysis based on steady state rate equations. The emission cross section of the infrared emission at 1546 nm is larger (about 6.7 × 10− 21 cm2), which is suitable for making fiber amplifier. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
In the past decades, the Er3+ ions doped materials have effectively been studied for lasers and optical amplifiers. Most recently, their UC luminescence exhibits extensive applications in color display, fluorescent labeling, white light simulation, biomedical diagnostics, optical temperature sensors [1–7], and so on. Comparing with telluride, fluoride and sulfide glasses, phosphate glass has larger phonon energy. In general, UC emission of the phosphate glass is weaker. But the phosphate glass owns the advantages of the large solubility to rare earth ions, high energy transfer efficiency from Yb3+ to Er3+ and large emission cross section [8,9]. Consequently, the Er3+ ions doped phosphate glass is still a promising material. It is well known that UC emission is closely related to pump power and local environment temperature. The temperature dependence of UC emission had widely been studied [10–12]. The power dependence of UC emission intensity had been discussed in details [14]. Recently, the anomalous power dependence of UC emissions has also been investigated by Chen [7]. Until now, the report on the pump power dependence of the intensity ratio of UC emissions in the phosphate glass is less. The work will be helpful for improving the utilization of pump light and increasing the UC emission intensity. In this letter, the Er3+/Yb3+ co-doped phosphate glass was prepared. The UC emission at the different pump powers had been investigated.
The phosphate glass with a composition of 50P2O5–30CaO–12Na2O– 1Al2O3–2AgO–1Er2O3–4Yb2O3 (mol%) was prepared by high-temperature melting method. The start raw materials, consisting of reagent grade NH4H2PO4, CaCO3, NaH2PO4, Al2O3, AgNO3, Er2O3 and Yb2O3, were mixed thoroughly and melted at 1400 °C for 1 h in a corundum crucible. The melting glass was poured onto a preheated stainless-steel plate in air. Then the glass sample was heated at 400 °C for 4 h to release the thermal stress. Finally, the sample was incised and surface-polished for optical measurements. The photoluminescence spectra were measured with a model F111AI fluorescence spectrophotometer at 975 nm laser diode (LD) excitation. The visible light and near infrared luminescence were detected by photomultiplier tube detector and Ge detector, respectively.
⁎ Corresponding author. Fax: +86 22 2350 1743. E-mail address: [email protected] (F. Song). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.12.011
3. Results and discussion Fig. 1 shows the UC luminescence in the 500–700 nm regions at different pump powers. The 523 nm, 546 nm green emissions and the 659 nm red emission are due to the transition of Er3+ ions: 2H11/2 → 4I15/2, 4 S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively. With the increasing of pump power, the intensities of two green emissions and the red emission increase obviously. The intensities of two green emissions improve faster than that of the red emission. At low pump power, the intensity of red emission is larger than those of the two green emissions. When the pump power is higher, the intensity of the 546 nm green emission is larger than that of the red emission. The intensity ratios of I523/I659 and I546/I659 are shown in the inset of Fig. 2. From the inset of Fig. 1, it is very obvious that
C. Ming et al. / Optics Communications 284 (2011) 1868–1871
Fig. 1. Photoluminescence spectra in the 500–700 nm ranges of Er3+/Yb3+ co-doped phosphate glass under different powers. The inset is the power spectra of the green emissions.
the intensity of the 546 nm green light is stronger than that of the 523 nm green light under 250 mW pump power. However, when the pump power is beyond 312 mW, the intensity of the 523 nm green light is larger than that of the 546 nm green light. Fig. 2 shows the log–log plots for the dependence of the green and red emission intensities on pump power. According to the formula [13]: m
Iup = P ;
ð1Þ
where Iup is the UC emission intensity, P is the pump laser power, and m represents the number of laser photons absorbed when emitting an UC photon. The m values of the 523 nm, 546 nm green emissions, the total green emission and the 659 nm red emissions are 2.33, 1.82, 2.08 and 1.74 at the low pump power. But under the high power, they are 3.23, 2.38, 2.84 and 1.51, respectively. The results show that the green emissions vary from two-photon to three-photon processes. The energy level diagrams of Yb3+ and Er3+, as well as the proposed UC processes under the excitation of 975 nm LD are shown in Fig. 3. For the two green emissions, their population processes are
1869
Fig. 3. Energy level diagram of Er3+ and Yb3+ ions, as well as the proposed UC mechanism at 975 nm LD excitation.
similar. Firstly, by energy transfer(ET) 1: 2F5/2 (Yb3+) + 4I15/2 (Er3+) → F7/2 (Yb3+) + 4I11/2 (Er3+), and ground state absorption (GSA): 4I15/2 (Er3+) + hv → 4I11/2 (Er3+), Er3+ ions are pumped to the 4I11/2 state. Then by excited state absorption(ESA) 1: 4I11/2 (Er3+) + hv → 4F7/2 (Er3+), ET2: 2F5/2 (Yb3+)+ 4I11/2 (Er3+)→ 2F7/2 (Yb3+)+ 4F7/2 (Er3+), and cross-relaxation (CR) 1: 4I11/2 (Er3+)+ 4I11/2 (Er3+)→ 4I15/2 (Er3+)+ 4 F7/2 (Er3+), the ions transfer to the 4F7/2 state. Finally, Er3+ ions at 4F7/2 are relaxed very rapidly to 2H11/2 and 4S3/2 states by non-radiative transition, from which the two green emissions arise. The population processes of the 659 nm red emission can be described as follows: Er3+ ions at the ground state transfer to the 4I11/2 state by ET1 and GSA. Subsequently, the ions in the 4I11/2 state relax to the 4I13/2 state by non-radiative transition. Finally, by ESA2: 4I13/2 (Er3+) + hv → 4F9/2 (Er3+) and ET3: 2F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+), the ions are pumped to the 4F9/2 state. Meanwhile, Er3+ ions at the 4F7/2, 2H11/2, 4S3/2 are also relaxed to the 4F9/2 state by non-radiative transition. To verify and make a theoretical interpretation of the dependence relation between emission intensity and pump power, we utilized the following rate equations: 2
dNYb1 = dt = −AYb1 NYb1 + σYb NYb0 I = hv−k12 NYb1 NEr2
ð2Þ
−k14 NYb1 NEr4 −k13 NYb1 NEr3 −k15 NYb1 NEr5 NYb0 = NYb −NYb1
ð3Þ
dNEr3 = dt = −AEr3 NEr3 −σ 3 NEr3 I = hv−k13 NYb1 NEr3 + w43 NEr4
ð4Þ
dNEr4 = dt = −AEr4 NEr4 −σ 2 NEr2 I = hv + k12 NYb1 NEr2
ð5Þ
−ðwuc + k44 NEr4 ÞNEr4 −w43 NEr4 wuc = k14 NYb1 + k44 NEr4 + σ 4 I = hv dNEr5 = dt = −AEr5 NEr5 + σ 3 NEr3 I = hv + k13 NYb1 NEr3
ð6Þ
−k15 NYb1 NEr5 −σ 5 NEr5 I = hv dNEr6 = dt = −AEr6 NEr6 −w6 NEr6 + wuc NEr4 –k68 NEr6 NEr8 Fig. 2. Dependence of the intensity of the UC green and red emissions as function of the pump power. The inset is the intensity ratios of A:I523/I546, B:I523/I659, C:I546/I659 and D:(I523 + I546)/I659 at different pump powers.
+
wuc Nup
ð7Þ
dNEr7 = dt = −AEr7 NEr7 + 2k68 NEr6 NEr8 + wEr67 NEr6
ð8Þ
dNEr8 = dt = −AEr8 NEr8 −k68 NEr6 NEr8 + ð wEr6 −wEr67 ÞNEr6
ð9Þ
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C. Ming et al. / Optics Communications 284 (2011) 1868–1871
NEr2 = NEr −NEr3 −NEr4 −NEr5 −NEr6 −Nup
ð10Þ
NEr6 = NEr7 + NEr8
ð11Þ
where NYb and NEr are the total ytterbium and erbium concentrations, NYbi and NEri represent the population density of the corresponding levels shown in Fig. 3, kij and wij are the coefficients of energy transfer and non-radiative transition probabilities between the i and j level, wuc is the UC non-radiative transition probability of the 4I11/2 energy level, Ai corresponds to the spontaneous radiative probabilities of level i, the I, σYb and σi symbolize the laser intensity, the absorption cross section of Yb3+ and the absorption cross section of Er3+, finally Nup and wup are the population density and non-radiative probabilities of the above energy levels of 4F7/2. Under steady-state excitation, the equation can be obtained from Eq. (2): NYb1 = σ Yb I NYb0 = hvðAYb1 + k12 NEr2 + k14 NEr4 + k13 NEr3 + k15 NEr5 Þ∝I:
ð12Þ From Eq. (5), NEr4 = ðσ 2 NEr2 I = hv + k12 NYb1 NEr2 Þ = ðAEr4 + w43 + wuc + k44 NEr4 Þ; ð13Þ under low pump power, (AEr4 + w43) is the dominant depletion mechanism of level 4I11/2. It is reasonable for (AEr4 + w43) ≫ (wuc + k44NEr4). Thus, the following expression can be obtained: NEr4 = ðσ 2 NEr2 I = hv + k12 NYb1 NEr2 Þ = ðAEr4 + w43 Þ∝I:
ð14Þ
According to Eq. (4), we get NEr3 = w43 NEr4 = ðAEr3 + σ 3 I = hv + k13 NYb1 Þ:
ð15Þ
Considering the smaller value of AEr3 (40.70 s− 1 calculated by the Judd–Ofelt theory), the item of (σ3I/hv + k13NYb1) cannot be ignored. Thus, we get NE3 ∝ Im (0 b m b 1). From Eq. (6), we obtain the following equation: NEr5 = NEr3 ðσ 3 I = hv + k13 NYb1 Þ = ðAEr5 + σ 5 I = hv + k15 NYb1 Þ:
ð16Þ
At low pump power, the item of (σ5I/hv + k15NYb1) can be neglected. We obtain NEr5 ∝ NEr3I ∝ Im (1 b m b 2). And when the power is higher, (σ5I/hv + k15NYb1) cannot be neglected. Thus, the value of m will become smaller. These conclusions are in good agreement with the experimental results. The m values are1.74 and 1.51 under the low and high pump power, which can be attributed to the competition between UC processes and linear decay for the depletion of the 4I13/2 state [14]. Meanwhile, the CR of the 4F9/2 state can also cause m to become small. From Eq. (7), we can obtain the following formula: NEr6
= wup NEr4 + wup Nup = ðAEr6 + wEr6 + k68 NEr8 ÞÞ h = ðk14 NYb1 + k44 NEr4 + σ 4 I = hvÞNEr4 + wup Nup
From Eqs. (8) and (9), we obtain the following equations: NEr7 = ð2k68 NEr8 + wEr67 ÞNEr6 = AEr7 ;
ð18Þ
NEr8 = NEr6 ðwEr6 −wEr67 Þ = ðAEr8 + k68 NE6 Þ:
ð19Þ
According to the above two equations, we can know that the m value of NEr6 is larger than that of NEr8, but is less than that of NEr7. Under the low power, the experiment values of the 523 nm and 546 nm green emission are 2.33 and 1.82. But the values are 3.23 and 2.38 at the high pump power, respectively. Combining Eqs. (11), (15), (16) and (17), we get the following expression: NEr7 + NEr8 = ðAEr5 + σ 5 I = hv + k15 NYb1 ÞðAEr3 + σ 3 I = hv + k13 NYb1 Þ× NEr5 k14 NYb1 + k44 NEr4 + σ 4 I = hv + wup Nup : ð20Þ w43 ðσ 3 I = hv + k13 NYb1 ÞðAEr6 + k68 NEr8 + wEr6 Þ
Under low pump power, we can neglect the minor items: k44NEr4, wupNup, (σ5I/hv + k15NYb1) and k68NE8, and then the Eq. (20) can be expressed as NEr7 + NEr8 A ðk N + σ 4 I = hvÞðAEr3 + σ 3 I = hv + k13 NYb1 Þ ; = Er5 14 Yb1 w43 ðAEr6 + wEr6 Þðσ 3 I = hv + k13 NYb1 Þ NEr5 ð21Þ where AEr3, AEr5, AEr6, w43 and wEr6 are not related to the pump power. It is obvious that Eq. (21) is an increasing function about I. Thus, with the increasing pump power, the ratio of (NEr7 + NEr8)/NEr5 will become larger and larger. It is the reason that the intensity ratio of (I523 + I546)/I659 increases continuously. According to Eqs. (18) and (19), both NEr7 and NEr8 mainly come from the non-radiative transition of 4F7/2 energy level (NEr6). However, when the pump power is improved, the increasing rate of NEr7 is faster than that of NEr6; NEr8 is just contrary. Thus, with the increasing pump power, the intensity ratio of I523/I546 increases monotonously. Strong 1546 nm NIR fluorescence was observed under the 975 nm LD excitation (output power 373 mW). The emission cross section is calculated by two ways: (I) McCumber theory [15], σ eM ðλÞ = σ a ðλÞexp½ðελ−hcÞ = KTλ;
ð22Þ
where σa(λ) is the absorption cross section, k and h and the Boltzmann constant and the Planck constant, and ε is the free energy required to excite one Er3+ ion from the 4I15/2 state to 4 I13/2 state at temperature T. (II) Füchtbauer Ladenburg (FL) method [16], h i 5 2 σ eF = λ IðλÞ = 8πn cτ∫λI ðλÞdλ ;
ð23Þ
ð17Þ
= AEr6 + wEr6 + k68 NEr8 Þ; Because of the small energy gap between 4F7/2 and 2H11/2, the nonradiative transition from 4F7/2 to 2H11/2 is very large. AEr6 + wEr6 ≫ k68NEr8, and wucNEr4 ≫ wupNup at low pump power, consequently, we can obtain NEr6 ∝ INEr4 ∝ I2. The experimental m value of the total green emission is 2.08. But at high power, wupNup cannot be neglected. Generally Nup ∝ Im (m ≥ 3), thus for NEr6, it is possible that the m value is more than 2, which is in agreement with the experiment value 2.84.
where I(λ) is the emission intensity, n is the refractive index of the glass sample, c is the velocity of light in the vacuum, and τ is the lifetime of the 4I13/2 energy level of Er3+, which is calculated by the Judd–Ofelt theory. The results are shown in Fig. 4 and Table 1. As shown in Fig. 4, the emission sections e1 is calculated by the absorption spectrum and the emission sections e2 is obtained by the emission spectrum. It is possible that the two emission cross sections are not superposition. But the values of the two emission cross sections are almost the same, which indicates that the calculated results are correct.
C. Ming et al. / Optics Communications 284 (2011) 1868–1871
1871
Table 1 Comparison of the optical parameters of Er3 + 4I13/2 → 4I15/2 transition in different matrix materials. Host materials
σe (10− 21cm2)
τ (ms)
△λ (nm)
σe△λ
References
Phosphate glass ZBLAN Modified silicate Phosphate GC CaF2
σeM = 6.7, σeF = 6.6 5.1 7.5 7.0 5.8
19.2 9.5 11.0 8.1 7.6
50.3 65.0 53.0 54.0 70.0
336.2 331.0 398.0 378.0 406.0
a b b c c
a
Our work, bReference [17], cReference [18].
that of the red emission under high power. By rate equation, we attempted to interpret the above phenomenon. McCumber theory and FL method were used to calculate the emission cross section of 1546 nm NIR luminescence. The emission cross section of our sample is large (about 6.7 × 10− 21 cm2), which is suitable for making fiber amplifier. Fig. 4. Absorption and emission cross sections of the Er3+/Yb3+ sample at 1546 nm band. a — The absorption cross section, e1 — the emission cross section calculated by McCumber theory, and e2 — the emission cross section calculated by FL method.
The effective bandwidth △λeff is calculated by: Δλeff = ∫I ðλÞdðλÞ = I λp ;
ð24Þ
where I(λp) is the emission intensity at the peak wavelength. The correlate values for the 4I13/2 → 4I15/2 transition of Er3+ in different matrix hosts are listed in Table 1. The value of σe△λ of our sample is commensurate with the typical ZBLAN glass and the fluoride glass [17,18]. Consequently, the Er3+/Yb3+ co-doped phosphate glass is a promising host material for fiber amplifier. 4. Conclusions In summary, Er3+/Yb3+ co-doped phosphate glass was prepared by high-temperature melting method. The UC and NIR luminescence were investigated in details. By measuring the pump power spectra of the UC emission, we observe that the green emissions and the red emission are two-photon and one/two-photon processes under lower pump power. But the 523 nm green emission and the total green emission become three-photon processes at higher power. With the increasing pump power, the intensity ratios of I523/I659, I546/I659 and I523/I546 gradually increase, and the intensity of 523 nm green emission is stronger than
Acknowledgments This work was supported by the Natural Nature Science Foundation of China (No. 60778038, 90923035 and 50702026), and the Program for Innovative Research Team in University. References [1] N. Rakov, C. de Araújo, Y. Messaddeq, M.A. Aegeter, Appl. Phys. Lett. 70 (1997) 3084. [2] G.S. Maciel, A. Biswas, R. Kapoor, P.N. Prasad, Appl. Phys. Lett. 76 (2000) 1978. [3] W.S. Tsang, W.M. Yu, C.L. Mak, W.L. Tsui, K.H. Wong, H.K. Hui, J. Appl. Phys. 91 (2002) 1871. [4] S.A. Wade, S.F. Collins, G.W. Baxter, J. Appl. Phys. 94 (2003) 4743. [5] J. Fernández, R. Balda, A. Mendioroz, A.J.G. Adeva, J. Phys. Condens. Matter 13 (2001) 10347. [6] R. Balda, A.J.G. Adeva, J. Fernández, J.M.F. Navarro, J. Opt. Soc. Am. B 21 (2004) 744. [7] G.Y. Chen, Y.G. Zhang, G. Somesfalean, Z.G. Zhang, Appl. Phys. Lett. 89 (2006) 163105. [8] F. Song, G.Y. Zhang, M.R. Shang, Appl. Phys. Lett. 7 (2001) 1748. [9] X.C. Yu, F. Song, W.T. Wang, L.J. Luo, C.G. Ming, Opt. Commun. 282 (2009) 2045. [10] B. Dong, D.P. Liu, X.J. Wang, Appl. Phys. Lett. 90 (2007) 181117. [11] C.R. Li, S.F. Li, B. Dong, Sensor Actuators B 134 (2008) 313. [12] L. Han, F. Song, S.Q. Chen, Appl. Phys. Lett. 93 (2008) 011110. [13] F. Pandozzi, F. Vetrone, J.C. Boyer, R. Naccache, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem. B 109 (2005) 17400. [14] M. Pollnau, D.R. Gamelin, S.R. Lüthi, H.U. Güdel, Phys. Rev. B 61 (2000) 3337. [15] D.E. McCumber, Phys. Rev. A 136 (1964) A954. [16] B.M. Walsh, N.P. Barnes, B.J. di Bartolo, J. Appl. Phys. 83 (1998) 2772. [17] A. Jha, S. Shen, M. Naftaly, Phys. Rev. B 62 (2000) 6215. [18] X.C. Yu, F. Song, W.T. Wang, J. Appl. Phys. 104 (2008) 113105.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Optical character of Er3+/Yb3+ co-doped P2O5–CaO–Na2O–Al2O3–AgO phosphate glass Chengguo Ming, Feng Song ⁎, Yin Yu, Gong Zhang, Qingru Wang, Hua Yu, Tongqing Sun, Jianguo Tian Photonics Center, College of Physical Science, Nankai University, Tianjin 300071, China The Key Laboratory of Weak Light Nonlinear Photonics , Ministry of Education, Nankai University, Tianjin 300457, China
a r t i c l e
i n f o
Article history: Received 3 July 2010 Received in revised form 1 December 2010 Accepted 3 December 2010 Available online 24 December 2010 Keywords: Er3+/Yb3+ co-doped Rate equation Luminescence
a b s t r a c t The up-conversion (UC) and near infrared (NIR) luminescence of Er3+/Yb3+ co-doped phosphate glass are investigated. In the UC emission range, the 523 nm, 546 nm green emissions and the 659 nm red emission are observed. With the increasing pump power, the intensity ratios of I523/I659, I546/I659 and I523/I546 increase gradually. The phenomenon is reasonably interpreted by theoretical analysis based on steady state rate equations. The emission cross section of the infrared emission at 1546 nm is larger (about 6.7 × 10− 21 cm2), which is suitable for making fiber amplifier. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
In the past decades, the Er3+ ions doped materials have effectively been studied for lasers and optical amplifiers. Most recently, their UC luminescence exhibits extensive applications in color display, fluorescent labeling, white light simulation, biomedical diagnostics, optical temperature sensors [1–7], and so on. Comparing with telluride, fluoride and sulfide glasses, phosphate glass has larger phonon energy. In general, UC emission of the phosphate glass is weaker. But the phosphate glass owns the advantages of the large solubility to rare earth ions, high energy transfer efficiency from Yb3+ to Er3+ and large emission cross section [8,9]. Consequently, the Er3+ ions doped phosphate glass is still a promising material. It is well known that UC emission is closely related to pump power and local environment temperature. The temperature dependence of UC emission had widely been studied [10–12]. The power dependence of UC emission intensity had been discussed in details [14]. Recently, the anomalous power dependence of UC emissions has also been investigated by Chen [7]. Until now, the report on the pump power dependence of the intensity ratio of UC emissions in the phosphate glass is less. The work will be helpful for improving the utilization of pump light and increasing the UC emission intensity. In this letter, the Er3+/Yb3+ co-doped phosphate glass was prepared. The UC emission at the different pump powers had been investigated.
The phosphate glass with a composition of 50P2O5–30CaO–12Na2O– 1Al2O3–2AgO–1Er2O3–4Yb2O3 (mol%) was prepared by high-temperature melting method. The start raw materials, consisting of reagent grade NH4H2PO4, CaCO3, NaH2PO4, Al2O3, AgNO3, Er2O3 and Yb2O3, were mixed thoroughly and melted at 1400 °C for 1 h in a corundum crucible. The melting glass was poured onto a preheated stainless-steel plate in air. Then the glass sample was heated at 400 °C for 4 h to release the thermal stress. Finally, the sample was incised and surface-polished for optical measurements. The photoluminescence spectra were measured with a model F111AI fluorescence spectrophotometer at 975 nm laser diode (LD) excitation. The visible light and near infrared luminescence were detected by photomultiplier tube detector and Ge detector, respectively.
⁎ Corresponding author. Fax: +86 22 2350 1743. E-mail address: [email protected] (F. Song). 0030-4018/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2010.12.011
3. Results and discussion Fig. 1 shows the UC luminescence in the 500–700 nm regions at different pump powers. The 523 nm, 546 nm green emissions and the 659 nm red emission are due to the transition of Er3+ ions: 2H11/2 → 4I15/2, 4 S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively. With the increasing of pump power, the intensities of two green emissions and the red emission increase obviously. The intensities of two green emissions improve faster than that of the red emission. At low pump power, the intensity of red emission is larger than those of the two green emissions. When the pump power is higher, the intensity of the 546 nm green emission is larger than that of the red emission. The intensity ratios of I523/I659 and I546/I659 are shown in the inset of Fig. 2. From the inset of Fig. 1, it is very obvious that
C. Ming et al. / Optics Communications 284 (2011) 1868–1871
Fig. 1. Photoluminescence spectra in the 500–700 nm ranges of Er3+/Yb3+ co-doped phosphate glass under different powers. The inset is the power spectra of the green emissions.
the intensity of the 546 nm green light is stronger than that of the 523 nm green light under 250 mW pump power. However, when the pump power is beyond 312 mW, the intensity of the 523 nm green light is larger than that of the 546 nm green light. Fig. 2 shows the log–log plots for the dependence of the green and red emission intensities on pump power. According to the formula [13]: m
Iup = P ;
ð1Þ
where Iup is the UC emission intensity, P is the pump laser power, and m represents the number of laser photons absorbed when emitting an UC photon. The m values of the 523 nm, 546 nm green emissions, the total green emission and the 659 nm red emissions are 2.33, 1.82, 2.08 and 1.74 at the low pump power. But under the high power, they are 3.23, 2.38, 2.84 and 1.51, respectively. The results show that the green emissions vary from two-photon to three-photon processes. The energy level diagrams of Yb3+ and Er3+, as well as the proposed UC processes under the excitation of 975 nm LD are shown in Fig. 3. For the two green emissions, their population processes are
1869
Fig. 3. Energy level diagram of Er3+ and Yb3+ ions, as well as the proposed UC mechanism at 975 nm LD excitation.
similar. Firstly, by energy transfer(ET) 1: 2F5/2 (Yb3+) + 4I15/2 (Er3+) → F7/2 (Yb3+) + 4I11/2 (Er3+), and ground state absorption (GSA): 4I15/2 (Er3+) + hv → 4I11/2 (Er3+), Er3+ ions are pumped to the 4I11/2 state. Then by excited state absorption(ESA) 1: 4I11/2 (Er3+) + hv → 4F7/2 (Er3+), ET2: 2F5/2 (Yb3+)+ 4I11/2 (Er3+)→ 2F7/2 (Yb3+)+ 4F7/2 (Er3+), and cross-relaxation (CR) 1: 4I11/2 (Er3+)+ 4I11/2 (Er3+)→ 4I15/2 (Er3+)+ 4 F7/2 (Er3+), the ions transfer to the 4F7/2 state. Finally, Er3+ ions at 4F7/2 are relaxed very rapidly to 2H11/2 and 4S3/2 states by non-radiative transition, from which the two green emissions arise. The population processes of the 659 nm red emission can be described as follows: Er3+ ions at the ground state transfer to the 4I11/2 state by ET1 and GSA. Subsequently, the ions in the 4I11/2 state relax to the 4I13/2 state by non-radiative transition. Finally, by ESA2: 4I13/2 (Er3+) + hv → 4F9/2 (Er3+) and ET3: 2F5/2 (Yb3+) + 4I13/2 (Er3+) → 2F7/2 (Yb3+) + 4F9/2 (Er3+), the ions are pumped to the 4F9/2 state. Meanwhile, Er3+ ions at the 4F7/2, 2H11/2, 4S3/2 are also relaxed to the 4F9/2 state by non-radiative transition. To verify and make a theoretical interpretation of the dependence relation between emission intensity and pump power, we utilized the following rate equations: 2
dNYb1 = dt = −AYb1 NYb1 + σYb NYb0 I = hv−k12 NYb1 NEr2
ð2Þ
−k14 NYb1 NEr4 −k13 NYb1 NEr3 −k15 NYb1 NEr5 NYb0 = NYb −NYb1
ð3Þ
dNEr3 = dt = −AEr3 NEr3 −σ 3 NEr3 I = hv−k13 NYb1 NEr3 + w43 NEr4
ð4Þ
dNEr4 = dt = −AEr4 NEr4 −σ 2 NEr2 I = hv + k12 NYb1 NEr2
ð5Þ
−ðwuc + k44 NEr4 ÞNEr4 −w43 NEr4 wuc = k14 NYb1 + k44 NEr4 + σ 4 I = hv dNEr5 = dt = −AEr5 NEr5 + σ 3 NEr3 I = hv + k13 NYb1 NEr3
ð6Þ
−k15 NYb1 NEr5 −σ 5 NEr5 I = hv dNEr6 = dt = −AEr6 NEr6 −w6 NEr6 + wuc NEr4 –k68 NEr6 NEr8 Fig. 2. Dependence of the intensity of the UC green and red emissions as function of the pump power. The inset is the intensity ratios of A:I523/I546, B:I523/I659, C:I546/I659 and D:(I523 + I546)/I659 at different pump powers.
+
wuc Nup
ð7Þ
dNEr7 = dt = −AEr7 NEr7 + 2k68 NEr6 NEr8 + wEr67 NEr6
ð8Þ
dNEr8 = dt = −AEr8 NEr8 −k68 NEr6 NEr8 + ð wEr6 −wEr67 ÞNEr6
ð9Þ
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C. Ming et al. / Optics Communications 284 (2011) 1868–1871
NEr2 = NEr −NEr3 −NEr4 −NEr5 −NEr6 −Nup
ð10Þ
NEr6 = NEr7 + NEr8
ð11Þ
where NYb and NEr are the total ytterbium and erbium concentrations, NYbi and NEri represent the population density of the corresponding levels shown in Fig. 3, kij and wij are the coefficients of energy transfer and non-radiative transition probabilities between the i and j level, wuc is the UC non-radiative transition probability of the 4I11/2 energy level, Ai corresponds to the spontaneous radiative probabilities of level i, the I, σYb and σi symbolize the laser intensity, the absorption cross section of Yb3+ and the absorption cross section of Er3+, finally Nup and wup are the population density and non-radiative probabilities of the above energy levels of 4F7/2. Under steady-state excitation, the equation can be obtained from Eq. (2): NYb1 = σ Yb I NYb0 = hvðAYb1 + k12 NEr2 + k14 NEr4 + k13 NEr3 + k15 NEr5 Þ∝I:
ð12Þ From Eq. (5), NEr4 = ðσ 2 NEr2 I = hv + k12 NYb1 NEr2 Þ = ðAEr4 + w43 + wuc + k44 NEr4 Þ; ð13Þ under low pump power, (AEr4 + w43) is the dominant depletion mechanism of level 4I11/2. It is reasonable for (AEr4 + w43) ≫ (wuc + k44NEr4). Thus, the following expression can be obtained: NEr4 = ðσ 2 NEr2 I = hv + k12 NYb1 NEr2 Þ = ðAEr4 + w43 Þ∝I:
ð14Þ
According to Eq. (4), we get NEr3 = w43 NEr4 = ðAEr3 + σ 3 I = hv + k13 NYb1 Þ:
ð15Þ
Considering the smaller value of AEr3 (40.70 s− 1 calculated by the Judd–Ofelt theory), the item of (σ3I/hv + k13NYb1) cannot be ignored. Thus, we get NE3 ∝ Im (0 b m b 1). From Eq. (6), we obtain the following equation: NEr5 = NEr3 ðσ 3 I = hv + k13 NYb1 Þ = ðAEr5 + σ 5 I = hv + k15 NYb1 Þ:
ð16Þ
At low pump power, the item of (σ5I/hv + k15NYb1) can be neglected. We obtain NEr5 ∝ NEr3I ∝ Im (1 b m b 2). And when the power is higher, (σ5I/hv + k15NYb1) cannot be neglected. Thus, the value of m will become smaller. These conclusions are in good agreement with the experimental results. The m values are1.74 and 1.51 under the low and high pump power, which can be attributed to the competition between UC processes and linear decay for the depletion of the 4I13/2 state [14]. Meanwhile, the CR of the 4F9/2 state can also cause m to become small. From Eq. (7), we can obtain the following formula: NEr6
= wup NEr4 + wup Nup = ðAEr6 + wEr6 + k68 NEr8 ÞÞ h = ðk14 NYb1 + k44 NEr4 + σ 4 I = hvÞNEr4 + wup Nup
From Eqs. (8) and (9), we obtain the following equations: NEr7 = ð2k68 NEr8 + wEr67 ÞNEr6 = AEr7 ;
ð18Þ
NEr8 = NEr6 ðwEr6 −wEr67 Þ = ðAEr8 + k68 NE6 Þ:
ð19Þ
According to the above two equations, we can know that the m value of NEr6 is larger than that of NEr8, but is less than that of NEr7. Under the low power, the experiment values of the 523 nm and 546 nm green emission are 2.33 and 1.82. But the values are 3.23 and 2.38 at the high pump power, respectively. Combining Eqs. (11), (15), (16) and (17), we get the following expression: NEr7 + NEr8 = ðAEr5 + σ 5 I = hv + k15 NYb1 ÞðAEr3 + σ 3 I = hv + k13 NYb1 Þ× NEr5 k14 NYb1 + k44 NEr4 + σ 4 I = hv + wup Nup : ð20Þ w43 ðσ 3 I = hv + k13 NYb1 ÞðAEr6 + k68 NEr8 + wEr6 Þ
Under low pump power, we can neglect the minor items: k44NEr4, wupNup, (σ5I/hv + k15NYb1) and k68NE8, and then the Eq. (20) can be expressed as NEr7 + NEr8 A ðk N + σ 4 I = hvÞðAEr3 + σ 3 I = hv + k13 NYb1 Þ ; = Er5 14 Yb1 w43 ðAEr6 + wEr6 Þðσ 3 I = hv + k13 NYb1 Þ NEr5 ð21Þ where AEr3, AEr5, AEr6, w43 and wEr6 are not related to the pump power. It is obvious that Eq. (21) is an increasing function about I. Thus, with the increasing pump power, the ratio of (NEr7 + NEr8)/NEr5 will become larger and larger. It is the reason that the intensity ratio of (I523 + I546)/I659 increases continuously. According to Eqs. (18) and (19), both NEr7 and NEr8 mainly come from the non-radiative transition of 4F7/2 energy level (NEr6). However, when the pump power is improved, the increasing rate of NEr7 is faster than that of NEr6; NEr8 is just contrary. Thus, with the increasing pump power, the intensity ratio of I523/I546 increases monotonously. Strong 1546 nm NIR fluorescence was observed under the 975 nm LD excitation (output power 373 mW). The emission cross section is calculated by two ways: (I) McCumber theory [15], σ eM ðλÞ = σ a ðλÞexp½ðελ−hcÞ = KTλ;
ð22Þ
where σa(λ) is the absorption cross section, k and h and the Boltzmann constant and the Planck constant, and ε is the free energy required to excite one Er3+ ion from the 4I15/2 state to 4 I13/2 state at temperature T. (II) Füchtbauer Ladenburg (FL) method [16], h i 5 2 σ eF = λ IðλÞ = 8πn cτ∫λI ðλÞdλ ;
ð23Þ
ð17Þ
= AEr6 + wEr6 + k68 NEr8 Þ; Because of the small energy gap between 4F7/2 and 2H11/2, the nonradiative transition from 4F7/2 to 2H11/2 is very large. AEr6 + wEr6 ≫ k68NEr8, and wucNEr4 ≫ wupNup at low pump power, consequently, we can obtain NEr6 ∝ INEr4 ∝ I2. The experimental m value of the total green emission is 2.08. But at high power, wupNup cannot be neglected. Generally Nup ∝ Im (m ≥ 3), thus for NEr6, it is possible that the m value is more than 2, which is in agreement with the experiment value 2.84.
where I(λ) is the emission intensity, n is the refractive index of the glass sample, c is the velocity of light in the vacuum, and τ is the lifetime of the 4I13/2 energy level of Er3+, which is calculated by the Judd–Ofelt theory. The results are shown in Fig. 4 and Table 1. As shown in Fig. 4, the emission sections e1 is calculated by the absorption spectrum and the emission sections e2 is obtained by the emission spectrum. It is possible that the two emission cross sections are not superposition. But the values of the two emission cross sections are almost the same, which indicates that the calculated results are correct.
C. Ming et al. / Optics Communications 284 (2011) 1868–1871
1871
Table 1 Comparison of the optical parameters of Er3 + 4I13/2 → 4I15/2 transition in different matrix materials. Host materials
σe (10− 21cm2)
τ (ms)
△λ (nm)
σe△λ
References
Phosphate glass ZBLAN Modified silicate Phosphate GC CaF2
σeM = 6.7, σeF = 6.6 5.1 7.5 7.0 5.8
19.2 9.5 11.0 8.1 7.6
50.3 65.0 53.0 54.0 70.0
336.2 331.0 398.0 378.0 406.0
a b b c c
a
Our work, bReference [17], cReference [18].
that of the red emission under high power. By rate equation, we attempted to interpret the above phenomenon. McCumber theory and FL method were used to calculate the emission cross section of 1546 nm NIR luminescence. The emission cross section of our sample is large (about 6.7 × 10− 21 cm2), which is suitable for making fiber amplifier. Fig. 4. Absorption and emission cross sections of the Er3+/Yb3+ sample at 1546 nm band. a — The absorption cross section, e1 — the emission cross section calculated by McCumber theory, and e2 — the emission cross section calculated by FL method.
The effective bandwidth △λeff is calculated by: Δλeff = ∫I ðλÞdðλÞ = I λp ;
ð24Þ
where I(λp) is the emission intensity at the peak wavelength. The correlate values for the 4I13/2 → 4I15/2 transition of Er3+ in different matrix hosts are listed in Table 1. The value of σe△λ of our sample is commensurate with the typical ZBLAN glass and the fluoride glass [17,18]. Consequently, the Er3+/Yb3+ co-doped phosphate glass is a promising host material for fiber amplifier. 4. Conclusions In summary, Er3+/Yb3+ co-doped phosphate glass was prepared by high-temperature melting method. The UC and NIR luminescence were investigated in details. By measuring the pump power spectra of the UC emission, we observe that the green emissions and the red emission are two-photon and one/two-photon processes under lower pump power. But the 523 nm green emission and the total green emission become three-photon processes at higher power. With the increasing pump power, the intensity ratios of I523/I659, I546/I659 and I523/I546 gradually increase, and the intensity of 523 nm green emission is stronger than
Acknowledgments This work was supported by the Natural Nature Science Foundation of China (No. 60778038, 90923035 and 50702026), and the Program for Innovative Research Team in University. References [1] N. Rakov, C. de Araújo, Y. Messaddeq, M.A. Aegeter, Appl. Phys. Lett. 70 (1997) 3084. [2] G.S. Maciel, A. Biswas, R. Kapoor, P.N. Prasad, Appl. Phys. Lett. 76 (2000) 1978. [3] W.S. Tsang, W.M. Yu, C.L. Mak, W.L. Tsui, K.H. Wong, H.K. Hui, J. Appl. Phys. 91 (2002) 1871. [4] S.A. Wade, S.F. Collins, G.W. Baxter, J. Appl. Phys. 94 (2003) 4743. [5] J. Fernández, R. Balda, A. Mendioroz, A.J.G. Adeva, J. Phys. Condens. Matter 13 (2001) 10347. [6] R. Balda, A.J.G. Adeva, J. Fernández, J.M.F. Navarro, J. Opt. Soc. Am. B 21 (2004) 744. [7] G.Y. Chen, Y.G. Zhang, G. Somesfalean, Z.G. Zhang, Appl. Phys. Lett. 89 (2006) 163105. [8] F. Song, G.Y. Zhang, M.R. Shang, Appl. Phys. Lett. 7 (2001) 1748. [9] X.C. Yu, F. Song, W.T. Wang, L.J. Luo, C.G. Ming, Opt. Commun. 282 (2009) 2045. [10] B. Dong, D.P. Liu, X.J. Wang, Appl. Phys. Lett. 90 (2007) 181117. [11] C.R. Li, S.F. Li, B. Dong, Sensor Actuators B 134 (2008) 313. [12] L. Han, F. Song, S.Q. Chen, Appl. Phys. Lett. 93 (2008) 011110. [13] F. Pandozzi, F. Vetrone, J.C. Boyer, R. Naccache, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem. B 109 (2005) 17400. [14] M. Pollnau, D.R. Gamelin, S.R. Lüthi, H.U. Güdel, Phys. Rev. B 61 (2000) 3337. [15] D.E. McCumber, Phys. Rev. A 136 (1964) A954. [16] B.M. Walsh, N.P. Barnes, B.J. di Bartolo, J. Appl. Phys. 83 (1998) 2772. [17] A. Jha, S. Shen, M. Naftaly, Phys. Rev. B 62 (2000) 6215. [18] X.C. Yu, F. Song, W.T. Wang, J. Appl. Phys. 104 (2008) 113105.