Energy-transfer upconversion of rare earth ions in ionic crystals Case of Tm3+-Ho3+-codoped LiYF4 c

Journal of Alloys and Compounds408–412(2006)

680–686

Energy-transfer upconversion of rare earth ions in ionic crystals:

Case of Tm3+/Ho3+-codoped LiYF4crystals

Taiju Tsuboi?,Hideyuki Murayama

Faculty of Engineering,Kyoto Sangyo University,Kamigamo,Kita-ku,Kyoto603-8555,Japan

Received30July2004;accepted25November2004

Available online13June2005

Abstract

Up-conversion spectra of Tm3+and Ho3+ions in LiYF4crystals have been investigated at various temperatures between10and320K. The photoluminescence is investigated under excitation with Xe-lamp and laser diode.In addition to blue up-converted emission of Tm3+ and Ho3+and green up-converted Ho3+emission,anti-Stokes emission bands are observed at687and703nm under excitation in the3H4 state of Tm3+with785nm laser diode.These bands are observed above200K,and their intensities increase exponentially with increasing temperature.They are attributed to endothermic Tm3+emission due to the transition to the3H6ground state from the upper3F3state,which is thermally populated from the3H4state.Discussion is given on the optical process of green up-converted Ho3+emission,which is generated by the785nm laser diode excitation.

?2005Elsevier B.V.All rights reserved.

Keywords:Luminescence;Rare-earth ions;Upconversion;Tm3+;Ho3+;LiYF4

1.Introduction

Tm3+/Ho3+-codoped crystals have been of interest because the infrared lasers of about2?m wavelength are important for medical applications[1–4].Tm3+-doped crys-tals show an additional infrared laser oscillation at about 1.47?m.Optical communication has been currently per-formed using lasers operating in the C-band(1.53–1.67?m) region where silica?bers have the ultra low loss.The wave-length region of1.45–1.50?m is also a low-loss region.This wavelength region will be important in near future as it will enable us to extend the optical communication bandwidth to S-band(1.48–1.53?m)region[5,6].Therefore,Tm3+-doped materials are suggested to be necessary as optical ampli?ca-tion device in the future optical communications.

There are many kinds of host materials such as oxides, chlorides and?uorides for doping rare-earth ions.Of them the oxides have been intensively studied.Unlike the oxides,

?Corresponding author.Tel.:+81757051899;fax:+81757051899.

E-mail address:tsuboi@cc.kyoto-su.ac.jp(T.Tsuboi).intense emission is obtained in?uorides because the oxides have much higher phonon energy than the?uorides.The high phonon energy easily gives rise to non-radiative multi-phonon transition from the emitting state to the nearest low-energy state,resulting in degradation of photolumines-cence.Additionally the?uorides are more transparent in the ultra violet spectral region than the oxides,i.e.not only the blue luminescence but also ultra violet luminescence is observed in?uorides.This indicates that the?uorides are more useful to obtain the blue and ultra violet emis-sions than the oxides.Such a short-wavelength lumines-cence is necessary for the optical memory,optical recording and full-color display.Therefore,we investigate the?u-oride host material such as LiYF4(YLF)in the present study.

The Tm3+-and Tm3+/Ho3+-codoped crystals are also of interest because they generate ultraviolet,blue,green and red up-converted emissions by infrared laser pumping.Of various Tm3+-doped crystals,Tm3+-doped and Tm3+/Ho3+-codoped YLF crystals are materials well known for the high ef?cient up-conversion process,indicated by blue and red

0925-8388/$–see front matter?2005Elsevier B.V.All rights reserved. doi:10.1016/j.jallcom.2004.11.079

T.Tsuboi,H.Murayama /Journal of Alloys and Compounds 408–412(2006)680–686681

emissions [7–9].Laser action in the visible has been obtained under infrared laser diode (LD)excitation at room tem-perature.Spectroscopic study of the upconversion helps to achieve high ef?cient laser action.Here we report the opti-cal properties of Tm 3+/Ho 3+-codoped YLF crystals under excitation with 785nm LD.Taking into account the results obtained by excitation with 460,680and 780nm light of Xe lamp,we suggest a new optical process for the anti-Stokes 687and 703nm emissions generated by 785nm LD excitation.

There are various up-conversion processes in rare-earth-doped materials.The simple process is the two-,three-or multi-photon absorptions where two,three or n photons with the same photon energy h νare absorbed by one rare-earth ion simultaneously and excited to the excited state at 2h ν,3h νor nh ν,respectively,and then luminescence with energy of 2h ν,3h νor nh νis generated by one-photon process.The second type of up-conversion process is via excited state absorption.For example,a Tm ion (Tm1)is excited to the 3H 4state by 785nm laser,followed the relaxation to the 3H 5state non-radiatively,then the second 785nm laser light gives rise to the transition from the 3H 5state to the 1G 4state (the excited state absorption).As a result the upconversion occurs by the transition from the 1G 4state to the ground state (as shown in the left-hand side of Fig.1).Alternatively,the nearby Tm ion (Tm2)is possible to be excited to the 3H 4state,followed the photon emission by the reversed transition.The Tm1is excited to the 1G 4state by absorbing the emitted photon from Tm2through the cross relaxation (see the right-hand side of Fig.1).Such an energy transfer between the two Tm 3+ions is possible in heavily Tm 3+-doped crystals.In the case of co-doped materials (i.e.materials doped with different rare-earth ions like Tm 3+and Ho 3+),the cross relaxation process is frequently observed.In this paper we study which kind of up-conversion processes occurs in Tm/Ho-codoped YLF

crystals.

Fig.1.Two kinds of up-conversion processes by two-photon absorption via (1)the excited state absorption in a Tm 3+ion (Tm1)and (2)the cross-relaxation between two Tm 3+ions (Tm1and Tm2)and the excited state absorption in Tm1.

2.Experimental procedure

Single crystals of Ho 3+/Tm 3+-codoped YLF crystals were grown in a standard RF-heating Czochralski furnace.An irid-ium crucible was put in the furnace.Two kinds of crystals were studied here;one is a very-lightly Ho 3+-doped crys-tal (called TmYLF,hereafter)with 6%Tm 3+and 0.01%Ho 3+ions,the other is YLF crystal with 8%Tm 3+and 0.5%Ho 3+ions (called HoYLF).The concentrations of Tm 3+and Ho 3+ions are 1.12×1021and 7.00×1019cm ?3in HoYLF,respectively.The emission and excitation spectra were measured with a Spex Fluorolog-3?uorophotometer in 350–1600nm spectral region.The excitation was under-taken by 450W Xe-lamp.The spectral resolution was set to be 0.2nm.Crystals were also excited with 785nm LD with maximum power of 42mW to investigate the upconversion.Absorption spectra of the crystal were measured with a Cary-5E spectrophotometer.Emission and absorption spectra were measured at various temperatures between 10and 320K.

3.Experimental results and discussion

Tm 3+ions have absorption bands at about 1600–1780,1120–1230,770–800,680–700,645–670,450–480,350–365,285–290,280–284,270–275and 255–264nm,which are due to the 4f 12→4f 12electronic transitions from the 3H 6ground state to the 3F 4,3H 5,3H 4,3F 3,3F 2,1G 4,1D 2,1I 6

,3P 0,3P 1and 3P 2states of Tm 3+,respectively [9–12].Tm 3+gives rise to many emission bands in infrared

to ultra violet spectral region.Of them,there is a broad emission band with several components at 1400–1500nm [13],as observed in various Tm 3+-doped materials [5,6].Fig.2shows the excitation spectrum for 1451nm emission of TmYLF at 285K,which is measured in 250–850nm region.Many excitation bands are observed in Fig.2.All the excitation bands correspond to the absorption bands of Tm 3+.The excited states are indicated for each excitation band in Fig.2

.

Fig.2.Excitation spectrum for 1451nm Tm 3+emission in TmYLF crystal at 285K.

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680–686

Fig.3.(a)Up-conversion emission spectra,in 340–600nm spectral region,of TmYLF and HoYLF crystals excited with 785nm laser diode with 35mW at 290K.(b)Up-conversion emission spectra,in 600–900nm spectral region,of TmYLF and HoYLF crystals excited with 785nm laser diode with 35mW at 290K.An intense line at 785nm is due to the excitation laser diode.

Fig.3(a)and (b)shows the ultraviolet to near-infrared emission spectra of TmYLF and HoYLF crystals excited in the 3H 4state of Tm 3+with 785nm LD at room temperature.No upconversion are observed at 300–350nm,but many up-converted emission bands are observed at wavelength longer than 350nm.Fig.4shows the enlarged spectrum of Fig.3(a)in the blue region.Nguyen et al.reported the blue emission bands appearing at 443–458nm region in Tm 3+-doped YLF crystal by two-color excitation with 780.78and 648.77nm lights at room temperature [14].The spectrum is consistent with the spectrum of TmYLF shown in https://www.360docs.net/doc/0f9459715.html,paring the TmYLF spectrum with the HoYLF spectrum,we can

attribute

Fig.4.Enlarged spectrum of Fig.3(a)in a 430–470nm spectral region.

the emission bands at 456nm to Ho 3+and the other bands to Tm 3+as shown in Fig.4.

The emission bands with peaks at 360,449,477,666,687,703,806and 814nm are due to Tm 3+ions,while the emis-sion bands at 383.5,385,406,418,456,520,541,549,579,649.5,and 750nm are due to Ho 3+ions.The 360,449,477,483.5,666,806and 814nm Tm 3+bands are attributed to the transitions 1D 2→3H 6,1D 2→3F 4,1G 4→3H 6,1G 4→3H 6,3H 4→3H 6,and 3H 4→3H 6

,respectively.The 385,406,418,456,483.5,520,541,549,579,649.5,and 750nm Ho 3+bands are attributed to the transitions 5G 4→5I 8,3D 3→5I 6,5G 5→5I 8,5D 3→5I 5,5D 3→5I 4,3H 5→5I 6

,5D 2→5I 8,

5S 2→5I 8,5G 4→5I 6,5F 5

→5I 8,and 5I 4→5I 8,respec-tively.These assignments are made by comparison of the up-conversion spectrum with the energy levels of Tm 3+and Ho 3+ions as shown in Fig.5.For example,if the energy of emission band peak coincides with that of excited state of Tm 3+or Ho 3+,the emission band is assigned to the transition from the excitated state to the ground state.The assignments are also made by comparison with the usual photolumines-cence (down-conversion)measurement using Xe lamp light with different excitation wavelengths such as 260,360,460and 680nm.

Weak emission band is observed at 885–895nm in HoYLF in Fig.3(b).It consists of three components at 887,890and 893nm.This emission band is due to the transition 5I 5→5I

8

https://www.360docs.net/doc/0f9459715.html,parison of up-conversion spectrum of HoYLF excited by 35-mW 785nm LD (right-hand side of the ?gure)with the energy levels of Tm 3+and Ho 3+ions.The up-conversion intensity is shown in log-scale.A sharp peak at 12739cm ?1is due to the 785nm LD used for excitation.

T.Tsuboi,H.Murayama/Journal of Alloys and Compounds408–412(2006)680–686

683

Fig.6.Peak intensities of the360,449,687and814nm Tm3+emission bands in TmYLF crystal plotted against the785nm laser diode intensity at 290K.

of Ho3+.In Fig.3(b),two weak emission bands are observed

at687and703nm by785nm LD excitation.These emission

bands were not observed by the down-conversion measure-

ment with,e.g.360,460and680nm light from Xe lamp.We

discuss the687and703nm emission bands later.In TmYLF

the549nm emission band is observed as the Ho3+emission

in TmYLF.The intensity is comparable with the650–670nm

Tm3+emission intensity although the Ho3+concentration is

much smaller than the Tm3+one in TmYLF,indicating ef?-

cient energy transfer from Tm3+to Ho3+.

Fig.6plots the intensity of the360,449,687and814nm

Tm3+emissions observed in TmYLF against the785nm LD

power.The360,449,687and814nm emissions take slopes

of3.0,3.0,1.2and1.0in the log–log scale.This indicates

that the360and449nm emission is due to the upconver-

sion by three-photon process.The360and449nm emission

bands arise from the transition from the1D2state to the3H6

ground state and3F4state,respectively,as mentioned above.

To excite into the1D2state by785nm photon,the follow-

ing processes are necessary.The?rst photon excites Tm3+

ion from the ground state to the3H4state from which Tm3+

relaxes to the3H5state.The second785nm photon excites

from the3H5state into the1G4state from which Tm3+relaxes

to the3F3state.The third785nm photon excites from the 3F3state into the1D2state.On the other hand,the814nm emission is con?rmed to arise from the one-photon down

conversion.It was observed that the up-converted687and

703nm emissions take the same slope of1.0.

We plot the intensities of the other emission bands,which

were observed in TmYLF and HoYLF crystals,against LD

power in Figs.7and8,respectively.The slope in the log–log

plotted power dependence is1.98,1.87and1.86for the477,

483and489nm emissions,1.93,1.93,1.91,1.96and2.02for

the517,520,528,541and549nm emissions,1.59,1.52,1.56

and1.59for the648,653,663and666nm emissions,1.41

and1.42for the642and656nm emissions,1.8,1.9,2.0and

2.1for the690,705,740and750nm emissions,respectively.

These up-conversion emission bands have the slope values of

much higher than1.0.The results of Fig.7are the same as

the Fig.7.Peak intensities of various emission bands in TmYLF crystal plotted against the785nm laser diode intensity at290K.

results of Fig.8except for the360and386nm emissions.The 360nm Tm3+and386nm Ho3+emissions take slope of3in TmYLF,while both of the two emissions take slope of2at low LD power range and slope of3at high power range in HoYLF as shown in Fig.8.This indicates that different optical process occurs to give rise to these up-converted emissions between the TmYLF and HoYLF crystals.We consider this optical process as follows.

The concentration of Ho3+ions is higher in HoYLF than in TmYLF.The360nm Tm3+emission arises from the transi-tion from the1D2state at about27800cm?1above the ground state.Therefore,at least three785nm photons(with photon energy of12740cm?1)are needed to excite into the1D2state as shown in Fig.9(b).This interpretation is consistent with the slope of3obtained for the360nm emission in TmYLF.In HoYLF,however,the360nm emission was obtained by two-photon absorption process at low785nm LD power range. Why only two photons are possible to excite into the1D2 state in HoYLF?

The?rst785nm photon excites Tm3+ion from the ground state to the3H4state.Ho3+ion has the5I4state which is quite close to the3H4state.Therefore,the energy transfer occurs easily from Tm3+to Ho3+because Tm3+ions are more possible to locate near Ho3+sites in heavily Tm3+

-doped Fig.8.Peak intensities of various emission bands in HoYLF crystal plotted against the785nm laser diode intensity at290K.A letter“s”means the value of slope for a straight line drawn in the log–log scale.

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680–686

Fig.9.(a)The360nm Tm3+up-converted emission and386nm Ho3+up-converted emission by two-photon process with785nm LD photon by energy transfer between Tm3+and Ho3+ions.(b)The360nm Tm3+up-converted emission by three-photon process(see text).

(8%)HoYLF crystal than in the very lightly Ho3+-doped (0.01%)TmYLF crystal.The second785nm photon excites Ho3+from the excited5I4state to the3G4state of Ho3+,from which the386nm Ho3+emission is generated(see Fig.9(a)). At the same time the3F2state of Tm3+locates close to the 5I4state.Thus,the energy transfer occurs to the3F2state,

followed the excitation from the3F2state to the1D2state of Tm3+by the second785nm photon(as shown in Fig.9(a)). In this way we can understand the presence of the386and 360nm upconversions by two-photon absorption process.

The687and703nm emission bands show the slope of almost1.00as mentioned above.The slope of1means that excitation with one-photon absorption process gives rise to emission.Why the up-converted687and703nm emis-sions are possible by the one-photon excitation process?We explain this unusual result as follows.

Fig.10shows the temperature dependence of the687and 703nm emission bands.It is observed when crystal is heated from15K up to320K that these bands appear above200K. The band intensities increase exponentially with temperature in a form of a i exp(? E/kT)as shown in Fig.11.The best?t was observed at E=1700cm?1and a2/a1=1.77where a1 and a2are parameters for the687and703nm emission bands, respectively.This indicates that the two emission bands arise from the thermally excited states.

Fig.12shows the3H6→3H4,3F3and3F2absorption spectra of655–805nm region at15and290K,together with the spectrum of the687and703nm emission

bands.Fig.10.The687and703nm emission bands of TmYLF,which was excited with785nm laser diode,at various temperatures between215and296

K. Fig.11.Temperature dependence of the intensities of the687and703nm emission bands observed in Fig.10.Solid curves are the best?t to the experimental points,which were derived using a form of a i exp(? E/kT) (see

text).

Fig.12.Absorption spectra of TmYLF at15and290K,together with the spectrum of the687and703nm emission bands obtained by785nm laser diode excitation at290K.

T.Tsuboi,H.Murayama/Journal of Alloys and Compounds408–412(2006)680–686

685

Fig.13.Schematic energy level diagram of the3F3,3H4and3H6states of Tm3+and the optical transitions of the absorption bands at687,703,780 and786nm at290K and the687and703nm emission bands obtained by 785nm laser diode excitation.

The687nm band peak wavelength coincides with the wave-length of the3H6→3F3absorption band peak.Absorption band at703nm is observed at room temperature.Its inten-sity decreases with decreasing temperature and the703nm absorption band is not observed at low temperatures such as15K.This means that the703nm absorption band is a hot band which is due to transition from a thermally excited Stark substate of the3H6ground state(see Fig.13).The wavelength of the703nm absorption band peak coincides with that of the 703nm emission band peak.This coincidence allows us to attribute the703nm emission to the reverse electronic tran-sition of the703nm absorption(Fig.13).

The3F3state is at about1700cm?1above the3H4state which is excited by the785nm LD.The energy gap between the3F3and3H4states agrees with E value derived from the experimental results.Taking into account these results and considering that the LD irradiation gives rise to heating of crystal,it is con?rmed that the687and703nm emission bands are caused by transition from the3F3state which is thermally populated from the3H4state as shown in Fig.11. The687and703nm emissions are attributed to the transition from the3F3state to the Stark levels at0and300cm?1in the 3H6ground state,respectively.The presence of the300cm?1 Stark level is con?rmed by the appearance of the703nm hot absorption band.

An intense Ho3+up-conversion band is observed at549nm in TmYLF by785nm LD excitation as shown in Fig.3(a). Their slope values in the log–log plotted power dependence are quite close to2.0,indicating these emissions are due to the two-photon excitation.The LD light excites Tm3+to the 3H4state.Ho3+is not excited by the785nm LD because Ho3+has no absorption band around785nm.Therefore,the 549nm Ho3+emission band is caused by energy transfer

from Fig.14.Two possible energy transfers from Tm3+to Ho3+by non-resonant process in the up-converted549nm Ho3+emission generated by excitation in the3H4state of Tm3+with785nm laser diode.The dotted arrow means the non-radiative relaxation.

Tm3+to Ho3+.Two processes are conceivable for the energy transfer,which are due to the non-resonant energy transfer[8] assisted by creation or annihilation of phonons.The energies of the5I5and5I4states of Ho3+are close to that of the3H4 state of Tm3+as shown in Fig.14.Therefore,the energy in the 3H4state of Tm3+excited by the?rst785nm LD photon is

transferred to the5I5and5I4state,subsequently excited to the upper states5G5and5G4of Ho3+by the second785nm pho-ton,respectively(see Fig.14(a)and(b)).In fact,two-photon excited up-converted419and386nm emission bands,which are due to the5G5→5I8and5G4→5I8transitions of Ho3+, are observed in HoYLF crystal,respectively,although these emission intensities are not as intense as the other upconver-sion.Therefore,the two processes of Fig.14(a)and(b)occur in HoYLF.It is dif?cult to determine at this moment which process occurs more strongly and more ef?ciently.

4.Summary

Photoluminescence and up-conversion spectra of Tm3+ and Ho3+ions in LiYF4crystals with6%Tm3+and0.01% Ho3+ions and with8%Tm3+and0.5%Ho3+ions were investigated in350–1600nm spectral region at various tem-peratures between10and320K.The photoluminescence was investigated using excitation with460,680and780nm light of Xe-lamp,while up-conversion and down-conversion was investigated by785nm laser diode with maximum power of35mW.The1150–1250nm1G4→3H4emission of Tm3+was observed under the460nm excitation but not observed under the680and780nm excitation.In addition to the386and419nm Ho3+,477–489nm Tm3+,541–549nm Ho3+,648–666nm Tm3+up-converted emissions due to two-photon excitation,anti-Stokes emission bands were observed at687and703nm under excitation in the3H4state of Tm3+with785nm laser diode.The687and703nm bands were observed to appear above200K,and their intensities

686T.Tsuboi,H.Murayama/Journal of Alloys and Compounds408–412(2006)680–686

increased exponentially with increasing temperature.They were attributed to endothermic Tm3+emission due to the transition to the3H6ground state from the upper3F3state, which is thermally populated from the3H4state.Optical pro-cesses by energy transfer from Tm3+to Ho3+were suggested for the green up-converted Ho3+emission. Acknowledgments

We thank Prof.K.Shimamura for providing crystals which were used in the present work.The present work is partially supported by the Grant-in-Aid for Scienti?c Research from the Japan Society for Promotion of Science.

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