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Red to blue upconversion luminescence in Tm3+ doped ZrF4–ZnF2–AlF3–BaF2–YF3 optical glass
Microelectronics Journal 34 (2003) 849–854 www.elsevier.com/locate/mejo
Red to blue upconversion luminescence in Tm3þ doped ZrF4 –ZnF2 –AlF3 –BaF2 –YF3 optical glass C.H. Kam, S. Buddhudu* Photonics Research Group, Microelectronics Division, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore, 639798 Received 2 December 2002; revised 17 February 2003; accepted 25 February 2003
Abstract We report on the preparation and optical spectroscopy analysis of a new fluoride glass in the chemical composition of 20ZrF4 – 30ZnF2 – 25AlF3 – 10BaF2 –15YF3 with Tm3þ as the luminescent ions. Under an UV source, this material has displayed an intense blue emission colour. Upon excitation with a red wavelength at 688 nm (3H6 ! 3F3), this glass has shown two upconverted blue emissions at 452 nm (1D2 ! 3F4) and 474 nm (1G4 ! 3H6), respectively. Possible mechanisms involved in such upconverted blue emissions are explained via ground state absorption and excited state absorption processes through an energy-level structure diagram of Tm3þ(4f12). By the successful application of Judd – Ofelt calculations, the luminescence results have successfully been analysed. Besides obtaining a clear understanding of the optical characteristics of this glass, we have also measured its physical properties such as the refractive indices at three different wavelengths in order to evaluate its light dispersion performance, glass density and other related parameters as well. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Tm3þ; Optical glass-upconversion; Blue emission
1. Introduction Solid state materials allowing generation of primary light colours of light (red, green and blue) are required for image devices that need high spatial resolution with wide colour image gamut, and also for optoelectronic devices and high density optical storage systems. For the last decade, upconversion luminescence have been investigated in optical materials because of the basic research interest in the phenomenon as well as of the need for solid state short wavelength lasers. The mechanisms for producing upconverted fluorescence are: a two-step absorption, energy transfer upconversion and photon avalanche. Among rare earth ions, the Tm3þ (4f12) ion is very attractive since it has two stable excited levels, 1D2 and 1G4 to emit red upconverted blue luminescence [1 –5]. In order to search for chemically stable and more efficient upconversion materials, different glass compositions have been investigated. Fluoride glasses have been studied intensively as high performance new materials for different applications * Corresponding author. Tel.: þ 65-6790-6189; fax: þ 65-6790-4161. E-mail address: [email protected] (S. Buddhudu).
due to their high transparency and relatively low phonon energy in the heavy metal glass family. Earlier, in the literature, the method of preparation and a prominent green upconversion emission from erbium ion doped ZrF4 – ZnF2 – AlF3 – BaF2 – YF3 (ZZABY) optical glasses have been reported [6,7]. Because of the fact that in recent times, considerable importance has been attached to the study of upconversion emission from rare earths doped solid state materials towards the development of upconverted visible laser systems, we have made an attempt here to evaluate the feasibility of Tm3þ doped ZZABY optical glass from the measurement of its optical absorption spectrum, upconversion photoluminescence (PL) and measured lifetimes. Judd Ofelt calculations have also been made to correlate the upconversion luminescence and fluorescence results with the absorption data.
2. Experimental studies Thulium glass samples were prepared from high purity (99.99%, Aldrich) spectral grade chemicals of ZnF2, AlF3, BaF2, YF3 and TmF3 as the starting materials. Since the ZrF4
0026-2692/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0026-2692(03)00132-0
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chemical could not be obtained in fluoride form, its oxide form (ZrO2) was converted into its fluoride (ZrF4) by fluorination of ZrO2 with NH4HF2 at 300– 400 for 5 h. The remaining raw chemicals were also mixed with 10 –20% of NH4HF2 in order to convert residual oxides if any into fluorides. The thulium glass samples thus obtained have the chemical composition of 20ZrF 4 – 30ZnF 2 – 25AlF3 – 10BaF2 – 14YF3 – 1TmF3 and were labelled as Tm3þ: ZZABY glasses for convenient reference. A host reference glass without the thulium ions abbreviated in short form as ZZABY (20ZrF4 – 30ZnF2 –25AlF3 – 10BaF2 –15YF3) was also prepared. The well-mixed chemical batches were put in alumina crucibles heated at 350 8C for an hour and then melted at 900 8C for half an hour in N2 atmosphere. These melts were quenched between two smooth surfaced brass plates resulting in circular-shaped glass disks 2 –3 cm in diameter with uniform thickness of 0.3 cm. These thulium glasses were annealed at 500 8C for half an hour. Room temperature absorption spectrum (250 – 1000 nm) was carried out on an absorption spectrophotometer of the model HP-89090A. The glass transition temperature ðTg Þ and glass crystallization temperature ðTx Þ were measured on a Perkin Elmer DSC-7 and the data are given in Table 1. Based on these measurements, the glass relative thermal stability factor ðTx 2 Tg ¼ 137Þ was calculated as was done earlier [8]. Thulium glass refractive indices (nC ; nd and nF ) were obtained on an Abbe refractometer at three spectral lines of lC ¼ 656:3 nm; ld ¼ 589:3 nm and lF ¼ 486:1 nm; respectively, in evaluating the glass Abbe value ðv ¼ 61Þ; non-linearity refractive index ðn2 ¼ 1:122Þ and coefficient ðg ¼ 3:164Þ; reflection loss ðR ¼ 4%Þ and these are presented in Table 1. The mean dispersion ðnF 2 nC ¼ 0:008Þ value of this glass having a lower refractive index ðnd ¼ 1:486Þ demonstrates its potential use as the core material of optical fibre glasses. The mathematical expressions of these parameters were obtained from the articles reported earlier [9 – 12]. The density ðd ¼ 4:364Þ of the glass was determined at room temperature using Table 1 Different physical and optical properties of Tm3þ:ZZABY optical glass Glass average molecular weight (M) (g) Glass density (d) (g/cm3) Glass luminescent ions ðNTm3þ Þ (1020 ions/cm3) Refractive index ðnd Þ (at 589.3 nm) Refractive index ðnC Þ (at 656.3 nm) Refractive index ðnF Þ (at 486.1 nm) Glass reflection loss (R) (%) Abbe value ðvd Þ Non-linearity refractive index ðn2 Þ (10213 esu) Non-linearity refractive index coefficient ðgÞ (1015cm2 W) ˚) Tm3þ ion (rp ) (A ˚) Tm3þ –Tm3þ inter ionic distance (ri ) (A Glass transition temperature Tg (8C) Crystallization temperature Tx (8C) Glass relative stability Tx 2 Tg
125.67 4.364 2.16 1.486 1.482 1.490 4 61 1.122 3.164 1.447 3.591 320 457 137
the Archimedes technique with CCl4 as an immersion liquid. Table 1 presents the values of the different properties of the Tm3þ glasses studied. Both the red to blue (452, 474 nm) upconversion emission spectra and the normal emission spectra were recorded at room temperature on a SPEX Fluorolog3-21 spectrophotometer. The excitation source was a 450Watt CW Xenon short arc, mounted vertically in an aircooled housing; electromagnetic radiation collection and focusing by off-axis mirror for maximum efficiency at all wavelengths were used. Luminescence lifetimes were measured with a SPEX 1934D3 phosphorimeter fitted with a 50 W pulse lamp as the excitation source with a flash duration of , 3 ms (FWHM) for a resolution of 0.01 ms. The average lifetimes ðtÞ of the upconversion emission transitions and the normal emission transitions were computed from the experimental data by means of a double exponential as was carried out earlier in the literature [13,14] upon the usage of the Origin (6.0) program.
3. Results and discussion Absorption spectrum of Tm3þ:ZZABY (Fig. 1) has revealed eight absorption bands and these were assigned to the respective electronic transitions [15 – 16] 3
H6 ! 3P2 (260 nm), 3P1 (271 nm), 3P0 (282 nm), 1D2 (357 nm) 3 H6 ! 1G4 (471 nm), 3F2 (644 nm), 3F3 (688 nm), 3H4 (790 nm) Transitions for ions in solids are primarily electric and magnetic dipole in character. Judd and Ofelt [17,18] have shown that the intensity of the transition between manifolds of 4f configurations could be expressed as a simple sum of the transition matrix elements and a set of phenomenological Judd –Ofelt parameters. Following the procedures reported earlier in literature [19,20] the absorption bands intensities (f £ 1026) and Judd – Ofelt intensity parameters ðVl¼2;4;6 £ 10220 Þ of Tm3þ:ZZBAY glass, have been evaluated by performing a least squares fit method and the results are presented in Table 2. As explained in Ref. [19], we have satisfactorily correlated the experimental absorption intensities with the theoretical results with a smaller value of root-mean square (rms) parameter ðd ¼ ^0:026Þ demonstrating the goodness of the fitting between the experiment and theoretical absorption spectral oscillator strengths. Fig. 2 shows the UV excitation spectrum of Tm3þ:ZZBAY glass with an emission at 452 nm. We have observed three weaker UV excitation bands at 259 nm (3H6 ! 3P2), 273 nm (3H6 ! 3P1), 285 nm (3H6 ! 3P0) and an intense excitation band at 355 nm (3H6 ! 1D2), respectively. Fig. 3 presents the normal emission spectrum of Tm3þ glass studied at different UV excitation wavelengths. Fig. 4 explains the mechanism behind the normal blue emission process at four different UV excitation wavelengths.
C.H. Kam, S. Buddhudu / Microelectronics Journal 34 (2003) 849–854
851
Fig. 1. Absorption spectrum of Tm3þ:ZZABY optical glass.
The dashed lines show non-radiative decay onto the metastate 1D2 level and from here a radiative emission (1D2 ! 3F4) occurs. Fig. 5 shows two red (688 nm) upconverted blue emissions at 452 nm (1D2 ! 3F4) and
474 nm (1G4 ! 3H6), respectively and Fig. 6 explains the red upconversion into blue luminescence in terms of the energy level scheme. From this figure, we can clearly understand the origin of the upconversion of red light into
Table 2 Absorption transitions, wavelengths, energies, line strengths, spectral intensities and intensity differences of Tm3þ:ZZABY optical glass Absorption transitions
3
H6 ! 3H4 H6 ! 3F3 3 H6 ! 3F2 3 H6 ! 1G4 3 H6 ! 1D2 3 H6 ! 3P0 3 H6 ! 3P1 3 H6 ! 3P2 3
Wave length (nm)
789 688 662 469 357 280 275 262
Energy (cm21)
12,673 14,514 15,112 21,306 28,014 35,641 36,295 38,175
Line strength Sed (1022)
286.26 94.66 14.96 26.59 51.14 4.38 7.19 39.65
Spectral intensities fexp (106)
fcal (106)
1.820 1.500 0.265 0.600 1.560 0.150 0.260 1.655
1.831 1.518 0.249 0.626 1.583 0.173 0.288 1.672
Difference ðDf Þ2
1.21 £ 1024 3.24 £ 1024 2.56 £ 1024 6.76 £ 1024 5.29 £ 1024 5.29 £ 1024 7.84 £ 1024 2.89 £ 1024
Intensity related rms deviation parameter d ¼ SðDf Þ2 =ðN 2 nÞð¼ ^0:026Þ where N is number of absorption bands, n is number of fitting parameters. Intensity (Judd–Ofelt) parameters: V2 ¼ 3.11 £ 10220 cm2, V4 ¼ 1.45 £ 10220 cm2, V6 ¼ 0.58 £ 10220 cm2.
Fig. 2. UV-excitation spectrum of Tm3þ:ZZABY optical glass upon monitoring the blue emission ðlemi ¼ 452 nmÞ:
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Fig. 3. Fluorescent blue emission of Tm3þ:ZZABY optical glass at different UV-excitation wavelengths.
shorter blue wavelengths. The mechanism is a multistep excitation by the excited state absorption (ESA) process. An excited Tm3þ ion reaches the 3F3 level upon pumping with red light (688 nm) under ground state absorption (GSA) and then relaxes to the 3F4 level non-radiatively. The excited Tm3þ ion in the 3F4 level could then be raised to the 1D2 level by the absorption of a second excitation photon and subsequently a bright upconverted blue emission at 452 nm due to the transition of 1D2 ! 3F4. The ion in the 3F4 level might also relax to the 3H4 level via non-radiative transitions through cross relaxation processes due to the ion – ion interactions in the host matrix. If the second excitation photon is absorbed while the luminescent Tm3þ ion is in the 3H4 level, then a blue emission can occur at 474 nm because the transition of 1G4 ! 3H6 becomes possible [21]. Fig. 7 compares the decay curves of the upconverted blue and normal blue emissions at the stated pump wavelengths. In the case of upconverted blue emission its lifetime is at least three times smaller than
the normal blue emission as indicated in Fig. 7 and it clearly confirms the fact that the lifetime depends on both the pump wavelength used and also the mechanism employed in the generation of blue emission from the thulium glass. The experimentally measured emission lifetimes are little smaller compared to the radiative lifetimes evaluated from the absorption measurements. This could be because the contributions from the multiphonon relaxation are expected to play a considerable role and therefore the measured luminescence decay time would be quenched by multiphonon relaxation. The non-radiative transition probability 21 ðWnr ¼ t21 m 2 tr Þ values of the blue emission both in the upconversion and normal emission cases have been estimated following the procedure given in Ref. [22], where tr is the inverse of the radiative transition probability calculated from Judd – Ofelt theory and tm is the measured lifetime. In the case of the uponconverted blue emission, the non-radiative transition probability ðWnr Þ value is around 4 times larger compared to the normal blue emission process
Fig. 4. Energy level and blue mechanisms of Tm3þ:ZZABY optical glass at different UV excitation wavelengths.
C.H. Kam, S. Buddhudu / Microelectronics Journal 34 (2003) 849–854
853
Fig. 5. Upconversion blue emissions of Tm3þ:ZZABY optical glass upon excitation with a red wavelength ðlexci ¼ 688 nmÞ:
Fig. 6. Energy level and blue upconversion emission mechanisms of Tm3þ:ZZABY optical glass by excited state absorption. GSA: ground state absorption; MPR: multiphonon relaxation; ESA: excited state absorption.
Fig. 7. Decay curves of upconverted blue emission (at lexc ¼ 688 nmÞ and normal emission (at lexc ¼ 355 nmÞ of Tm3þ:ZZABY optical glass.
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Table 3 Luminescence properties of Tm3þ:ZZABY optical glass Upconverted blue emissions at lexci ¼ 688 nm (a) Upconverted intense blue emission peak position (lp ; nm) (1D2 ! 3F4) FWHM (Dlp ; nm) Measured lifetime tm (ms) Theoretical lifetime tR (ms) Emission cross section ðsep Þ (cm2 £ 10220) Non-radiative transition probability ðWnr Þ £ 103 (b) Upconverted weaker blue emission peak position (lp ; nm) (1G4 ! 3H6) FWHM (Dlp ; nm) Measured lifetime tm (ms) Theoretical lifetime tR (ms) Emission cross section ðsep Þ (cm2 £ 10220) Fluorescent blue emissions at lexci ¼ 355 nm (c) Fluorescent blue emission peak position (lp ; nm) (1D2 ! 3F4) FWHM (Dlp ; nm) Measured lifetime tm (ms) Theoretical lifetime tR (ms) Emission cross section ðsep Þ (cm2 £ 10220) Non-radiative transition probability ðWnr Þ £ 103
452 9 34.97 60.66 7.97 12.11 474 2.5 14.20 31.15 85.45 452 11 99.96 148.54 2.28 3.27
under UV excitation as presented in Table 3 for a comparison. Stimulated emission cross-section values are computed for both blue upconversion emissions (452, 474 nm) and the normal blue emission (454 nm) based on their emission peak wavelengths ðlp Þ; effective emission peak widths ðDleff Þ (FWHM), lifetimes ðtm Þ and glass refractive index ðnd Þ values as was carried out earlier in literature for other types of optical glassy materials [23 – 26]. From the data presented in Table 3, it is observed that the blue upconversion (1D2 ! 3F4) at 452 nm has an emission cross-section that is about 12 times smaller compared to other blue emission (1G4 ! 3H6) at 474 nm. However when compared with compared with normal emission crosssection, its value is observed to be 3 times larger. This is because the emission processes are different and therefore such a trend is not unexpected. Thus, we have examined the potential of this material to be developed in bulk both as a normal blue fluorescent and more particularly as promising red upconverted blue luminescent optical glass of technological importance.
4. Conclusions A new multicomponent Tm 3þ:20ZrF 4 – 30ZnF2 – 25AlF3 – 10BaF2 –15YF3 optical glass was developed to generate prominent red upconverted blue emissions (452 and 474 nm) due to the electronic transitions of 1D2 ! 3F4 and 1G4 ! 3H6, respectively. By correlating the Judd Ofelt
absorption results with the measured luminescence data, radiative lifetime, non-radiative transition probability and cross sections of the upconverted blue emissions and also normal blue emissions of the thulium glass were computed. Both upconversion and normal blue emissions were explained through relevant energy level schemes. Our analysis suggest that Tm 3þ:ZZBAY glass could be considered as a promising material for the development of red upconverted blue luminescent optical system by the excited state absorption process.
References [1] E. Bielejec, E. Kisel, A. Silversmith, J. Lumin. 72–74 (1997) 62. [2] C.P. Wyss, M. Kherli, T. Huber, P.J. Morris, W. Luthy, H.P. Weber, A.I. Zagumennyi, Y.D. Zavartsev, P.A. Studenikin, I.A. Scherbakov, A.F. Zerrouk, J. Lumin. 82 (1999) 137. [3] M.F. Joubert, Opt. Mater. 11 (1999) 181. [4] M. Tsuda, K. Soga, H. Inoue, S. Inoue, A. Makishima, J. Appl. Phys. 85 (1999) 29. [5] J. Qiu, M. Shijaya, Y. Kawamoto, J. Appl. Phys. 86 (1999) 909. [6] J. Qiu, K. Maeda, R. Terai, Phys. Chem. Glasses 36 (1995) 70. [7] X. Zhang, K. Pita, S. Buddhudu, E. Daran, Y.L. Lam, X.R. Liu, Opt. Mater. 20 (2002) 21. [8] A. Elyamani, E. Snitzer, R. Pefcheck, G.H. Sighel Jr., J. Mater. Res. 7 (1992) 1541. [9] F. Gan, Optical and Spectroscopic Properties of Glass, Springer, Berlin, 1992, p. 62. [10] F. Gan, J. Non Cryst Solids 184 (1995) 9. [11] K. Annapurna, S. Buddhudu, J. Solid State Chem. 93 (1991) 454. [12] L. Samek, J. WaryLek, Opt. Engng 36 (1997) 1657. [13] S. Buddhudu, M. Morita, S. Murakami, D. Rau, J. Lumin 83 (1999) 4199. [14] T. Fuji, K. Dodaira, S. Kawauchi, S.N. Tanaka, H. Yamashita, M. Anpo, J. Phys. Chem. 101 (1997) 10631. [15] R.C. Powell, Physics of Solid State Laser Materials, AIP Press, New York, 1998, p. 309. [16] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4412. [17] B.R. Judd, Phys. Rev. 127 (1962) 750. [18] G.S. Ofelt, J. Chem. Phys. 37 (1962) 511. [19] C.E. Bonner Jr., P.M. Mwangi, S.J. Creekmore, L.J. Richardson, S. Stefanos, G.B. Loutts, B. Walsh, Opt. Mater. 20 (2002) 1. [20] B.M. Walsh, N.P. Barnes, B. DiBartolo, J. Appl. Phys. 83 (1998) 2772. [21] G. Ozen, A. Kermaoui, J.P. Denis, X. Wu, F. Pelle, B. Blanzat, J. Lumin. 63 (1995) 85. [22] K. Miazato, D.F. de Sousa, A. Delben, J.R. Delben, S.L. De Oliveira, L.A.O. Nunes, J. Non Cryst. Solids 273 (2000) 246. [23] S. Zaccaria, M. Casarin, A. Speghini, D. Ajo, M. Bettinelli, Spectrochim. Acta A55 (1) (1999) 171. [24] R. Francini, F. Giovenale, U.M. Grassano, P. Laporta, S. Taccheo, Opt. Mater. 13 (4) (2000) 417. [25] R. Balda, J. Fernandez, M. Sanz, A. de Pablos, J.M.F. Navarro, J. Mugnier, Phys. Rev. 61 (5) (2000) 3384. [26] Y.G. Choi, K.H. Kim, B.J. Park, J. Heo, Appl. Phys. Lett. 78 (9) (2001) 1249.
Red to blue upconversion luminescence in Tm3þ doped ZrF4 –ZnF2 –AlF3 –BaF2 –YF3 optical glass C.H. Kam, S. Buddhudu* Photonics Research Group, Microelectronics Division, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore, 639798 Received 2 December 2002; revised 17 February 2003; accepted 25 February 2003
Abstract We report on the preparation and optical spectroscopy analysis of a new fluoride glass in the chemical composition of 20ZrF4 – 30ZnF2 – 25AlF3 – 10BaF2 –15YF3 with Tm3þ as the luminescent ions. Under an UV source, this material has displayed an intense blue emission colour. Upon excitation with a red wavelength at 688 nm (3H6 ! 3F3), this glass has shown two upconverted blue emissions at 452 nm (1D2 ! 3F4) and 474 nm (1G4 ! 3H6), respectively. Possible mechanisms involved in such upconverted blue emissions are explained via ground state absorption and excited state absorption processes through an energy-level structure diagram of Tm3þ(4f12). By the successful application of Judd – Ofelt calculations, the luminescence results have successfully been analysed. Besides obtaining a clear understanding of the optical characteristics of this glass, we have also measured its physical properties such as the refractive indices at three different wavelengths in order to evaluate its light dispersion performance, glass density and other related parameters as well. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Tm3þ; Optical glass-upconversion; Blue emission
1. Introduction Solid state materials allowing generation of primary light colours of light (red, green and blue) are required for image devices that need high spatial resolution with wide colour image gamut, and also for optoelectronic devices and high density optical storage systems. For the last decade, upconversion luminescence have been investigated in optical materials because of the basic research interest in the phenomenon as well as of the need for solid state short wavelength lasers. The mechanisms for producing upconverted fluorescence are: a two-step absorption, energy transfer upconversion and photon avalanche. Among rare earth ions, the Tm3þ (4f12) ion is very attractive since it has two stable excited levels, 1D2 and 1G4 to emit red upconverted blue luminescence [1 –5]. In order to search for chemically stable and more efficient upconversion materials, different glass compositions have been investigated. Fluoride glasses have been studied intensively as high performance new materials for different applications * Corresponding author. Tel.: þ 65-6790-6189; fax: þ 65-6790-4161. E-mail address: [email protected] (S. Buddhudu).
due to their high transparency and relatively low phonon energy in the heavy metal glass family. Earlier, in the literature, the method of preparation and a prominent green upconversion emission from erbium ion doped ZrF4 – ZnF2 – AlF3 – BaF2 – YF3 (ZZABY) optical glasses have been reported [6,7]. Because of the fact that in recent times, considerable importance has been attached to the study of upconversion emission from rare earths doped solid state materials towards the development of upconverted visible laser systems, we have made an attempt here to evaluate the feasibility of Tm3þ doped ZZABY optical glass from the measurement of its optical absorption spectrum, upconversion photoluminescence (PL) and measured lifetimes. Judd Ofelt calculations have also been made to correlate the upconversion luminescence and fluorescence results with the absorption data.
2. Experimental studies Thulium glass samples were prepared from high purity (99.99%, Aldrich) spectral grade chemicals of ZnF2, AlF3, BaF2, YF3 and TmF3 as the starting materials. Since the ZrF4
0026-2692/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0026-2692(03)00132-0
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C.H. Kam, S. Buddhudu / Microelectronics Journal 34 (2003) 849–854
chemical could not be obtained in fluoride form, its oxide form (ZrO2) was converted into its fluoride (ZrF4) by fluorination of ZrO2 with NH4HF2 at 300– 400 for 5 h. The remaining raw chemicals were also mixed with 10 –20% of NH4HF2 in order to convert residual oxides if any into fluorides. The thulium glass samples thus obtained have the chemical composition of 20ZrF 4 – 30ZnF 2 – 25AlF3 – 10BaF2 – 14YF3 – 1TmF3 and were labelled as Tm3þ: ZZABY glasses for convenient reference. A host reference glass without the thulium ions abbreviated in short form as ZZABY (20ZrF4 – 30ZnF2 –25AlF3 – 10BaF2 –15YF3) was also prepared. The well-mixed chemical batches were put in alumina crucibles heated at 350 8C for an hour and then melted at 900 8C for half an hour in N2 atmosphere. These melts were quenched between two smooth surfaced brass plates resulting in circular-shaped glass disks 2 –3 cm in diameter with uniform thickness of 0.3 cm. These thulium glasses were annealed at 500 8C for half an hour. Room temperature absorption spectrum (250 – 1000 nm) was carried out on an absorption spectrophotometer of the model HP-89090A. The glass transition temperature ðTg Þ and glass crystallization temperature ðTx Þ were measured on a Perkin Elmer DSC-7 and the data are given in Table 1. Based on these measurements, the glass relative thermal stability factor ðTx 2 Tg ¼ 137Þ was calculated as was done earlier [8]. Thulium glass refractive indices (nC ; nd and nF ) were obtained on an Abbe refractometer at three spectral lines of lC ¼ 656:3 nm; ld ¼ 589:3 nm and lF ¼ 486:1 nm; respectively, in evaluating the glass Abbe value ðv ¼ 61Þ; non-linearity refractive index ðn2 ¼ 1:122Þ and coefficient ðg ¼ 3:164Þ; reflection loss ðR ¼ 4%Þ and these are presented in Table 1. The mean dispersion ðnF 2 nC ¼ 0:008Þ value of this glass having a lower refractive index ðnd ¼ 1:486Þ demonstrates its potential use as the core material of optical fibre glasses. The mathematical expressions of these parameters were obtained from the articles reported earlier [9 – 12]. The density ðd ¼ 4:364Þ of the glass was determined at room temperature using Table 1 Different physical and optical properties of Tm3þ:ZZABY optical glass Glass average molecular weight (M) (g) Glass density (d) (g/cm3) Glass luminescent ions ðNTm3þ Þ (1020 ions/cm3) Refractive index ðnd Þ (at 589.3 nm) Refractive index ðnC Þ (at 656.3 nm) Refractive index ðnF Þ (at 486.1 nm) Glass reflection loss (R) (%) Abbe value ðvd Þ Non-linearity refractive index ðn2 Þ (10213 esu) Non-linearity refractive index coefficient ðgÞ (1015cm2 W) ˚) Tm3þ ion (rp ) (A ˚) Tm3þ –Tm3þ inter ionic distance (ri ) (A Glass transition temperature Tg (8C) Crystallization temperature Tx (8C) Glass relative stability Tx 2 Tg
125.67 4.364 2.16 1.486 1.482 1.490 4 61 1.122 3.164 1.447 3.591 320 457 137
the Archimedes technique with CCl4 as an immersion liquid. Table 1 presents the values of the different properties of the Tm3þ glasses studied. Both the red to blue (452, 474 nm) upconversion emission spectra and the normal emission spectra were recorded at room temperature on a SPEX Fluorolog3-21 spectrophotometer. The excitation source was a 450Watt CW Xenon short arc, mounted vertically in an aircooled housing; electromagnetic radiation collection and focusing by off-axis mirror for maximum efficiency at all wavelengths were used. Luminescence lifetimes were measured with a SPEX 1934D3 phosphorimeter fitted with a 50 W pulse lamp as the excitation source with a flash duration of , 3 ms (FWHM) for a resolution of 0.01 ms. The average lifetimes ðtÞ of the upconversion emission transitions and the normal emission transitions were computed from the experimental data by means of a double exponential as was carried out earlier in the literature [13,14] upon the usage of the Origin (6.0) program.
3. Results and discussion Absorption spectrum of Tm3þ:ZZABY (Fig. 1) has revealed eight absorption bands and these were assigned to the respective electronic transitions [15 – 16] 3
H6 ! 3P2 (260 nm), 3P1 (271 nm), 3P0 (282 nm), 1D2 (357 nm) 3 H6 ! 1G4 (471 nm), 3F2 (644 nm), 3F3 (688 nm), 3H4 (790 nm) Transitions for ions in solids are primarily electric and magnetic dipole in character. Judd and Ofelt [17,18] have shown that the intensity of the transition between manifolds of 4f configurations could be expressed as a simple sum of the transition matrix elements and a set of phenomenological Judd –Ofelt parameters. Following the procedures reported earlier in literature [19,20] the absorption bands intensities (f £ 1026) and Judd – Ofelt intensity parameters ðVl¼2;4;6 £ 10220 Þ of Tm3þ:ZZBAY glass, have been evaluated by performing a least squares fit method and the results are presented in Table 2. As explained in Ref. [19], we have satisfactorily correlated the experimental absorption intensities with the theoretical results with a smaller value of root-mean square (rms) parameter ðd ¼ ^0:026Þ demonstrating the goodness of the fitting between the experiment and theoretical absorption spectral oscillator strengths. Fig. 2 shows the UV excitation spectrum of Tm3þ:ZZBAY glass with an emission at 452 nm. We have observed three weaker UV excitation bands at 259 nm (3H6 ! 3P2), 273 nm (3H6 ! 3P1), 285 nm (3H6 ! 3P0) and an intense excitation band at 355 nm (3H6 ! 1D2), respectively. Fig. 3 presents the normal emission spectrum of Tm3þ glass studied at different UV excitation wavelengths. Fig. 4 explains the mechanism behind the normal blue emission process at four different UV excitation wavelengths.
C.H. Kam, S. Buddhudu / Microelectronics Journal 34 (2003) 849–854
851
Fig. 1. Absorption spectrum of Tm3þ:ZZABY optical glass.
The dashed lines show non-radiative decay onto the metastate 1D2 level and from here a radiative emission (1D2 ! 3F4) occurs. Fig. 5 shows two red (688 nm) upconverted blue emissions at 452 nm (1D2 ! 3F4) and
474 nm (1G4 ! 3H6), respectively and Fig. 6 explains the red upconversion into blue luminescence in terms of the energy level scheme. From this figure, we can clearly understand the origin of the upconversion of red light into
Table 2 Absorption transitions, wavelengths, energies, line strengths, spectral intensities and intensity differences of Tm3þ:ZZABY optical glass Absorption transitions
3
H6 ! 3H4 H6 ! 3F3 3 H6 ! 3F2 3 H6 ! 1G4 3 H6 ! 1D2 3 H6 ! 3P0 3 H6 ! 3P1 3 H6 ! 3P2 3
Wave length (nm)
789 688 662 469 357 280 275 262
Energy (cm21)
12,673 14,514 15,112 21,306 28,014 35,641 36,295 38,175
Line strength Sed (1022)
286.26 94.66 14.96 26.59 51.14 4.38 7.19 39.65
Spectral intensities fexp (106)
fcal (106)
1.820 1.500 0.265 0.600 1.560 0.150 0.260 1.655
1.831 1.518 0.249 0.626 1.583 0.173 0.288 1.672
Difference ðDf Þ2
1.21 £ 1024 3.24 £ 1024 2.56 £ 1024 6.76 £ 1024 5.29 £ 1024 5.29 £ 1024 7.84 £ 1024 2.89 £ 1024
Intensity related rms deviation parameter d ¼ SðDf Þ2 =ðN 2 nÞð¼ ^0:026Þ where N is number of absorption bands, n is number of fitting parameters. Intensity (Judd–Ofelt) parameters: V2 ¼ 3.11 £ 10220 cm2, V4 ¼ 1.45 £ 10220 cm2, V6 ¼ 0.58 £ 10220 cm2.
Fig. 2. UV-excitation spectrum of Tm3þ:ZZABY optical glass upon monitoring the blue emission ðlemi ¼ 452 nmÞ:
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Fig. 3. Fluorescent blue emission of Tm3þ:ZZABY optical glass at different UV-excitation wavelengths.
shorter blue wavelengths. The mechanism is a multistep excitation by the excited state absorption (ESA) process. An excited Tm3þ ion reaches the 3F3 level upon pumping with red light (688 nm) under ground state absorption (GSA) and then relaxes to the 3F4 level non-radiatively. The excited Tm3þ ion in the 3F4 level could then be raised to the 1D2 level by the absorption of a second excitation photon and subsequently a bright upconverted blue emission at 452 nm due to the transition of 1D2 ! 3F4. The ion in the 3F4 level might also relax to the 3H4 level via non-radiative transitions through cross relaxation processes due to the ion – ion interactions in the host matrix. If the second excitation photon is absorbed while the luminescent Tm3þ ion is in the 3H4 level, then a blue emission can occur at 474 nm because the transition of 1G4 ! 3H6 becomes possible [21]. Fig. 7 compares the decay curves of the upconverted blue and normal blue emissions at the stated pump wavelengths. In the case of upconverted blue emission its lifetime is at least three times smaller than
the normal blue emission as indicated in Fig. 7 and it clearly confirms the fact that the lifetime depends on both the pump wavelength used and also the mechanism employed in the generation of blue emission from the thulium glass. The experimentally measured emission lifetimes are little smaller compared to the radiative lifetimes evaluated from the absorption measurements. This could be because the contributions from the multiphonon relaxation are expected to play a considerable role and therefore the measured luminescence decay time would be quenched by multiphonon relaxation. The non-radiative transition probability 21 ðWnr ¼ t21 m 2 tr Þ values of the blue emission both in the upconversion and normal emission cases have been estimated following the procedure given in Ref. [22], where tr is the inverse of the radiative transition probability calculated from Judd – Ofelt theory and tm is the measured lifetime. In the case of the uponconverted blue emission, the non-radiative transition probability ðWnr Þ value is around 4 times larger compared to the normal blue emission process
Fig. 4. Energy level and blue mechanisms of Tm3þ:ZZABY optical glass at different UV excitation wavelengths.
C.H. Kam, S. Buddhudu / Microelectronics Journal 34 (2003) 849–854
853
Fig. 5. Upconversion blue emissions of Tm3þ:ZZABY optical glass upon excitation with a red wavelength ðlexci ¼ 688 nmÞ:
Fig. 6. Energy level and blue upconversion emission mechanisms of Tm3þ:ZZABY optical glass by excited state absorption. GSA: ground state absorption; MPR: multiphonon relaxation; ESA: excited state absorption.
Fig. 7. Decay curves of upconverted blue emission (at lexc ¼ 688 nmÞ and normal emission (at lexc ¼ 355 nmÞ of Tm3þ:ZZABY optical glass.
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Table 3 Luminescence properties of Tm3þ:ZZABY optical glass Upconverted blue emissions at lexci ¼ 688 nm (a) Upconverted intense blue emission peak position (lp ; nm) (1D2 ! 3F4) FWHM (Dlp ; nm) Measured lifetime tm (ms) Theoretical lifetime tR (ms) Emission cross section ðsep Þ (cm2 £ 10220) Non-radiative transition probability ðWnr Þ £ 103 (b) Upconverted weaker blue emission peak position (lp ; nm) (1G4 ! 3H6) FWHM (Dlp ; nm) Measured lifetime tm (ms) Theoretical lifetime tR (ms) Emission cross section ðsep Þ (cm2 £ 10220) Fluorescent blue emissions at lexci ¼ 355 nm (c) Fluorescent blue emission peak position (lp ; nm) (1D2 ! 3F4) FWHM (Dlp ; nm) Measured lifetime tm (ms) Theoretical lifetime tR (ms) Emission cross section ðsep Þ (cm2 £ 10220) Non-radiative transition probability ðWnr Þ £ 103
452 9 34.97 60.66 7.97 12.11 474 2.5 14.20 31.15 85.45 452 11 99.96 148.54 2.28 3.27
under UV excitation as presented in Table 3 for a comparison. Stimulated emission cross-section values are computed for both blue upconversion emissions (452, 474 nm) and the normal blue emission (454 nm) based on their emission peak wavelengths ðlp Þ; effective emission peak widths ðDleff Þ (FWHM), lifetimes ðtm Þ and glass refractive index ðnd Þ values as was carried out earlier in literature for other types of optical glassy materials [23 – 26]. From the data presented in Table 3, it is observed that the blue upconversion (1D2 ! 3F4) at 452 nm has an emission cross-section that is about 12 times smaller compared to other blue emission (1G4 ! 3H6) at 474 nm. However when compared with compared with normal emission crosssection, its value is observed to be 3 times larger. This is because the emission processes are different and therefore such a trend is not unexpected. Thus, we have examined the potential of this material to be developed in bulk both as a normal blue fluorescent and more particularly as promising red upconverted blue luminescent optical glass of technological importance.
4. Conclusions A new multicomponent Tm 3þ:20ZrF 4 – 30ZnF2 – 25AlF3 – 10BaF2 –15YF3 optical glass was developed to generate prominent red upconverted blue emissions (452 and 474 nm) due to the electronic transitions of 1D2 ! 3F4 and 1G4 ! 3H6, respectively. By correlating the Judd Ofelt
absorption results with the measured luminescence data, radiative lifetime, non-radiative transition probability and cross sections of the upconverted blue emissions and also normal blue emissions of the thulium glass were computed. Both upconversion and normal blue emissions were explained through relevant energy level schemes. Our analysis suggest that Tm 3þ:ZZBAY glass could be considered as a promising material for the development of red upconverted blue luminescent optical system by the excited state absorption process.
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