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Journal of Luminescence 106 (2004) 15–20

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0022-2313/

doi:10.1016

Upconverted VUV luminescence of Nd3+ and Er3+ doped intoLiYF4 crystals under XeF-laser excitation

D. Loa, V.N. Makhovb,*, N.M. Khaidukovc, J.C. Krupad, J.Y. Geslande

aPhysics Department, Chinese University of Hong Kong, Hong Kong SAR, ChinabLebedev Physical Institute, Leninsky Prospect 53, Moscow 119991, Russia

cKurnakov Institute of General and Inorganic Chemistry, Moscow 117907, Russiad Institut de Physique Nucl!eaire, Orsay Cedex 91406, France

eUniversit!e du Maine, Le Mans 72017, France

Received 15 October 2002; received in revised form 25 June 2003; accepted 25 June 2003

Abstract

The upconverted VUV (185 nm) and UV (230 and 260 nm) luminescence due to 5d–4f radiative transitions in Nd3+

ions doped into a LiYF4 crystal has been obtained under excitation by 351/353 nm radiation from a XeF excimer laser.

The maximum upconversion efficiency, defined as the ratio of intensity for 5d–4f luminescence to overall intensity for

5d–4f and 4f–4f luminescence from the 4D3/2 Nd3+ level, has been estimated to be about 70% under optimal focusing

conditions for XeF laser radiation. A redistribution of intensity between three main components of 5d–4f Nd3+

luminescence is observed under changing the excitation power density, which favors the most long-wavelength band

(260 nm) at higher excitation density level. The effect is interpreted as being due to excited state absorption of radiation

emitted. The upconverted VUV and UV luminescence from the high-lying 2F(2)7/2 4f level of Er3+ doped into a LiYF4

crystal has also been obtained under XeF-laser excitation the most intense line being at 280 nm from the spin-allowed

transition to the 2H(2)11/2 4f level of Er3+, but the efficiency of upconversion for Er3+ emission is low, less than 5%.

r 2003 Elsevier B.V. All rights reserved.

Keywords: 5d–4f and 4f–4f luminescence; Nd3+; Er3+; Fluoride crystals; Upconversion pumping

1. Introduction

Wide band-gap crystals doped with rare earth(RE) ions Nd3+, Er3+ and Tm3+ exhibiting theinterconfigurational 5d–4f transitions are promis-ing candidates for laser action in the VUV range[1]. However, up to now in the VUV only the172 nm laser emission from Nd3+-doped LaF3

onding author. Fax: +7-95-938-2251.

address: makhov@sci.lebedev.ru (V.N. Makhov).

$ - see front matter r 2003 Elsevier B.V. All rights reserve

/S0022-2313(03)00129-7

crystals has been reported [2,3]. The main problemarising in such experiments is the formation ofboth stable and transient color centers with strongabsorption on the wavelengths of the 5d–4ftransitions in the RE3+ ions. The effect is due tothe excited state absorption (ESA) of the pumpradiation, which promotes the electron from theRE3+ 5d state to the conduction band (two-stepphotoionization of the RE3+ ions), resulting in thecreation of color centers [4]. On the other hand, inprinciple the strong ESA on the laser wavelength

d.

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D. Lo et al. / Journal of Luminescence 106 (2004) 15–2016

can completely prevent any laser effect if the ESAcross-section exceeds the stimulated emission crosssection. In other words, ESA is probably the mainobstacle for laser action in the systems based on5d–4f transitions in RE ions doped into wideband-gap crystals.The effect of ESA on the pump wavelength can

be considerably less important in the upconversionpumping schemes for the VUV solid-state laserswhen the photon energy of the pump radiationmight be not enough for photoionizing the RE3+

ion from the upper laser (5d) state. In particular,the observation of VUV/UV 5d–4f emission ofNd3+ under two-step excitation at 355 nm with apulsed frequency tripled Nd:YAG laser has beenreported for LiYF4, LiLuF4, BaY2F8 andNa0.4Y0.6F2.2 doped with Nd

3+ [5–7]. Howeverupconversion pumping at the 355 nm radiation hasbeen considered to be inadequate for obtaininglaser action on 5d–4f transitions in Nd3+ due torather small cross-section (o10�20 cm2) of the4D3/2-5d transition which is involved inthe second stage of the two-step pumping forthe fluoride crystals doped with Nd3+ [6]. In thepresent paper the two-step scheme of pumping forthe VUV/UV emission from Nd3+ doped intoLiYF4, SrF2 and BaF2 crystals as well as fromEr3+ doped into LiYF4 crystal has been studiedunder excitation by 351/353 nm radiation from aXeF excimer laser.

2. Experiment

A XeF excimer laser AQX-150 (MPB Technol-ogies Inc.) operating at 351/353 nm was used forthe upconverted luminescence experiments. Thelaser emits about 60% of its radiation into the351 nm line whereas the rest 40% in the 353 nmline. For exciting the studied crystals, the pumpbeam from the XeF laser was introduced throughthe MgF2 window into the vacuum samplechamber and was focused into a 0.3� 30mm2

spot by using a CaF2 cylindrical lens with a focallength of 10 cm. The spectrum of radiation emittedfrom the crystal was analyzed with a 0.4m Czerny-Turner vacuum monochromator (SpectraPro VM-504, Acton Research Corporation) and a gated

diode array detector (IVUV-700, Princeton Instru-ments). The measurements of emission timeevolution were carried out with a photomultipliertube (EMI 9781B) installed in a separate exit armof the monochromator behind the sodium-salici-late-coated window for VUV-to-visible light con-version. The fluorescence decay curves wererecorded using a digital LeCroy 9362C 1.5GHzoscilloscope. The energy in the 28 ns pulse of theXeF laser was variable from 50 to 120mJdepending on the operation conditions.Single crystals of LiYF4 doped with 1.2 at%

Nd3+ or with 3.0 at% Er3+ were grown by theCzochralski method as described in Ref. [8]. Singlecrystals of SrF2 doped with 0.25wt% Nd

3+ andsingle crystals of BaF2 doped with 0.5wt% Nd

3+

were grown by the Bridgman method in the StateInstitute of Rare Metals. For the experiments thepolished rods with a length of 10mm were used.The entrance slit of the VUV monochromatorsituated at a distance of about 50 cm from thecrystal served as a pinhole to collect radiationemitted from the crystal along the axis of laser-pumped volume. All the experiments were per-formed at room temperature.

3. Results and discussion

3.1. Neodymium doped crystals

The upconverted VUV (185 nm) and UV (230and 260 nm) luminescence due to 5d–4f radiativetransitions in Nd3+ ions doped into a LiYF4crystal has been obtained under excitation by theXeF excimer laser (Fig. 1). The first photon fromthe laser excites the Nd3+ ion onto the 4D5/2 levels,i.e. to slightly higher-lying levels as compared tothe levels excited by the third harmonic ofNd:YAG laser. However, due to very small energyseparation between the 4D5/2 and the

4D3/2 levels,this excited state of the Nd3+ ion quickly relaxesnon-radiatively to the 4D3/2 level, the lowestStark component of which is located inNd3+:LiYF4 at 28095 cm

�1 [9]. The secondphoton from the laser after the absorption bythe excited Nd3+ ion directly populates the 5dlevels of Nd3+ since the zero-phonon line, or the

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Fig. 1. Emission spectrum of the 1.2 at% Nd3+:LiYF4 crystal excited by 351/353 nm radiation from the XeF excimer laser. The

assignments of emission lines to corresponding 5d–4f and 4f–4f transitions in the Nd3+ ion are also shown in the figure (T ¼ 300K).

Fig. 2. Emission spectra of the 1.2 at% Nd3+:LiYF4 crystal

excited by the XeF laser under optimal and non-optimal

focusing conditions (T ¼ 300K).

D. Lo et al. / Journal of Luminescence 106 (2004) 15–20 17

edge of f–d transitions for Nd3+ in LiYF4 issituated at about 56180 cm�1 [10].The intensity of VUV/UV emission strongly

depends on the focusing conditions, i.e. on theexcitation power density (Fig. 2). The maximumup-conversion efficiency, defined as the ratio ofintensity for 5d–4f luminescence to overall inten-sity for 5d–4f and 4f–4f luminescence from theNd3+ 4D3/2 level, has been obtained to be about70% under optimal focusing conditions for XeFlaser radiation. This means that 70% of Nd3+ ionsexcited to the 4D5/2 level after the absorption of thefirst laser photon are excited to 5d levels on thesecond stage of two-step excitation. By taking intoaccount the parameters of the XeF laser, theexperiment geometry and the data concerning 4f–4f absorption coefficients for Nd3+ in LiYF4 fromLSB’s Database Lasers [11], our rough estimationshows that a concentration of the Nd3+ ionsexcited in the 4D3/2 state up to 7� 10

17 cm�3 canbe created in the laser-pumped volume when thelaser beam is properly focused. The second stageof two-step excitation promotes 70% of these ionsfrom the 4D3/2 state to 5d states, i.e. creates apopulation density DNB5� 1017 cm�3 on the 5dlevel of Nd3+. The latter result is not consistent

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with absorption cross-section of o10�20 cm2

reported in Ref. [6] for the 4D3/2-5d transition.For the 5d–4f VUV emission of Nd3+ in LiYF4the stimulated emission cross-section sð0Þ at the185 nm line center has been estimated in Ref. [12]to beB2� 10�17 cm2. Accordingly the gain for the5d–4f laser transition in Nd3+:LiYF4 that can beobtained in our scheme of two-step exitation bythe XeF laser is a ¼ sð0Þ � DNB10 cm�1, but thisis the estimated upper limit of the gain. In our casethe actually obtainable net gain would be smallerand is not sufficient for obtaining the superradiantlasing effect, that is, emission line narrowing orreduction of the emission duration, without the aidof an optical cavity.In principle, populating 5d levels in Nd3+ during

the second stage can arise from both direct excitationof Nd3+ ions being in the 4D3/2 state and cross-relaxation between neighboring Nd3+ ions excited tothe 4D3/2 level. The mechanism of excitation can berecognized by the profile of the decay kinetics for the5d emission since cross-relaxation suggests ratherprolonged duration of the process due to a relativelylong lifetime for the Nd3+ 4D3/2 state. This emittinglevel lifetime for 1.2at% Nd3+:LiYF4 has beenmeasured to be of the order of 1ms. On the otherhand, the time-resolved measurements (Fig. 3) have

Fig. 3. Time evolution curves for 5d-4f Nd3+ emissions at 185 (2) anlevel at 410 nm (4) in the 1.2 at% Nd3+:LiYF4 crystal excited by

(T ¼ 300K).

also shown that decay kinetics of Nd3+ 5d emissionfollows well to a single-exponential profile withlifetime B30ns after the excitation by the laserpulse with duration B27ns and does not depend onthe excitation power density caused by focusingconditions.A remarkable intensity redistribution between

the three main components of Nd3+ 5d–4f lumi-nescence is observed under changing the excitationpower density, which favors the most long-wave-length band (260nm) at a higher density level, thatis when the upconversion efficiency is maximum.This feature can be explained by ESA for radiationemitted. For the shortest wavelength emission bandat 185nm, the photon energy can be large enoughfor photoionizing the Nd3+ ions excited into the 5dlevels. In other words such photons can beeffectively absorbed by the excited Nd3+ ionsand, as a result, the part of the most high-energyphotons disappears from the emission spectrumgiving rise to a visual redistribution of emittedphotons in favor of the long-wavelength part of thespectrum. The effect should be more pronouncedfor higher ESA rate, that is, at higher excitationpower density. For the long-wavelength emissionbands the effect of ESA can be also present, but inthis case ESA results in excitation of Nd3+ ions

d 260 (3) nm as well as for 4f-4f Nd3+ emission from the 4D3/2pulsed 351/353 nm radiation (1) from the XeF excimer laser

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D. Lo et al. / Journal of Luminescence 106 (2004) 15–20 19

into higher-lying 5d levels without any photoioni-zation since the photon energy is not enough forphotoionizing Nd3+. Such a channel of ESA resultsin only the repopulation of the lowest 5d Nd3+

level after fast non-radiative relaxation without anyinfluence on the Nd3+ 5d–4f emission spectrum.The upconverted 5d–4f luminescence from

Nd3+ has been also obtained for the Nd3+:SrF2and Nd3+:BaF2 crystals (not shown) but theefficiency of upconversion is considerably lowerfor these crystals than for Nd3+:LiYF4. The lowerupconversion efficiency for Nd3+:SrF2 andNd3+:BaF2 crystals is probably due to lowerconcentration of Nd3+ and more poor opticalquality of these crystals as compared withNd3+:LiYF4. As expected, no upconverted 5d–4fluminescence of Nd3+ has been observed forthe Nd3+:LaF3 crystal, similarly to the resultsobtained in Ref. [5], since the energy of two-stepexcitation is not enough to excite the Nd3+ ion inLaF3 even to the lowest 5d state.

3.2. Erbium doped crystals

The upconverted VUV and UV luminescencefrom the high-lying 2F(2)7/2 4f level of Er

3+ doped

Fig. 4. Emission spectrum of the 3.0 at% Er3+:LiYF4 crystal excit

assignments of emission lines to corresponding 4f–4f transitions in th

into LiYF4 crystal has been also obtained underXeF-laser excitation (Fig. 4). As expected the mostintense emission line is observed at 280 nm for thespin-allowed transition to the 2H(2)11/2 Er

3+ level.The photon energy of the XeF laser is not enoughfor a two-step excitation of Er3+ to 5d states sincethe band edge of f–d spin-forbidden transitions forEr3+ in LiYF4 is measured to be near 60980 cm

�1

[13]. The two-step excitation of Er3+ into the2F(2)7/2 level by the XeF laser photons requires anintermediate non-radiative relaxation stage afterthe absorption of the first 351/353 nm photon.Photons with such an energy can excite Er3+ up tothe 4G7/2 state having the energy levels within therange 28252–28352 cm�1 for, as an example, LaF3[14]. The photon energy from the XeF lasermatches well the transition 4G11/2–

2F(2)7/2 [14],i.e. a non-radiative relaxation from the 4G7/2 to the4G11/2 levels should occur before the second stageof the two-step excitation of Er3+ ions. Thekinetics of such a relaxation is definitely ratherfast because the maximal energy gap between the4G9/2 and the

4G11/2 levels which should beovercome is not more than 900 cm�1. Due tomuch weaker absorption cross-sections for the4f–4f transitions than for the 4f–5d ones which are

ed by 351/353 nm radiation from the XeF excimer laser. The

e Er3+ ion are also shown in the figure (T ¼ 300K).

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involved in the second stage of the two-stepexcitation, the efficiency of upconverted excitationof VUV/UV emission from the 2F(2)7/2 Er

3+ levelis less than 5%, i.e. it is much lower than in thecase of upconverted excitation of 5d–4f emissionfrom Nd3+.

4. Conclusions

The upconverted VUV (185 nm) and UV (230and 260 nm) luminescence due to 5d–4f radiativetransitions in Nd3+ ions doped into a LiYF4crystal can be easily obtained under excitation by351/353 nm radiation from a XeF excimer laseras a result of a two-step mechanism: 4I9/2-4D5/2/

4D3/2-5d. The maximal upconversion effi-ciency reached is about 70%. However, in ourcase, the pumping conditions have not beenoptimal enough to observe a lasing effect in thesuperradiant emission mode. A remarkable redis-tribution of intensity among the three maincomponents of 5d–4f Nd3+ luminescence towardsthe long-wavelength band at 260 nm is observedunder changing the excitation power density. Thiseffect is due to the influence of ESA on emittedradiation. The upconverted VUV and UV lumi-nescence from the high-lying 2F(2)7/2 4f level ofEr3+ doped into a LiYF4 crystal has been alsoobtained under XeF-laser excitation, but theefficiency of upconversion for Er3+ emission islow, less than 5%. The latter process requires anintermediate non-radiative relaxation for theexcited Er3+ ions from the 4G7/2 to the

4G11/2states before the second stage of the two-stepexcitation process.

Acknowledgements

The support by Earmarked Research Grant No.4024/98E by the Government of Hong Kong SAR,INTAS Grant 99–01350 and Russian FederalProgram ‘‘Integration’’ (Grant B0049) is gratefullyacknowledged. One of the authors (V.M.) wassupported by a C.N. Yang fellowship when theexperiments in this work were performed.

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