Author name used in this publication: C. L. MakAuthor name used in this publication: W. L. TsuiAuthor name used in this publication: K. H. Wong2001-2002 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishe

Hui, HK

Mak, CL

Tsang, WS

Tsui, WL

Wong, KH

Yu, WM

Journal of Applied Physics

Author name used in this publication: C. L. MakAuthor name used in this publication: W. L. TsuiAuthor name used in this publication: K. H. Wong2001-2002 > Academic research: refereed > Publication in refereed journalVersion of RecordPublishedVoR allowe

PolyU Institutional Repository

JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 4 15 FEBRUARY 2002DowEvidence of the influence of phonon density on Tm3¿ upconversionluminescence in tellurite and germanate glassesW. S. Tsang, W. M. Yu, C. L. Mak,a) W. L. Tsui, K. H. Wong, and H. K. HuiDepartment of Applied Physics and Materials Research Centre, The Hong Kong Polytechnic University,Hung Hom, Hong Kong~Received 20 June 2001; accepted for publication 29 October 2001!Spectral properties of blue upconversion luminescences in Tm31 doped tellurite ~PWT, PbF2-WO3-TeO2! glasses and germanate ~PWG, PbF2-WO3-GeO2! glasses pumped by a tunable dye laser werestudied at room temperature. Two emission bands centered at 453 and 477 nm, corresponding to the1D2→3H4 and 1G4→3H6 transitions of Tm31 ions respectively, were observed. The two-photonabsorption mechanism responsible for the 477 nm luminescence was confirmed by a quadraticdependence of luminescent intensities on the excitation power. Tellurite glasses showed a weakerupconversion luminescence than germanate glasses. This observation was inconsistent with theprediction from the phonon sideband measurement. In this article, Raman spectroscopy andtransmittance measurement were employed to investigate the origin of the difference inupconversion luminescences in the two glasses. Compared with phonon sideband spectroscopy,Raman spectroscopy extracts more information, including both phonon energy and phonon density,and therefore, is a more effective analytical tool for understanding upconversion luminescence. Ourresults showed that the phonon energy as well as phonon density of the host glass is important indetermining the upconversion efficiency. © 2002 American Institute of Physics.@DOI: 10.1063/1.1429762#I. INTRODUCTIONThe study of blue lasers for use in color display, opticalrecording, biomedical diagnostics and optical communica-tion applications has become a focus of research in recentyears. The search for compact and efficient solid-state bluelasers has stimulated renewed interest in upconversionphenomena.1 Upconversion involves the absorption of two ormore photons by a rare-earth ion followed by emission of aphoton of higher energy. Tm31 doped in many host materialsis known to exhibit upconversion behavior. Among differenthost materials, glass is the most attractive one. It is transpar-ent over a wide optical range and a relatively large amount ofrare-earth ions can be easily and homogeneously incorpo-rated into its matrix. Furthermore, glass has good mechanicalproperties and can be used to fabricate even fibers. In a re-cent paper, Kishimoto and Hirao2 reported that phonon side-band spectroscopy could be used to study the efficiency ofupconversion luminescence in these doped glasses. Theyshowed that glass matrix with higher phonon energy usuallyhas a lower upconversion efficiency resulting in a weakerupconversion luminescence. However, germanate glass has ahigher phonon energy than that of tellurite glass and yet itpossesses a better upconversion efficiency.2 In fact, the pho-non energies of these two glasses are very close to eachother. Hence, the consideration of phonon energies alonemay not be sufficient to explain the upconversion efficien-cies.a!Author to whom correspondence should be addressed; electronic mail:apaclmak@polyu.edu.hk1870021-8979/2002/91(4)/1871/4/$19.00nloaded 23 Mar 2011 to 158.132.161.9. Redistribution subject to AIP licRaman scattering, which studies the interaction betweenphotons and high frequency optical phonons caused by vi-bration of molecules and atoms in matter, is a nondestructiveand convenient diagnostic tool for material characterizationand analysis. Recently, Chen et al. reported the Raman andupconversion luminescence spectra of Tm31 doped in ger-manate glass.3 The Raman peaks of the glass matrix werequite distinguishable. However, the researchers did not cor-relate the Raman spectra with the upconversion lumines-cence spectra. In this article, upconversion luminescences ofTm31 doped tellurite ~PWT, PbF2-WO3-TeO2! glasses andgermanate ~PWG, PbF2-WO3-GeO2! glasses were measured.Previous phonon sideband measurements2 predicted that ger-manate glasses should have a weaker upconversion lumines-cence than that of tellurite glasses. Our results, however, re-vealed the opposite. In order to better understand theefficiency of upconversion luminescence in these twoglasses, Raman spectroscopy and transmittance measurementwere employed. The upconversion luminescent intensities ofthese two glasses were correlated with the results of Ramanspectroscopy and transmittance measurements. By compar-ing the Raman peaks of germanium oxide and tellurium ox-ide, the effects of phonon density of the host matrix on theupconversion efficiency can then be determined. Since theRaman spectroscopic studies can extract more informationthan phonon sideband measurement, we have shown that Ra-man spectroscopy is a superior technique than phonon side-band spectroscopy in studying the upconversion lumines-cence of these glasses.1 © 2002 American Institute of Physicsense or copyright; see http://jap.aip.org/about/rights_and_permissions1872 J. Appl. Phys., Vol. 91, No. 4, 15 February 2002 Tsang et al.DowII. EXPERIMENTThe starting materials for preparing the Tm31 dopedPWT and PWG glasses were TeO2 , GeO2 , PbF2 , WO3 andTm2O3. The composition of the glasses is listed in Table I.The oxides were first mixed thoroughly by ball milling for 2h with ethyl alcohol. The mixtures were then sintered in analumina crucible at 1200 °C for 15 ~PWT glasses! or 20~PWG glasses! min. Throughout the sintering processing, thecrucibles were covered with alumina lids to reduce volatil-ization loss. The molten PWT and PWG glasses werequenched by being pressed rapidly between two stainlesssteel molds which were preheated in a furnace at 350 °C for15 min. The glasses were subsequently annealed for 1 h atjust below the glass transition temperatures of 350 and400 °C for PWT and PWG glasses, respectively. Finally, thesamples were allowed to cool to room temperature in air.The room temperature Raman spectra were measured ata backscattering geometry using an Ar-ion laser ~CoherentInnova 70! operating at 488 nm as the excitation source.Spectra were recorded by a double grating spectrometer~Spex 1403! equipped with standard photon countingequipment. A 4-~dicyanomethylene!-2-methyl-6-~p-dimethyl-aminostyryl!-4H-pyran dye laser pumped by the same Ar-ionlaser was used in our upconversion luminescence study.Three different dye laser wavelengths ~648, 650 and 652 nm!were used and their output powers were monitored with apower meter ~Ophir 10A!. The transmittance measurementwas obtained by an UV-VIS scanning spectrophotometer~Shimadzu, UV-2101!.III. RESULTS AND DISCUSSIONThe room temperature blue upconversion emission spec-tra of PWT2 and PWG2 glasses excited with 200 mW laserlight of different wavelengths are presented in Fig. 1. Twoemission bands at 453 and 477 nm, corresponding to the1D2→3H4 and 1G4→3H6 transitions, respectively, were ob-served. It is noticed that the 477 nm upconversion lumines-cent intensity maximizes at the pump wavelength of 650 nm,demonstrating a resonance effect for both glasses. The 453nm luminescent peaks of germanate glasses show a similarresonance phenomenon. This, however, is not observed intellurite glasses. Apart from the 453 and 477 nm emissionbands, a weak emission at ;457 nm is also detected. Thispeak has not been reported in similar glass systems.2,3 Sincethe energy of this emission band does not match any transi-tion within the energy levels of Tm31 ion, we believe thatthis emission band is not due to intra-ion transition of Tm31ion. PWT and PWG glasses of lower Tm31 doping concen-tration ~PWT1 and PWG1! show similar emission profiles.TABLE I. Composition of the glasses prepared.Sample code Composition ~mol %!PWT 2 49.9TeO2-39.92PbF2- 9.98WO3-0.2Tm2O3PWG2 46GeO2-49PbF2- 4.8WO3-0.2Tm2O3PWT 1 52TeO2-42PbF2- 5.9WO3-0.1Tm2O3PWG 1 50GeO2-40PbF2- 9.9WO3-0.1Tm2O3nloaded 23 Mar 2011 to 158.132.161.9. Redistribution subject to AIP licThe relative intensities of these emission bands for PWG andPWT glasses are shown in Fig. 2. The PWG glasses, in gen-eral, have stronger upconversion emission intensities ~> oneorder of magnitude! compared with PWT glasses. This ob-servation is similar to results obtained in tellurite- andgermanate-based oxide glasses.2Figure 3 shows the intensity of the 477 nm emissionband as a function of excitation power. We see that the up-conversion emission intensity varies quadratically with exci-FIG. 1. Upconversion luminescence of PWT2 and PWG2 glasses measuredwith excited wavelength of ~a! 648 nm, ~b! 650 nm and ~c! 652 nm.FIG. 2. The relative intensities of ~a! PWG1, ~b! PWG2, ~c! PWT1 and ~d!PWT2 glasses excited by 200 mW at 650 nm.ense or copyright; see http://jap.aip.org/about/rights_and_permissions1873J. Appl. Phys., Vol. 91, No. 4, 15 February 2002 Tsang et al.Dowtation intensity for both PWT and PWG glasses. This resultsuggests that the 1G4 level is populated through a two-photon process.4,5 The energy levels of Tm31 ions and thetwo-step upconversion process are shown in Fig. 4. Themechanism is a multi-step excitation by the excited stateabsorption process.6 An excited Tm31 ion in the 3F2 levelwith 650 nm excitation relaxes to the 3F4 level via nonradi-ative transitions. The excited Tm31 ion in the 3F4 level isthen raised to 1D2 level by the absorption of a second exci-tation photon. Blue emission centered at 453 nm then resultsfrom relaxation to 3H4 level. On the other hand, the ion inthe 3F4 level may also relax to the 3H4 level via nonradiativetransitions or cross-relaxation process. If the ion is in the 3H4level and a second excitation photon is absorbed, anotherblue emission from 1G4 level centered at 477 nm is gener-ated. Here, the strong resonance effect observed in 477 nmemission band is due to the energy difference between the3H4 and 1G4 levels ~515 392 cm21!, which is strongly reso-nant to 650 nm. A similar resonance effect for the 453 nmemission band in germanate glasses is expected since thedifference of energies between the level 3F4 and 1D2 is15 225 cm21 ~657 nm!, very close to the 650 nm excitationFIG. 3. Relationship between intensity of 477 nm emission band vs excita-tion power for ~a! PWG2 and ~b! PWT2 glasses. The excitation wavelengthis 650 nm. The slopes of curves ~a! and ~b! are equal to 2.FIG. 4. Energy level diagram and the upconversion mechanism of Tm31ion. The energy values are obtained from Ref. 11.nloaded 23 Mar 2011 to 158.132.161.9. Redistribution subject to AIP licenergy. However, the absence of resonance effect in the 453nm luminescence band in tellurite glasses needs further in-vestigation.The efficiency of this upconversion emission depends onthe lifetime of the intermediate excited state, t, whose mag-nitude is governed by the radiative and nonradiative decayprocesses as71t5WRD1WP1WE , ~1!where WRD is the radiative decay rate, WP is the multi-phonon relaxation rate and WE is the energy transfer rate dueto the cross- relaxation process between ions. In the weak-coupling case of rare-earth luminescence centers, WP fol-lows an exponential gap law given by7WP5CF exp~\vP /kT !exp~\vP /kT !21Gpe2aDE ~2!In Eq. ~2!, p5DE/\vP is the phonon order, a52ln(e)/\vP where e accounts for the exact nature of theion-phonon coupling and is insensitive as the log dependencein the formula, and C is a constant dependent on the host.This multi-phonon decay of rare-earth ions in glass, in gen-eral, depends on the maximum phonon energy of the hostglass as we observe its exponential dependence in Eq. ~2!. Asa result, the lifetime of the intermediate excited state, andthus the intensity of the luminescence, decreases with in-creasing phonon energy. Based on phonon sidebandmeasurement,2 germanium-based oxide glasses with largerphonon energies should suffer a lower efficiency, i.e., aweaker luminescence. However, this prediction is inconsis-tent with our measurement as shown in Fig. 2. Kishimotoet al. also observed a similar effect in their glass systems,2suggesting that this disagreement was due to the differencein the absorption profiles of these glasses. Figure 5 shows thetransmittance spectra of the PWT2 and PWG2 glasses withabsorption edges at ;400 nm and 350 nm, respectively. At477 nm, the PWG glass has a transmittance of only abouttwice that of PWT glass. Therefore, we believe that the dif-ference in the absorption profiles of the two glasses cannotcompletely account for their difference in their luminescentintensities.In order to explain this discrepancy, Raman spectroscopywas employed to investigate the phonon spectra of PWT andPWG glasses. Figure 6 illustrates the room temperature Ra-FIG. 5. Transmittance curves of PWG2 and PWT2 glasses.ense or copyright; see http://jap.aip.org/about/rights_and_permissions1874 J. Appl. Phys., Vol. 91, No. 4, 15 February 2002 Tsang et al.Dowman spectra of PWT and PWG glasses. Our Raman spectraof PWG glasses agree well with that reported in a previousstudy.3 Three Raman bands at 345, 808 and 920 cm21, origi-nated from the Pb–F, Ge–O and W–O vibrational bonds,respectively,8,9 are observed. Similar spectra are observed inPWT glasses, with the 808 cm21 peak ~Ge–O mode! beingreplaced by a 741 cm21 peak that corresponds to a Te–Omode. In fig. 6, the W–O peak is used as an internal refer-ence to obtain a relative Raman spectra. As we have pointedout, the upconversion luminescent lifetime, and thus the lu-minescent intensity, decreases with increasing energy lost bymulti-phonon scattering. However, as the energies of thephonon are very near indicating that the p in different glasssystems are in close proximity to each other, we believe thatthe phonon density, in this case, might become important indetermining WP . The constant C, if we look at it in moredetail, also depends on the phonon density of the matrix.Based on Debye approximation, C}r(v)5/3 where r(v) isthe phonon density of state of the matrix.10 Therefore a largerphonon density will result in a larger C, implying a greaterWP and a weaker upconversion luminescence. As shown inFig. 6, the Ge–O peak in PWG glasses ~808 cm21! has aweaker intensity and smaller area than the Te–O peak ~741cm21! in PWT glasses. This indicates that the phonon den-FIG. 6. Raman spectra of ~a! PWT2 and PWG1 glasses, ~b! PWT1 andPWG2 glasses excited at 488 nm.nloaded 23 Mar 2011 to 158.132.161.9. Redistribution subject to AIP licsity of the Te–O mode in PWT glasses is much higher thanthat of Ge–O mode in PWG glasses. On the other hand, theGe–O mode has a larger energy than the Te–O mode. Al-though the PWT glass has smaller phonon energy than thePWG glass, it exhibits a lower luminescent intensity becauseof its increased multi-phonon scattering due to larger phonondensity. We expect that this phonon density effect will be-come more pronounced when the difference in phonon ener-gies is small as in our case. This interpretation based onphonon density has not been used in previous studies.2,3,7Analysis of vibrational energy as well as the phonon densitythrough Raman spectroscopy, as a result, is of great signifi-cance in understanding upconversion luminescence.IV. CONCLUSIONThe blue upconversion luminescences in Tm31 dopedtellurite and germanate glasses have been studied using of atunable dye laser as a pumping source. Tellurite glasses haveshown a much weaker upconversion luminescence than ger-manate glasses. This contradicts the prediction by phononenergy consideration. We have correlated the luminescent ef-ficiency of these two glasses with Raman spectroscopy andtransmittance measurement. Our results suggest that in theseglass systems, not only is the phonon energy of the host glassimportant in determining the luminescent efficiency, but alsothe phonon density. Since Raman spectroscopy can extractmore information ~both phonon energy and phonon density!than phonon sideband spectroscopy, it is a more effectivetool for investigating upconversion luminescence.ACKNOWLEDGMENTSThis research was partially supported by the Center ofSmart Materials of the Hong Kong Polytechnic Universityand a grant from the Hong Kong Polytechnic University~Code No. GYC-17!. W.S.T. was supported by a M. Phil.studentship of the Hong Kong Polytechnic University. Theauthors would like to gratefully acknowledge P. M. Hui forhis useful comments on this work, and W. Y. Kwan and K. T.Tam for their kind help in the experiments.1 F. Gan, Optical and Spectroscopic Properties of Glass ~Springer, Berlin,Shanghai Scientific and Technical, Shanghai, 1992!, pp. 27 and 176.2 S. Kishimoto and K. Hirao, J. Appl. Phys. 80, 1965 ~1996!.3 B. Chen, X. Kong, S. Ye, and W. Qin, Chinese J. Lumin. 20, 2 ~1999!.4 G. Ozen, A. Kermaoui, J. P. Denis, X. Wu, F. Pelle, and B. Blanzat, J.Lumin. 63, 85 ~1995!.5 M. Pollnau, D. R. Gamelin, S. R. Luthi, and H. U. Gudel, Phys. Rev. B 61,3337 ~2000!.6 K. Hirao, K. Tamai, S. Tanabe, and N. Soga, J. Non-Cryst. Solids 160, 261~1993!.7 Z. Pan, S. H. Morgan, K. Dyer, A. Ueda, and H. Liu, J. Appl. Phys. 79,8906 ~1996!.8 N. N. Greenwood, E. J. F. Ross, and B. P. Straughan, Index of VibrationalSpectra of Inorganic and Organometallic Compounds ~CRC, Cleveland,1972 and 1975!, Vols. 1 and 2.9 A. Margaryan and M. Piliavin, Germanate Glasses: Structure, Spectros-copy, and Properties ~Artech, London, 1993!, p. 34.10 R. C. Powell, Physics of Solid-State Laser Materials ~Springer, New York,1998!, Chap. 4.11 A. A. Kaminskii, Laser Crystals: Their Physics and Properties ~Springer,Berlin, 1990!, p. 146.ense or copyright; see http://jap.aip.org/about/rights_and_permissions

Evidence of the influence of phonon density on Tm³⁺ upconversion luminescence in tellurite and germanate glasses

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