site-selective upconversion luminescence of ho3+ -doped caf2 crystals

XIAO ZHANG et al.: Upconversion Luminescence of Ho3+-Doped CaF, Crystals 559

phys. stat. sol. (b) 184, 559 (1994)

Subject classification: 78.55; 71.55; S9.12

GRSM, Lahoratoire d'EnergPtique et d'optique, UniversitP de Reims ')

Site-Selective Upconversion Luminescence of Ho3 +-Doped CaF, Crystals

BY XIAO ZHANG, J.-P. JOUART, M. BOUFFARD, and G. MARY

With red dye laser excitation, site-selective upconversion properties of three Ho3+ centres (C,,, C,,, and dimer) in CaF, : 0.1 at% Ho3+ crystals are studied. The energy level diagrams for the three centres are deduced from the spectral data. The excited state absorption from the 51, level plays a very important role in the upconversion coming from C,, and C3" centres, whereas the ground state absorption of 'I, dominates for that from the dimer centre. The upconversion of C,, and C,, centres is explained by a sequential two-photon excitation mechanism. For the dimer centre, the predominant upconversion mechanism is energy transfer.

Nous avons Ctudie les fluorescences anti-Stokes bleue et verte, dues aux trois principaux centres (deux ions Ho3+ isoles et une paire d'ions Ho3+ associes) de CaF, :0,1 at% Ho3+ excites dans le rouge par un laser a colorant. Les niveaux d'energie de chacun de ces trois centres ont ete etablis. Les fluorescences anti-Stokes provenant des ions Ho3+ isoles resultent d'une absorption dans leur etat excite 'I,, alors que celles des paires resultent d'une relaxation croisee entre les ions Ho3+ voisins.

1. Introduction

The laser-induced site-selective fluoresence is a well-established method in solid state luminescence. This method has been widely applied in analyzing various luminescence centres of trivalent rare-earth ions in fluorite-type crystals (MF, with M = Ca, Sr, Ba, and Cd) [l to 61. In this kind of crystals, the trivalent rare-earth ions (Re3') occupy a divalent site and, therefore, a charge compensator is required. The site symmetry of Re3+ depends strongly on the nature and relative position of the compensation species, and consequently the Re3+ free-ion levels are further split by the crystal field of the site. As a result, each distinct site gives rise to its own excitation and emission spectra, and one can investigate them individually with the aid of laser-selective excitation technology [ 11. This kind of work has been carried out by many authors, and especially by Wright and his colleagues [l , 2,7,8].

The upconversion is an unusual luminescence mode in which a long-wave radiation is converted into a short-wave radiation. This can be achieved through several multiphoton processes: the successive energy transfer process proposed by Auzel [9], the sequential two-photon absorption by a single centre suggested by Bloembergen [lo], the cooperative effects between different sites explained by Ovsyankin and Feofilov [ 111, and the direct two-photon transition [12]. Among these mechanisms, the direct two-photon transition is less important, because the cross section for the two-photon absorption is usually very low and therefore the transition probability is much lower than that of the other three. The remaining three mechanisms have been widely used to explain the experimental observations.

') B.P. 347, F-51062 Reims Cedex, France.

560 XIAO ZHANG, J.-P. JOUART, M. BOUFFARD, and G. MARY

The upconversion phenomenon has been observed and studied in particular in fluoride crystals and fluoride glasses owing to the relatively small phonon energy in this kind of materials. In this paper, we present a spectroscopic study of the site-selective upconversion luminescence of Ho3+ centres in the CaF, crystal.

In CaF,, the usual site symmetries of rare-earth ions are tetragonal (C,J and trigonal ((2,") referring to a fluorine interstitial compensator as the nearest and next-nearest neighbour, respectively [13]. With increasing concentration, dimer and other higher-order clusters are formed. The optical properties of the CaF, : Ho3+ crystals were investigated thirty years ago at the Lebedev Institute in Moscow. A laser action at 18141 cm-' (5S,-518 channel of C,, centre) has been obtained at 77 K by Voronko et al. [16]. The upconversion fluorescence of Ho3+ ions in CaF, has been reported by several authors [2,3,17 to 191. However, as far as we know, the upconversion properties of Ho3+ ions for the C,, symmetry site have not been reported up to now. In a previous work, Seelbinder and Wright [2] have reported the upconversion fluorescence arising from dimer centres. Recently, Mujaji et al. [3] have observed very weak upconversion fluorescence from the C,, centre in slightly doped (0.005 atyo) CaF, and SrF, crystals, while no upconversion has been detected by them for the C,, centre in both crystals. In our work for a crystal doped with 0.1 at% Ho3+, upconversion luminescence from all the three centres (C4", C,,, and dimer) in the CaF, crystal were observed. The emission intensities of the C,, centre are comparable with those of the other two centres. Since general agreement with the results reported before has been observed for C,, and cluster centres, here we present mainly the results for the C,, centre.

2. Experimental

The CaF,:0.1 at% Ho3+ crystal used in this study was purchased from the Optovac Inc. The absorption spectrum was obtained at room temperature by using a Perkin-Elmer Lambda-9 spectrophotometer, with a crystal of 5 mm in length,

A Spectra-Physics 375 tunable dye laser pumped by a cw argon ion laser (Spectra- Physics 2000) was used to excite selectively different centres in this crystal. The dye used was Kiton Red (Exciton Inc.). With an output power up to 400mW the laser beam is continuously tuned from 620 to 660nm and covers all the absorption bands 518 + 5F,. The laser beam was focused into the crystal with a 50 mm focal lens. The luminescence from the sample was dispersed by a Coderg three-grating monochromator and detected by a cooled photomultiplier (EM1 9558 QB). For lifetime measurements, the laser beam was modulated by a mechanical chopper (or a Pockels cell), and a Metrix oscillograph (0x750-2) was used to record the rise and decay signals. The results were analyzed by an on-line computer. All the measurements were carried out at liquid nitrogen temperature (77 K).

3. Results and Discussion

3.1 Optical spectra

The room temperature absorption spectrum of CaF,:0.1 at% Ho3+ between 300 and 700 nm is given in Fig. 1. Absorption bands arising from the 518 ground multiplet to most of the upper-lying manifolds were detected. The transition assignments are shown in Fig. 1. The inset depicts the red absorption band ,I8 + 5F, through which the laser was tuned.

Upon red laser excitation, we have identified upconversion fluorescence from three distinct centres in the crystal. These centres are assigned as tetragonal (C4,), trigonal (C,"), and

Site-Selective Upconversion Luminescence of Ho3 +-Doped CaF, Crystals 561

1 I I

with the corresponding inter-Stark transitions (the Stark sublevels of each multiplet are numbered in order of increasing energy).

T a b l e 1 Emission lines of Ho3+ centres in the CaF,:O.l a t% Ho3+ crystal. The wave numbers are given in air

transition C,, centre CdV centre dimer centre

wave inter- number Stark (cm- I ) transitions

5F, + 51, 20808 5-1 20787 4-1 20754 3-1 20744 5-3 20730 2-1 20719 1-1 20705 4-4

wave number (cm - ')

20786 20773 20761 20146 20741 20 729 20719

inter- Stark transitions

2- 1 5 -6 3-3 4-4 1-2 3-4 3- 5

wave intcr- number Stark (cm - ') transitions

20825 6 1 20809 5-1, 6-2 20790 4-1 20775 3-1, 4-2 20764 2-1 20761 3 -2, 6-3 20744 5-3


T a b l e 1 (continued)

transition

'F, -+ '1,

'F,, 5S2 + 51,

'F, + '1,

C,, centre

wave inter- number Stark (cm- ') transitions

C,, centre

wave number (cm-')

dimer centre


wave inter- number Stark (cm I ) transitions

20693 20673 20656

18775 18 752 18 725 18713 18686 18658 18639 18634 18 607 18 594 18 569 18543 18506 18391 18355 18 203 18152 18 146

15650 15630 15615 1 5 607 15591 15585 15567 1 5 544

20 704 20658 20312 20296 20272

18837 18753 18 728 18656 18 646 18616 18603 18587 18 569 18543 18533 18 530 18 520 18487 18475 18279 18238 18 222 18 I83 18 166 18 141 18 125 18089

15 684 15 623 15612 15610 15 529 15 525 15151 15133 15121

2-3,4-6 1-3,2-5 2-8 2-9 2-10

8-2 7-1 6-1 3-1 6-3 2- 1 1-1, 5 - 3 4-3 5-4 4-5 2-3 3-5 1-3 1-4 1-5 7-8 6-9 5-7 3-8 3-9 2-8, 1-7 2-9 1-10

4- 1 3-2 2- 1 2-2 2-3 1-3 3-8,2-7 1-8 2-9

20 72 1 20711 20707 20700 20675 20660 20580 20536 20354 20318 20293

18 800 18 729 18 708 18693 18 673 18657 18 642 18 626 18 620 18616 18 605 18 602 18593 18585 18578 18569 18557 18 549 18543 18530 18523 18513 18 508 18487 18343 18203 18 168 18 143 15670 15 664 15654 15 642 15 630 15 627 15615 15 608 15 599

1-1,5-4 3-3 1-2,5-5 2--3 2 -4, 6-6 2-5 2-8 1-8 5-9 3-9,4-10 2-10

7-1 6- 1 5-1 5-2 4- 1 3-1,4-2 2-1,3-2 2-2,6-5 5-4 1-1 5-5 1-2 3-3 4-4 2-3,6-6 3-4,4-5 5 -6 5-7 6-8 1-4 4-6, 5-8 1-5,4-7 3-6 4-8 7-9 3-9 2-10 1-10 5-1 4-1 5-2 3-1 2-1 3-2 2-2 1-1 4-3

Site-Selective Upconversion Luminescence of Ho3 +-Doped CaF, Crystals

~


transition C,, centre ~

wave inter- number Stark (cm- ') transitions

5F, --t ,I,

,F,, 5S2 --t ,I, 13398 13384 13 379 13364 13357 13354 13351 13348 13336 13334 13332 13330 13327 13316 13 298 13 296 13295 13 289

4-5 5-9 4-8 1 - 1 2-2 3-3 1-3 3-4 2-5, 3--5 1-5 2-6,3-6 1-6,3-7 1-7 I-8,2-8 3 -9 2-9 1-9 1-10

C,, centre -

~

wave number (cm-')


15095

13 426 1341 1 13 398 13394 13383 13369 13 357 13344 13341 13330 13328 13326 13307 13269 13 264 13 256 13252 1 3 244 13 238 13188

1 10

5- 1 4 -1, 5-2 3-1,5-4 4-3 4-4 3-4 2-1 1-1 2- 2 2-4 1-2 1-3 2-5 1-6 4 8 4-9 3-8 1-7 3-10 1-9

563

dimer centre

wave inter- number Stark (cm- I ) transitions

15594 15585 15576 15566 15521

13 426 13414 13390 13378 13 374 13370 13 362 13359 13 346 13341 13336 13333 13327 13320 13316 13310 13304 13 292 13284 13270

1-2 5-4 4-4 2-3, 5 5 1-4, 5-6

5-1 5-2 4- 1 4-2 3-1 5-6 3-2, 5-7 2-1 2-2,4-4 4-5 3-3.4 6 1-1 4-7 1-2,2-3 2-4 2-5, 3-7 2-6 1-3 1-5 1-7

The excitation spectrum for the red emission from the C,, centre (Fig. 3a) principally results from the ground state absorption 51, + 'F,, whereas that for the green emission (Fig. 3b) consists of many sharp lines located between 15600 and 15380 cm-', besides the lines appeared in Fig. 3a. The additional lines situated between 15600 and 15439 cm-' (Fig. 3b) correspond to the excited state absorption 51, --f 5F3. Notice that the three lines at 15591, 15569, and 15566cm-', which coincide in both the ground state and excited state transitions, are much more intense than the others. The remaining lines observed below 15439 cm-', which cannot be assigned to '1, + 'F, or '1, + ,F, transitions, can be attributed to the ,I , + 5G, transition. The excitation spectra for the blue and infrared emissions (750 nm) are similar to that for the green emission. This result implies that the 5F3 and ,F,, 'S, levels are populated by the same excitation process. Similar results are observed for the C,, centre (Fig. 4). For the dimer centre, however, the excitation spectra for Stokes and anti-Stokes luminescences have the same structure (Fig. 5). These different observations are owing to different upconversion mechanisms, which will be discussed in Section 3.2. The main excitation lines and the corresponding inter-Stark transitions are assigned in Table 2. We have also detected excitation lines between 15800 and 16200 cm- for the C,, and C,, centres. These spectra are not yet well-characterized. The sharp lines located between 16000 and 16200cm-I for the C,, centre probably correspond to the 51, + 5F,, ,K, transitions.

? Y physica (b) 18412


a

2 0 150 20350 20550

5 F 4 , '52 '18 b

I I

18050 18250 18450 10650 18850

wave number [ cm-1) - Fig. 2a, b

T a b l e 2 Excitation lines of Ho3+ centres in the CaF, :0.1 at% Ho3+ crystal. The wave numbers are given in air

~

dimer centre transition C3" centre CdV centre

wave inter- wave inter- wave inter- number Stark number Stark number Stark (cm - ') transitions (cm ~ I) transitions (cm I ) transitions

51, + 5F, 15650 1-5 15754 1-5 15670 1-5 15630 1-4 15684 1 4 15664 1-4 15615 1 2 15623 2-3 15655 2 5 15607 1-1 15612 1-2 15642 1 3 15591 2-3 15610 2 2 15630 1 2 15568 4-5, 3-4 15525 3 1 15628 2 3 15544 3 1 15615 2 2


C

- J I ~

1 5 000 15200 15 400 15 600

w a v e n u m b e r (cm-l I- Fig. 2. a) Blue, b) green, c) red, and d) infrared emission spectra for the Ho3+C3" centre in CaF, : 0.1 a t % Ho3+ at 77 K. For all the spectra the excitation line is tuned at 15591 cm-'


transition C,, centre C,, centre dimer centre

wave inter- wave inter- wave number Stark number Stark number (cm- ') transitions (cm- ') transitions (cm ~ ')

51, + 5 ~ , I5 608 15 599 15594 15585 15576 15566 15521


1-1 3--4 2- 1 4--5 3-3,4-4 3-2 4- 1


Table 2 (continued)

transition C,, centre

wave inter- number Stark (em ') transitions

'1, + 'F, 15591 3-5 15581 1-4 15569 3 - 4 15566 7-5 15552 5-4 15547 6-4 15535 3-3 15531 4-3 15525 1-2 15518 2-2 15511 3-2,7-3 15509 10-4 15506 2-1 15500 3-1, 8-3 15491 6--2 15474 10-3 15439 10-1

CdV centre dimer centre

wave inter- wave inter- number Stark number Stark (cm- ') transitions (em- ') transitions

15670 1-5 15654 2-5 15602 1-4 15597 6-5 15586 1-3 15528 1-2 15525 8-5 15512 2-2 15501 4-2 15484 1-1 15476 5-2

Table 3 Crystal-field splitting of the Ho3 'C,,, C,,, and dimer centres in CaF, : 0.1 at% Ho3 + at 77 K

level Stark level positions (em-')

5Fi, 'S, 5F3

C,, centre

0, 27, 64, 81, 179, 213, 367, 417, 424 5205, 5213, 5219, 5223, 5235, 5239, 5242, 5254, 5274, 5279 15607, 15615, 15618, 15630, 15650 18569, 18570, 18572, 18634, 18658, 18665, 18713, 18752, 18775 20719, 20730, 20754, 20787, 20809

C,, centrc

0, 2, 83, 117, 126, 156, 463, 474, 490, 514 5259, 5275, 5277, 5287, 5309, 5333, 5359, 5405, 5415, 5419 15608, 15612, 15625, 15684, 15754 18603, 18616, 18656, 18670, 18686, 18729, 18753, 18839 20743, 20786, 20845, 20862, 20929

dimer centre 0, 15, 64, 87, 103, 150, 159, 185, 455, 472 5283, 5295, 5322, 5326, 5332, 5338, 5346 15608, 15630, 15642, 15664, 15670 18616, 18642, 18657, 18673, 18708, 18729, 18800 20721, 20764, 20775, 20790, 20809, 20825


r 0, m o . m a r m

m r

b

a

I I

15780 15590 1 5 COO -wave number( cm-11

I I

15780 15590 1 5 COO -wave number( cm-11

Fig. 3. Excitation spectra of the a) red emission line at 15544 cm-' and b) green emission line at 18569cm-', for the Ho3+C3,, centre in CaF,:0.1 at% Ho3+ at 77 K

The sublevels of the multiplets 51,, 'I,, 'F,, 'F,, 'S2, and 'F, for the three centres were deduced from our excitation and emission spectra and are shown in Table 3. Our results are in good agreement with those of Mujaji et al. [3].

3.2 Upconversion processes

The dependence of anti-Stokes fluorescence intensity on the incident excitation power has been performed for all the three centres. The log-log dependence basically shows two sldpes, as shown in Fig. 6 for the C,, centre. The slopes are 1.7 and 1.1 at low and high excitation powers, respectively. Similar results were observed for the C,, and dimer centres with line slopes of 2.0, 1.2 for C4" and 1.9, 1.1 for dimer.


I

b

a

-wave number ( cm-1)

Fig. 4. Excitation spectra of the a) red emission line at 15133 cm-' and b) green emission line at 18602 cm- ' , for the Ho3+C4\ centre in CaF,:O.l a t% Ho3+ a t 77 K

At low power, a quadratic dependence was observed and the upconversion mechanism is principally a two-photon process. This can be either a sequential excitation process or an energy transfer process. The presence of the lines 51, + 'F5 and 51, + 5F3 in the excitation spectra for the anti-Stokes emission of C,, and C,, centres (see Fig. 3 and 4) suggests a two-step excitation process via the intermediate level 51, (Fig. 7a). For the dimer centre, the same excitation spectra were obtained for Stokes and anti-Stokes emissions and all the observed lines correspond to the ground state transition (see Fig. 5 and Table Z),


I I

15780 15 620 15 L60 -wave number [ cm-1 1

Fig. 5. Excitation spectra of the a) red emission line at 15521 cm-' and b) green emission line at 18656 cm-', for the Ho3+ dimer centre in CaF,:0.1 a t % Ho3+ at 77 K

indicating that the dominant process is a cross relaxation between neighbouring Ho3+ ions (Fig. 7b).

The dependence of anti-Stokes emission intensity on the laser power becomes linear at high power. This phenomenon has been also observed with Er3i [20] and interpreted as a 'saturation effect when the excited state absorption exceeds the spontaneous decay of the intermediate level. This is proved by the decrease of the rise time of the anti- Stokes luminescence when the laser power exceeds a threshold of about 60 mW (when the power is lower than this value, the rise time is basically constant). Similar results were


Fig.6. Log-log dependence of the green emission intensity on the incident laser power for the Ho3+C,, centre at 77 K (em. = 18569 cm-', ex. = 15591 cm-')

loo 10' l o2 lo3 Laser power (mw) -

obtained for the C,, centre. The lifetimes of the green emission for the three centres are about 1.3 ms (C4J, 0.8 ms (C3"), and 0.5 ms (dimer). The results for the two single centres agree with those observed for Stokes luminescence [19], whereas that for dimer was not reported before.

Fig. 7. Upconversion mechanisms for the Ho3+ different centres in CaF,. a) Two-step excitation for the C,, and C,, centres, b) energy transfer for the dimer

Site-Selective Upconvcrsion Luminescence of Ho3 +-Doped CaF, Crystals 571

4. Conclusion

Upconversion luminescence has been observed from three distinct Ho3+ sites (C,,, C,,, and dimer) in the CaF, : 0.1 a t% Ho3+ crystal with red cw dye laser excitation. The upconversion of C,, centre, whose intensity is comparable to that of the other two centres, is reported in detail for the first time. For C4", C,, centres, the excited state absorption from the 51, first excited state is efficient compared with the ground state absorption and the upconversion is explained by a two-step excitation process. For the dimer centre, on the contrary, only the ground state absorption is responsible for both the Stokes and anti-Stokes emissions and the dominant process is energy transfer. With increasing laser power, a saturation effect is observed for all the three centres.

References

[I] D. R. TALLANT and J. C. WRIGHT, J. chem. Phys. 63, 2074 (1975). [2] M. B. SEELBINDER and J. C. WRIGHT. Phys. Rev. B 20, 4308 (1979). [3] M. MUJAJI, G. D. JONES, and R. W. G. SYME, Phys. Rev. B 46, 14398 (1992). [4] N. J. COCKROFT, G. D. JONES, and R. W. G. SYME, J. chem. Phys. 92, 2166 (1990). [5] A. LEZAMA, M. ORIA, and C. B. DE ARAUJO, Phys. Rev. B 33, 4493 (1986). [6] J. P. JOUART, C. BISSIEUX, G. MARY, and M. EGEE. J. Phys. C 18, 1539 (1985). [7] M. P. MILLER and J. C. WRIGHT, J. chem. Phys. 68, 1548 (1978). [8] D. S. MOORE and J. C. WRIGHT, J. chem. Phys. 74, 1626 (1981). [9] F. AUZEL, C.R. Acad. Sci. (France) 262, 1016 (1966); 263, 819 (1966).

[lo] N. BLOEMBLRGEN, Phys. Rev. Letters 2, 84 (1959). [ l l ] V. V. OVSYANKIN and P. P. FEOFILOV, Soviet Phys.-J. exper. theor. Phys. Letters 4, 317 (1966). [12] J. D. AXE, JR., Phys. Rev. 136, A 42 (1964). [13] J. M. BAKER, in: Crystals with the Fluorite Structure, Oxford University Press. 1974 (p. 341). [14] V. V. OSIKO, Soviet Phys.-Solid State 7, 1047 (1965). [15] J. CORISH, C. R. A. CATLOW, P. W. M. JACOBS, and S. H. ONG, Phys. Rev. I? 25, 6425 (1982). [16] Yu. K. VORONKO, A. A. KAMINSKII, V. V. OSIKO. and A. M. PKOKHOROV, Soviet Phys.-J. exper.

[17] M. R. BROWN and W. A. SHAND, Phys. Letters 11, 219 (1964). [18] D. N. RAO. J. PRASAD, and P. N. PRASAD, Phys. Rev. B 28, 20 (1983). [I91 S. H . TANG, H. Y . ZHANG, M. H. KUOK, and S. C. KEE, phys. stat. sol. (b) 168, 351 (1991). [20] J . P. JOUART, C. BISSIEUX, and G. MARY, J. Phys. C 20, 2019 (1987).

theor. Phys. Letters 1, 3 (1965).

(Received December 8, 1993; in revised form Murch 17, 1994)

site-selective upconversion luminescence of ho3+ -doped caf2 crystals

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