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Strong cooperative upconversion luminescence of ytterbium doped oxyfluoride nanophase vitroceramics
Solid State Communications 136 (2005) 313–317 www.elsevier.com/locate/ssc
Strong cooperative upconversion luminescence of ytterbium doped oxyfluoride nanophase vitroceramics Xiaobo Chena,*, Zengfu Songb a
Applied Optics Beijing Area Major Laboratory, Analysis and Testing Center, Beijing Normal University, Beijing 100875, China b School of Physics, Peking University, Beijing 100871, China Received 6 April 2005; received in revised form 21 June 2005; accepted 18 August 2005 by P. Sheng Available online 31 August 2005
Abstract The strong 479.1 nm blue cooperative upconversion luminescence of ytterbium Yb3C ion doped oxyfluoride nanophase vitroceramics (Yb:FOV) is studied in this article. It is found that the 479.1 nm blue cooperative upconversion luminescence strength of Yb(5):FOV is 230 times greater than that of fluoride glass Yb(3):ZBLAN. The large enhancement on cooperative upconversion blue luminescence of this article results from the comprehensive improvement on the aspects of better coupled chance of the Yb3C–Yb3C cluster, less cross-relaxation, better concentration contribution of Yb3C activator, non-saturation, and better upconversion luminescence efficiency. q 2005 Elsevier Ltd. All rights reserved. PACS: 78.55.Km; 42.70.Ka; 42.62.Fi Keywords: A. Nanostructures; D. Optical properties; E. Luminescence
Optical physics [1,2] is the foundation of natural science, in which upconversion is a interesting field [3–19]. Cooperative upconversion [3–11] is a kind of upconversion mechanism, which points out a good method to achieve integrated blue light that is of particular urgent [2]. A number of ytterbium–europium Yb3C–Eu3C or Yb3C– thulium Tm 3C or Yb3C –terbium Tb 3C cooperative upconversion researches have been carried out up to now [3,6,10,11]. However, cooperative upconversion was very low efficient in past times. The direct Yb3C–Yb3C cooperative upconversion blue luminescence are correctly achieved recently [6,9,10]. And it has been proved that the cooperative upconversion green luminescence results from the Yb3C–Tb3C cooperative effect [6] and so on [3–10]. The cooperative upconversion blue luminescence of the * Corresponding author. Tel.: C86 10 58 808 268; fax: C86 10 58 800 076. E-mail address: [email protected] (X.B. Chen).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.08.019
direct Yb3C–Yb3C cluster exhibits good attraction initially [6,9,10]. Meanwhile, upconversion has interesting applications in laser [17,18], 3-d volumetric display, wave-guide, guard-against-forge applications, imaging, data-storage, (temperature) sensors and so on [16]. It is significant to study oxyfluoride nanophase vitroceramics material because the upconversion luminescence is enhanced excellently [12–15]. The oxyfluoride nanophase vitroceramics is a kind of vitroceramics material whose nanocrystalline is inlayed within the glass material [12,15]. In this Yb3C-doped oxyfluoride nanophase vitroceramics sample, the Yb3C ions are mainly located in fluoride nanocrystals that are embedded in an oxide vitreous matrix. The fluoride nanocrystals provide a low phonon energy environment to benefit upconversion have large luminescent efficiency. In this paper, six Yb3C-doped or Yb3CHo3C-codoped oxyfluoride nanophase vitroceramics (FOV) or fluoride glass (ZBLAN) material are used to proceed our
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X.B. Chen, Z.F. Song / Solid State Communications 136 (2005) 313–317
upconversion experiments. FOV is made from the oxide of Silicon SiO2, and the fluoride of plumbum PbF2, zinc ZnF2, lutetium LuF3, ytterbium YbF3, and holmium HoF3. Two of FOV samples are doped singly with Yb3C ion at concentrations of 5 and 0.5 mol%, which are denoted as [a] Yb(5):FOV and [b] Yb(0.5):FOV, respectively. The other three Yb3CHo3C-codoped FOV samples are denoted as [c] Yb(5)Ho(0.1):FOV, [d] Yb(5)Ho(0.5):FOV, and [e] Yb(1)Ho(0.5):FOV. A zirconium Zr barium Ba lanthanum La aluminum Al and sodium Na system (ZBLAN) fluoride glass sample [f] Yb(3):ZBLAN is used in our experiments to compare the upconversion luminescent ability between the FOV and ZBLAN material. The optical absorption of all samples is measured by using a spectrophotometer (Shimadsu, UV-365). The absorption coefficient at 960 nm wavelength of samples [a] Yb(5):FOV, [b] Yb(0.5):FOV, [c] Yb(5)Ho(0.1):FOV, [d] Yb(5)Ho(0.5):FOV, [e] Yb(1) Ho(0.5):FOV and [f] Yb(3):ZBLAN is 2.41, 0.242, 2.41, 2.40, 0.482, 0.717 cmK1, respectively. The Fig. 1 shows the absorption coefficient of samples [a] Yb(5):FOV, [c] Yb(5)Ho(0.1):FOV, [d] Yb(5)Ho(0.5):FOV, and [f] Yb(3):ZBLAN. The result exhibits that the absorption of Yb3C ion has a sharp main peak at 975.1 nm (10,255 cmK1) with two sub-peak at 927.0 nm (10,787 cmK1 ) and 952.0 nm (10,504 cmK1). It is clear that the accuracy and repeatability of our experiment is very good. The upconversion luminescence is measured based on a set of the experimental device [5], in which the pumping source is a 960 nm continuous diode-laser and the core fluorescence-detecting component is the fluorescence spectro-photometer (JY-ISA, Fluorolog-Tau-3 with double grating monochromator). The collection direction of the
Fig. 1. The absorption coefficient around 960 nm of [a] Yb(5):FOV, [d] Yb(5)Ho(0.5):FOV, [c] Yb(5)Ho(0.1):FOV, and [f] Yb(3): ZBLAN samples.
fluorescence is perpendicular to the propagation direction of the pumping laser. The upconversion luminescence, which is only emitted from the surface layer of the sample is collected into monochromator and measured in our experiment. The experimental conditions are kept the same, so as to ensure that the relative fluorescence intensities of the measured curves are comparable. The upconversion spectra of samples [a] Yb(5):FOV and [b] Yb(0.5):FOV are measured first when excited by a 227 mW 960 nm continuous diode-laser. The results are shown in Fig. 2. In order to compare cooperative upconversion luminescence ability between FOV and ZBLAN, the upconversion luminous spectra of [a] Yb(5):FOV and [f] Yb(3):ZBLAN are measured comparably also, which is shown in Fig. 3. It is clear that the cooperative upconversion luminescence of [a] Yb(5):FOV is 230 times greater than that of [f] Yb(3):ZBLAN. By changing the diode-laser power from 5.4 to 227 mW, the variation of the 479.1 nm upconversion luminescence intensity as a function of pump laser power is measured carefully, as shown in Fig. 4. Meanwhile, the upconversion luminous spectra of [c] Yb(5)Ho(0.1):FOV, [d] Yb(5)Ho(0.5):FOV and [e] Yb(1)Ho(0.5):FOV samples within 450–505 nm wavelength range are measured, as shown in Fig. 2 also. The waveform variation with laser power is measured also, which shows that there are no observable changes in either the positions or profiles when laser power is changed. The interesting phenomena concerning the 479.1 nm cooperative upconversion blue luminescence of the Yb:
Fig. 2. Cooperative upconversion luminescence spectra of [a] Yb(5):FOV, [b] Yb(0.5):FOV, [c] Yb(5)Ho(0.1):FOV, [d] Yb(5) Ho(0.5):FOV and [e] Yb(1)Ho(0.5):FOV samples induced by a 960 nm laser. The intensities of all five curves are normalized and comparable directly.
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Fig. 3. Cooperative upconversion luminescence spectra of [a] Yb(5):FOV (solid-line) and [f] Yb(3):ZBLAN (dashed line) induced by a 960 nm laser. The data for [f] Yb(3):ZBLAN is enlarged by a factor of 230. The intensities of both curves are comparable directly.
FOV are discovered in this article. First, the 479.1 nm upconversion luminescence is a two-photon upconversion luminescence. The integrated luminescence intensity F as a function of the excitation laser power P for both samples [a]
Fig. 4. Cooperative upconversion luminescence intensity as a function of 960 nm diode-laser power for samples Yb(5):FOV and Yb(0.5):FOV. The slopes for double logarithmic plots log F–log P of samples Yb(5):FOV and Yb(0.5):FOV are 1.885 and 2.029, respectively.
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Yb(5):FOV and [b] Yb(0.5):FOV are shown in Fig. 4 through a double logarithmic plot. These data in double logarithmic plots log F–log P clearly follow several very good straight lines. The slopes are obtained through leastsquare fitting, which are 1.885 and 2.029, respectively. It is clear that both slopes are near to 2. These experimental results indicate that the observed blue luminescence results from the two-photon upconversion process [1–7]. The 479.1 nm upconversion blue luminescence is closely related to the concentration of Yb3C ions. From Fig. 2, it is obvious that the 479.1 nm upconversion luminescence intensity of [a] Yb(5):FOV is about 100 times greater than that of [b] Yb(0.5):FOV. However, the Yb 3C ion concentration of [a] Yb(5):FOV is only 10 times greater than that of [b] Yb(0.5):FOV. Similarly, the 462.5 nm upconversion blue luminescence sub-peak intensity of [d] Yb(5)Ho(0.5):FOV is nearly 25 times greater than that of [e] Yb(1)Ho(0.5):FOV although there is an obvious influence of Ho3C ion. This illustrates that the 479.1 nm upconversion luminescence is proportional to the square of the Yb3C ion concentration. Thus it can be believed that the 479.1 nm upconversion luminescence is not oriented from linear process involving single Yb3C ion system, because the upconversion luminescence intensity of a single Yb3C ion system must be linearly proportional to the Yb3C ion concentration. As is well known, if the two-photon upconversion luminescence originated from a two-Yb3C ion system, the upconversion luminescence intensity would be usually proportional to the square of the Yb3C ions’ concentration. Therefore, this 479.1 nm blue upconversion luminescence should only come from either energy transfer between Yb3C ions or the coupled state of the Yb3C–Yb3C cluster. Furthermore, it is reasonable to believe that the 479.1 nm upconversion luminescence is a kind of cooperative upconversion luminescence originated from the coupled states of the Yb3C–Yb3C clusters [6,9,10]. The main peak of this cooperative upconversion luminescence lies at about 479.1 nm with two sub-peaks at about 462.5 and 475.5 nm. According to the absorption of [a] Yb(5):FOV, [c] Yb(5)Ho(0.1):FOV, and [d] Yb(5)Ho(0.5):FOV, the absorption stark peak positions of Yb3C ion 2F5/2 level of the FOV are (Yb3CK1main: 975.1 nm, 10,255 cmK1), (Yb3CK2sub: 952.0 nm, 10,504 cmK1) and (Yb3CK3sub: 927.0 nm, 10,787 cmK1). The Yb3C ion is easily excited to its 2F5/2 level from ground 2F7/2 level by absorbing a 960 nm 10,417 cmK1 laser photon. Therefore, if two Yb3C ions happen to form a cluster, the coupled state may be located at 20,759 cmK1 that is made up by the Yb3C ion’s sub-states (Yb3CK1main: 975.1 nm, 10,255 cmK1) and (Yb3CK2sub: 952.0 nm, 10,504 cmK1), which is very close to the measured main peak (Yb3CYb3CKAmain: 479.1 nm, 20,872 cmK1) of the cooperative upconversion luminescence. In a similar manner, the coupled state may be made up by the sub-states (Yb3CK3sub: 927.0 nm, 10,787 cmK1) and (Yb3CK3sub: 927.0 nm, 10,787 cmK1) to position at
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21,574 cmK1 also, which is quite close to the measured subpeak (Yb3CYb3C–Csub: 462.5 nm, 21,622 cmK1) of the cooperative upconversion luminescence. Then, the coupled state may be made up by the Yb3C ion’s sub-states (Yb3CK 2sub: 952.0 nm, 10,504 cmK1) and (Yb3CK2sub: 952.0 nm, 10,504 cmK1) to position at 21,008 cmK1, or made up by the sub-states (Yb3CK1main: 975.1 nm, 10,255 cmK1) and (Yb3CK3sub: 927.0 nm, 10,787 cmK1) to position at 21,042 cmK1, both of which are quite close to the measured sub-peak (Yb3CYb3C–Bsub: 475.5 nm, 21,030 cmK1) of the cooperative upconversion luminescence. The schematic diagram of energy level structure is shown in Fig. 5. So, it is believed from an energy-match consideration that the 479.1 nm fluorescence is a kind of cooperative upconversion luminescence [6,9,10,19], which is proved by the dynamic measurement also [6,9,10]. The upconversion processes are as follows. First, each isolated Yb3C ion absorbs a pump photon and is excited to the 2F5/2 state from the 2F7/2 ground state. Then the two Yb3C ions which are both at the 2F5/2 excited state form a coupled cluster state 2F25/2F5/2 under the laser pumping. The 479.1 nm cooperative upconversion luminescence is then radiated from this excited coupled state 2F25/2F5/2(Yb3CYb3C) to the ground coupled state 2F7/2 2 F7/2(Yb3CYb3C). Moreover, the measured upconversion luminescence main-peak 479.1 nm in this article is coincided with all the published correct papers [6,9,10], whose upconversion luminescence main-peaks of the direct Yb3C–Yb3C cluster are all located at about the 476–480 nm wavelength range. The main peak we have observed is different from the characteristic fluorescence main-peak of Tb3C ion located at about 495–504 nm wavelength range [1]. It has been proved also that the cooperative upconversion green luminescences results from the Yb3C– Tb3C cooperative effect [6] and so on [3–10]. All of these indicate that the large cooperative upconversion blue luminescence of the direct Yb3C–Yb3C cluster discovered 3
K7, 5G4 5
3+
3+ 2
5
2
Yb Yb : F5/2 F5/2 3
G5
F1, 5G6
K8, 5F2, 5F3
5
F4, 5S2 5
Cooperative Upconversion Luminescence
F5
5
2
I4
5
I5
5
I6
5
I7
F5/2
Normal Pumping Laser Yb3+Yb3+:2F7/22F7/2 3+
3+
Yb Yb cluster
2
F7/2
960nm 3+
Yb ion
Luminescence 5
I8 Ho3+ ion
Fig. 5. The schematic diagram of energy level structure.
in this article is clearly seen and stable. As is well known, the concept of cooperative upconversion of Yb3C–Tm3C or Yb3C–Tb3C was proposed even before Reisfeld [1]. However, the near-resonant back energy-transfer from Tm3C or Tb3C ion restricts the enhancement of this cooperative upconversion luminescence, particularly for huge phonon energy material [9–12]. Even the sol–gel material can provide good coupled chance for Yb3C ions to form the Yb3C–Yb3C clusters [10], the further improvement is urgently needed because the phonon energy of this material is high and its electron–phonon interaction is strong. The large cooperative upconversion blue luminescence of the Yb3C–Yb3C clusters obtained in this article results from the better coupled chance of the Yb3C–Yb3C clusters [19] within the FOV material and the less phononrelaxation and cross-relaxation. Finally, it is important to find that the cooperative upconversion blue luminescence ability is much enhanced by our oxyfluoride nanophase vitroceramics. As we know, the fluoride ZBLAN glass is one of the best efficiency materials for upconversion luminescence. The upconversion efficiency has been studied widespread. The upconversion luminescence efficiency of the Yb:FOV could be known easily by comparing upconversion luminescence ability between the FOV and the ZBLAN material. It is clear that the 479.1 nm cooperative upconversion blue luminescence strength of [a] Yb(5):FOV is 230 times greater than that of [f] Yb(3):ZBLAN according to the results of Fig. 3 of this article. The reason for this improvement is that the FOV material provides much better opportunities for Yb3C ions to directly form better Yb3C–Yb3C clusters than the ZBLAN glass so that it can radiate much stronger cooperative upconversion blue luminescence, because rare-earth ions are enrich into the fluoride nanocrystalline in this FOV material that results in the Yb3C ions are nearer than that in homogeneous glasses [12,15]. The cooperative upconversion luminescence of YbHo: FOV emerges two obvious additional peaks that are not present in that of Yb:FOV, which are positioned at (484.6 nm, 20,637 cmK1) and (491.2 nm, 20,357 cmK1). They are very near to the 5I8/5F3 absorption peaks (484.2 nm, 20,646 cmK1) and (490.8 nm, 20,377 cmK1) of Ho3C ion [1]. Thus it is believed that the upconversion luminescence (484.6 nm, 20,637 cmK1) and (491.2 nm, 20,357 cmK1) are 5F3/5I8 fluorescence transition of Ho3C ion. The former is a transition to the lower Starkstate of 5I8 level, while the latter is a transition to the higher Stark-state of 5I8 level. It is clear that the codoped Ho3C ion causes the obvious reduction in the cooperative blue upconversion luminescence of the coupled states of the Yb3C–Yb3C clusters. It could be found from Fig. 2 that the 479 nm upconversion luminescence of Yb(5)Ho(0.1):FOV and Yb(5)Ho(0.5):FOV is about 40.8 and 3.01% to that of Yb(5):FOV, respectively. This illustrates that the codoped Ho3C ion obviously decreases the cooperative upconversion
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luminescence of the Yb3C–Yb3C clusters. In fact, there is a resonant energy transfer from the 2F25/2F5/2 couple-state of the Yb3C–Yb3C clusters to the 3K8, 5F2 and 5F3 levels of the Ho3C ion because their energy levels match very well. The energy of the 3K8, 5F2 and 5F3 levels would relax to the 5F4 and 5S2 levels in a fast rate since the 5F4 and 5S2 levels are a little lower. Thus the energy of the Yb3C–Yb3C clusters will transfer unidirectionally to the Ho3C ion. It may be a reason why the cooperative upconversion luminescence was thought as kind of very small upconversion fluorescence before. This implies that the impurities mixed during the manufacture procedure would serious reduce the cooperative upconversion luminescence intensity due to crossrelaxation. It is significant also for this article to find that pure Yb3C ion-doped sample, which has as little impurity as possible to reduce cross-relaxation, could achieve higher cooperative upconversion luminescence of the Yb3C–Yb3C clusters. In conclusion, the large enhancement on cooperative upconversion blue luminescence, that is the cooperative upconversion blue luminescence strength of Yb(5):FOV is 230 times greater than that of fluoride glass Yb(3):ZBLAN, results from the comprehensive improvement on the aspects of better coupled chance of the Yb3C–Yb3C cluster, less cross-relaxation, better concentration contribution of Yb3C activator, non-saturation, and better upconversion luminescence efficiency.
Acknowledgements The project is supported by the National Natural Science Foundation of China (Grant No. 19874033 and 10174008) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Chinese State Education Ministry. The authors thank so much for Prof Ou Wen, Prof Qing Cai and Prof Rongsheng Chen at Beijing Glass
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Research Institute and Prof N. Sawanobori at Sumita Optical Glass Inc. of Japan for providing samples.
References [1] R. Reisfeld, Lasers and Excited States of Rare-earth, Springer, Berlin, 1977. [2] J.E. Chen, S.Y. Wu, G.Z. Yang, P.X. Ye, G.Y. Chen, D.H. Wang, Z.Z. Gan, Optical Physics Science, Science Press, Beijing, 1995 (in Chinese). [3] G.S. Maciel, A. Biswas, P.N. Prasad, Opt. Commun. 178 (2000) 65. [4] R.T. Wegh, A. Meijerink, Chem. Phys. Lett. 246 (1995) 495. [5] X.B. Chen, Y.X. Nie, W.M. Du, N. Sawanobori, Opt. Commun. 184 (2000) 289. [6] J.L. Adam, N. DuhamelHenry, J.Y. Allain, J. Non-Cryst. Solids 213 (1997) 245. [7] X.B. Chen, M.X. Li, N. Sawanobori, Z. Zhe, Y.X. Nie, Acta Phys. Sin. 49 (2000) 2482 (in Chinese). [8] X.B. Chen, Y.X. Nie, O. Wen, Chin. Phy. Lett. 18 (2001) 280. [9] P. Goldner, F. Pelle, D. Meichenin, F. Auzel, J. Lumin. 71 (1997) 142. [10] G.S. Maciel, A. Biswas, R. Kapoor, P.N. Prasad, Appl. Phys. Lett. 76 (2000) 1978. [11] I.R. Martin, A.C. Yanes, J. Mendez-Ramos, M.E. Torres, V.D. Rodriguez, J. Appl. Phys. 89 (2001) 2520. [12] Y.H. Wang, J. Ohwaki, Appl. Phys. Lett. 63 (1993) 3268. [13] H.W. Song, B.J. Sun, T. Wang, S.Z. Lu, L.M. Yang, B.J. Chen, X.J. Wang, X.G. Kong, Solid State Commun. 132 (6) (2004) 409. [14] H.T. Sun, L.Y. Zhang, S.L. Zhao, J.J. Zhang, C.L. Yu, D.B. He, Z.C. Duan, L.L. Hu, Z.H. Jiang, Solid State Commun. 133 (6) (2005) 357. [15] F. Lahoz, I.R. Martin, J.M. Calvilla-Quintero, Appl. Phys. Lett. 86 (5) (2005) 051106. [16] F. Auzel, Chem. Rev. 104 (1) (2004) 139. [17] S. Sanders, R.G. Waarts, D.G. Mehuys, D.F. Wetch, Appl. Phys. Lett. 67 (1995) 1815. [18] D.M. Baney, G. Rankin, K.W. Chang, Appl. Phys. Lett. 69 (12) (1996) 1662. [19] M.P. Hehlen, H.U. Gu¨del, J. Chem. Phys. 98 (3) (1993) 1768.
Strong cooperative upconversion luminescence of ytterbium doped oxyfluoride nanophase vitroceramics Xiaobo Chena,*, Zengfu Songb a
Applied Optics Beijing Area Major Laboratory, Analysis and Testing Center, Beijing Normal University, Beijing 100875, China b School of Physics, Peking University, Beijing 100871, China Received 6 April 2005; received in revised form 21 June 2005; accepted 18 August 2005 by P. Sheng Available online 31 August 2005
Abstract The strong 479.1 nm blue cooperative upconversion luminescence of ytterbium Yb3C ion doped oxyfluoride nanophase vitroceramics (Yb:FOV) is studied in this article. It is found that the 479.1 nm blue cooperative upconversion luminescence strength of Yb(5):FOV is 230 times greater than that of fluoride glass Yb(3):ZBLAN. The large enhancement on cooperative upconversion blue luminescence of this article results from the comprehensive improvement on the aspects of better coupled chance of the Yb3C–Yb3C cluster, less cross-relaxation, better concentration contribution of Yb3C activator, non-saturation, and better upconversion luminescence efficiency. q 2005 Elsevier Ltd. All rights reserved. PACS: 78.55.Km; 42.70.Ka; 42.62.Fi Keywords: A. Nanostructures; D. Optical properties; E. Luminescence
Optical physics [1,2] is the foundation of natural science, in which upconversion is a interesting field [3–19]. Cooperative upconversion [3–11] is a kind of upconversion mechanism, which points out a good method to achieve integrated blue light that is of particular urgent [2]. A number of ytterbium–europium Yb3C–Eu3C or Yb3C– thulium Tm 3C or Yb3C –terbium Tb 3C cooperative upconversion researches have been carried out up to now [3,6,10,11]. However, cooperative upconversion was very low efficient in past times. The direct Yb3C–Yb3C cooperative upconversion blue luminescence are correctly achieved recently [6,9,10]. And it has been proved that the cooperative upconversion green luminescence results from the Yb3C–Tb3C cooperative effect [6] and so on [3–10]. The cooperative upconversion blue luminescence of the * Corresponding author. Tel.: C86 10 58 808 268; fax: C86 10 58 800 076. E-mail address: [email protected] (X.B. Chen).
0038-1098/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2005.08.019
direct Yb3C–Yb3C cluster exhibits good attraction initially [6,9,10]. Meanwhile, upconversion has interesting applications in laser [17,18], 3-d volumetric display, wave-guide, guard-against-forge applications, imaging, data-storage, (temperature) sensors and so on [16]. It is significant to study oxyfluoride nanophase vitroceramics material because the upconversion luminescence is enhanced excellently [12–15]. The oxyfluoride nanophase vitroceramics is a kind of vitroceramics material whose nanocrystalline is inlayed within the glass material [12,15]. In this Yb3C-doped oxyfluoride nanophase vitroceramics sample, the Yb3C ions are mainly located in fluoride nanocrystals that are embedded in an oxide vitreous matrix. The fluoride nanocrystals provide a low phonon energy environment to benefit upconversion have large luminescent efficiency. In this paper, six Yb3C-doped or Yb3CHo3C-codoped oxyfluoride nanophase vitroceramics (FOV) or fluoride glass (ZBLAN) material are used to proceed our
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X.B. Chen, Z.F. Song / Solid State Communications 136 (2005) 313–317
upconversion experiments. FOV is made from the oxide of Silicon SiO2, and the fluoride of plumbum PbF2, zinc ZnF2, lutetium LuF3, ytterbium YbF3, and holmium HoF3. Two of FOV samples are doped singly with Yb3C ion at concentrations of 5 and 0.5 mol%, which are denoted as [a] Yb(5):FOV and [b] Yb(0.5):FOV, respectively. The other three Yb3CHo3C-codoped FOV samples are denoted as [c] Yb(5)Ho(0.1):FOV, [d] Yb(5)Ho(0.5):FOV, and [e] Yb(1)Ho(0.5):FOV. A zirconium Zr barium Ba lanthanum La aluminum Al and sodium Na system (ZBLAN) fluoride glass sample [f] Yb(3):ZBLAN is used in our experiments to compare the upconversion luminescent ability between the FOV and ZBLAN material. The optical absorption of all samples is measured by using a spectrophotometer (Shimadsu, UV-365). The absorption coefficient at 960 nm wavelength of samples [a] Yb(5):FOV, [b] Yb(0.5):FOV, [c] Yb(5)Ho(0.1):FOV, [d] Yb(5)Ho(0.5):FOV, [e] Yb(1) Ho(0.5):FOV and [f] Yb(3):ZBLAN is 2.41, 0.242, 2.41, 2.40, 0.482, 0.717 cmK1, respectively. The Fig. 1 shows the absorption coefficient of samples [a] Yb(5):FOV, [c] Yb(5)Ho(0.1):FOV, [d] Yb(5)Ho(0.5):FOV, and [f] Yb(3):ZBLAN. The result exhibits that the absorption of Yb3C ion has a sharp main peak at 975.1 nm (10,255 cmK1) with two sub-peak at 927.0 nm (10,787 cmK1 ) and 952.0 nm (10,504 cmK1). It is clear that the accuracy and repeatability of our experiment is very good. The upconversion luminescence is measured based on a set of the experimental device [5], in which the pumping source is a 960 nm continuous diode-laser and the core fluorescence-detecting component is the fluorescence spectro-photometer (JY-ISA, Fluorolog-Tau-3 with double grating monochromator). The collection direction of the
Fig. 1. The absorption coefficient around 960 nm of [a] Yb(5):FOV, [d] Yb(5)Ho(0.5):FOV, [c] Yb(5)Ho(0.1):FOV, and [f] Yb(3): ZBLAN samples.
fluorescence is perpendicular to the propagation direction of the pumping laser. The upconversion luminescence, which is only emitted from the surface layer of the sample is collected into monochromator and measured in our experiment. The experimental conditions are kept the same, so as to ensure that the relative fluorescence intensities of the measured curves are comparable. The upconversion spectra of samples [a] Yb(5):FOV and [b] Yb(0.5):FOV are measured first when excited by a 227 mW 960 nm continuous diode-laser. The results are shown in Fig. 2. In order to compare cooperative upconversion luminescence ability between FOV and ZBLAN, the upconversion luminous spectra of [a] Yb(5):FOV and [f] Yb(3):ZBLAN are measured comparably also, which is shown in Fig. 3. It is clear that the cooperative upconversion luminescence of [a] Yb(5):FOV is 230 times greater than that of [f] Yb(3):ZBLAN. By changing the diode-laser power from 5.4 to 227 mW, the variation of the 479.1 nm upconversion luminescence intensity as a function of pump laser power is measured carefully, as shown in Fig. 4. Meanwhile, the upconversion luminous spectra of [c] Yb(5)Ho(0.1):FOV, [d] Yb(5)Ho(0.5):FOV and [e] Yb(1)Ho(0.5):FOV samples within 450–505 nm wavelength range are measured, as shown in Fig. 2 also. The waveform variation with laser power is measured also, which shows that there are no observable changes in either the positions or profiles when laser power is changed. The interesting phenomena concerning the 479.1 nm cooperative upconversion blue luminescence of the Yb:
Fig. 2. Cooperative upconversion luminescence spectra of [a] Yb(5):FOV, [b] Yb(0.5):FOV, [c] Yb(5)Ho(0.1):FOV, [d] Yb(5) Ho(0.5):FOV and [e] Yb(1)Ho(0.5):FOV samples induced by a 960 nm laser. The intensities of all five curves are normalized and comparable directly.
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Fig. 3. Cooperative upconversion luminescence spectra of [a] Yb(5):FOV (solid-line) and [f] Yb(3):ZBLAN (dashed line) induced by a 960 nm laser. The data for [f] Yb(3):ZBLAN is enlarged by a factor of 230. The intensities of both curves are comparable directly.
FOV are discovered in this article. First, the 479.1 nm upconversion luminescence is a two-photon upconversion luminescence. The integrated luminescence intensity F as a function of the excitation laser power P for both samples [a]
Fig. 4. Cooperative upconversion luminescence intensity as a function of 960 nm diode-laser power for samples Yb(5):FOV and Yb(0.5):FOV. The slopes for double logarithmic plots log F–log P of samples Yb(5):FOV and Yb(0.5):FOV are 1.885 and 2.029, respectively.
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Yb(5):FOV and [b] Yb(0.5):FOV are shown in Fig. 4 through a double logarithmic plot. These data in double logarithmic plots log F–log P clearly follow several very good straight lines. The slopes are obtained through leastsquare fitting, which are 1.885 and 2.029, respectively. It is clear that both slopes are near to 2. These experimental results indicate that the observed blue luminescence results from the two-photon upconversion process [1–7]. The 479.1 nm upconversion blue luminescence is closely related to the concentration of Yb3C ions. From Fig. 2, it is obvious that the 479.1 nm upconversion luminescence intensity of [a] Yb(5):FOV is about 100 times greater than that of [b] Yb(0.5):FOV. However, the Yb 3C ion concentration of [a] Yb(5):FOV is only 10 times greater than that of [b] Yb(0.5):FOV. Similarly, the 462.5 nm upconversion blue luminescence sub-peak intensity of [d] Yb(5)Ho(0.5):FOV is nearly 25 times greater than that of [e] Yb(1)Ho(0.5):FOV although there is an obvious influence of Ho3C ion. This illustrates that the 479.1 nm upconversion luminescence is proportional to the square of the Yb3C ion concentration. Thus it can be believed that the 479.1 nm upconversion luminescence is not oriented from linear process involving single Yb3C ion system, because the upconversion luminescence intensity of a single Yb3C ion system must be linearly proportional to the Yb3C ion concentration. As is well known, if the two-photon upconversion luminescence originated from a two-Yb3C ion system, the upconversion luminescence intensity would be usually proportional to the square of the Yb3C ions’ concentration. Therefore, this 479.1 nm blue upconversion luminescence should only come from either energy transfer between Yb3C ions or the coupled state of the Yb3C–Yb3C cluster. Furthermore, it is reasonable to believe that the 479.1 nm upconversion luminescence is a kind of cooperative upconversion luminescence originated from the coupled states of the Yb3C–Yb3C clusters [6,9,10]. The main peak of this cooperative upconversion luminescence lies at about 479.1 nm with two sub-peaks at about 462.5 and 475.5 nm. According to the absorption of [a] Yb(5):FOV, [c] Yb(5)Ho(0.1):FOV, and [d] Yb(5)Ho(0.5):FOV, the absorption stark peak positions of Yb3C ion 2F5/2 level of the FOV are (Yb3CK1main: 975.1 nm, 10,255 cmK1), (Yb3CK2sub: 952.0 nm, 10,504 cmK1) and (Yb3CK3sub: 927.0 nm, 10,787 cmK1). The Yb3C ion is easily excited to its 2F5/2 level from ground 2F7/2 level by absorbing a 960 nm 10,417 cmK1 laser photon. Therefore, if two Yb3C ions happen to form a cluster, the coupled state may be located at 20,759 cmK1 that is made up by the Yb3C ion’s sub-states (Yb3CK1main: 975.1 nm, 10,255 cmK1) and (Yb3CK2sub: 952.0 nm, 10,504 cmK1), which is very close to the measured main peak (Yb3CYb3CKAmain: 479.1 nm, 20,872 cmK1) of the cooperative upconversion luminescence. In a similar manner, the coupled state may be made up by the sub-states (Yb3CK3sub: 927.0 nm, 10,787 cmK1) and (Yb3CK3sub: 927.0 nm, 10,787 cmK1) to position at
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21,574 cmK1 also, which is quite close to the measured subpeak (Yb3CYb3C–Csub: 462.5 nm, 21,622 cmK1) of the cooperative upconversion luminescence. Then, the coupled state may be made up by the Yb3C ion’s sub-states (Yb3CK 2sub: 952.0 nm, 10,504 cmK1) and (Yb3CK2sub: 952.0 nm, 10,504 cmK1) to position at 21,008 cmK1, or made up by the sub-states (Yb3CK1main: 975.1 nm, 10,255 cmK1) and (Yb3CK3sub: 927.0 nm, 10,787 cmK1) to position at 21,042 cmK1, both of which are quite close to the measured sub-peak (Yb3CYb3C–Bsub: 475.5 nm, 21,030 cmK1) of the cooperative upconversion luminescence. The schematic diagram of energy level structure is shown in Fig. 5. So, it is believed from an energy-match consideration that the 479.1 nm fluorescence is a kind of cooperative upconversion luminescence [6,9,10,19], which is proved by the dynamic measurement also [6,9,10]. The upconversion processes are as follows. First, each isolated Yb3C ion absorbs a pump photon and is excited to the 2F5/2 state from the 2F7/2 ground state. Then the two Yb3C ions which are both at the 2F5/2 excited state form a coupled cluster state 2F25/2F5/2 under the laser pumping. The 479.1 nm cooperative upconversion luminescence is then radiated from this excited coupled state 2F25/2F5/2(Yb3CYb3C) to the ground coupled state 2F7/2 2 F7/2(Yb3CYb3C). Moreover, the measured upconversion luminescence main-peak 479.1 nm in this article is coincided with all the published correct papers [6,9,10], whose upconversion luminescence main-peaks of the direct Yb3C–Yb3C cluster are all located at about the 476–480 nm wavelength range. The main peak we have observed is different from the characteristic fluorescence main-peak of Tb3C ion located at about 495–504 nm wavelength range [1]. It has been proved also that the cooperative upconversion green luminescences results from the Yb3C– Tb3C cooperative effect [6] and so on [3–10]. All of these indicate that the large cooperative upconversion blue luminescence of the direct Yb3C–Yb3C cluster discovered 3
K7, 5G4 5
3+
3+ 2
5
2
Yb Yb : F5/2 F5/2 3
G5
F1, 5G6
K8, 5F2, 5F3
5
F4, 5S2 5
Cooperative Upconversion Luminescence
F5
5
2
I4
5
I5
5
I6
5
I7
F5/2
Normal Pumping Laser Yb3+Yb3+:2F7/22F7/2 3+
3+
Yb Yb cluster
2
F7/2
960nm 3+
Yb ion
Luminescence 5
I8 Ho3+ ion
Fig. 5. The schematic diagram of energy level structure.
in this article is clearly seen and stable. As is well known, the concept of cooperative upconversion of Yb3C–Tm3C or Yb3C–Tb3C was proposed even before Reisfeld [1]. However, the near-resonant back energy-transfer from Tm3C or Tb3C ion restricts the enhancement of this cooperative upconversion luminescence, particularly for huge phonon energy material [9–12]. Even the sol–gel material can provide good coupled chance for Yb3C ions to form the Yb3C–Yb3C clusters [10], the further improvement is urgently needed because the phonon energy of this material is high and its electron–phonon interaction is strong. The large cooperative upconversion blue luminescence of the Yb3C–Yb3C clusters obtained in this article results from the better coupled chance of the Yb3C–Yb3C clusters [19] within the FOV material and the less phononrelaxation and cross-relaxation. Finally, it is important to find that the cooperative upconversion blue luminescence ability is much enhanced by our oxyfluoride nanophase vitroceramics. As we know, the fluoride ZBLAN glass is one of the best efficiency materials for upconversion luminescence. The upconversion efficiency has been studied widespread. The upconversion luminescence efficiency of the Yb:FOV could be known easily by comparing upconversion luminescence ability between the FOV and the ZBLAN material. It is clear that the 479.1 nm cooperative upconversion blue luminescence strength of [a] Yb(5):FOV is 230 times greater than that of [f] Yb(3):ZBLAN according to the results of Fig. 3 of this article. The reason for this improvement is that the FOV material provides much better opportunities for Yb3C ions to directly form better Yb3C–Yb3C clusters than the ZBLAN glass so that it can radiate much stronger cooperative upconversion blue luminescence, because rare-earth ions are enrich into the fluoride nanocrystalline in this FOV material that results in the Yb3C ions are nearer than that in homogeneous glasses [12,15]. The cooperative upconversion luminescence of YbHo: FOV emerges two obvious additional peaks that are not present in that of Yb:FOV, which are positioned at (484.6 nm, 20,637 cmK1) and (491.2 nm, 20,357 cmK1). They are very near to the 5I8/5F3 absorption peaks (484.2 nm, 20,646 cmK1) and (490.8 nm, 20,377 cmK1) of Ho3C ion [1]. Thus it is believed that the upconversion luminescence (484.6 nm, 20,637 cmK1) and (491.2 nm, 20,357 cmK1) are 5F3/5I8 fluorescence transition of Ho3C ion. The former is a transition to the lower Starkstate of 5I8 level, while the latter is a transition to the higher Stark-state of 5I8 level. It is clear that the codoped Ho3C ion causes the obvious reduction in the cooperative blue upconversion luminescence of the coupled states of the Yb3C–Yb3C clusters. It could be found from Fig. 2 that the 479 nm upconversion luminescence of Yb(5)Ho(0.1):FOV and Yb(5)Ho(0.5):FOV is about 40.8 and 3.01% to that of Yb(5):FOV, respectively. This illustrates that the codoped Ho3C ion obviously decreases the cooperative upconversion
X.B. Chen, Z.F. Song / Solid State Communications 136 (2005) 313–317
luminescence of the Yb3C–Yb3C clusters. In fact, there is a resonant energy transfer from the 2F25/2F5/2 couple-state of the Yb3C–Yb3C clusters to the 3K8, 5F2 and 5F3 levels of the Ho3C ion because their energy levels match very well. The energy of the 3K8, 5F2 and 5F3 levels would relax to the 5F4 and 5S2 levels in a fast rate since the 5F4 and 5S2 levels are a little lower. Thus the energy of the Yb3C–Yb3C clusters will transfer unidirectionally to the Ho3C ion. It may be a reason why the cooperative upconversion luminescence was thought as kind of very small upconversion fluorescence before. This implies that the impurities mixed during the manufacture procedure would serious reduce the cooperative upconversion luminescence intensity due to crossrelaxation. It is significant also for this article to find that pure Yb3C ion-doped sample, which has as little impurity as possible to reduce cross-relaxation, could achieve higher cooperative upconversion luminescence of the Yb3C–Yb3C clusters. In conclusion, the large enhancement on cooperative upconversion blue luminescence, that is the cooperative upconversion blue luminescence strength of Yb(5):FOV is 230 times greater than that of fluoride glass Yb(3):ZBLAN, results from the comprehensive improvement on the aspects of better coupled chance of the Yb3C–Yb3C cluster, less cross-relaxation, better concentration contribution of Yb3C activator, non-saturation, and better upconversion luminescence efficiency.
Acknowledgements The project is supported by the National Natural Science Foundation of China (Grant No. 19874033 and 10174008) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars of Chinese State Education Ministry. The authors thank so much for Prof Ou Wen, Prof Qing Cai and Prof Rongsheng Chen at Beijing Glass
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Research Institute and Prof N. Sawanobori at Sumita Optical Glass Inc. of Japan for providing samples.
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