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0022-2313/$ - se

doi:10.1016/j.jlu

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Journal of Luminescence 119–120 (2006) 28–32

www.elsevier.com/locate/jlumin

Two-photon excitation to 4f65d configuration of Gd3þ

in LiGdF4 crystal by KrF excimer laser

D. Loa, V.N. Makhovb,�, N.M. Khaidukovc, J.C. Krupad

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

cKurnakov Institute of General and Inorganic Chemistry, 119991 Moscow, RussiadInstitut de Physique Nucleaire, 91406 Orsay Cedex, France

Available online 30 January 2006

Abstract

The two-photon excitation to high-lying levels of Gd3þ via the intermediate 6DJ levels in LiGdF4 and GdF3 crystals

has been studied under pumping by 248 nm radiation from a KrF excimer laser. Luminescence in the UV (204 nm),

orange (570–640 nm) and IR (720–810 nm) regions, which is due to 6GJ–8S7=2,

6GJ–6PJ and

6GJ–6IJ transitions in Gd3þ,

respectively, has been observed from the LiGdF4 crystal at room temperature. No radiative transitions from the 4f7 6GJ

levels of Gd3þ have been detected from the GdF3 crystal. It was assumed that at low temperatures the two- photon

excitation of the LiGdF4 crystal with a KrF laser could result in the observation of short-wavelength upconverted VUV

luminescence at �127 nm due to 4f65d–4f78S7=2 transitions in Gd3þ .

r 2006 Elsevier B.V. All rights reserved.

PACS: 78.55.Hx; 71.55.Ht; 42.72.Bj

Keywords: 5d–4f and 4f–4f luminescence; Trivalent gadolinium ion; Fluoride crystals; Upconverted UV/VUV luminescence; LiGdF4;

GdF3

1. Introduction

Wide band-gap crystals doped with rare earth(RE) ions, which exhibit radiative interconfigura-tional 4fn�15d–4fn transitions, are well-knownsolid-state VUV emitters and are considered aspromising candidates for laser action in the VUV

e front matter r 2006 Elsevier B.V. All rights reserve

min.2005.12.007

ing author. Fax: +7095 9382251.

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

range [1]. However, up to now, only 172 nm laseremission from Nd3þ-doped LaF3 crystals has beenreported for this region [2,3]. The upconversion(two-photon) pumping seems to be more favour-able because it is expected that in such pumpingschemes the effect of excited state absorption [4]is less important than in the case of one-photonoptical pumping. VUV/deep UV upconversionemission sources are also of practical interest,in particular, for sub-micron photolithography.

d.

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150 200 250 300 3500.0

4.0x103

8.0x103

1.2x1046PJ

8S7/2

6IJ8S7/2

6DJ8S7/2

6GJ8S7/2

scatteredlaser

:25

λex= 248 nm

T=300 K

LiGdF4

Inte

nsity

, cou

nts

Wavelength, nm

Fig. 1. UV emission spectrum of the LiGdF4 crystal excited by

248nm radiation from a KrF excimer laser at 300K. The

assignments of emission lines to corresponding 4f7–4f7 transi-

tions in the Gd3þ ion are shown.

D. Lo et al. / Journal of Luminescence 119– 120 (2006) 28–32 29

In this context, VUV/UV 4f25d–4f3 luminescenceof Nd3þ under two-photon excitation at 355 nmwith a pulsed-frequency-tripled Nd:YAG laser forLiYF4, LiLuF4, BaY2F8 and Na0:4Y0:6F2:2 dopedwith Nd3þ [5–7] and under two-photon excitationat 351 nm with a XeF excimer laser for Nd3þ-doped LiYF4 [8] has been detected.

Recently, rather short-wavelength VUV lumi-nescence at 125–127 nm, which is due to 4f65d–4f7

interconfigurational transitions in the Gd3þ ion,has been discovered in the LiGdF4 and GdF3

crystals under one-photon excitation [9]. In thepresent paper the two-photon excitation to high-lying levels of Gd3þ in LiGdF4 and GdF3 crystalshas been studied under pumping by 248 nmradiation from a KrF excimer laser.

2. Experiment

A KrF excimer laser AQX-150 (MPB Technol-ogies Inc.) operating at 248 nm was used for thetwo-photon excitation experiments. Luminescenceemitted from the samples was analyzed bya 0.4m Czerny–Turner vacuum monochromator(SpectraPro VM-504, Acton Research Corpora-tion) equipped with a gated diode array detector(IVUV-700, Princeton Instruments). The instru-mental resolution of the spectrometer was 0.2 nm.The energy in the 25 ns pulse of the KrF laser was150–170mJ.

Single crystals of LiGdF4 and GdF3 weregrown by the Czochralski method as described inRef. [10]. For the experiments polished cubicsamples with a characteristic length of 5mm wereused. All the experiments were performed at roomtemperature.

3. Results and discussion

Under excitation of the LiGdF4 crystal by248 nm radiation from a KrF laser, the UVluminescence is observed (Fig. 1), including theupconverted emission at 204 nm which is due toelectronic transitions from the 4f7 6GJ levels to the4f7 8S7=2 ground state in the Gd3þ ion [11]. Thepresence of the 204 nm emission in the spectrum

undoubtedly evidences that two-photon excitationto high-lying Gd3þ levels takes place in theLiGdF4 crystal under KrF laser pumping. Radia-tive transitions from low-lying levels withinGd3þ4f7 configuration, namely from 6DJ,

6IJ and6PJ to the ground state, are also observed in thespectrum. As expected, the most intense emissionis observed at 311 nm from the lowest excitedGd3þ 6PJ levels separated from the groundstate by the gap �32; 000 cm�1. The energy gapsbetween 6DJ and 6IJ multiplets and between 6IJand 6PJ multiplets are around 2800 cm�1, whereasthe cut-off phonon energy for the LiGdF4 crystalis �560 cm�1 [12], i.e. these gaps can be bridged by5-phonon relaxation process resulting in highenough rate of non-radiative relaxation from therespective levels. The rate of 6I7=2 !

6PJ non-radiative relaxation has been estimated to exceedthe probability of radiative decay of the 6I7=2 levelto the ground state [13], which results in additionalpopulation of the lowest excited 6PJ levels.In the long-wavelength range, orange (570–

640 nm) and IR (720–810 nm) luminescence is alsoobserved under KrF laser excitation (Fig. 2),which is due to 6GJ–

6PJ and 6GJ–6IJ transitions

in Gd3þ, respectively [11]. These visible and IRemissions as well as the UV (204 nm) luminescencecan be observed, in principle, under excitationboth to the high-lying 4f7 levels (higher than6G7=2) and to the Gd3þ 4f65d levels.

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550 600 650 700 750 8000.0

5.0x103

1.0x104

1.5x104

6PJ8S7/2

6PJ6S7/2

6G7/2

6P3/26P5/2

6GJ6P7/2

2nd order

*5

6G7/26IJ

6P7/2

λex= 248 nm

T=300 K

LiGdF4

Inte

nsity

, cou

nts

Wavelength, nm

Fig. 2. Emission spectrum in the long-wavelength region of the

LiGdF4 crystal excited by 248 nm radiation from a KrF excimer

laser at 300K. The assignments of emission lines to correspond-

ing 4f7–4f7 transitions in the Gd3þ ion are shown.

0

10

20

30

40

50

60

70

80

Gd3+ (LiGdF4)

4f65d

6FJ

6GJ

6DJ6IJ

6PJ

8S7/2

Ene

rgy,

cm

-1

Fig. 3. The energy level scheme illustrating the processes under

two-photon excitation of the Gd3þ ion in LiGdF4. Bold lines

show KrF laser pumping, thin solid lines indicate observed

radiative transitions and dotted lines are non-radiative transi-

tions.

D. Lo et al. / Journal of Luminescence 119– 120 (2006) 28–3230

The scheme of radiative and non-radiativetransitions in Gd3þ for the LiGdF4 crystal isshown in Fig. 3, by taking into account theenergies of Gd3þ4f7 levels from Ref. [14]. Ob-viously, the two-photon excitation by 248 nmradiation should result in excitation of Gd3þ ionsto the excited 4f65d configuration. The fact is thatthe laser photon energy matches well the energy ofthe 8S7=2–

6D7=2 transition and so one can expectthat the 6D7=2 level is populated by KrF laserradiation with high enough efficiency. Since noquantitative data concerning the probabilities ofrespective transitions in Gd3þ are available inthe literature, no quantitative estimations ofpossible population density for the 6D7=2 levelunder KrF laser pumping can be made forthe present. After the excitation by the firstphoton to this energy level the Gd3þ ion shouldquickly relax to the lower lying 6D9=2 level,which has long enough lifetime. The energy ofthe second laser photon is quite sufficient to excitethe Gd3þ ion from the 6D9=2 level to the lowestlevel of the 4f65d configuration, the energy ofwhich is �79; 400 cm�1 in LiGdF4 [9]. On the otherhand, although the transition from the 6D9=2 levelto the lowest level of the 4f65d configuration isparity-allowed, it is spin-forbidden because thespin of the lowest 4f65d level of the Gd3þ ion is thesame as that of the ground 8S7=2 state, and sothe very high efficiency of such a scheme of two-photon excitation cannot be expected.

It should be noted that the high-lying 4f7 levels ofGd3þ ion in LiGdF4 have been identified up to theenergies �67; 500 cm�1 [11,15]. It is also expectedthat many high-lying levels of the Gd3þ4f7 config-uration overlap with the Gd3þ4f65d levels [16]. Thisshould result in non-radiative relaxation from theexcited Gd3þ4f65d levels to high-lying 4f7 levelswith consequent non-radiative multi-phonon relaxa-tion to the lowest 4f7 6GJ levels which have ratherlarge energy gap �8000 cm�1 to the next lower 6DJ

levels. Accordingly the multi-phonon relaxation ratefrom the 6GJ levels is very small and radiativetransitions from the 6GJ levels to low-lying 4f7 levelscan be observed. Indeed, UV (204nm), orange(570–640nm) and IR (720–810nm) luminescence

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D. Lo et al. / Journal of Luminescence 119– 120 (2006) 28–32 31

which is due to 6GJ–8S7=2,

6GJ–6PJ, and

6GJ–6IJ

transitions, respectively, is detected from theLiGdF4 crystal under KrF laser excitation. Thelatter two radiative transitions will give rise toadditional population of the 6PJ and 6IJ levelsresulting in the increase of intensity for respectiveradiative transitions to the ground state.

The measurements of emission spectra underKrF laser excitation have also been performedfor the GdF3 crystal. No UV emission around204 nm as well as no orange and IR luminescencewhich can be due to radiative transitions fromthe Gd3þ4f7 6GJ levels is detected in this crystal(the spectrum is not shown). Only emissions fromthe Gd3þ levels with the energy lower than thelaser photon energy are observed, i.e. only one-photon excitation of Gd3þ to the 6DJ levels hasbeen obtained for GdF3 crystal under 248 nmirradiation. The energy of the lowest Gd3þ4f75dlevel in GdF3 is higher (483; 000 cm�1 [9]) than inLiGdF4, i.e. the photon energy of KrF laser isnot sufficient for two-photon excitation of Gd3þ

to 4f75d configuration in GdF3. Although somehigh-energy Gd3þ4f7 levels definitely exist, whichmatch well the two-photon excitation scheme by248 nm photons within the energy levels of 4f7

configuration, the probability for the 4f7–4f7

transitions from the 6D9=2 level in the secondstage of such two-photon excitation is very lowbecause all the high-lying Gd3þ4f7 levels withthe corresponding energies are doublet or quartet[17], i.e. the respective 4f7–4f7 transitions arespin-forbidden. This results in very inefficienttwo-photon excitation of high-lying Gd3þ4f7 levelsin GdF3.

Taking into account that the VUV luminescenceat �127 nm from the LiGdF4 crystal caused bythe 4f65d–4f7 8S7=2 interconfigurational transi-tions in Gd3þ has high enough intensity only attemperatures up to about 100K and is completelyquenched near 200K [9], obviously upconvertedVUV luminescence could not be observed underour experimental conditions (at room tempera-ture). However, if the two-photon excitation of theLiGdF4 crystal with a KrF laser was performed atlow temperatures, this short-wavelength VUVluminescence due to 4f65d–4f7 8S7=2 transitionsin Gd3þ could be obtained.

4. Conclusions

The possibility of two-photon excitation tothe 4f65d configuration of Gd3þ via intermediate4f7 6DJ levels in the LiGdF4 crystal by 248 nmradiation from a KrF excimer laser has beendemonstrated. The radiative transitions fromthe high-lying 4f7 6GJ levels of Gd3þ in the UV(204 nm), orange (570–640 nm) and IR (720–810 nm) regions which are due to 6GJ–

8S7=2,6GJ–

6PJ, and 6GJ–6IJ transitions, respectively,

have been detected from the LiGdF4 crystal atroom temperature. One can expect that the sameexperiment performed at low temperature couldresult in upconversion excitation of VUV lumines-cence at �127 nm, which is due to interconfigura-tional 4f65d–4f7 8S7=2 transitions in the Gd3þ ion.This would allow the realization of the shortwavelength solid-state VUV emitter, which couldbe optically pumped with a KrF excimer laser bytwo-photon excitation.

Acknowledgments

The support by Earmarked Research Grant ofHong Kong SAR CUHK 4234/03E and RFBRGrant 05-02-17306 is gratefully acknowledged.

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