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Red-to-blue up-conversion spectroscopy of Tm3+ in SrF2, CaF2, BaF2 and CdF2
Optical Materials 14 (2000) 73±79
www.elsevier.nl/locate/optmat
Red-to-blue up-conversion spectroscopy of Tm3 in SrF2, CaF2, BaF2 and CdF2 M. Bouard a,*, J.P. Jouart a, M.-F. Joubert b b
a Laboratoire dÕEnerg etique et dÕOptique, UTAP, Universit e de Reims Champagne-Ardenne, BP 1039, 51687 Reims Cedex, France Laboratoire de Physico-Chimie des Mat eriaux Luminescents, UMR n 5620 du CNRS, Universit e de Lyon 1, 43 bd 11 Novembre 1918, 69622 Villeurbanne Cedex, France
Received 15 February 1999; accepted 21 September 1999
Abstract Up-conversion spectroscopy at low temperature is used to probe the symmetry of the sites occupied by the Tm3 ion in MF2 :TmF3 (with M Sr, Ca, Ba ) and in CdF2 : TmF3 , NaF. We detected single Tm3 centers of cubic, trigonal, tetragonal and orthorhombic symmetries and established energy level diagrams. Single Tm3 centers of cubic symmetry tend to predominate in CaF2 :TmF3 as the Tm3 concentration exceeds 1 at.% Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 78.55 Keywords: Up-conversion; Thulium; Fluorite-type crystals; Excited-state absorption; Selective excitation
1. Introduction Incorporated in small quantities inside transparent materials (crystals or glasses) and excited by an adapted laser beam, the trivalent rare earth (RE) ions usually generate photons of lower energies (down-conversion) but also higher energies (up-conversion) than those of the incident photons. In the case of erbium, excited-state absorption (ESA) from a metastable electronic level at low concentration (<0.1 at.%) and excited-state transfer at high concentration (>0.1 at.%) are both
* Corresponding author. Tel.: +33-326-053251; fax: +33-326053250. E-mail address: michel.bou[email protected] (M. Bouard).
main up-conversion processes [1,2]. In the case of thulium, on the other hand, ESA remains dominant when the concentration increases and photon avalanche up-conversion involving both ESA and cross relaxations between excited and nonexcited Tm3 ions were observed [3]. Trivalent RE-doped MF2 (with M Sr, Ca, Ba, Cd) is a suitable material characterized by low energy phonons [4] and large transfer coecients between the RE ions [5,6]. Most of the RE were extensively studied in it, except Tm3 which gave rise only to some reports [7±13]. In MF2 , the trivalent RE ion occupies a M2 site, thus requiring a charge-compensating interstitial Fÿ i ion which can modify the geometry and the constitution of the center by taking a position close to the RE ion. Any modi®cation in the local surroundings of the RE ion results in a lift of the degeneracy of certain
0925-3467/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 9 ) 0 0 1 1 0 - X
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M. Bouard et al. / Optical Materials 14 (2000) 73±79
electronic levels which split in several Stark sublevels. Perturbations of trigonal and tetragonal symmetries induced by a Fÿ i ion are usually observed in trivalent RE-doped MF2 , however the cubic crystal-®eld is not perturbed as the Fÿ i ion is away from the RE ion. In the study we achieve, the main Tm3 centers of MF2 :TmF3 are selectively excited by an upconversion technique at liquid nitrogen temperature, under high excitation densities, in the red spectral range, obtained with a single focused laser beam. Under these conditions, each Tm3 center exhibits noticeable blue, red and ir ¯uorescences (Fig. 1) when the laser wave number is resonantly tuned on an ESA from a metastable electronic level 3 F4 or 3 H4 , in the wings of the ground-state absorption 3 H6 ® 3 F2 . The Tm3 energy level diagrams derived from an analysis of our excitation and emission spectra are presented for each center whose we look for to identify the Tm site symmetry according to the selection rules predicted by the group theory.
2. Experimental Three CaF2 crystals doped with 0.05, 0.1 and 2 at.% TmF3 ; three SrF2 crystals doped with 0.1, 1 and 2 at.% TmF3 ; one BaF2 crystal doped with 0.1 at.% TmF3 ; one CdF2 crystal double-doped with 0.1 at.% TmF3 and 1 at.% NaF were studied. The CdF2 crystal was grown at the University of Reims using the Bridgman method, the other crystals were synthesized by Optovac. The crystals are unoriented. The laser excitation technique used to perform the up-conversion experiments has already been described [14]. The ¯uorescence spectra were recorded at 77 K on a Coderg three-grating monochromator working from 12 200 to 22 700 cmÿ1 . A tuneable cw dye laser working with the kiton red from 15 200 to 15 900 cmÿ1 was used to excite the Tm3 ions. The excitation spectrum of each center was obtained by monitoring the intensity of a peculiar ¯uorescence line as a function of the laser wave number. Some absorption spectra were also carried out at 14 K with a CARY spectrophotometer at the University of Lyon 1.
3. Results and discussion Each crystal exhibits, under red laser excitation, numerous ¯uorescence lines between 12 200 and 22 700 cmÿ1 . These lines were used to establish the energy levels of the Tm3 ion. Two types of spectra were recorded for the blue ¯uorescence: the ®rst type exhibits only lines between 20 000 and 20 800 cmÿ1 originating from 1 G4 , while the second type exhibits in addition lines between 21 700 and 22 700 cmÿ1 originating from 1 D2 . 3.1. Evidence for cubic (Oh ) Tm3 centers in SrF2 , CaF2 , BaF2 and CdF2
Fig. 1. Energy level diagram for the Tm3 ion in MF2 showing ESA and ¯uorescence transitions studied in this paper.
In the ®rst type spectrum, the 1 D2 ® 3 F4 transition is completely missing closely following the forbidden character of electric-dipole transitions between the crystal-®eld levels of a Oh symmetry center. The observed lines are in agreement with
M. Bouard et al. / Optical Materials 14 (2000) 73±79
75
Fig. 2. 3 F4 !1 G4 excitation spectrum of the Tm3 (Oh ) center of CaF2 : 2 at.% TmF3 at 77 K.
the magnetic-dipole selection rules : the main transitions 1
G4 A1g !3 F4 T1g
and 1
G4 A1g !3 H5 T1g
are those for which DJ 0 or 1. The red emission (1 G4 ® 3 F4 ) is one order of magnitude stronger
than the blue emission (1 G4 ® 3 H6 ). The emitting G4 level is populated through a two-step absorption process [14]: ®rst absorption in a vibronic sideband of 3 F2 followed by relaxation to the metastable 3 F4 level, second absorption (resonant) from 3 F4 to 1 G4 . The 3 F4 ® 1 G4 excitation spectrum for the red (or blue) ¯uorescence from the cubic Tm3 center of CaF2 (Fig. 2) consists of two sharp lines located at 15 267 and 15 788 cmÿ1 : the ®rst one, very weak, 1
Table 1 A1g ® T1g lines positions (in cmÿ1 ) of the Tm3 (Oh ) center of MF2 and splittings due to a crystal ®eld perturbation of C3V symmetry for CaF2 :Tm, SrF2 :Tm, BaF2 :Tm and to a crystal ®eld perturbation of C2V symmetry for CdF2 :Tm, Na Crystal
Tm3 ±Fÿ separation (nm)
CdF2 :Tm, Na
0.233
CaF2 :Tm
Excitation 3 F4 (A1g ) ® 1 G4 (T1g ) 8 < 15 806 15 802 15 800 : 15 783
Fluorescence 1 G4 (A1g ) ® 3 F4 (T1g ) 8 < 14 949 14 930 14 946 : 14 925
Fluorescence 1 G4 (A1g ) ® 3 H5 (T1g ) 8 < 12 244 12 217 12 225 : 12 216
0.236
15 796 15 788 15 763
14 979 14 957 14 944
12 256
SrF2 :Tm
0.251
15 777 15 759 15 740
15 031 15 015 14 989
12 335
BaF2 :Tm
0.268
15 750 15 735 15 719
15 078 15 064 15 042
12 397
12 267 12 252
12 330 12 318
12 394 12 383
76
M. Bouard et al. / Optical Materials 14 (2000) 73±79
3.2. Evidence for trigonal (C3v ) Tm3 centers in SrF2 and BaF2
corresponds to A1g ® A1g (a small perturbation possibly due to a neighbouring Tm3 ion removes the interdiction), the second one, strong, corresponds to A1g ® T1g (allowed). Two broad lines attributed to vibronic side bands are also observed towards the higher energies compared to A1g ® A1g . Similar spectra (lowest and highest energy lines in brackets) were recorded for CdF2 (15 254, 15 802 cmÿ1 ), SrF2 (15 294, 15 759 cmÿ1 ) and BaF2 (15 317, 15 735 cmÿ1 ). We note regular shifts of excitation and ¯uorescence lines from cubic Tm3 centers as a function of the Tm3 ±Fÿ separation (Table 1).
A (Tm3 , Fÿ i ) center of trigonal symmetry is identi®ed in SrF2 and BaF2 (Table 2). The trigonal symmetry is due to an interstitial Fÿ i ion located in the position (a/2, a/2, a/2) with respect to the Tm3 ion (Fig. 3). In this case, the Stark sublevel T1g splits into two sublevels A2 and E, giving rise to two lines A1 ® A2 and A1 ® E located on both sides of each line A1g ® T1g (Table 1). An example is given Fig. 4. We are not unfortunately in a position to attribute each line
Table 2 Energy levels (in cmÿ1 ) and irreducible representations of the Tm3 (C3V ) center of MF2 Multiplets
CaF2
SrF2
1
D2
28 203 28 159 28 146
28 200 28 180 28 131
1
G4
21 330 21 297 20 804
A1
28 184 28 156 28 109
A1 E E
21 318 21 281 20 830
A1
21 297 21 266 20 862
A1
E E
15 282 15 247
E E
3
F2
15 315 15 280
3
F3
14 762 14 691
H4
12 872 12 844 12 747 12 629
3
12 639 3
H5
3
F4
8552 8537
5860 5830 5534 3
H6
14 656
A1 A2
8512 8500 8411 8403
E A2 A1
6158 6148 6094 5841 5799 5541
A2
435 224 138 118 0
226 158 0
BaF2 A1 E E
12 724 12 620
A1 A2
8479 8468
E A2 A1
6105 6044 5819 5782 5547
E A2 A1
391 200 A2
104 0
A2
M. Bouard et al. / Optical Materials 14 (2000) 73±79
77
Fig. 3. Oh , C4v , C3v and C2v centers of MF2 : Tm studied in this paper.
to its own irreducible representation. Nevertheless, the emergence of such splittings in the excitation and ¯uorescence spectra, the three sublevels for the 1 D2 multiplet (Table 2) and the lack of lines corresponding to the electric-dipole A1 $ A2 transitions, clearly indicate that the Tm3 ion occupies a trigonal site in SrF2 and BaF2 . Some irreducible representations are given in the Table 2. 3.3. Evidence for tetragonal (C4v ) Tm3 center in CaF2 In CaF2 , for Tm3 concentrations not exceeding 0.1 at.%, the Tm3 ion preferentially occupies a tetragonal (C4v ) center. The trigonal (C3V ) center is in the minority, the energy levels of this center are given in the Table 2. The spectroscopic properties of C4V center have been reported before [13]. An interstitial Fÿ i ion in the position (a/2, 0, 0) is responsible for the C4V symmetry (Fig. 3). The absorption spectrum of CaF2 : 2 at.% Tm3 (Fig. 5)
Fig. 4. Part of the 3 F4 !1 G4 excitation spectra of SrF2 :TmF3 at 77 K for (a) the Tm3 (C3V ) center and (b) the Tm3 (Oh ) center.
corresponding to the magnetic-dipole transition H6 ® 3 H5 consists of three major lines : in accordance with [13], the line located at 8430 cmÿ1 is ascribed to the 3 H6 (A2g ) ® 3 H5 (T2g ) transition from centers of cubic (Oh ) symmetry, the other lines being probably due to cluster centers which have been revealed by NMR studies [15], no absorption line due to centers of tetragonal (C4V ) symmetry is visible on this spectrum. It is noteworthy that in CaF2 : 2 at.% Tm3 the Oh center concentration is much higher than those of the 3
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M. Bouard et al. / Optical Materials 14 (2000) 73±79
Fig. 5. 3 H6 !3 H5 absorption spectrum of CaF2 : 2 at.% TmF3 at 14 K.
tetragonal center. The transient eects were studied while chopping the laser beam with a Pockels cell: the rise and decay of the ¯uorescence from 1 G4 depend on the nature of the center: Oh (sr 110 ms, sd 3:6 ms), C4v (sr 40 ms, sd 1:1 ms).
appreciable change of position for the eight nearest neighbour (nn) Fÿ ions surrounding the Tm3 ion, and supporting our attributions concerning the Oh Tm3 centers.
4. Conclusion 3.4. Evidence for orthorhombic (C2v ) Tm3 center in CdF2 In order to con®rm our interpretation, we have studied a CdF2 crystal double-doped with 0.1 at.% TmF3 and 1 at.% NaF. As might be expected, a (Tm3 , Na ) center of orthorhombic (C2v ) symmetry and a Tm3 center of cubic symmetry were identi®ed in it. A substitutional Na ion located in the position (a/2, a/2, 0) with respect to the Tm3 ion (Fig. 3) induces the weak perturbation of C2V symmetry which splits the level T1g of Oh center into three sublevels corresponding to irreducible representations A2 , B1 and B2 . Three lines were eectively observed near the A1g ® T1g lines (Table 1 and Fig. 6) in the excitation and ¯uorescence spectra of the (Tm3 , Na ) center. We verify that the (Tm3 , Na ) centerÕs energy levels are close to those of the Oh center (Table 1), testifying to no
Three types of single Tm3 centers (Oh , C3v , C4v ) were identi®ed in MF2 :TmF3 (with M Sr, Ca, Ba). Energy levels were established for each of them, except the Oh center for which the 3 H4 level position has not been determined. The (Tm3 , Fÿ i ) centers of C4v and C3v symmetries are in the majority in MF2 : the C4v center in CaF2 (for Tm3 concentrations not exceeding 0.1 at.%), the C3v center in SrF2 and BaF2 . These symmetries are due to an interstitial Fÿ i ion located in nn and next nearest neighbour (nnn) position, respectively, with respect to the Tm3 ion. In CaF2 , the Oh center concentration strongly increases relative to those of the C4v center, as the Tm3 concentration exceeds 1 at.%. (Tm3 , Na ) centers of C2v symmetry and Tm3 centers of Oh symmetry are also identi®ed in CdF2 : TmF3 ,NaF.
M. Bouard et al. / Optical Materials 14 (2000) 73±79
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References [1] J.P. Van Der Ziel, F.W. Ostermayer Jr., L.G. Van Uitert, Phys. Rev. B 2 (1970) 4432±4441. [2] F. Auzel, Proc. IEEE 61 (1973) 758. [3] S. Guy, M.F. Joubert, B. Jacquier, J. Alloys. Compounds 275-277 (1998) 186±190. [4] W. Hayes, Crystals with the Fluorite Structure, Clarendon, Oxford, 1974. [5] S.A. Pollack, D.B. Chang, J. Appl. Phys. 64 (1988) 2885± 2893. [6] P. Xie, S.C. Rand, Opt. Lett. 15 (1990) 848±850. [7] L. Esterowitz, J. Noonan, A. Schnitzler, Appl. Phys. Lett. 8 (1966) 271±273. [8] K. Muto, J. Phys. Chem. Solids 34 (1973) 2029±2035. [9] T. Szczurek, M. Schlesinger, Phys. Rev. B 34 (1986) 6109± 6111. [10] M. Schlesinger, T. Szczurek, Phys. Rev. B 35 (1987) 8341± 8347. [11] R.M. Macfarlane, Opt. Lett. 18 (1993) 829±831. [12] J.P. Jouart, M. Bouard, G. Klein, G. Mary, J. Lumin. 60/ 61 (1994) 93±96. [13] N.M. Strickland, G.D. Jones, Phys. Rev. 56 (1997) 10 916± 10 929. [14] J.P. Jouart, C. Bissieux, G. Mary, J. Phys. C 20 (1987) 2019±2026. [15] R.J. Booth, B.R. McGarvey, Phys. Rev. B 21 (1980) 1627± 1635.
Fig. 6. Part of the 3 F4 !1 G4 excitation spectra of (a) the Tm3 (C2V ) center of CdF2 :TmF3 , NaF (b) the Tm3 (C3V ) center of SrF2 :TmF3 and (c) the Tm3 (C3V ) center of BaF2 :TmF3 at 77 K.
www.elsevier.nl/locate/optmat
Red-to-blue up-conversion spectroscopy of Tm3 in SrF2, CaF2, BaF2 and CdF2 M. Bouard a,*, J.P. Jouart a, M.-F. Joubert b b
a Laboratoire dÕEnerg etique et dÕOptique, UTAP, Universit e de Reims Champagne-Ardenne, BP 1039, 51687 Reims Cedex, France Laboratoire de Physico-Chimie des Mat eriaux Luminescents, UMR n 5620 du CNRS, Universit e de Lyon 1, 43 bd 11 Novembre 1918, 69622 Villeurbanne Cedex, France
Received 15 February 1999; accepted 21 September 1999
Abstract Up-conversion spectroscopy at low temperature is used to probe the symmetry of the sites occupied by the Tm3 ion in MF2 :TmF3 (with M Sr, Ca, Ba ) and in CdF2 : TmF3 , NaF. We detected single Tm3 centers of cubic, trigonal, tetragonal and orthorhombic symmetries and established energy level diagrams. Single Tm3 centers of cubic symmetry tend to predominate in CaF2 :TmF3 as the Tm3 concentration exceeds 1 at.% Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 78.55 Keywords: Up-conversion; Thulium; Fluorite-type crystals; Excited-state absorption; Selective excitation
1. Introduction Incorporated in small quantities inside transparent materials (crystals or glasses) and excited by an adapted laser beam, the trivalent rare earth (RE) ions usually generate photons of lower energies (down-conversion) but also higher energies (up-conversion) than those of the incident photons. In the case of erbium, excited-state absorption (ESA) from a metastable electronic level at low concentration (<0.1 at.%) and excited-state transfer at high concentration (>0.1 at.%) are both
* Corresponding author. Tel.: +33-326-053251; fax: +33-326053250. E-mail address: michel.bou[email protected] (M. Bouard).
main up-conversion processes [1,2]. In the case of thulium, on the other hand, ESA remains dominant when the concentration increases and photon avalanche up-conversion involving both ESA and cross relaxations between excited and nonexcited Tm3 ions were observed [3]. Trivalent RE-doped MF2 (with M Sr, Ca, Ba, Cd) is a suitable material characterized by low energy phonons [4] and large transfer coecients between the RE ions [5,6]. Most of the RE were extensively studied in it, except Tm3 which gave rise only to some reports [7±13]. In MF2 , the trivalent RE ion occupies a M2 site, thus requiring a charge-compensating interstitial Fÿ i ion which can modify the geometry and the constitution of the center by taking a position close to the RE ion. Any modi®cation in the local surroundings of the RE ion results in a lift of the degeneracy of certain
0925-3467/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 3 4 6 7 ( 9 9 ) 0 0 1 1 0 - X
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M. Bouard et al. / Optical Materials 14 (2000) 73±79
electronic levels which split in several Stark sublevels. Perturbations of trigonal and tetragonal symmetries induced by a Fÿ i ion are usually observed in trivalent RE-doped MF2 , however the cubic crystal-®eld is not perturbed as the Fÿ i ion is away from the RE ion. In the study we achieve, the main Tm3 centers of MF2 :TmF3 are selectively excited by an upconversion technique at liquid nitrogen temperature, under high excitation densities, in the red spectral range, obtained with a single focused laser beam. Under these conditions, each Tm3 center exhibits noticeable blue, red and ir ¯uorescences (Fig. 1) when the laser wave number is resonantly tuned on an ESA from a metastable electronic level 3 F4 or 3 H4 , in the wings of the ground-state absorption 3 H6 ® 3 F2 . The Tm3 energy level diagrams derived from an analysis of our excitation and emission spectra are presented for each center whose we look for to identify the Tm site symmetry according to the selection rules predicted by the group theory.
2. Experimental Three CaF2 crystals doped with 0.05, 0.1 and 2 at.% TmF3 ; three SrF2 crystals doped with 0.1, 1 and 2 at.% TmF3 ; one BaF2 crystal doped with 0.1 at.% TmF3 ; one CdF2 crystal double-doped with 0.1 at.% TmF3 and 1 at.% NaF were studied. The CdF2 crystal was grown at the University of Reims using the Bridgman method, the other crystals were synthesized by Optovac. The crystals are unoriented. The laser excitation technique used to perform the up-conversion experiments has already been described [14]. The ¯uorescence spectra were recorded at 77 K on a Coderg three-grating monochromator working from 12 200 to 22 700 cmÿ1 . A tuneable cw dye laser working with the kiton red from 15 200 to 15 900 cmÿ1 was used to excite the Tm3 ions. The excitation spectrum of each center was obtained by monitoring the intensity of a peculiar ¯uorescence line as a function of the laser wave number. Some absorption spectra were also carried out at 14 K with a CARY spectrophotometer at the University of Lyon 1.
3. Results and discussion Each crystal exhibits, under red laser excitation, numerous ¯uorescence lines between 12 200 and 22 700 cmÿ1 . These lines were used to establish the energy levels of the Tm3 ion. Two types of spectra were recorded for the blue ¯uorescence: the ®rst type exhibits only lines between 20 000 and 20 800 cmÿ1 originating from 1 G4 , while the second type exhibits in addition lines between 21 700 and 22 700 cmÿ1 originating from 1 D2 . 3.1. Evidence for cubic (Oh ) Tm3 centers in SrF2 , CaF2 , BaF2 and CdF2
Fig. 1. Energy level diagram for the Tm3 ion in MF2 showing ESA and ¯uorescence transitions studied in this paper.
In the ®rst type spectrum, the 1 D2 ® 3 F4 transition is completely missing closely following the forbidden character of electric-dipole transitions between the crystal-®eld levels of a Oh symmetry center. The observed lines are in agreement with
M. Bouard et al. / Optical Materials 14 (2000) 73±79
75
Fig. 2. 3 F4 !1 G4 excitation spectrum of the Tm3 (Oh ) center of CaF2 : 2 at.% TmF3 at 77 K.
the magnetic-dipole selection rules : the main transitions 1
G4 A1g !3 F4 T1g
and 1
G4 A1g !3 H5 T1g
are those for which DJ 0 or 1. The red emission (1 G4 ® 3 F4 ) is one order of magnitude stronger
than the blue emission (1 G4 ® 3 H6 ). The emitting G4 level is populated through a two-step absorption process [14]: ®rst absorption in a vibronic sideband of 3 F2 followed by relaxation to the metastable 3 F4 level, second absorption (resonant) from 3 F4 to 1 G4 . The 3 F4 ® 1 G4 excitation spectrum for the red (or blue) ¯uorescence from the cubic Tm3 center of CaF2 (Fig. 2) consists of two sharp lines located at 15 267 and 15 788 cmÿ1 : the ®rst one, very weak, 1
Table 1 A1g ® T1g lines positions (in cmÿ1 ) of the Tm3 (Oh ) center of MF2 and splittings due to a crystal ®eld perturbation of C3V symmetry for CaF2 :Tm, SrF2 :Tm, BaF2 :Tm and to a crystal ®eld perturbation of C2V symmetry for CdF2 :Tm, Na Crystal
Tm3 ±Fÿ separation (nm)
CdF2 :Tm, Na
0.233
CaF2 :Tm
Excitation 3 F4 (A1g ) ® 1 G4 (T1g ) 8 < 15 806 15 802 15 800 : 15 783
Fluorescence 1 G4 (A1g ) ® 3 F4 (T1g ) 8 < 14 949 14 930 14 946 : 14 925
Fluorescence 1 G4 (A1g ) ® 3 H5 (T1g ) 8 < 12 244 12 217 12 225 : 12 216
0.236
15 796 15 788 15 763
14 979 14 957 14 944
12 256
SrF2 :Tm
0.251
15 777 15 759 15 740
15 031 15 015 14 989
12 335
BaF2 :Tm
0.268
15 750 15 735 15 719
15 078 15 064 15 042
12 397
12 267 12 252
12 330 12 318
12 394 12 383
76
M. Bouard et al. / Optical Materials 14 (2000) 73±79
3.2. Evidence for trigonal (C3v ) Tm3 centers in SrF2 and BaF2
corresponds to A1g ® A1g (a small perturbation possibly due to a neighbouring Tm3 ion removes the interdiction), the second one, strong, corresponds to A1g ® T1g (allowed). Two broad lines attributed to vibronic side bands are also observed towards the higher energies compared to A1g ® A1g . Similar spectra (lowest and highest energy lines in brackets) were recorded for CdF2 (15 254, 15 802 cmÿ1 ), SrF2 (15 294, 15 759 cmÿ1 ) and BaF2 (15 317, 15 735 cmÿ1 ). We note regular shifts of excitation and ¯uorescence lines from cubic Tm3 centers as a function of the Tm3 ±Fÿ separation (Table 1).
A (Tm3 , Fÿ i ) center of trigonal symmetry is identi®ed in SrF2 and BaF2 (Table 2). The trigonal symmetry is due to an interstitial Fÿ i ion located in the position (a/2, a/2, a/2) with respect to the Tm3 ion (Fig. 3). In this case, the Stark sublevel T1g splits into two sublevels A2 and E, giving rise to two lines A1 ® A2 and A1 ® E located on both sides of each line A1g ® T1g (Table 1). An example is given Fig. 4. We are not unfortunately in a position to attribute each line
Table 2 Energy levels (in cmÿ1 ) and irreducible representations of the Tm3 (C3V ) center of MF2 Multiplets
CaF2
SrF2
1
D2
28 203 28 159 28 146
28 200 28 180 28 131
1
G4
21 330 21 297 20 804
A1
28 184 28 156 28 109
A1 E E
21 318 21 281 20 830
A1
21 297 21 266 20 862
A1
E E
15 282 15 247
E E
3
F2
15 315 15 280
3
F3
14 762 14 691
H4
12 872 12 844 12 747 12 629
3
12 639 3
H5
3
F4
8552 8537
5860 5830 5534 3
H6
14 656
A1 A2
8512 8500 8411 8403
E A2 A1
6158 6148 6094 5841 5799 5541
A2
435 224 138 118 0
226 158 0
BaF2 A1 E E
12 724 12 620
A1 A2
8479 8468
E A2 A1
6105 6044 5819 5782 5547
E A2 A1
391 200 A2
104 0
A2
M. Bouard et al. / Optical Materials 14 (2000) 73±79
77
Fig. 3. Oh , C4v , C3v and C2v centers of MF2 : Tm studied in this paper.
to its own irreducible representation. Nevertheless, the emergence of such splittings in the excitation and ¯uorescence spectra, the three sublevels for the 1 D2 multiplet (Table 2) and the lack of lines corresponding to the electric-dipole A1 $ A2 transitions, clearly indicate that the Tm3 ion occupies a trigonal site in SrF2 and BaF2 . Some irreducible representations are given in the Table 2. 3.3. Evidence for tetragonal (C4v ) Tm3 center in CaF2 In CaF2 , for Tm3 concentrations not exceeding 0.1 at.%, the Tm3 ion preferentially occupies a tetragonal (C4v ) center. The trigonal (C3V ) center is in the minority, the energy levels of this center are given in the Table 2. The spectroscopic properties of C4V center have been reported before [13]. An interstitial Fÿ i ion in the position (a/2, 0, 0) is responsible for the C4V symmetry (Fig. 3). The absorption spectrum of CaF2 : 2 at.% Tm3 (Fig. 5)
Fig. 4. Part of the 3 F4 !1 G4 excitation spectra of SrF2 :TmF3 at 77 K for (a) the Tm3 (C3V ) center and (b) the Tm3 (Oh ) center.
corresponding to the magnetic-dipole transition H6 ® 3 H5 consists of three major lines : in accordance with [13], the line located at 8430 cmÿ1 is ascribed to the 3 H6 (A2g ) ® 3 H5 (T2g ) transition from centers of cubic (Oh ) symmetry, the other lines being probably due to cluster centers which have been revealed by NMR studies [15], no absorption line due to centers of tetragonal (C4V ) symmetry is visible on this spectrum. It is noteworthy that in CaF2 : 2 at.% Tm3 the Oh center concentration is much higher than those of the 3
78
M. Bouard et al. / Optical Materials 14 (2000) 73±79
Fig. 5. 3 H6 !3 H5 absorption spectrum of CaF2 : 2 at.% TmF3 at 14 K.
tetragonal center. The transient eects were studied while chopping the laser beam with a Pockels cell: the rise and decay of the ¯uorescence from 1 G4 depend on the nature of the center: Oh (sr 110 ms, sd 3:6 ms), C4v (sr 40 ms, sd 1:1 ms).
appreciable change of position for the eight nearest neighbour (nn) Fÿ ions surrounding the Tm3 ion, and supporting our attributions concerning the Oh Tm3 centers.
4. Conclusion 3.4. Evidence for orthorhombic (C2v ) Tm3 center in CdF2 In order to con®rm our interpretation, we have studied a CdF2 crystal double-doped with 0.1 at.% TmF3 and 1 at.% NaF. As might be expected, a (Tm3 , Na ) center of orthorhombic (C2v ) symmetry and a Tm3 center of cubic symmetry were identi®ed in it. A substitutional Na ion located in the position (a/2, a/2, 0) with respect to the Tm3 ion (Fig. 3) induces the weak perturbation of C2V symmetry which splits the level T1g of Oh center into three sublevels corresponding to irreducible representations A2 , B1 and B2 . Three lines were eectively observed near the A1g ® T1g lines (Table 1 and Fig. 6) in the excitation and ¯uorescence spectra of the (Tm3 , Na ) center. We verify that the (Tm3 , Na ) centerÕs energy levels are close to those of the Oh center (Table 1), testifying to no
Three types of single Tm3 centers (Oh , C3v , C4v ) were identi®ed in MF2 :TmF3 (with M Sr, Ca, Ba). Energy levels were established for each of them, except the Oh center for which the 3 H4 level position has not been determined. The (Tm3 , Fÿ i ) centers of C4v and C3v symmetries are in the majority in MF2 : the C4v center in CaF2 (for Tm3 concentrations not exceeding 0.1 at.%), the C3v center in SrF2 and BaF2 . These symmetries are due to an interstitial Fÿ i ion located in nn and next nearest neighbour (nnn) position, respectively, with respect to the Tm3 ion. In CaF2 , the Oh center concentration strongly increases relative to those of the C4v center, as the Tm3 concentration exceeds 1 at.%. (Tm3 , Na ) centers of C2v symmetry and Tm3 centers of Oh symmetry are also identi®ed in CdF2 : TmF3 ,NaF.
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Fig. 6. Part of the 3 F4 !1 G4 excitation spectra of (a) the Tm3 (C2V ) center of CdF2 :TmF3 , NaF (b) the Tm3 (C3V ) center of SrF2 :TmF3 and (c) the Tm3 (C3V ) center of BaF2 :TmF3 at 77 K.