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Up-converting Yb3+–Er3+ co-doped amorphous fluoride thin films prepared by pulsed-laser deposition for visible light source
PERGAMON
Solid State Communications 120 (2001) 211±214
www.elsevier.com/locate/ssc
Up-converting Yb 31 ±Er 31 co-doped amorphous ¯uoride thin ®lms prepared by pulsed-laser deposition for visible light source Guanshi Qin a,b,*, Weiping Qin a,b, Shihua Huang a,b, Changfeng Wu a,b, Baojiu Chen a,b, Shaozhe Lu a,b, E. Shulin a,b a
b
Laboratory of excited states processes, Chinese Academy of Sciences, Changchun 130021, People's Republic of China Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130021, People's Republic of China Received 9 July 2001; accepted 30 August 2001 by E.L. Ivchenko
Abstract Yb 31 ±Er 31 co-doped amorphous Zn0.3Al0.25Pb0.3Li0.12xYbyErxF2.0512x13y ®lms have been prepared by pulsed-laser deposition (PLD). Violet, green, red and ultraviolet (UV) up-conversion emissions were observed under the infrared excitation at 950 nm. In comparison with the up-conversion of the target, the violet and UV emissions are enhanced greatly. The enhancement is attributed to the enrichment of Yb concentration in the sample during PLD. q 2001 Published by Elsevier Science Ltd. PACS: 78.55; 81.15I Keywords: A. Thin ®lms; C. Scanning and transmission electron microscopy; E. Luminescence
1. Introduction Some rare earth doped crystals or glasses have demonstrated up-converted luminescence in the visible and ultraviolet (UV) region while they were pumped at wavelengths accessible to semiconductor lasers [1,2]. Up-conversion light sources made from bulk crystals and ®bers are quite attractive; however, high costs associated with the material preparation and the device packaging may limit their applications. Films, on the other hand, combine the advantages of bulk material and the compactness of ®ber, offering a kind of low cost up-conversion devices. To deposit and fabricate up-converted wave-guides on semiconductor substrates is possible, that would allow to directly integrate on a same chip the up-conversion device with the pump source and other optoelectronic components [3,4]. PLD is a recently developed technique, which is compatible with deposition in gas environments and has been * Corresponding author. Address: Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130021, People's Republic of China. Tel.: 186-431593-7564; fax: 186-431-595-5378. E-mail addresses: [email protected] (G. Qin), [email protected] (G. Qin).
used successfully to grow a wide variety of multi-component thin ®lms [5]. Glasses are suitable for preparing active wave-guides, however, studies devoted to amorphous ®lms grown by PLD were quite limited [6±8]. We reported in this paper the up-conversion emission properties of the Yb 31 ±Er 31 co-doped Zn0.3Al0.25Pb0.3Li0.12xYbyErxF2.0512x13y (ZAPL) ®lms. With the excitation of a 950 nm-laser-diode (LD), violet, green, red and UV emissions were observed. The mechanism responsible for the enhancement of the violet and UV emission was discussed. 2. Results and discussion 953.6 nm, from a Raman shifter pumped by the second harmonic of an Nd:YAG pulsed laser (pulse width 10 ns, repetition rate 10 Hz) was used as the light source for PLD. The Stokes-shifted line with the peak power of 10 4 W was focused onto the target [9] (amorphous ZAPL with x 0:01; y 0:1) in air at an incidence angle of about 458. With such low power, PLD was produced mainly by the intense absorption of Yb 31 ions in the target. The substrate (SiO2 glass or GaAs slice) was hold at 10±20 mm in front of the target at room temperature. During the deposition, orange light emission-laser ablation was observed. The
0038-1098/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0038-109 8(01)00380-5
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G. Qin et al. / Solid State Communications 120 (2001) 211±214
Fig. 1. Up-conversion emission spectra of Zn0.3Al0.25Pb0.3Li0.09Yb0.1Er0.01F2.37, (a) the starting bulk material; (b) the PLD ®lm.
®lm is amorphous and the particle size is about 0.1±1 mm measured with a transmission electron microscope (JEM2010). The up-converted emission spectra were measured under a 950 nm LD excitation and were detected with a Fluorescence Spectrophotometer (Hitachi F-4500). Fig. 1(b) represents the up-conversion emission spectrum of the ®lm. Similar results were obtained for the ®lm deposited on the GaAs substrate. The emissions in the UV and visible range correspond to the following transitions: 4 G11=2 ! 4 I15=2 (,380 nm); 2 H9=2 ! 4 I15=2 (,408 nm); 4 F3=2;5=2 ! 4 I15=2 (,452 nm); 4 G11=2 ! 4 I13=2 (,504 nm); 2 H11=2 ! 4 I15=2 (,520 nm); 4 S3=2 ! 4 I15=2 (,550 nm); and 4 F9=2 ! 4 I15=2 (,650 nm). For unsaturated up-conversion, emission intensity, Is, is proportional to I n, where I is the intensity of the excitation light and the integer n is the number of photons absorbed per up-converted photon emitted [10]. Intensity dependence of
the up-conversion emissions are shown in Fig. 2: n 3:02 for the emission at 408 nm; n 1:91 for the 520 and 540 bands; and n 2:01 for the 652 nm emission. In Yb 31 ±Er 31 co-doped systems, different processes may result in up-conversion [11]. These processes include multistep excited state absorption (ESA), energy transfer (ET) between excited neighboring Er 31 ions and APTE (Addition de photons par transfer d'E4nergie) between Yb 31 and Er 31. Among these processes, APTE is the most ef®cient. Infrared to green ( 2H11/2, 4S3/2 ! 4I15/2 transitions) and to red (4 F9=2 ! 4 I15=2 transition) up-converted emissions in Yb 31 ±Er 31 codoped systems have been widely investigated [12]. Here, we focus our attention on the UV (,380 nm) and the violet (,408 nm) emissions. As shown in Fig. 3, for the violet emission, there are two processes in populating the state 2H9/2: (1) Energy transfer 2 F5=2 ! 2 F7=2 (Yb 31): 4 F9=2 ! 2 H9=2 (Er 31) (ET1); and (2)
Fig. 2. The dependence of the up-conversion luminescence intensity on the pump power for the red green and violet emissions.
G. Qin et al. / Solid State Communications 120 (2001) 211±214
213
Fig. 3. The schematic diagram of Yb 31-sensitized Er 31 up-conversion in Zn0.3Al0.25Pb0.3Li0.09Yb0.1Er0.01F2.37 ®lm under 950 nm excitation.
Energy transfer 2 F5=2 ! 2 F7=2 (Yb 31): 4 S3=2 ! 2 G7=2 (Er 31) (ET2), followed by fast cascading relaxation to the 4G11/2 and 2H9/2 states. The emission intensities of the UV and violet relative to that of the red and green in our amorphous ®lm are enhanced greatly, in comparison with the up-conversion emissions in other Yb 31 ±Er 31 co-doped systems reported previously [11] and the target itself (Fig. 1(a)). The processes ET1 and ET2 populate the 2H9/2 and 4G11/2 states. At the same time, they quench the red and green up-conversion emissions, respectively. Two possible mechanisms may enhance ET1 and ET2: (1) Enrichment of Yb 31 concentration in the ®lm during PLD and (2) Localization of light in the particles with the size comparable to the wavelength of the incident light [12]. To clarify which mechanism is dominant, we carried out the following experiment. Another sample (ZAPL1) was prepared with a YAG:Nd (1.06 mm) laser from the same target. The size of the particles is the same as that of the sample ZAPL, while its up-conversion emission spectrum shows no difference with the target (Fig. 1(a)). Were the mechanism (2) in effect, the up-conversion spectrum of ZAPL1 would coincide with that of ZAPL. The negative result exclude (2) from the enhancement mechanism. If the energy density of the laser is chosen properly, the ®lms should have the same composition as the target [5]. That must be the case for ZAPL1. However, in preparation of ZAPL, 953.6 nm is within the Yb 31 absorption spectrum, that makes the Yb 31 concentration enriched in the ®lm. Let NG and NR be the populations of the ( 4S3/2, 2H11/2) and 4 F9/2 states, N be the number of excited Yb 31 ions in the sample. The energy transfer rates of ET1 and ET2 can be expressed as w1NNR and w2NNG, where w1 and w2 are the average transfer rates between a pair of an excited Yb 31 ion and an Er ion on the states ( 4S3/2, 2H11/2) or 4F9/2, respec-
tively. At steady state, the populations of 2H9/2 (NV) and 4 G11/2 (NUV) can be written as NV
w1 NR 1 bw2 NG N gV
NUV
w2 NG N gUV
where g V and g UV are the transition rates of 2H9/2 and 4G11/2, and b is the ratio of the population on 4G11/2 that relaxed nonradiatively to 2H9/2. From these equations, we have NV w N =N 1 bw2 1 R G N NG gV NUV w 2 N NG gUV The states ( 4S3/2, 2H11/2) and 4F9/2 are populated by two step energy transfer processes, the ratio NG/NR does not change much with the Yb 31 concentration. In optically thin sample, like our ®lm, the number of excited Yb ions is proportional to its concentration. Therefore, the violet and UV up-conversion emission intensities relative to the green or red would also be proportional to the Yb concentration. Thus, we may conclude that the mechanism (1) makes the UV and violet up-conversions enhanced. In addition, our result also indicates that PLD is an ef®cient way to prepare high concentration materials, which was forbidden by limited solubility in solid state reaction. 3. Conclusion In conclusion, up-converting thin ®lms for visible light source were prepared by PLD. Under a 950 nm LD excitation, intense violet, green, red and considerably strong UV
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G. Qin et al. / Solid State Communications 120 (2001) 211±214
emissions were observed at room temperature. In comparison with the target, violet and UV emissions are enhanced. The enhancement is the result of the enrichment of Yb concentration during PLD.
[5] [6]
Acknowledgements This research was supported by State Key Project of Basic Research (973 G1998061309) and Natural Science Foundation of China (59872042).
[7]
[8]
References [1] F.E. Auzel, D. Pecile, Comparison and ef®ciency of materials for summation of photons assisted by energy transfer, J. Lumin. 8 (1973) 32. [2] Shihua Huang, Shui T. Lai, Liren Lou, Weiyi Jia, W.M. Yen, Up-conversion in LaF3:Tm 31, Phys. Rev. B 24 (1981) 59. [3] L.S. Hung, G.R. Paz-Pujalt, T.N. Blanton, D.D. Tuschel, Up-converting Tm-doped BaYYbF8 optical waveguides epitaxially grown by MBE, Appl. Phys. Lett. 66 (1995) 2454. [4] M. Lui, R.A. McFarlane, D. Yap, D. Lederman, Up-conver-
[9] [10] [11] [12]
sion luminescence of Er-doped ZnF2 channel waveguides grown by MBE, Electron. Lett. 29 (1993) 172. D.H. Lowndes, D.B. Geohegan, A.A. Puretzky, D.P. Norton, C.M. Rouleau, Synthesis of novel thin-®lm materials by pulsed laser deposition, Science 273 (1996) 898. R. Serna, J.M. Balleteros, M. Jimenez de Castro, J. Solis, C.N. Afonso, Optically active Er±Yb doped glass ®lms prepared by pulsed laser deposition, J. Appl. Phys. 84 (1998) 2352. K.E. Youden, T. Gravatt, R.W. Enson, H.N. Rutt, R.S. Deol, G. Wylanzowski, Pulsed laser deposition of Ga±La±S chalcogenide glass thin ®lm optical waveguides, Appl. Phys. Lett. 63 (1993) 1061. R. Serna, C.N. Afonso, In situ growth of optically active erbium doped Al2O3 thin ®lms by pulsed laser deposition, Appl. Phys. Lett. 69 (1996) 1514. M. Sural, A. Ghosh, New ¯uoride glasses in the system ZnF2 ± AlF3 ±PbF2 ±LiF, J. Mater. Sci. Lett. 19 (2000) 41. M. Pollnau, D.R. Gamelin, Power dependence of up-conversion luminescence in lanthanide and transition-metal±ion systems, Phys. Rev. B 61 (2000) 337. G. Huber, E. Henmann, T. Sandrock, K. Petermann, Upconversion process in laser crystals, J. Lumin. 72 (1997) 1. Yoh Mita, Y. Wang, S. Shionoya, High brightness blue and green light sources pumped with a 980 nm emitting laser diode, Appl. Phys. Lett. 62 (1993) 802.
Solid State Communications 120 (2001) 211±214
www.elsevier.com/locate/ssc
Up-converting Yb 31 ±Er 31 co-doped amorphous ¯uoride thin ®lms prepared by pulsed-laser deposition for visible light source Guanshi Qin a,b,*, Weiping Qin a,b, Shihua Huang a,b, Changfeng Wu a,b, Baojiu Chen a,b, Shaozhe Lu a,b, E. Shulin a,b a
b
Laboratory of excited states processes, Chinese Academy of Sciences, Changchun 130021, People's Republic of China Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130021, People's Republic of China Received 9 July 2001; accepted 30 August 2001 by E.L. Ivchenko
Abstract Yb 31 ±Er 31 co-doped amorphous Zn0.3Al0.25Pb0.3Li0.12xYbyErxF2.0512x13y ®lms have been prepared by pulsed-laser deposition (PLD). Violet, green, red and ultraviolet (UV) up-conversion emissions were observed under the infrared excitation at 950 nm. In comparison with the up-conversion of the target, the violet and UV emissions are enhanced greatly. The enhancement is attributed to the enrichment of Yb concentration in the sample during PLD. q 2001 Published by Elsevier Science Ltd. PACS: 78.55; 81.15I Keywords: A. Thin ®lms; C. Scanning and transmission electron microscopy; E. Luminescence
1. Introduction Some rare earth doped crystals or glasses have demonstrated up-converted luminescence in the visible and ultraviolet (UV) region while they were pumped at wavelengths accessible to semiconductor lasers [1,2]. Up-conversion light sources made from bulk crystals and ®bers are quite attractive; however, high costs associated with the material preparation and the device packaging may limit their applications. Films, on the other hand, combine the advantages of bulk material and the compactness of ®ber, offering a kind of low cost up-conversion devices. To deposit and fabricate up-converted wave-guides on semiconductor substrates is possible, that would allow to directly integrate on a same chip the up-conversion device with the pump source and other optoelectronic components [3,4]. PLD is a recently developed technique, which is compatible with deposition in gas environments and has been * Corresponding author. Address: Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130021, People's Republic of China. Tel.: 186-431593-7564; fax: 186-431-595-5378. E-mail addresses: [email protected] (G. Qin), [email protected] (G. Qin).
used successfully to grow a wide variety of multi-component thin ®lms [5]. Glasses are suitable for preparing active wave-guides, however, studies devoted to amorphous ®lms grown by PLD were quite limited [6±8]. We reported in this paper the up-conversion emission properties of the Yb 31 ±Er 31 co-doped Zn0.3Al0.25Pb0.3Li0.12xYbyErxF2.0512x13y (ZAPL) ®lms. With the excitation of a 950 nm-laser-diode (LD), violet, green, red and UV emissions were observed. The mechanism responsible for the enhancement of the violet and UV emission was discussed. 2. Results and discussion 953.6 nm, from a Raman shifter pumped by the second harmonic of an Nd:YAG pulsed laser (pulse width 10 ns, repetition rate 10 Hz) was used as the light source for PLD. The Stokes-shifted line with the peak power of 10 4 W was focused onto the target [9] (amorphous ZAPL with x 0:01; y 0:1) in air at an incidence angle of about 458. With such low power, PLD was produced mainly by the intense absorption of Yb 31 ions in the target. The substrate (SiO2 glass or GaAs slice) was hold at 10±20 mm in front of the target at room temperature. During the deposition, orange light emission-laser ablation was observed. The
0038-1098/01/$ - see front matter q 2001 Published by Elsevier Science Ltd. PII: S 0038-109 8(01)00380-5
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G. Qin et al. / Solid State Communications 120 (2001) 211±214
Fig. 1. Up-conversion emission spectra of Zn0.3Al0.25Pb0.3Li0.09Yb0.1Er0.01F2.37, (a) the starting bulk material; (b) the PLD ®lm.
®lm is amorphous and the particle size is about 0.1±1 mm measured with a transmission electron microscope (JEM2010). The up-converted emission spectra were measured under a 950 nm LD excitation and were detected with a Fluorescence Spectrophotometer (Hitachi F-4500). Fig. 1(b) represents the up-conversion emission spectrum of the ®lm. Similar results were obtained for the ®lm deposited on the GaAs substrate. The emissions in the UV and visible range correspond to the following transitions: 4 G11=2 ! 4 I15=2 (,380 nm); 2 H9=2 ! 4 I15=2 (,408 nm); 4 F3=2;5=2 ! 4 I15=2 (,452 nm); 4 G11=2 ! 4 I13=2 (,504 nm); 2 H11=2 ! 4 I15=2 (,520 nm); 4 S3=2 ! 4 I15=2 (,550 nm); and 4 F9=2 ! 4 I15=2 (,650 nm). For unsaturated up-conversion, emission intensity, Is, is proportional to I n, where I is the intensity of the excitation light and the integer n is the number of photons absorbed per up-converted photon emitted [10]. Intensity dependence of
the up-conversion emissions are shown in Fig. 2: n 3:02 for the emission at 408 nm; n 1:91 for the 520 and 540 bands; and n 2:01 for the 652 nm emission. In Yb 31 ±Er 31 co-doped systems, different processes may result in up-conversion [11]. These processes include multistep excited state absorption (ESA), energy transfer (ET) between excited neighboring Er 31 ions and APTE (Addition de photons par transfer d'E4nergie) between Yb 31 and Er 31. Among these processes, APTE is the most ef®cient. Infrared to green ( 2H11/2, 4S3/2 ! 4I15/2 transitions) and to red (4 F9=2 ! 4 I15=2 transition) up-converted emissions in Yb 31 ±Er 31 codoped systems have been widely investigated [12]. Here, we focus our attention on the UV (,380 nm) and the violet (,408 nm) emissions. As shown in Fig. 3, for the violet emission, there are two processes in populating the state 2H9/2: (1) Energy transfer 2 F5=2 ! 2 F7=2 (Yb 31): 4 F9=2 ! 2 H9=2 (Er 31) (ET1); and (2)
Fig. 2. The dependence of the up-conversion luminescence intensity on the pump power for the red green and violet emissions.
G. Qin et al. / Solid State Communications 120 (2001) 211±214
213
Fig. 3. The schematic diagram of Yb 31-sensitized Er 31 up-conversion in Zn0.3Al0.25Pb0.3Li0.09Yb0.1Er0.01F2.37 ®lm under 950 nm excitation.
Energy transfer 2 F5=2 ! 2 F7=2 (Yb 31): 4 S3=2 ! 2 G7=2 (Er 31) (ET2), followed by fast cascading relaxation to the 4G11/2 and 2H9/2 states. The emission intensities of the UV and violet relative to that of the red and green in our amorphous ®lm are enhanced greatly, in comparison with the up-conversion emissions in other Yb 31 ±Er 31 co-doped systems reported previously [11] and the target itself (Fig. 1(a)). The processes ET1 and ET2 populate the 2H9/2 and 4G11/2 states. At the same time, they quench the red and green up-conversion emissions, respectively. Two possible mechanisms may enhance ET1 and ET2: (1) Enrichment of Yb 31 concentration in the ®lm during PLD and (2) Localization of light in the particles with the size comparable to the wavelength of the incident light [12]. To clarify which mechanism is dominant, we carried out the following experiment. Another sample (ZAPL1) was prepared with a YAG:Nd (1.06 mm) laser from the same target. The size of the particles is the same as that of the sample ZAPL, while its up-conversion emission spectrum shows no difference with the target (Fig. 1(a)). Were the mechanism (2) in effect, the up-conversion spectrum of ZAPL1 would coincide with that of ZAPL. The negative result exclude (2) from the enhancement mechanism. If the energy density of the laser is chosen properly, the ®lms should have the same composition as the target [5]. That must be the case for ZAPL1. However, in preparation of ZAPL, 953.6 nm is within the Yb 31 absorption spectrum, that makes the Yb 31 concentration enriched in the ®lm. Let NG and NR be the populations of the ( 4S3/2, 2H11/2) and 4 F9/2 states, N be the number of excited Yb 31 ions in the sample. The energy transfer rates of ET1 and ET2 can be expressed as w1NNR and w2NNG, where w1 and w2 are the average transfer rates between a pair of an excited Yb 31 ion and an Er ion on the states ( 4S3/2, 2H11/2) or 4F9/2, respec-
tively. At steady state, the populations of 2H9/2 (NV) and 4 G11/2 (NUV) can be written as NV
w1 NR 1 bw2 NG N gV
NUV
w2 NG N gUV
where g V and g UV are the transition rates of 2H9/2 and 4G11/2, and b is the ratio of the population on 4G11/2 that relaxed nonradiatively to 2H9/2. From these equations, we have NV w N =N 1 bw2 1 R G N NG gV NUV w 2 N NG gUV The states ( 4S3/2, 2H11/2) and 4F9/2 are populated by two step energy transfer processes, the ratio NG/NR does not change much with the Yb 31 concentration. In optically thin sample, like our ®lm, the number of excited Yb ions is proportional to its concentration. Therefore, the violet and UV up-conversion emission intensities relative to the green or red would also be proportional to the Yb concentration. Thus, we may conclude that the mechanism (1) makes the UV and violet up-conversions enhanced. In addition, our result also indicates that PLD is an ef®cient way to prepare high concentration materials, which was forbidden by limited solubility in solid state reaction. 3. Conclusion In conclusion, up-converting thin ®lms for visible light source were prepared by PLD. Under a 950 nm LD excitation, intense violet, green, red and considerably strong UV
214
G. Qin et al. / Solid State Communications 120 (2001) 211±214
emissions were observed at room temperature. In comparison with the target, violet and UV emissions are enhanced. The enhancement is the result of the enrichment of Yb concentration during PLD.
[5] [6]
Acknowledgements This research was supported by State Key Project of Basic Research (973 G1998061309) and Natural Science Foundation of China (59872042).
[7]
[8]
References [1] F.E. Auzel, D. Pecile, Comparison and ef®ciency of materials for summation of photons assisted by energy transfer, J. Lumin. 8 (1973) 32. [2] Shihua Huang, Shui T. Lai, Liren Lou, Weiyi Jia, W.M. Yen, Up-conversion in LaF3:Tm 31, Phys. Rev. B 24 (1981) 59. [3] L.S. Hung, G.R. Paz-Pujalt, T.N. Blanton, D.D. Tuschel, Up-converting Tm-doped BaYYbF8 optical waveguides epitaxially grown by MBE, Appl. Phys. Lett. 66 (1995) 2454. [4] M. Lui, R.A. McFarlane, D. Yap, D. Lederman, Up-conver-
[9] [10] [11] [12]
sion luminescence of Er-doped ZnF2 channel waveguides grown by MBE, Electron. Lett. 29 (1993) 172. D.H. Lowndes, D.B. Geohegan, A.A. Puretzky, D.P. Norton, C.M. Rouleau, Synthesis of novel thin-®lm materials by pulsed laser deposition, Science 273 (1996) 898. R. Serna, J.M. Balleteros, M. Jimenez de Castro, J. Solis, C.N. Afonso, Optically active Er±Yb doped glass ®lms prepared by pulsed laser deposition, J. Appl. Phys. 84 (1998) 2352. K.E. Youden, T. Gravatt, R.W. Enson, H.N. Rutt, R.S. Deol, G. Wylanzowski, Pulsed laser deposition of Ga±La±S chalcogenide glass thin ®lm optical waveguides, Appl. Phys. Lett. 63 (1993) 1061. R. Serna, C.N. Afonso, In situ growth of optically active erbium doped Al2O3 thin ®lms by pulsed laser deposition, Appl. Phys. Lett. 69 (1996) 1514. M. Sural, A. Ghosh, New ¯uoride glasses in the system ZnF2 ± AlF3 ±PbF2 ±LiF, J. Mater. Sci. Lett. 19 (2000) 41. M. Pollnau, D.R. Gamelin, Power dependence of up-conversion luminescence in lanthanide and transition-metal±ion systems, Phys. Rev. B 61 (2000) 337. G. Huber, E. Henmann, T. Sandrock, K. Petermann, Upconversion process in laser crystals, J. Lumin. 72 (1997) 1. Yoh Mita, Y. Wang, S. Shionoya, High brightness blue and green light sources pumped with a 980 nm emitting laser diode, Appl. Phys. Lett. 62 (1993) 802.