- Email: [email protected]
Ultraviolet upconversion luminescence in Er3+-doped Y2O3 excited by 532 nm CW compact solid-state laser
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1137–1139
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Ultraviolet upconversion luminescence in Er3+-doped Y2O3 excited by 532 nm CW compact solid-state laser Feng Qin a, Yangdong Zheng a, Ying Yu a, Changbin Zheng a, Huijuan Liang a, Zhiguo Zhang a,, Lingling Xu b a b
Department of Physics, Harbin Institute of Technology, 150001 Harbin, People’s Republic of China Department of Physics, Harbin Normal University, 150080 Harbin, People’s Republic of China
a r t i c l e in fo
abstract
Article history: Received 9 December 2008 Received in revised form 26 April 2009 Accepted 6 May 2009 Available online 15 May 2009
Ultraviolet upconversion emissions at 262, 276, 308 and 320 nm were observed from Er3+-doped Y2O3 with a 532 nm continuous wave compact solid-state laser excitation. Power-dependence analysis demonstrates that two-photon upconversion process populates the 4D5/2, 2H9/2 and 2P3/2 states. The energy transfer upconversion (ETU) plays an important role in populating 4D5/2 and 2P3/2 states. It appears that 2P3/2 state population originates from ETU 2H11/2+2H11/2-4I13/2+2P3/2, moreover, a subsequent excited state absorption (ESA) from the 4I9/2 level. & 2009 Elsevier B.V. All rights reserved.
Keywords: Upconversion UV emission Er3+
1. Introduction Energy upconversion phenomena have garnered significant attention in the last few decades [1,2], and intense interest in upconversion studies results from wide potential applications of short-wavelength solid-state lasers, such as high density optical data storage, high-resolution printing, medical diagnostics, color displays and environmental monitoring [3–6]. Er3+ ion has rich energy levels in infrared (IR)–ultraviolet (UV) range that makes it more suitable for upconversion studies. Recently, upconversion luminescence in Er3+-doped host materials were studied extensively, resulting in the development of upconversion laser in visible region in Er3+-doped CaF2, LiYF4 (YLF), Y3Al5O12 (YAG), YAlO3 (YAP) and fluorozirconate fibers (ZBLAN) [7–11]. Currently, most upconversion studies in Er3+-doped materials were mainly focused on infrared and visible regions, and only a few papers were devoted to the studies of UV and violet upconversion fluorescence. Excited by IR diode laser, argon laser and pulse dye laser, UV upconversion emissions were observed in Er3+-doped host material [5,12–17]. Low cost IR diode laser has low photon energy, therefore, to produce one UV photon, the trivalent Erbium ion has to absorb at least three IR photons. That is to say, IR diode laser is low efficient for UV upconversion emission most of the time. Thus, shorter wavelength visible laser excitation is expected to play an important role for obtaining UV upconversion. Meanwhile, the high cost, complex structures and
Corresponding author. Tel.: +8645186402639; fax: +8645186412828.
E-mail address: [email protected] (Z. Zhang). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.05.006
large volume of traditional argon laser and pulsed dye laser greatly restrict the development of such lasers in practical application. Recently, high power and cost-effective commercial compact 532 nm continuous wave (CW) solid-state laser is readily available; therefore the cost of UV laser instrumentation can be greatly reduced. Here we report, for the first time to our knowledge, on the generation of blue, violet and ultraviolet upconversion luminescence in the range 250–480 nm excited by the laser in Y2O3:Er3+. Yttrium oxide, as a promising upconverted host material, has low phonon energy (phonon cut off about 600 cm 1) and excellent physical properties (high melting point, phase stability and low thermal expansion) [17–19]. Thus, relatively low nonradiative relaxation rate for rare-earth ions in Y2O3 due to low energy of phonon can greatly enhance the upconversion emission. Furthermore, compared with ubiquitous and popular laser medium YAG (Y3Al5O12) and YAP (YAlO3), Y2O3 has higher thermal conductivity, similar thermal expansion coefficient and much broader transparency range from 0.23 to 8 mm [17,20]. Meanwhile, a broad fluorescent spectrum ranging from UV to IR can be produced by intra-4f transitions of Er3+ ions. Consequently, Er3+-ion-doped Y2O3 constitutes one of the most efficient UC systems.
2. Results and discussion In present work, yttrium (III) oxide (Y2O3) powder doped with 3 mol% Er3+ was prepared by simple sol–gel method [21]. Subsequently, the nanocrystal was pressed into pellet under 10 t of pressure, and then fired in air at 1300 1C for 32 h to form bulk
ARTICLE IN PRESS 1138
F. Qin et al. / Journal of Luminescence 129 (2009) 1137–1139
sample. The photomultiplier-tube-equipped monochromator (Zolix SBP300) with a spectral resolution of 3 nm was used to measure upconversion luminescence spectra of the sample excited by 532 nm laser (CLO Laser DPGL-500L). To detect the upconversion luminescence efficiently, spectrum in the range 240–340 nm (Fig. 1a) was detected by UV photomultiplier tube (PMT, Hamamatsu R7154) and spectrum in the region of 300–500 nm (Fig. 1b) was detected by photomultiplier tube (Zolix CR131) instead. When the sample was pumped by the 532 nm laser that is resonant with the transitions of 4I15/2-4S3/2, 2H11/2, resulting upconversion signals were analyzed by Zolix SBP300 as shown in Fig. 1. Actually the peak at 320 nm is much weaker than at 325 nm in Fig. 1a due to the spectral response characteristics of PMT R7154, which can be confirmed in Fig. 1b. Strong emissions at 320, 409 and 475 nm can be easily assigned to the transitions from 2P3/ 4 2 state to the ground state I15/2 and the first two excited states 4 I13/2 and 4I11/2 [5]. Emissions at 276 and 340 nm correspond to the transitions 2H9/2-4I15/2 and 2H9/2-4I13/2 of the Er3+ ions [15]. Moreover, the luminescence around 262 and 308 nm can only be assigned to the transitions from 4D5/2 to 4I15/2 and 4I13/2. The energy-level diagram of Er3+ ions and corresponding transitions are shown in Fig. 2 [22]. Upconversion emissions from 4D5/2 state were detected distinctly for the first time, and such emission
→ 4I15/2
400
2P
2H 9/2
250
150
D5/2 → 4I13/2
→ 4I15/2
200
5/2 4D
100
4
Intensity (a.u.)
300
3/2
→ 4I15/2
350
50 0 240
260
280 300 Wavelength (nm)
320
340
15/2
2
2
G11/2 → 4I15/2
4F
7/2 →
4I
4
H9/2 → 4I11/2 2
4I 2H
2
50
9/2 →
100
13/2
150 P3/2 → 4I15/2
Intensity (a.u.)
200
P3/2 → 4I11/2
250
G9/2 → 4I15/2
4 2P 3/2 → I13/2
300
0
x10
x5
x1 350
400 Wavelength (nm)
provides us a further understanding about the characters of high excited states of trivalent erbium ion. To understand the energy upconversion phenomena well, intensity dependence of upconverted luminescence on laser energy was measured and the results are shown in Fig. 3 in double logarithmic scale. A plot of signal intensity versus laser power exhibited a slope of 1.8, 1.5, 1.6 and 1.6 for the 262, 276, 409 and 320 nm bands, respectively. Thus, we can propose that two pump laser photons are involved in populating 4D5/2, 2H9/2 and 2 P3/2 states. Meanwhile, the same slope for both 409 and 320 nm peaks suggest the same way to populate corresponding upper levels. Therefore, we get a conclusion that both of the two emission bands are origin from 2P3/2 state. The population for high energy levels by low energy photon pumping is well known and usually occurs by two independent processes, energy transfer upconversion (ETU) and excited state absorption (ESA). ESA process involves in a single ion and thus it is the main possible upconversion process that occurs in the materials with low dopant concentrations, while ETU involves in two ions and will be dominant in the materials with high doping concentrations due to shortening the average distance between dopant ions and enhancing the interionic interaction [17]. In the experiment, laser brings Er3+ ions into thermalized levels (4S3/2, 2 H11/2) and simultaneously with the help of phonons both of the two states (4S3/2 and 2H11/2) are populated. And then one ion in 2 H11/2 state returns to 4I15/2 state and immediately transfers its energy to the neighboring ion in the same state and excites it to the upper 4D5/2 state. The ETU process can be expressed as: 2H11/ 2 4 4 2+ H11/2- I15/2+ D5/2. Similarly, another four possible ETU processes lined out in Fig. 2, 2H11/2+2H11/2-4I13/2+2P3/2, 2H11/2+2H11/ 4 4 4 S3/2+4S3/2-4I15/2+2H9/2, 4S3/2+4S3/2-4I9/2+2G9/2, 2- I9/2+ G11/2, can also take place due to those four pairs of energy level gaps matching well, which populate 2P3/2, 4G11/2, 2H9/2 and 2G9/2 states, respectively. Moreover, the ions in 4G11/2 state also populate 2G9/2 state by rapid nonradiative relaxation, which can explain the strong emission at 419 nm from 2G9/2 state to the ground state 4 I15/2. The weak emission from 4D5/2, 2H9/2, 4G11/2 and 2G9/2 states suggests that the efficiency of ETU is not as high as ESA. The luminescence from 4D5/2 and 4G11/2 states exhibit still weaker than that from 2H9/2 and 2G9/2 states, which indicates that ETU process originates from 2H11/2 state (2H11/2+2H11/2-4I13/2+2P3/2 and 2H11/2+2H11/2-4I9/2+4G11/2) is rather less efficient than that originates from 4S3/2 state (4S3/2+4S3/2-4I15/2+2H9/2 and 4S3/2+4S3/ 4 2 2 2- I9/2+ G9/2). However, strong emission from P3/2 state implies that there must be other mechanism populating 2P3/2 state in addition to the process of ETU (2H11/2+2H11/2-4I13/2+2P3/2) originates from 2H11/2 state. It happened that the energy level gap between 2P3/2 and 4I9/2 states is consistent with the energy of a pumping photon. Consequently, the population of 4I9/2 state due to energy transfer populating 2G9/2 and 4G11/2 states enables the ESA process from 4I9/2 to 2P3/2, which provides another way for populating 2P3/2 state and therefore the strong emissions at 320, 409 and 475 nm from the state of 2P3/2 can thus be explained. Here we provide a new method to populate 2P3/2 state efficiently. However, to understand the mechanism well, more detailed investigation should be done in future works. Moreover, a weak emission at 490 nm corresponding to the transition of 4F7/2-4I15/2 is also observed. Proposed upconversion mechanisms to produce UV fluorescent radiations are shown in Fig. 2.
450
500
Fig. 1. Measured upconverted fluorescent spectrum in the wavelength range of 240–340 nm (a) and 300–500 nm (b) in Y2O3 powders doped with 3 mol% Er3+.
3. Conclusion Ultraviolet and violet upconversion luminescence has been studied under 532 nm CW laser excitation in Y2O3:Er3+. Based on
ARTICLE IN PRESS F. Qin et al. / Journal of Luminescence 129 (2009) 1137–1139
1139
4D 7/2 4 D5/2 2H 9/2
2P 3/2
409 475
340
532
3x104
308
4x104
4
G11/2 G9/2
Energy (cm-1)
2
4F 7/2 2
2x104
H11/2 4S 3/2 4 F9/2
1x104
4
I9/2
4
I11/2
0 Er3+
390nm 419nm 490nm
320nm
276nm
262nm
532nm
4I 13/2
4I 15/2
Er3+
Fig. 2. Energy level diagrams of the Er3+ ions as well as the proposed UC mechanism to produce the UV emission bands.
Intensity (a.u.)
105
262 nm 276 nm 320 nm 409 nm
electrons and lattice vibrations. Therefore, similar experiments can be conducted in Er3+-doped transparent materials and lowphonon host materials for high efficient UV upconversion in future work.
Slope = 1.6 ± 0.2 Slope = 1.6 ± 0.2
Acknowledgements
104 This work was supported by the 863 Hi-Tech Research and Development Program of the People’s Republic of China.
Slope = 1.5 ± 0.2 103
References
Slope = 1.8 ± 0.2 102 250
300 350 Pump power (mW)
400
450
Fig. 3. (Color online) Pump power dependence of the fluorescent bands centered at 262, 276, 320 and 409 nm. The diagram is in a double logarithmic scale.
the detailed discussion and arguments developed above, possible upconversion mechanisms were proposed. The ETU processes are responsible for the population of 4D5/2, 2H9/2, 4G11/2 and 2G9/2 states. Additionally, for the strong emission from 2P3/2 state, besides the ETU process (2H11/2+2H11/2-4I13/2+2P3/2), we believe there is a strong ESA process (4I9/2-2P3/2) populating such state. Our results demonstrate that the 532 nm CW compact solid-state laser is a high efficient pumping source and it is a more suitable choice for pumping UV all-solid-state lasers. However, it should be noted that no UV emission in the range 240–340 nm has previously been reported in Y2O3:Er3+, which may be explained by the fact that Er3+ ion concentrations as high as 3 mol% were used in our experiment, thus inducing strong quenching of the 2H11/2 and 4S3/2 states. As is known, the 4f electrons of rare-earth ions have localized states and exhibit weak coupling to ligand
[1] M. Takahashi, M. Shojiya, R. Kanno, Y. Kawamoto, K. Kadono, T. Ohtsuki, Peyghambarian, J. Appl. Phys. 81 (1997) 2940. [2] M. Tsuda, K. Soga, H. Inoue, S. Inoue, A. Makishima, J. Appl. Phys. 85 (1999) 29. ˜ oz-Yagu¨e, Phys. Rev. [3] X. Zhang, C. Serrano, E. Daran, F. Lahoz, G. Lacoste, A. Mun B 62 (2000) 4446. [4] W. Lu¨thy, H.P. Weber, Infrared Phys. 32 (1991) 283. [5] C.L. Pope, B.R. Reddy, S.K. Nash-Stevenson, Opt. Lett. 22 (1997) 295. [6] N.N. Zu, H.G. Yang, Z.W. Dai, Physica B 403 (2008) 174. [7] S.A. Pollack, D.B. Chang, N.L. Moise, J. Appl. Phys. 60 (1986) 4077. [8] P. Xie, S.C. Rand, Opt. Lett. 17 (1992) 1198. [9] W.Q. Shi, M. Bass, M. Birnbaum, J. Opt. Soc. Am. B 7 (1990) 1456. [10] A.J. Silversmith, W. Lenth, R.M. Macfarlane, Appl. Phys. Lett. 51 (1987) 1977. [11] S. Georgescu, O. Toma, SPIE 5581 (2004) 245. [12] H. Xu, Z. Dai, Z. Jiang, Eur. Phys. J. D 17 (2001) 79. [13] B.R. Reddy, S.K. Nash-Stevenson, J. Appl. Phys. 76 (1994) 3896. [14] S. Georgescu, V. Lupei, A. Petraru, C. Hapenciuc, C. Florea, C. Naud, C. Porte, J. Lumin. 93 (2001) 281. [15] D.N. Patel, R.B. Reddy, S.K. Nash-Stevenson, Appl. Opt. 37 (1998) 7805. [16] H.L. Xu, Z.K. Jiang, Phys. Rev. B 66 (2002) 35103. [17] X. Wang, G.Y. Shan, K. Chao, Y.L. Zhang, R. Liu, L.Y. Feng, Q.H. Zeng, Y.J. Sun, Y.C. Liu, X.G. Kong, Mater. Chem. Phys. 99 (2006) 370. [18] J.A. Capobianco, J.C. Boyer, F. Vetrone, A. Speghini, M. Bettinelli, Chem. Mater. 14 (2002) 2915. [19] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem. B 106 (2002) 5622. [20] Y. Guyot, R. Moncoge´, L.D. Merkle, A. Pinto, B. Mclntosh, H. Verdun, Opt. Mater. 5 (1996) 127. [21] G.Y. Chen, Y.G. Zhang, G. Somesfalean, Z.G. Zhang, Q. Sun, F.P. Wang, Appl. Phys. Lett. 89 (2006) 163105. [22] J.R. Quagliano, M.F. Reid, F.S. Richardson, M.E. Hills, M.D. Seltzer, S.B. Stevens, C.A. Morrison, T.H. Allik, Phys. Rev. B 48 (1993) 15561.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Ultraviolet upconversion luminescence in Er3+-doped Y2O3 excited by 532 nm CW compact solid-state laser Feng Qin a, Yangdong Zheng a, Ying Yu a, Changbin Zheng a, Huijuan Liang a, Zhiguo Zhang a,, Lingling Xu b a b
Department of Physics, Harbin Institute of Technology, 150001 Harbin, People’s Republic of China Department of Physics, Harbin Normal University, 150080 Harbin, People’s Republic of China
a r t i c l e in fo
abstract
Article history: Received 9 December 2008 Received in revised form 26 April 2009 Accepted 6 May 2009 Available online 15 May 2009
Ultraviolet upconversion emissions at 262, 276, 308 and 320 nm were observed from Er3+-doped Y2O3 with a 532 nm continuous wave compact solid-state laser excitation. Power-dependence analysis demonstrates that two-photon upconversion process populates the 4D5/2, 2H9/2 and 2P3/2 states. The energy transfer upconversion (ETU) plays an important role in populating 4D5/2 and 2P3/2 states. It appears that 2P3/2 state population originates from ETU 2H11/2+2H11/2-4I13/2+2P3/2, moreover, a subsequent excited state absorption (ESA) from the 4I9/2 level. & 2009 Elsevier B.V. All rights reserved.
Keywords: Upconversion UV emission Er3+
1. Introduction Energy upconversion phenomena have garnered significant attention in the last few decades [1,2], and intense interest in upconversion studies results from wide potential applications of short-wavelength solid-state lasers, such as high density optical data storage, high-resolution printing, medical diagnostics, color displays and environmental monitoring [3–6]. Er3+ ion has rich energy levels in infrared (IR)–ultraviolet (UV) range that makes it more suitable for upconversion studies. Recently, upconversion luminescence in Er3+-doped host materials were studied extensively, resulting in the development of upconversion laser in visible region in Er3+-doped CaF2, LiYF4 (YLF), Y3Al5O12 (YAG), YAlO3 (YAP) and fluorozirconate fibers (ZBLAN) [7–11]. Currently, most upconversion studies in Er3+-doped materials were mainly focused on infrared and visible regions, and only a few papers were devoted to the studies of UV and violet upconversion fluorescence. Excited by IR diode laser, argon laser and pulse dye laser, UV upconversion emissions were observed in Er3+-doped host material [5,12–17]. Low cost IR diode laser has low photon energy, therefore, to produce one UV photon, the trivalent Erbium ion has to absorb at least three IR photons. That is to say, IR diode laser is low efficient for UV upconversion emission most of the time. Thus, shorter wavelength visible laser excitation is expected to play an important role for obtaining UV upconversion. Meanwhile, the high cost, complex structures and
Corresponding author. Tel.: +8645186402639; fax: +8645186412828.
E-mail address: [email protected] (Z. Zhang). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.05.006
large volume of traditional argon laser and pulsed dye laser greatly restrict the development of such lasers in practical application. Recently, high power and cost-effective commercial compact 532 nm continuous wave (CW) solid-state laser is readily available; therefore the cost of UV laser instrumentation can be greatly reduced. Here we report, for the first time to our knowledge, on the generation of blue, violet and ultraviolet upconversion luminescence in the range 250–480 nm excited by the laser in Y2O3:Er3+. Yttrium oxide, as a promising upconverted host material, has low phonon energy (phonon cut off about 600 cm 1) and excellent physical properties (high melting point, phase stability and low thermal expansion) [17–19]. Thus, relatively low nonradiative relaxation rate for rare-earth ions in Y2O3 due to low energy of phonon can greatly enhance the upconversion emission. Furthermore, compared with ubiquitous and popular laser medium YAG (Y3Al5O12) and YAP (YAlO3), Y2O3 has higher thermal conductivity, similar thermal expansion coefficient and much broader transparency range from 0.23 to 8 mm [17,20]. Meanwhile, a broad fluorescent spectrum ranging from UV to IR can be produced by intra-4f transitions of Er3+ ions. Consequently, Er3+-ion-doped Y2O3 constitutes one of the most efficient UC systems.
2. Results and discussion In present work, yttrium (III) oxide (Y2O3) powder doped with 3 mol% Er3+ was prepared by simple sol–gel method [21]. Subsequently, the nanocrystal was pressed into pellet under 10 t of pressure, and then fired in air at 1300 1C for 32 h to form bulk
ARTICLE IN PRESS 1138
F. Qin et al. / Journal of Luminescence 129 (2009) 1137–1139
sample. The photomultiplier-tube-equipped monochromator (Zolix SBP300) with a spectral resolution of 3 nm was used to measure upconversion luminescence spectra of the sample excited by 532 nm laser (CLO Laser DPGL-500L). To detect the upconversion luminescence efficiently, spectrum in the range 240–340 nm (Fig. 1a) was detected by UV photomultiplier tube (PMT, Hamamatsu R7154) and spectrum in the region of 300–500 nm (Fig. 1b) was detected by photomultiplier tube (Zolix CR131) instead. When the sample was pumped by the 532 nm laser that is resonant with the transitions of 4I15/2-4S3/2, 2H11/2, resulting upconversion signals were analyzed by Zolix SBP300 as shown in Fig. 1. Actually the peak at 320 nm is much weaker than at 325 nm in Fig. 1a due to the spectral response characteristics of PMT R7154, which can be confirmed in Fig. 1b. Strong emissions at 320, 409 and 475 nm can be easily assigned to the transitions from 2P3/ 4 2 state to the ground state I15/2 and the first two excited states 4 I13/2 and 4I11/2 [5]. Emissions at 276 and 340 nm correspond to the transitions 2H9/2-4I15/2 and 2H9/2-4I13/2 of the Er3+ ions [15]. Moreover, the luminescence around 262 and 308 nm can only be assigned to the transitions from 4D5/2 to 4I15/2 and 4I13/2. The energy-level diagram of Er3+ ions and corresponding transitions are shown in Fig. 2 [22]. Upconversion emissions from 4D5/2 state were detected distinctly for the first time, and such emission
→ 4I15/2
400
2P
2H 9/2
250
150
D5/2 → 4I13/2
→ 4I15/2
200
5/2 4D
100
4
Intensity (a.u.)
300
3/2
→ 4I15/2
350
50 0 240
260
280 300 Wavelength (nm)
320
340
15/2
2
2
G11/2 → 4I15/2
4F
7/2 →
4I
4
H9/2 → 4I11/2 2
4I 2H
2
50
9/2 →
100
13/2
150 P3/2 → 4I15/2
Intensity (a.u.)
200
P3/2 → 4I11/2
250
G9/2 → 4I15/2
4 2P 3/2 → I13/2
300
0
x10
x5
x1 350
400 Wavelength (nm)
provides us a further understanding about the characters of high excited states of trivalent erbium ion. To understand the energy upconversion phenomena well, intensity dependence of upconverted luminescence on laser energy was measured and the results are shown in Fig. 3 in double logarithmic scale. A plot of signal intensity versus laser power exhibited a slope of 1.8, 1.5, 1.6 and 1.6 for the 262, 276, 409 and 320 nm bands, respectively. Thus, we can propose that two pump laser photons are involved in populating 4D5/2, 2H9/2 and 2 P3/2 states. Meanwhile, the same slope for both 409 and 320 nm peaks suggest the same way to populate corresponding upper levels. Therefore, we get a conclusion that both of the two emission bands are origin from 2P3/2 state. The population for high energy levels by low energy photon pumping is well known and usually occurs by two independent processes, energy transfer upconversion (ETU) and excited state absorption (ESA). ESA process involves in a single ion and thus it is the main possible upconversion process that occurs in the materials with low dopant concentrations, while ETU involves in two ions and will be dominant in the materials with high doping concentrations due to shortening the average distance between dopant ions and enhancing the interionic interaction [17]. In the experiment, laser brings Er3+ ions into thermalized levels (4S3/2, 2 H11/2) and simultaneously with the help of phonons both of the two states (4S3/2 and 2H11/2) are populated. And then one ion in 2 H11/2 state returns to 4I15/2 state and immediately transfers its energy to the neighboring ion in the same state and excites it to the upper 4D5/2 state. The ETU process can be expressed as: 2H11/ 2 4 4 2+ H11/2- I15/2+ D5/2. Similarly, another four possible ETU processes lined out in Fig. 2, 2H11/2+2H11/2-4I13/2+2P3/2, 2H11/2+2H11/ 4 4 4 S3/2+4S3/2-4I15/2+2H9/2, 4S3/2+4S3/2-4I9/2+2G9/2, 2- I9/2+ G11/2, can also take place due to those four pairs of energy level gaps matching well, which populate 2P3/2, 4G11/2, 2H9/2 and 2G9/2 states, respectively. Moreover, the ions in 4G11/2 state also populate 2G9/2 state by rapid nonradiative relaxation, which can explain the strong emission at 419 nm from 2G9/2 state to the ground state 4 I15/2. The weak emission from 4D5/2, 2H9/2, 4G11/2 and 2G9/2 states suggests that the efficiency of ETU is not as high as ESA. The luminescence from 4D5/2 and 4G11/2 states exhibit still weaker than that from 2H9/2 and 2G9/2 states, which indicates that ETU process originates from 2H11/2 state (2H11/2+2H11/2-4I13/2+2P3/2 and 2H11/2+2H11/2-4I9/2+4G11/2) is rather less efficient than that originates from 4S3/2 state (4S3/2+4S3/2-4I15/2+2H9/2 and 4S3/2+4S3/ 4 2 2 2- I9/2+ G9/2). However, strong emission from P3/2 state implies that there must be other mechanism populating 2P3/2 state in addition to the process of ETU (2H11/2+2H11/2-4I13/2+2P3/2) originates from 2H11/2 state. It happened that the energy level gap between 2P3/2 and 4I9/2 states is consistent with the energy of a pumping photon. Consequently, the population of 4I9/2 state due to energy transfer populating 2G9/2 and 4G11/2 states enables the ESA process from 4I9/2 to 2P3/2, which provides another way for populating 2P3/2 state and therefore the strong emissions at 320, 409 and 475 nm from the state of 2P3/2 can thus be explained. Here we provide a new method to populate 2P3/2 state efficiently. However, to understand the mechanism well, more detailed investigation should be done in future works. Moreover, a weak emission at 490 nm corresponding to the transition of 4F7/2-4I15/2 is also observed. Proposed upconversion mechanisms to produce UV fluorescent radiations are shown in Fig. 2.
450
500
Fig. 1. Measured upconverted fluorescent spectrum in the wavelength range of 240–340 nm (a) and 300–500 nm (b) in Y2O3 powders doped with 3 mol% Er3+.
3. Conclusion Ultraviolet and violet upconversion luminescence has been studied under 532 nm CW laser excitation in Y2O3:Er3+. Based on
ARTICLE IN PRESS F. Qin et al. / Journal of Luminescence 129 (2009) 1137–1139
1139
4D 7/2 4 D5/2 2H 9/2
2P 3/2
409 475
340
532
3x104
308
4x104
4
G11/2 G9/2
Energy (cm-1)
2
4F 7/2 2
2x104
H11/2 4S 3/2 4 F9/2
1x104
4
I9/2
4
I11/2
0 Er3+
390nm 419nm 490nm
320nm
276nm
262nm
532nm
4I 13/2
4I 15/2
Er3+
Fig. 2. Energy level diagrams of the Er3+ ions as well as the proposed UC mechanism to produce the UV emission bands.
Intensity (a.u.)
105
262 nm 276 nm 320 nm 409 nm
electrons and lattice vibrations. Therefore, similar experiments can be conducted in Er3+-doped transparent materials and lowphonon host materials for high efficient UV upconversion in future work.
Slope = 1.6 ± 0.2 Slope = 1.6 ± 0.2
Acknowledgements
104 This work was supported by the 863 Hi-Tech Research and Development Program of the People’s Republic of China.
Slope = 1.5 ± 0.2 103
References
Slope = 1.8 ± 0.2 102 250
300 350 Pump power (mW)
400
450
Fig. 3. (Color online) Pump power dependence of the fluorescent bands centered at 262, 276, 320 and 409 nm. The diagram is in a double logarithmic scale.
the detailed discussion and arguments developed above, possible upconversion mechanisms were proposed. The ETU processes are responsible for the population of 4D5/2, 2H9/2, 4G11/2 and 2G9/2 states. Additionally, for the strong emission from 2P3/2 state, besides the ETU process (2H11/2+2H11/2-4I13/2+2P3/2), we believe there is a strong ESA process (4I9/2-2P3/2) populating such state. Our results demonstrate that the 532 nm CW compact solid-state laser is a high efficient pumping source and it is a more suitable choice for pumping UV all-solid-state lasers. However, it should be noted that no UV emission in the range 240–340 nm has previously been reported in Y2O3:Er3+, which may be explained by the fact that Er3+ ion concentrations as high as 3 mol% were used in our experiment, thus inducing strong quenching of the 2H11/2 and 4S3/2 states. As is known, the 4f electrons of rare-earth ions have localized states and exhibit weak coupling to ligand
[1] M. Takahashi, M. Shojiya, R. Kanno, Y. Kawamoto, K. Kadono, T. Ohtsuki, Peyghambarian, J. Appl. Phys. 81 (1997) 2940. [2] M. Tsuda, K. Soga, H. Inoue, S. Inoue, A. Makishima, J. Appl. Phys. 85 (1999) 29. ˜ oz-Yagu¨e, Phys. Rev. [3] X. Zhang, C. Serrano, E. Daran, F. Lahoz, G. Lacoste, A. Mun B 62 (2000) 4446. [4] W. Lu¨thy, H.P. Weber, Infrared Phys. 32 (1991) 283. [5] C.L. Pope, B.R. Reddy, S.K. Nash-Stevenson, Opt. Lett. 22 (1997) 295. [6] N.N. Zu, H.G. Yang, Z.W. Dai, Physica B 403 (2008) 174. [7] S.A. Pollack, D.B. Chang, N.L. Moise, J. Appl. Phys. 60 (1986) 4077. [8] P. Xie, S.C. Rand, Opt. Lett. 17 (1992) 1198. [9] W.Q. Shi, M. Bass, M. Birnbaum, J. Opt. Soc. Am. B 7 (1990) 1456. [10] A.J. Silversmith, W. Lenth, R.M. Macfarlane, Appl. Phys. Lett. 51 (1987) 1977. [11] S. Georgescu, O. Toma, SPIE 5581 (2004) 245. [12] H. Xu, Z. Dai, Z. Jiang, Eur. Phys. J. D 17 (2001) 79. [13] B.R. Reddy, S.K. Nash-Stevenson, J. Appl. Phys. 76 (1994) 3896. [14] S. Georgescu, V. Lupei, A. Petraru, C. Hapenciuc, C. Florea, C. Naud, C. Porte, J. Lumin. 93 (2001) 281. [15] D.N. Patel, R.B. Reddy, S.K. Nash-Stevenson, Appl. Opt. 37 (1998) 7805. [16] H.L. Xu, Z.K. Jiang, Phys. Rev. B 66 (2002) 35103. [17] X. Wang, G.Y. Shan, K. Chao, Y.L. Zhang, R. Liu, L.Y. Feng, Q.H. Zeng, Y.J. Sun, Y.C. Liu, X.G. Kong, Mater. Chem. Phys. 99 (2006) 370. [18] J.A. Capobianco, J.C. Boyer, F. Vetrone, A. Speghini, M. Bettinelli, Chem. Mater. 14 (2002) 2915. [19] F. Vetrone, J.C. Boyer, J.A. Capobianco, A. Speghini, M. Bettinelli, J. Phys. Chem. B 106 (2002) 5622. [20] Y. Guyot, R. Moncoge´, L.D. Merkle, A. Pinto, B. Mclntosh, H. Verdun, Opt. Mater. 5 (1996) 127. [21] G.Y. Chen, Y.G. Zhang, G. Somesfalean, Z.G. Zhang, Q. Sun, F.P. Wang, Appl. Phys. Lett. 89 (2006) 163105. [22] J.R. Quagliano, M.F. Reid, F.S. Richardson, M.E. Hills, M.D. Seltzer, S.B. Stevens, C.A. Morrison, T.H. Allik, Phys. Rev. B 48 (1993) 15561.