Twentyfold blue upconversion emission enhancement through thermal effects in Pr 3+ /Yb 3+ -codoped fluoroindate glasses excited at 1.064 m A. S. Oliveira, E. A. Gouveia, M. T. de Araujo, A. S. Gouveia-Neto, Cid B. de Araújo, and Y. Messaddeq Citation: Journal of Applied Physics 87, 4274 (2000); doi: 10.1063/1.373065 View online: http://dx.doi.org/10.1063/1.373065 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/87/9?ver=pdfcov Published by the AIP Publishing [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: 186.217.234.225 On: Tue, 14 Jan 2014 13:11:52 http://scitation.aip.org/content/aip/journal/jap?ver=pdfcov http://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/1744363738/x01/AIP-PT/JAP_CoverPg_101613/aipToCAlerts_Large.png/5532386d4f314a53757a6b4144615953?x http://scitation.aip.org/search?value1=A.+S.+Oliveira&option1=author http://scitation.aip.org/search?value1=E.+A.+Gouveia&option1=author http://scitation.aip.org/search?value1=M.+T.+de+Araujo&option1=author http://scitation.aip.org/search?value1=A.+S.+Gouveia-Neto&option1=author http://scitation.aip.org/search?value1=Cid+B.+de+Ara�jo&option1=author http://scitation.aip.org/search?value1=Y.+Messaddeq&option1=author http://scitation.aip.org/content/aip/journal/jap?ver=pdfcov http://dx.doi.org/10.1063/1.373065 http://scitation.aip.org/content/aip/journal/jap/87/9?ver=pdfcov http://scitation.aip.org/content/aip?ver=pdfcov JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 9 1 MAY 2000 [This a Twentyfold blue upconversion emission enhancement through thermal effects in Pr 3¿ÕYb3¿-codoped fluoroindate glasses excited at 1.064 mm A. S. Oliveira, E. A. Gouveia, M. T. de Araujo, and A. S. Gouveia-Netoa) Departamento de Fı´sica, Universidade Federal de Alagoas, Maceio´ 57072/970 AL, Brazil Cid B. de Araújo Departamento de Fı´sica, Universidade Federal de Pernambuco, Recife 50670/901, PE, Brazil Y. Messaddeq UNESP, Araraquara 14800/900, SP, Brazil ~Received 4 January 2000; accepted for publication 2 February 2000! Infrared-to-visible upconversion emission enhancement through thermal effects in Yb31-sensitized Pr31-doped fluoroindate glasses excited at 1.064mm is investigated. A twentyfold increase in the 485 nm blue emission intensity as the sample temperature was varied from 20 to 260 °C was observed. The visible upconversion fluorescence enhancement is ascribed to the temperature dependent multiphonon-assisted anti-Stokes excitation of the ytterbium sensitizer and excited-state absorption of the praseodymium acceptor. A model based upon conventional rate equations considering a temperature dependent effective absorption cross section for the2F7/2→2F5/2 transition of the Yb31 and1G4→3P0 excited-state absorption of the Pr31, agrees very well with the experimental results. ©2000 American Institute of Physics.@S0021-8979~00!08209-8# f h u e in bl u a pe h ro i e o y ria t o s h d en c ith he in s a s in g of the e by ses ra- ble ing of a- ed al- op- om- bly mal ss of ns 64 by the - tion ma I. INTRODUCTION Much interest has recently been devoted to the search all-solid-state blue light sources for applications in hig density optical data reading and storage, undersea comm cations, and optical displays. An auspicious approach ploits infrared-to-visible frequency upconversion Pr31-doped materials pumped by commercially obtaina infrared sources. Blue laser operation using frequency conversion in the praseodymium-doped system has alre been demonstrated in the single- and double-pum configuration.1–4 However, for the majority of rare-eart single-doped media the infrared-to-visible upconversion p cess has been proven inefficient, particularly for excitation the 1.0–1.1mm wavelength region, where high power las sources are readily available. The realization Yb31-sensitized materials, exploiting the efficient energ transfer mechanism between the sensitizer and pairs or t of rare-earth ions, has allowed a substantial improvemen the upconversion efficiency process in Tm31,5 Er31,6 and Pr31 5,7–10 doped systems. Nevertheless, new approache increase the upconversion efficiency are still under searc In ytterbium-sensitized rare-earth doped materials, un infrared nonresonant excitation, with the pump-photon ergy lower than the2F7/2→2F5/2 transition of the Yb31 ion, the population of the acceptor visible emitting levels is a complished via an indirect pumping process w multiphonon-assisted anti-Stokes excitation of t sensitizer,11 followed by energy transfer to the acceptor the ground state and a subsequent energy-transfer proce or absorption from the excited state. The excited-state a!Author to whom correspondence should be addressed; electronic artur@fis.ufal.br 4270021-8979/2000/87(9)/4274/5/$17.00 rticle is copyrighted as indicated in the article. Reuse of AIP content is sub 186.217.234.225 On: Tue or - ni- x- e p- dy d - n r f - ds n to . er - - s to b- sorption can be either resonant or multiphonon-assisted a the case herein reported. Therefore, the effective pumpin the acceptor’s emitting levels is strongly dependent upon phonon population in the host material. In this work, w demonstrate both experimentally and theoretically that heating nonresonant infrared excited fluoroindate glas codoped with praseodymium and ytterbium in the tempe ture region of 20–260 °C, one obtains up to 20 times visi upconversion emission enhancement. II. EXPERIMENTAL The experimental investigation was carried out us fluoroindate glass samples with mol % composition (33.52 x)InF3 – 20ZnF2 – 20SrF2 – 16BaF2 – 6GaF3–2NaF– 0.5PrF32xYbF3. Fluoroindate glasses12 have recently been the subject of much interest owing to their potential applic tion in photonic devices based on rare-earth dop materials.10,13 The material presents very good optical qu ity, is stable against atmospheric moisture, exhibits low tical attenuation from 250 nm to 8mm, and due to the low maximum phonon-energy of;510 cm21,14 is expected to present significantly lower nonradiative decay rates as c pared to fluorozirconate glasses~;590 cm21!. The inclusion of GdF3 and NaF in the glass composition has considera reduced the devitrification process and improved the ther stability, permitting the realization of fluoroindate gla fiber.15 The samples had concentration of 5000 ppm/wt praseodymium and different concentrations of ytterbium io @5000~I!, 10000~II !, 15000~III ! and 20 000~IV ! ppm/wt#. The excitation source was a cw Nd:YAG laser operated at 1.0 mm. The pump beam was focused down into the samples a 5 cm focal length lens and the pump beam waist at samples location was;60 mm. The detection system con sisted of a scanning spectrograph with operating resolu il: 4 © 2000 American Institute of Physics ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: , 14 Jan 2014 13:11:52 ie m b ire on is pl na w . 5 n 0 o no ed ep d d ite es s- p 2 e in ot a tion cess ver- the y for om e - pon of one r of was sio the esent ta- 4275J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Oliveira et al. [This a of 0.5 nm equipped with a S-20 uncooled photomultipl tube coupled to a lock-in amplifier and computer. The te perature of the sample was increased from 20 to 260 °C placing it into an aluminum oven heated by resistive w elements. A copper-constantan thermocouple~reference at 0 °C! attached to one of the sample’s faces was used to m tor the temperature within;2 °C accuracy. III. RESULTS AND DISCUSSION Figure 1 illustrates the temperature evolution of the v ible upconversion emission of light emanating from sam ~IV ! at a fixed excitation power of 400 mW at 1.064mm. It can clearly be seen that the upconversion emission sig increased significantly as the sample’s temperature raised from 20 to 260 °C. The spectra depicted in Fig presented distinct emission bands centered around 485, 610, and 630 nm corresponding to the3P0→3H4 , 3P0 →3H5 , 1D2→3H4 , and3P0→3F2 transitions of Pr31 ions, respectively, with the blue signal obviously being the stro gest. The3P0→3H6 transition can also contribute to the 61 nm fluorescence. Pumping of the Pr31 excited-state visible emitting levels is accomplished through a combination phonon-assisted absorption, energy transfer, and pho assisted excited-state absorption processes, as portray the energy-level diagram depicted in Fig. 2. In the first st a pump photon at 1.064mm provokes a multiphonon-assiste anti-Stokes excitation of the Yb31 sensitizer from the2F7/2 ground-state to the2F5/2 excited-state level. The excite Yb31 transfers its energy to a neighbor Pr31 ion in the 3H4 ground state, exciting it to the1G4 level. This excited Pr31 ion undergoes a multiphonon-assisted anti-Stokes exc state absorption of a second pump photon, which promot to the3P0 upper emitting level. Finally, the excited Pr31 ion decays from3P0 either radiatively to generate the main vi ible fluorescence emission bands or nonradiatively to po late lower-lying luminescent levels, as indicated in Fig. The dependence of the blue emission intensity upon the citation intensity at room temperature and at 115 °C was vestigated and the results are presented in the log–log pl Fig. 3. It was observed that the blue emission exhibited FIG. 1. Temperature evolution of the frequency upconversion emis spectrum. Excitation power of 400 mW at 1.064mm. Sample IV. rticle is copyrighted as indicated in the article. Reuse of AIP content is sub 186.217.234.225 On: Tue r - y i- - e ls as 1 30, - f n- in , d- it u- . x- - of n approximately quadratic power law behavior~slope;2! with pump intensity. Within the excitation power range~up to 1.5 W! of our measurements, the results presented no indica of avalanche processes taking place. The avalanche pro is characterized by a nonlinear dependence of the upcon sion fluorescence emission upon the pump intensity with existence of a critical pumping threshold.8,16,17However, we have observed a slope decrease in the emission intensit pump powers above 700 mW at 115 °C, and 1.0 W at ro temperature~20 °C!. This behavior is in agreement with th power dependence of the3P0 population as will be demon strated later on in this work. The dependence of the blue emission at 485 nm u temperature was examined for a fixed excitation power 400 mW, and the results are presented in Fig. 4. As observes, the 485 nm signal intensity increased by a facto 20 in the temperature range of 20–260 °C. The 203 en- hancement factor in the upconversion emission intensity n FIG. 2. Energy-levels scheme indicating participation of phonons in absorption transitions. The solid lines connected by a dashed line repr the cross-relaxation process. FIG. 3. Log–log plot of blue emission intensity as a function of the exci tion power at room temperature~open squares! and at 115 °C~open circles!, for sample IV. ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: , 14 Jan 2014 13:11:52 48 ed w de he e tu e a- at t th e- er - - o e ce tu pu s t r d te, s for - nti- s ce, , orp- tains t in ve d a on nd is- o red ate al ver, e- ci 4276 J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Oliveira et al. [This a obtained by comparing the integrated spectrum around nm at 260 °C and the one at room temperature~20 °C!. As a matter of fact, we have not been able to quantify the infrar to-blue conversion efficiency owing to the very small lo power associated with the total visible fluorescence. Besi we would have to both filter out the blue emission from t rest and collect that fluorescence in all directions. Howev the enhancement is quite noticeable from the tempera evolution depicted in Fig. 1. This behavior can be explain as follows. The excitation of the Yb sensitizer from the2F7/2 ground state to the2F5/2 excited state requires the particip tion of at least two optical phonons in order to compens for the energy mismatch of;800 cm21 between the inciden photon and the ytterbium transition energy. Furthermore, praseodymium1G4→3P0 excited-state absorption also d mands at least three phonons in order to match the en difference of approximately 1450 cm21 between the pump photon energy and that of the Pr31 transition. As a conse quence, the population of the Pr31 excited-state3P0 level relies strongly on the phonon occupation number in the h matrix. The multiphonon-assisted absorption leads to an fective absorption cross section for both sensitizer and ac tor, which are increasing functions of the sample tempera giving rise to the enhancement of the emitting levels po lations. The results are analyzed using a model that include phonon-assisted transition in the Yb31 ion (2F7/2→2F5/2), energy transfer to Pr31(2F5/2→1G4), and subsequen phonon-assisted excited-state absorption to populate the3P0 level as portrayed in Fig. 2. Accordingly, the temperatu dependence of the 485 nm emission intensity is describe the following set of rate equations: ṅe5ngsge~T!F2neCS2n02 ne tS , ~1a! ṅ25neCS2n02n2s23~T!F2 n2 t2 , ~1b! ṅ35n2s23~T!F2 n3 t3 , ~1c! FIG. 4. Temperature dependence of the emission signal at 485 nm. Ex tion power of 400 mW at 1.064mm. Sample IV. rticle is copyrighted as indicated in the article. Reuse of AIP content is sub 186.217.234.225 On: Tue 5 - s, r, re d e e gy st f- p- re - a e by whereneCS2 is the sensitizer–acceptor energy-transfer ra tS , t2 , andt3 are the lifetimes of the levels2F5/2 ~level e!, 1G4 ~level 2!, and 3P0 ~level 3!, respectively, andF is the power flux. In Eqs.~1!, sge(T) and s23(T) represent the temperature dependent effective absorption cross section the Yb31 excitation and Pr31 excited-state absorption, re spectively, owing to the so called multiphonon-assisted a Stokes excitation process.11 The absorption cross section can be written in a general form as s~T!5s0@exp~hnphonon/kBT21!#2p, ~2! where s0 is the absorption cross section at resonan hnphononis the phonon energy,kB is the Boltzmann constant andT the absolute temperature. The exponentp accounts for the number of phonons taking part in the anti-Stokes abs tion processes. Combining the above equations, one ob the steady-state population of the blue emitting level as n3> t2t3tSs23~T!NANSCS2sge~T!F2 @11tSsge~T!F1tSNACS2# , ~3! where NA and NS are the Pr31 and Yb31 concentrations, respectively. In order to derive Eq.~3!, we have neglected the cross-relaxation mechanism (3P0 , 2F7/2)→(1G4 , 2F5/2) from the pair Pr31 – Yb31, for which one would expect tha the lifetime of the3P0 level depends upon the Yb31 concen- tration and such a behavior was not observed Pr31/Yb31-doped fluoroindate glasses.10 We could also ex- clude cooperative upconversion of Yb ions since we ha observed that the visible fluorescence intensity presente linear dependence with the concentration of Yb31. We have assumed that back transfer from1G4 to 2F5/2 does not play a significant role as in the case of ZBLAN.18 Finally, we esti- mateds23F!t2 21 ~fulfilled by our experimental conditions! using data from Ref. 19, which implies that a small fracti of Pr31 is excited, leading ton0'NA . The blue light inten- sity at 485 nm is then given byI 485nm5hn30A30n3 , where A30 is the radiative transition rate from level 3 to the grou state andn30 its frequency. To obtain the temperature dependence of the blue em sion intensity through Eq.~3! further considerations need t be made. The lifetime of the2F5/2 level is mainly radiative due to the large energy separation from ground state~10256 cm21!. The nonradiative transitions from1G4 ~9696 cm21! to lower-lying levels are expected to be small, as compa to the excited-state pumping rate1G4→3P0 and due to the low maximum-phonon energy associated with fluoroind hosts,14 which requires the participation of at least six optic phonons to bridge the energy gap~3245 cm21! connecting the levels1G4–3F3,4, resulting in negligible nonradiative transition probability.17 This means that the lifetimests and t2 are approximately temperature independent. Moreo the energy-transfer rateNSCS2 is very high for samples III and IV ~Ref. 10! and it is also temperature dependent b cause of the energy mismatch (DEe25560 cm21) between the 2F5/2 level of Yb31 and the1G4 level of Pr31, and this dependence can be accounted through exp(2DEe2 /kBT) ac- cording to Ref. 11. Finally, the lifetime of the3P0 is related to nonradiative transition probabilitiesWNR(T) through ta- ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: , 14 Jan 2014 13:11:52 d po f. t an 4. h p io e no pi ee o e ta e o gy io de h i- al . sing me pen- ntal en- um red on- lin- ti- on the the con- ature nsi- The ring ptor as ults im- e d m- ass in - i- s, by and and a, p 4277J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Oliveira et al. [This a t3 215( j A3 j1WNR~T!, ~4! and for low concentrations of rare-earth ions,WNR(T) is due to multiphonon relaxation processes, and can be relate the temperature through11,20 WNR~T!5WNR~0!b12exp~2hnshonon/kBT!c2p, ~5! whereWNR(0) is its value at zero temperature and the ex nent p is the phonon order linking the3P0 level ~20367 cm21! to the next lower energy level1D2 ~16942 cm21!. Using the experimental lifetimet3(28ms) for the 3P0 at room temperature10 and radiative transitions rate from Re 21 for our samples, one is left with a value ofWNR(300 K) 524 767 s21, which means that the3P0 level can populate the 1D2 level very efficiently. We have obtained the temperature dependence of blue emission intensity by adjusting the phonon energy, the result is illustrated by the solid line in the plot of Fig. As can be observed, indeed the theoretical model matc the experimental results very well. By using a similar a proach, we have recently described a fourfold upconvers emission enhancement in Er31/Yb31-codoped chalcogenid glasses and the theoretical model based upon multipho assisted anti-Stokes excitation of the nonresonant pum of the sensitizer has also proven to agree very well ind with the experimental data.22 The theoretical fitting of data depicted in Fig. 4, also permitted us to withdraw the value ;400 cm21 for the phonon mode participating in th multiphonon-assisted anti-Stokes excitation and excited-s absorption of the sensitizer and the acceptor, respectiv However, it can be inferred that there exists a deviation ;110 cm21 from the value for the maximum phonon ener associated with fluoroindate glasses.14 The deviation is at- tributed to the fact that in anti-Stokes sideband excitat processes,11 one has to consider an effective phonon mo which possesses lower energy than the cut-off one. The p non population distribution directly involved in the ant Stokes excitation mechanism is centered around the so c ‘‘effective-phonon-mode,’’ as has recently been realized23 FIG. 5. Temperature dependence of the blue emission efficiency for sam I–IV, at a fixed pump power, normalized to the Yb31 concentration. rticle is copyrighted as indicated in the article. Reuse of AIP content is sub 186.217.234.225 On: Tue to - he d es - n n- ng d f te ly. f n , o- led We have also performed the same set of experiments u samples I–III, and the results exhibited basically the sa behavior as far as blue emission intensity temperature de dence is concerned, as can be inferred from the experime data depicted in Fig. 5. The upconversion emission effici cies have followed the same trend with an overall maxim enhancement of approximately320 for all samples. How- ever, the lower ytterbium concentration samples requi higher pump powers, in order to obtain appreciable upc version fluorescence visible signals, as a result from the ear intensity dependence with Yb31 concentration. CONCLUSION In conclusion, the experimental and theoretical inves gation of thermally induced infrared-to-visible upconversi fluorescence emission enhancement in Yb31/Pr31-doped flu- oroindate glasses excited at 1.064mm was examined for the first time. Our results revealed a 20-fold enhancement in 485 nm blue emission intensity as the temperature of glass sample was varied in the 20–260 °C range. The up version emission enhancement was assigned to a temper dependent effective absorption cross section for the se tizer excitation and acceptor excited-state absorption. model based upon conventional rate equations conside the absorption cross sections of both sensitizer and acce as functions of the phonon population in the host matrix, h proven to agree very well with experimental data. The res indicated that the heating process can be exploited to prove power performance~times four output power increas and threshold reduction! of Er/Yb-doped fiber lasers pumpe by high power sources in the 1.0mm spectral region, and also enhance gain by 60% in a single-pass visible light a plification mechanism in Er/Yb-codoped chalcogenide gl pumped at 1.064mm, as has recently been demonstrated our lab.24,25 ACKNOWLEDGMENTS The financial support by FINEP~Financiadora de Estu dos e Projetos!, CNPq ~Conselho Nacional de Desenvolv mento Cientı´fico e Tecnolo´gico!, CAPES~Coordenadoria de Aperfeiçoamento de Pessoal de Ensino Superior!, PRONEX- UFPE/UFAL/UFPB, and PADCT/CNPq, Brazilian agencie is gratefully acknowledged. A. S. Oliveira was supported a graduate studentship from CAPES. 1J. Y. Allain, M. Monerie, and H. Poignant, Electron. Lett.26, 166~1990!. 2R. G. Smart, D. C. Hanna, A. C. Tropper, S. T. Davey, S. F. Carter, D. Szebesta, Electron. Lett.27, 1307~1994!. 3Y. Zhao and S. Poole, Electron. Lett.30, 967 ~1994!. 4Y. Zhao, S. Fleming, and S. Poole, Opt. Commun.114, 285 ~1995!. 5D. C. Hanna, R. M. Percival, I. R. Perry, R. G. Smart, J. E. Townsend, A. C. Tropper, Opt. Commun.78, 187 ~1990!. 6Y.-M. Hua, Q. Li, Y.-L. Chen, and Y.-X. Chen, Opt. Commun.88, 441 ~1992!. 7D. M. Baney, G. Rankin, and K. W. Chang, Appl. Phys. Lett.69, 1662 ~1996!. 8T. R. Gosnell, Electron. Lett.33, 411 ~1997!. 9D. M. Baney, G. Rankin, and K. W. Chang, Opt. Lett.21, 1372~1996!. 10W. Lozano B., C. B. de Arau´jo, C. E. Egalon, A. S. L. Gomes, B. J. Cost and Y. Messaddeq, Opt. Commun.153, 271 ~1998!; L. E. E. de Arau´jo, MSc. thesis, Universidade Federal de Pernambuco, Brazil, 1994. les ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: , 14 Jan 2014 13:11:52 O e on st. A. to, S. 4278 J. Appl. Phys., Vol. 87, No. 9, 1 May 2000 Oliveira et al. [This a 11F. Auzel, Phys. Rev. B13, 2809~1976!. 12Y. Messaddeq and M. Poulain, Mater. Sci. Forum67–68, 161 ~1989!. 13See, for example W. Lozano B, C. B. de Arau´jo, L. H. Acioli, and Y. Messaddeq, J. Appl. Phys.84, 2263~1998!; E. Martins, C. B. de Arau´jo, J. R. Delben, A. S. L. Gomes, B. J. da Costa, and Y. Messaddeq, Commun.158, 61 ~1998!; G. S. Maciel, L. de S. Menezes, C. B. d Araújo, and Y. Messaddeq, J. Appl. Phys.85, 6782~1999!, and references therein. 14R. M. Almeida, J. C. Pereira, Y. Messaddeq, and M. Aegerter, J. N Cryst. Solids161, 105 ~1993!. 15Fibers of fluoroindate glass have been developed by Le Ve`rre Floré ~France! since 1998—M. Poulain~private communications!. 16T. Sandrock, H. Scheife, E. Heumann, and G. Huber, Opt. Lett.22, 808 ~1997!. rticle is copyrighted as indicated in the article. Reuse of AIP content is sub 186.217.234.225 On: Tue pt. - 17J. S. Chivian, W. E. Case, and D. D. Eden, Appl. Phys. Lett.35, 124 ~1974!. 18P. Xie and T. R. Gosnell, Electron. Lett.31, 191 ~1995!. 19R. S. Quimby and B. Zheng, Appl. Phys. Lett.60, 1055~1992!. 20M. J. Weber, Phys. Rev. B8, 54 ~1973!. 21A. Flórez, O. L. Malta, Y. Messaddeq, and M. A. Aegerter, J. Non-Cry Solids213&214, 315 ~1997!. 22P. V. dos Santos, E. A. Gouveia, M. T. de Araujo, A. S. Gouveia-Neto, S. B. Sombra, and J. A. Medeiros Neto, Appl. Phys. Lett.74, 3607~1999!. 23F. Auzel and Y. H. Chen, J. Lumin.66Õ67, 224 ~1996!. 24C. J. da Silva, M. T. de Araujo, E. A. Gouveia, and A. S. Gouveia-Ne Opt. Lett.24, 1287~1999!. 25S. F. Felix, E. A. Gouveia, M. T. de Araujo, A. S. B. Sombra, and A. Gouveia-Neto, J. Lumin.~to be published!. ject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP: , 14 Jan 2014 13:11:52