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Yb3+ co-doped phosphate glass ceramics
Current Applied Physics 13 (2013) 351e354
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Color variety of up-conversion emission of Er3þ/Yb3þ co-doped phosphate glass ceramics Chengguo Ming a, *, Feng Song b, Xiaobin Ren a a b
Physics Department, School of Sciences, Tianjin University of Science & Technology, Weijin Road 94, Tianjin 300222, 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
a b s t r a c t
Article history: Received 27 July 2012 Received in revised form 19 August 2012 Accepted 20 August 2012 Available online 29 August 2012
The sample of Er3þ/Yb3þ co-doped phosphate glass ceramic was prepared. At 975 nm laser diode (LD) excitation, the strong up-conversion (UC) emissions were observed, which were the UC green emission at 510e570 nm and the UC red emission at 636e692 nm, respectively. At low pump power (126 mW), the red emission is primary, and the color purity Rcp is 0.81. With the increasing of pump power, the emission color gradually varies from red to green. The intensity of the green emission is stronger compared to that of the red emission at high power (868 mW), and the color purity Rcp is 0.76. Thus, this material can be applied to fluorescence anti-counterfeiting by the color variety of UC emission under different pump power. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Er3þ/Yb3þ co-doped Phosphate glass ceramic Up-conversion emission
1. Introduction Today, counterfeit products have pervaded various trades, such as agriculture, industry and banking business, and so on. Thus, the anti-counterfeiting industry is faced with a rigorous challenge. Many methods were applied to the anti-counterfeiting technologies, such as nuclear track anti-counterfeiting, micro-relief structures anti-counterfeiting and fluorescence anti-counterfeiting [1e 3], and so on. In many of these anti-counterfeiting technologies, the fluorescence plays a very important role. Specially, the ultraviolet (UV) and infrared inks (IR) labeling, which are invisible when illuminated using the visible light source, but is visible when illuminated with light in the UV and IR spectrum, respectively. For counterfeiters, they do not know the intrinsic mechanism. Therefore it is very difficult that counterfeiters forge the labeling. However, with the increased using of UV and IR inks, counterfeiters have been knowledgeable about their principle, and can reproduce the same or similar inks. And sometimes, the anti-counterfeiting ways should be open to all the people, such as the anticounterfeiting money. Thus, for the further development of fluorescent anti-counterfeit labeling, first, it is necessary that the anticounterfeiting materials have a special character, which can hardly be counterfeited even if counterfeiters know the specific phenomenon; secondly, the materials should not add the cost of * Corresponding author. Fax: þ86 22 2350 1743. E-mail address: [email protected] (C. Ming). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2012.08.011
the genuine products; third, the measure method should be simple, feasible and clear; finally, the materials must have good thermal and chemical stabilities. So far, many materials have been applied to anti-counterfeiting technologies, such as crystals, glasses and organic dyes, and so on. But they usually have their own shortcomings: poor thermal and chemical stability, low luminous efficiency and photo-bleaching, etc. Fortunately, the glass ceramic improves all these deficiencies, and simultaneously contains the merits of glass and crystal. The rare earth doped materials have abundant emission spectra in the visible light range under the UV or IR pump. Specially, the UC luminescent materials have attracted more and more attention [4e 9], in view of the lower cost of IR pump sources and the excellent properties of UC luminescence. It is well known that the luminescent colors of the optical materials are related to the excitation condition and local environment temperature. For example, the emission color of the ZnO nanoparticles varies with the excitation wavelength and the luminescent characters have been discussed in details [10]. The temperature dependence of the emission had been studied widely [11e14], and the power dependence of the emission intensity also had been studied [15]. Recently, the anomalous power dependence of UC emissions has been investigated by Chen et al. [8]. However, the UC luminescence intensity ratio changes with the pump power, little has been reported. In this letter, we had prepared the Er3þ/ Yb3þ co-doped phosphate glass ceramic, and investigated the UC emission intensity ratio for the pump power dependence in details.
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2. Experimental The glass with a composition of P2O5eCaOeNa2OeAl2O3eAgOe Er2O3eYb2O3 was prepared by melting quenching 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. The glass sample was heated at 400 C for 4 h to release the thermal stress, and 530 C for 5 h for crystallization. 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. The microstructure of sample was measured with an atomic force microscope (AFM). The X-ray diffraction (XRD) were obtained by a Bruker AXSB8 Discover model using CuKa radiation (l ¼ 0.154 nm). Before measuring, the sample was ground into fine powder, and the scan rate of 0.05 min1 was used to record a pattern in the 2q range of 10e60 . All measurements were taken at room temperature. 3. Results and discussion 3.1. Power spectra Atomic force microscope image of the glass ceramic is shown in Fig. 1. The scan range is 2.386 2.386 mm2. From Fig. 1, it is obvious that the average diameter of grains is about 50 nm, which shows the symmetrical surface morphology. Fig. 2 is the XRD pattern of sample, in which there are a few obvious diffraction peaks, they are attributed to the diffractions of AlPO4 and Ca3(PO4)2, and labeled in the Fig. 2. The UC fluorescence usually has several different emission bands under the excitation of IR source. When the different color lights mix each other, the material can display different color, and the color has a connection with the luminescent intensity ratio. Thus, for biological labeling, we hope that the material only emits single-color UC emission, avoiding the spectral cross-talk of the
Fig. 2. X-ray diffraction pattern of the glass ceramic sample.
main band and other bands [8]. But for fluorescence anticounterfeiting, the single-color material has been very common, and it can be easily forged by counterfeiters. Therefore, it is very useful that the material can emit multicolor emissions under the excitation of different pump sources. The multi-layer of protections plays an important role in the anti-counterfeiting industry. However, the complex technologies and detection methods bring lots of difficulties to counterfeiters, and they also bring the difficulties to anti-counterfeiting industries own. Thus, to develop fluorescence anti-counterfeiting labeling, it is important that the emission color can change with the external environment. Our sample emits just the UC red and green emissions in the visible band at 975 nm excitation. Fig. 3 shows the UC emission spectra of the sample in the 500e700 nm wavelength range at different excitation powers. The green emission at 510e570 nm should come from the transition of Er3þ ion: 2H11/2/4S3/2 / 4I15/2 , and the red emission at 636e692 nm is attributed to the transition of Er3þ ion: 4 F9/2 / 4I15/2. It is very obvious that the intensity of the UC red emission is stronger than that of the green emission at low power;
Fig. 1. Atomic force microscope image of the glass ceramic sample.
C. Ming et al. / Current Applied Physics 13 (2013) 351e354
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it is possible that the green color will increase. To sum up, with the increasing of pump power, the emission color changes from red to green, which proves the fluorescence color of our sample is tunable by changing the pump power, and there is a wider tunable range. The Fig. 4 shows the logelog plots for the dependence of green and red emission intensities on pump power. According to the formula [16]:
Iup fP m ; 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 green emission and the 659 nm red emissions are 2.30 and 1.55 at the low pump power; but under the high power, they is 3.45 and 1.34, respectively, which means that the green emissions vary from two-photon processes to three-photon processes, and one/two-photon processes are involved to the 4F9/2 state. 3.2. Mechanism of UC emissions Fig. 3. Photoluminescence spectra in the 500e700 nm wavelength range of Er3þ/Yb3þ co-doped phosphate glass under different power. The inset is the intensity ratio Igreen/ Ired as a function of pump power.
but at high power, the intensity of the red emission is weaker compared to that of the green emission. With the increasing of pump power, the intensity of the green emission becomes strong; the intensity of the red emission increases first and then decreases; the intensity ratio of the green emission to the red emission becomes larger and larger, which is shown in the inset of Fig. 3. Thus, the emission color of the sample is red and green to the naked eye at lower and higher pump power, respectively. The emission color of the sample can change by adjusting the excitation power. Ideal fluorescence labeling should have a strong, stable and narrow band emission. For our sample, the full bandwidths of the red and green emissions are about 56 and 60 nm, respectively. Such narrow bandwidths are comparable to the Quantum dots (QD) materials. So far, QD materials are a very ideal fluorescence labeling, but there are several main shortcomings: high auto-fluorescence, high production costs, complex production process and strong scattering of excitation light. Similarly, traditional organic dyes are easy to photo-bleaching, and the wide band emission results in the low signal to noise ratio. However, Er3þ/Yb3þ co-doped phosphate glass ceramics overcomes these shortcomings, and emits the narrow band, strong and stable UC red and green emissions, thus, it will be a promising fluorescence labeling. And the color of emission depends on the pump power, this character will have an important application on the fluorescence anti-counterfeiting technologies. Applying the color changing under different pump power to the fluorescence anti-counterfeiting labeling, it is important that the color variety should be obvious and distinguishable, that is, the ideal labeling can emit two different color emission at low and high pump power, and have a high color purity. The color purity RCP is defined as the ratio of intensity of the main light (the stronger emission) Imain and the intensity of the total visible light Itotal: RCP ¼ Imain/Itotal, the larger of RCP, the better of monochrome. From Fig. 2, under 126 mW pump power, the main emission band is red emission, the value of RCP is 0.81, which shows the main color of light is red; At 868 mW pump power, the main emission band is green emission, and the value of RCP is 0.76, which shows the green color of is primary. From the inset of Fig. 2, we know that the ratio of Igreen/Ired increases as the pump power. Thus at higher pump power,
The energy level diagram of Yb3þ and Er3þ is shown in Fig. 5, as well as the proposed UC processes under the excitation of 975 nm LD. The population processes of the green emission can be described as follows. By the energy transfer (ET) 1: 2F5/2 (Yb3þ) þ 4I15/2 (Er3þ) / 2F7/2 (Yb3þ) þ 4I11/2 (Er3þ), and the ground state absorption (GSA): 4I15/2 (Er3þ) þ hv / 4I11/2 (Er3þ), Er3þ ions in the ground state are pumped to the long-lived 4I11/2 state. By the excited state absorption (ESA1): 4I11/2 (Er3þ) þ hv / 4F7/2 (Er3þ), the energy transfer (ET) 2: 2F5/2 (Yb3þ) þ 4I11/2 (Er3þ) / 2F7/2 (Yb3þ) þ 4F7/2 (Er3þ), and the cross-relaxation (CR) 1: 4I11/2 (Er3þ) þ 4I11/2 (Er3þ) / 4I15/2 (Er3þ) þ 4F7/2 (Er3þ), the Er3þ ions in 4 I11/2 state can be pumped to the short-lived 4F7/2 state. Er3þ ions in 4 F7/2 are relaxed to 2H11/2 and 4S3/2 states by non-radiative transition, from where the green emission arises. The red luminescence at 659 nm comes from the transition of Er3þ ion: 4F9/2 / 4I15/2, whose UC processes can be described as follows. The Er3þ ions in the ground state transfer to 4I11/2 by ET1 and GSA. Subsequently, the Er3þ ions relax non-radiatively to the
Fig. 4. Dependence of intensity of the UC green and red emissions as function of the pump power. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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between UC processes and linear decay for the depletion of the 4I13/ 2 state [15]. With the increasing of pump power, the m value of green emission varies from 2.30 to 3.45. The phenomenon can be interpreted as follows. At higher power, the ions in the 4F9/2 state can be excited to higher energy levels, where the m value is usual larger than 3. And then the ions relax to 2H11/2 and 4S3/2 by nonradiative transition, resulting in the increasing of m value. 4. Conclusions In summary, the fluorescence color of Er3þ/Yb3þ co-doped phosphate glass ceramic sample can be changed by adjusting the pump power of 975 nm LD. At low power, the red emission is primary, and the color purity RCP is 0.81 under 126 mW power. With the increasing of power, the emission color of the sample can gradually vary from red to green. Finally, the emission color becomes green at high power, and the color purity RCP is 0.76 at 868 mW power. In view of the low cost of raw materials, simple production process and detection method, especially the unique emission principle, the material can be applied effectively to fluorescence anti-counterfeiting technologies. Acknowledgments Fig. 5. Energy level diagram of Er anism at 975 nm LD excitation.
3þ
3þ
and Yb
ions as well as the proposed UC mech-
long-lived 4I13/2 state. The Er3þ ions in 4I13/2 state are pumped to 4F9/ 4 3þ 4 3þ 2 2 state by ESA2: I13/2 (Er ) þ hv / F9/2 (Er ) and ET3: F5/2 (Yb3þ) þ 4I13/2 (Er3þ) / 2F7/2 (Yb3þ) þ 4F9/2 (Er3þ). Meanwhile, Er3þ ions in 4F7/2, 2H11/2, and 4S3/2 can also relax to 4F9/2 by non-radiative transition. According to the population processes of the UC green and red emissions, the emission intensity ratio of the green emission to red emission is close related to the 4I11/2 energy level. The reason is that some ions of 4I11/2 state can transfer to 4F7/2 state by ET2, ESA1 and CR1 and others will relax to 4I13/2 by non-radiative transition. The efficiency of ET2, ESA1 and CR1 depends on the population of 4I11/2 state. The more population, the higher the efficiency is. However, the non-radiative transition of 4I11/2 is not related to the population. At low pump power, the population of 4I11/2 state is very small, which results in the low efficiency of ET2, ESA1 and CR1. Most of the Er3þ ions in 4I11/2 state relax to 4I13/2 by non-radiative transition. Thus, the intensity of the UC red emission is larger compared to that of the green emission. But at high pump power, the opposite is true. Thus, the intensity of the green emission is stronger than that of the red emission. Especially, the ions in the 4F9/2 state can be excited to the higher excited state by the energy transition, like as ET4 and ESA3, which can result in the intensity of the red emission becomes weak and the intensity of the green emission rapidly increases. This is in good agreement with the experimental measurement, seen in Fig. 3. The m value of red emission is 1.55 and 1.34 at the low and high pump power, which can be attributed to the competition
This work was supported by the Natural Nature Science Foundation of China (No. 90923035 and No. 11104200), the Scientific Research Project of Tianjin Educational Committee (No. 20110906), and the Program for Changjiang Scholars and Innovative Research Team in University. References [1] Y.L. Wang, S.P. Xu, J.H. Lin, Radiat. Meas. 43 (2008) S659eS661. [2] P.W. Leech, H. Zeidler, Microelectron Eng. 65 (2003) 439e446. [3] Y.X. Zhang, K. Aslan, M.J.R. Previte, C.D. Geddes, Dyes Pigm. 77 (2008) 545e549. [4] W.J. Kong, J.N. Shan, Y.G. Ju, Mater. Lett. 64 (2010) 688e691. [5] M.S. Liao, L. Wen, H.Y. Zhao, Y.Z. Fang, H.T. Sun, L.L. Hu, et al., Mater. Lett. 61 (2007) 470e472. [6] F.G. Yang, G.T. Chen, Z.Y. You, Mater. Lett. 64 (2010) 824e826. [7] S.A. Wade, S.F. Collins, G.W. Baxter, J. Appl. Phys. 94 (2003) 4743e4756. [8] G.Y. Chen, Y.G. Zhang, G. Somesfalean, Z.G. Zhang, Q. Sun, F.P. Wang, Appl. Phys. Lett. 89 (2006) 163105. [9] F. Song, G.Y. Zhang, M.R. Shang, H. Tan, J. Yang, F.Z. Meng, Appl. Phys. Lett. 79 (2001) 1748e1750. [10] H.B. Zeng, G.T. Duan, Y. Li, S.K. Yang, X.X. Xu, W.P. Cai, Adv. Funct. Mater. 20 (2010) 561e572. [11] H.B. Zeng, Z.G. Li, W.P. Cai, B.Q. Cao, P.S. Liu, S.K. Yang, J. Phys. Chem. B 111 (2007) 14311e14317. [12] B. Dong, D.P. Liu, X.J. Wang, T. Yang, S.M. Miao, C.R. Li, et al., Appl. Phys. Lett. 90 (2006) 181117. [13] C.R. Li, S.F. Li, B. Dong, Z.F. Liu, C.L. Song, Q.X. Yu, Sensors and Actuators B 134 (2008) 313e316. [14] L. Han, F. Song, S.Q. Chen, C.G. Zou, X.C. Yu, J.G. Tian, et al., Appl. Phys. Lett. 93 (2008) 011110. [15] M. Pollnau, D.R. Gamelin, S.R. Lüthi, H.U. Güdel, Phys. Rev. B 61 (2000) 3337e3346. [16] F. Pandozzi, F. Vetrone, J. Boyer, R. Naccache, J. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem. B 109 (2005) 17400.
Contents lists available at SciVerse ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Color variety of up-conversion emission of Er3þ/Yb3þ co-doped phosphate glass ceramics Chengguo Ming a, *, Feng Song b, Xiaobin Ren a a b
Physics Department, School of Sciences, Tianjin University of Science & Technology, Weijin Road 94, Tianjin 300222, 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
a b s t r a c t
Article history: Received 27 July 2012 Received in revised form 19 August 2012 Accepted 20 August 2012 Available online 29 August 2012
The sample of Er3þ/Yb3þ co-doped phosphate glass ceramic was prepared. At 975 nm laser diode (LD) excitation, the strong up-conversion (UC) emissions were observed, which were the UC green emission at 510e570 nm and the UC red emission at 636e692 nm, respectively. At low pump power (126 mW), the red emission is primary, and the color purity Rcp is 0.81. With the increasing of pump power, the emission color gradually varies from red to green. The intensity of the green emission is stronger compared to that of the red emission at high power (868 mW), and the color purity Rcp is 0.76. Thus, this material can be applied to fluorescence anti-counterfeiting by the color variety of UC emission under different pump power. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Er3þ/Yb3þ co-doped Phosphate glass ceramic Up-conversion emission
1. Introduction Today, counterfeit products have pervaded various trades, such as agriculture, industry and banking business, and so on. Thus, the anti-counterfeiting industry is faced with a rigorous challenge. Many methods were applied to the anti-counterfeiting technologies, such as nuclear track anti-counterfeiting, micro-relief structures anti-counterfeiting and fluorescence anti-counterfeiting [1e 3], and so on. In many of these anti-counterfeiting technologies, the fluorescence plays a very important role. Specially, the ultraviolet (UV) and infrared inks (IR) labeling, which are invisible when illuminated using the visible light source, but is visible when illuminated with light in the UV and IR spectrum, respectively. For counterfeiters, they do not know the intrinsic mechanism. Therefore it is very difficult that counterfeiters forge the labeling. However, with the increased using of UV and IR inks, counterfeiters have been knowledgeable about their principle, and can reproduce the same or similar inks. And sometimes, the anti-counterfeiting ways should be open to all the people, such as the anticounterfeiting money. Thus, for the further development of fluorescent anti-counterfeit labeling, first, it is necessary that the anticounterfeiting materials have a special character, which can hardly be counterfeited even if counterfeiters know the specific phenomenon; secondly, the materials should not add the cost of * Corresponding author. Fax: þ86 22 2350 1743. E-mail address: [email protected] (C. Ming). 1567-1739/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cap.2012.08.011
the genuine products; third, the measure method should be simple, feasible and clear; finally, the materials must have good thermal and chemical stabilities. So far, many materials have been applied to anti-counterfeiting technologies, such as crystals, glasses and organic dyes, and so on. But they usually have their own shortcomings: poor thermal and chemical stability, low luminous efficiency and photo-bleaching, etc. Fortunately, the glass ceramic improves all these deficiencies, and simultaneously contains the merits of glass and crystal. The rare earth doped materials have abundant emission spectra in the visible light range under the UV or IR pump. Specially, the UC luminescent materials have attracted more and more attention [4e 9], in view of the lower cost of IR pump sources and the excellent properties of UC luminescence. It is well known that the luminescent colors of the optical materials are related to the excitation condition and local environment temperature. For example, the emission color of the ZnO nanoparticles varies with the excitation wavelength and the luminescent characters have been discussed in details [10]. The temperature dependence of the emission had been studied widely [11e14], and the power dependence of the emission intensity also had been studied [15]. Recently, the anomalous power dependence of UC emissions has been investigated by Chen et al. [8]. However, the UC luminescence intensity ratio changes with the pump power, little has been reported. In this letter, we had prepared the Er3þ/ Yb3þ co-doped phosphate glass ceramic, and investigated the UC emission intensity ratio for the pump power dependence in details.
352
C. Ming et al. / Current Applied Physics 13 (2013) 351e354
2. Experimental The glass with a composition of P2O5eCaOeNa2OeAl2O3eAgOe Er2O3eYb2O3 was prepared by melting quenching 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. The glass sample was heated at 400 C for 4 h to release the thermal stress, and 530 C for 5 h for crystallization. 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. The microstructure of sample was measured with an atomic force microscope (AFM). The X-ray diffraction (XRD) were obtained by a Bruker AXSB8 Discover model using CuKa radiation (l ¼ 0.154 nm). Before measuring, the sample was ground into fine powder, and the scan rate of 0.05 min1 was used to record a pattern in the 2q range of 10e60 . All measurements were taken at room temperature. 3. Results and discussion 3.1. Power spectra Atomic force microscope image of the glass ceramic is shown in Fig. 1. The scan range is 2.386 2.386 mm2. From Fig. 1, it is obvious that the average diameter of grains is about 50 nm, which shows the symmetrical surface morphology. Fig. 2 is the XRD pattern of sample, in which there are a few obvious diffraction peaks, they are attributed to the diffractions of AlPO4 and Ca3(PO4)2, and labeled in the Fig. 2. The UC fluorescence usually has several different emission bands under the excitation of IR source. When the different color lights mix each other, the material can display different color, and the color has a connection with the luminescent intensity ratio. Thus, for biological labeling, we hope that the material only emits single-color UC emission, avoiding the spectral cross-talk of the
Fig. 2. X-ray diffraction pattern of the glass ceramic sample.
main band and other bands [8]. But for fluorescence anticounterfeiting, the single-color material has been very common, and it can be easily forged by counterfeiters. Therefore, it is very useful that the material can emit multicolor emissions under the excitation of different pump sources. The multi-layer of protections plays an important role in the anti-counterfeiting industry. However, the complex technologies and detection methods bring lots of difficulties to counterfeiters, and they also bring the difficulties to anti-counterfeiting industries own. Thus, to develop fluorescence anti-counterfeiting labeling, it is important that the emission color can change with the external environment. Our sample emits just the UC red and green emissions in the visible band at 975 nm excitation. Fig. 3 shows the UC emission spectra of the sample in the 500e700 nm wavelength range at different excitation powers. The green emission at 510e570 nm should come from the transition of Er3þ ion: 2H11/2/4S3/2 / 4I15/2 , and the red emission at 636e692 nm is attributed to the transition of Er3þ ion: 4 F9/2 / 4I15/2. It is very obvious that the intensity of the UC red emission is stronger than that of the green emission at low power;
Fig. 1. Atomic force microscope image of the glass ceramic sample.
C. Ming et al. / Current Applied Physics 13 (2013) 351e354
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it is possible that the green color will increase. To sum up, with the increasing of pump power, the emission color changes from red to green, which proves the fluorescence color of our sample is tunable by changing the pump power, and there is a wider tunable range. The Fig. 4 shows the logelog plots for the dependence of green and red emission intensities on pump power. According to the formula [16]:
Iup fP m ; 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 green emission and the 659 nm red emissions are 2.30 and 1.55 at the low pump power; but under the high power, they is 3.45 and 1.34, respectively, which means that the green emissions vary from two-photon processes to three-photon processes, and one/two-photon processes are involved to the 4F9/2 state. 3.2. Mechanism of UC emissions Fig. 3. Photoluminescence spectra in the 500e700 nm wavelength range of Er3þ/Yb3þ co-doped phosphate glass under different power. The inset is the intensity ratio Igreen/ Ired as a function of pump power.
but at high power, the intensity of the red emission is weaker compared to that of the green emission. With the increasing of pump power, the intensity of the green emission becomes strong; the intensity of the red emission increases first and then decreases; the intensity ratio of the green emission to the red emission becomes larger and larger, which is shown in the inset of Fig. 3. Thus, the emission color of the sample is red and green to the naked eye at lower and higher pump power, respectively. The emission color of the sample can change by adjusting the excitation power. Ideal fluorescence labeling should have a strong, stable and narrow band emission. For our sample, the full bandwidths of the red and green emissions are about 56 and 60 nm, respectively. Such narrow bandwidths are comparable to the Quantum dots (QD) materials. So far, QD materials are a very ideal fluorescence labeling, but there are several main shortcomings: high auto-fluorescence, high production costs, complex production process and strong scattering of excitation light. Similarly, traditional organic dyes are easy to photo-bleaching, and the wide band emission results in the low signal to noise ratio. However, Er3þ/Yb3þ co-doped phosphate glass ceramics overcomes these shortcomings, and emits the narrow band, strong and stable UC red and green emissions, thus, it will be a promising fluorescence labeling. And the color of emission depends on the pump power, this character will have an important application on the fluorescence anti-counterfeiting technologies. Applying the color changing under different pump power to the fluorescence anti-counterfeiting labeling, it is important that the color variety should be obvious and distinguishable, that is, the ideal labeling can emit two different color emission at low and high pump power, and have a high color purity. The color purity RCP is defined as the ratio of intensity of the main light (the stronger emission) Imain and the intensity of the total visible light Itotal: RCP ¼ Imain/Itotal, the larger of RCP, the better of monochrome. From Fig. 2, under 126 mW pump power, the main emission band is red emission, the value of RCP is 0.81, which shows the main color of light is red; At 868 mW pump power, the main emission band is green emission, and the value of RCP is 0.76, which shows the green color of is primary. From the inset of Fig. 2, we know that the ratio of Igreen/Ired increases as the pump power. Thus at higher pump power,
The energy level diagram of Yb3þ and Er3þ is shown in Fig. 5, as well as the proposed UC processes under the excitation of 975 nm LD. The population processes of the green emission can be described as follows. By the energy transfer (ET) 1: 2F5/2 (Yb3þ) þ 4I15/2 (Er3þ) / 2F7/2 (Yb3þ) þ 4I11/2 (Er3þ), and the ground state absorption (GSA): 4I15/2 (Er3þ) þ hv / 4I11/2 (Er3þ), Er3þ ions in the ground state are pumped to the long-lived 4I11/2 state. By the excited state absorption (ESA1): 4I11/2 (Er3þ) þ hv / 4F7/2 (Er3þ), the energy transfer (ET) 2: 2F5/2 (Yb3þ) þ 4I11/2 (Er3þ) / 2F7/2 (Yb3þ) þ 4F7/2 (Er3þ), and the cross-relaxation (CR) 1: 4I11/2 (Er3þ) þ 4I11/2 (Er3þ) / 4I15/2 (Er3þ) þ 4F7/2 (Er3þ), the Er3þ ions in 4 I11/2 state can be pumped to the short-lived 4F7/2 state. Er3þ ions in 4 F7/2 are relaxed to 2H11/2 and 4S3/2 states by non-radiative transition, from where the green emission arises. The red luminescence at 659 nm comes from the transition of Er3þ ion: 4F9/2 / 4I15/2, whose UC processes can be described as follows. The Er3þ ions in the ground state transfer to 4I11/2 by ET1 and GSA. Subsequently, the Er3þ ions relax non-radiatively to the
Fig. 4. Dependence of intensity of the UC green and red emissions as function of the pump power. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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between UC processes and linear decay for the depletion of the 4I13/ 2 state [15]. With the increasing of pump power, the m value of green emission varies from 2.30 to 3.45. The phenomenon can be interpreted as follows. At higher power, the ions in the 4F9/2 state can be excited to higher energy levels, where the m value is usual larger than 3. And then the ions relax to 2H11/2 and 4S3/2 by nonradiative transition, resulting in the increasing of m value. 4. Conclusions In summary, the fluorescence color of Er3þ/Yb3þ co-doped phosphate glass ceramic sample can be changed by adjusting the pump power of 975 nm LD. At low power, the red emission is primary, and the color purity RCP is 0.81 under 126 mW power. With the increasing of power, the emission color of the sample can gradually vary from red to green. Finally, the emission color becomes green at high power, and the color purity RCP is 0.76 at 868 mW power. In view of the low cost of raw materials, simple production process and detection method, especially the unique emission principle, the material can be applied effectively to fluorescence anti-counterfeiting technologies. Acknowledgments Fig. 5. Energy level diagram of Er anism at 975 nm LD excitation.
3þ
3þ
and Yb
ions as well as the proposed UC mech-
long-lived 4I13/2 state. The Er3þ ions in 4I13/2 state are pumped to 4F9/ 4 3þ 4 3þ 2 2 state by ESA2: I13/2 (Er ) þ hv / F9/2 (Er ) and ET3: F5/2 (Yb3þ) þ 4I13/2 (Er3þ) / 2F7/2 (Yb3þ) þ 4F9/2 (Er3þ). Meanwhile, Er3þ ions in 4F7/2, 2H11/2, and 4S3/2 can also relax to 4F9/2 by non-radiative transition. According to the population processes of the UC green and red emissions, the emission intensity ratio of the green emission to red emission is close related to the 4I11/2 energy level. The reason is that some ions of 4I11/2 state can transfer to 4F7/2 state by ET2, ESA1 and CR1 and others will relax to 4I13/2 by non-radiative transition. The efficiency of ET2, ESA1 and CR1 depends on the population of 4I11/2 state. The more population, the higher the efficiency is. However, the non-radiative transition of 4I11/2 is not related to the population. At low pump power, the population of 4I11/2 state is very small, which results in the low efficiency of ET2, ESA1 and CR1. Most of the Er3þ ions in 4I11/2 state relax to 4I13/2 by non-radiative transition. Thus, the intensity of the UC red emission is larger compared to that of the green emission. But at high pump power, the opposite is true. Thus, the intensity of the green emission is stronger than that of the red emission. Especially, the ions in the 4F9/2 state can be excited to the higher excited state by the energy transition, like as ET4 and ESA3, which can result in the intensity of the red emission becomes weak and the intensity of the green emission rapidly increases. This is in good agreement with the experimental measurement, seen in Fig. 3. The m value of red emission is 1.55 and 1.34 at the low and high pump power, which can be attributed to the competition
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