Optical absorption and luminescence of Ho3+ ions in Bi2TeO5 single crystal

Optical Materials 29 (2007) 688–696 www.elsevier.com/locate/optmat

Optical absorption and luminescence of Ho3+ ions in Bi2TeO5 single crystal I. Fo¨ldva´ri

b

a,*

, A. Baraldi b, R. Capelletti b, N. Magnani b, R. Sosa F A. Munoz F c, L.A. Kappers d, A. Watterich a

a,c

,

a Research Institute for Solid State Physics and Optics, HAS, H-1121 Budapest, Konkoly-Thege u. 29-33, Hungary Physics Department and Consorzio Nazionale Interuniversitario di Scienze Fisiche della Materia (CNISM), University of Parma, 43100 Parma, Italy c Physics Department, Autonomous Metropolitan University at Iztapalapa, DF, Mexico d Physics Department and IMS, University of Connecticut, Storrs, CT 06269, USA

Received 1 September 2005; accepted 26 November 2005 Available online 18 January 2006

Abstract Bi2TeO5:Ho single crystals were grown by the Czochralski technique. Optical absorption spectra of Ho3+ have been monitored in the 4500–25,000 cm1 spectral and 9–300 K temperature range by high resolution Fourier Transform Spectroscopy. Luminescence excitation and emission spectra were measured at room temperature in the visible range. The intra-configurational f–f transitions of Ho3+ were identified. The experimental absorption data were analyzed by a single-ion model, and the crystal field parameters for a C2-symmetry site were determined. On the basis of these calculations and analogy with the case of Er-doped Bi2TeO5, it can be inferred that Ho– Bi substitution mostly takes place at the Bi(1) sites. The Judd–Ofelt analysis was performed by fitting the polarized absorption spectra. The calculated Judd–Ofelt parameters were used to evaluate the Ho3+ radiative transition probabilities and the branching ratios. The results are discussed in view of potential applications of Bi2TeO5:Ho3+ as a laser material.  2005 Elsevier B.V. All rights reserved. PACS: 42.70.a; 71.70.d; 78.30.j; 78.40.q Keywords: Bismuth tellurite; Ho3+; Laser material; Judd–Ofelt analysis

1. Introduction Bismuth tellurite (Bi2TeO5) is a non-linear optical crystal with interesting photorefractive properties that may be applied in holographic recording [1,2]. The Bi3+ and Te4+ lattice sites provide suitable environments for hosting several types of dopants [3,4]. The crystal structure of Bi2TeO5 is orthorhombic (Abm2) and can be interpreted as a 2 · 3 · 1 superstructure of the CaF2 structure with 1/ 6 empty oxygen positions [5]. These sites may be filled. As a consequence, traces of the oxidized Bi2TeO6 com-

*

Corresponding author. Tel.: +36 1 392 2627; fax: +36 1 392 2223. E-mail address: [email protected] (I. Fo¨ldva´ri).

0925-3467/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2005.11.018

pound, which is stable between 550 and 700 C, may be present. Furthermore, the additional oxygens may work in charge compensating some dopants. Chromium, for example, can enter the crystal as chromate anion (Cr6+) [6]. The undoped crystal is transparent in the 400– 7000 nm region [7]. Laser active dopants, as the trivalent rare earth ions, can be introduced into the Bi2TeO5 lattice with crystal/melt segregation coefficients close to unity [3,4]. They occupy different Bi(1), Bi(2), and Bi(3) lattice sites surrounded by 8, 7, and 7 oxygens, respectively (see terminology and structural details in Ref. [5]). Laser effect has not been shown in Bi2TeO5 so far, but the high incorporation level of the laser-active dopants, which could be achieved, and the non-linear character derived from the crystal structure

I. Fo¨ldva´ri et al. / Optical Materials 29 (2007) 688–696

make this crystal a potential self-frequency-doubling (SFD) laser host. SDF is limited to the IR–VIS conversion due to the 400 nm intrinsic absorption edge of the crystal. Among trivalent rare earths, Er and Ho have several metastable states (5 and 4, respectively), which make them promising for laser operation [8]. Until now, only the spectroscopic properties of Er3+ in Bi2TeO5 were studied in detail [9,10]. In the present work we extended the spectroscopic analysis to Ho-doped Bi2TeO5, by means of absorption and luminescence measurements. The expected complex spectra induced by a non-Kramers ion, as Ho3+ is, embedded in a low symmetry matrix were studied by applying the high resolution spectroscopy in the temperature range 9– 300 K. Crystal field calculations were performed to identify the Bi site occupied by Ho3+. The Judd–Ofelt model was used to evaluate the spontaneous emission probabilities and the branching ratios. Results are discussed in view of potential applications of Bi2TeO5:Ho3+ as a laser material. 2. Experimental Bi2TeO5 single crystals were grown by the diameter controlled Czochralski method. Holmium was added to the melt as Ho2O3: the concentrations were 103 and 102 Ho3+ ion/Bi2TeO5 mole, respectively. The growth was performed in a platinum crucible and in air atmosphere. The pulling rate was 0.8–1 mm/h along the [0 0 1] direction. High purity (5 N) TeO2 and Bi2O3 starting materials were used to prepare the Bi2TeO5 compound by a two-step solid phase reaction. The Bi2TeO5 growth is not an easy task, due to the complexity of the Bi2O3/TeO2 system phase diagram, the critical equilibrium with oxygen (Bi2TeO6 traces), and the component evaporation from the melt. The details of the improved growth technique are reported in Refs. [11,12]. Samples for spectroscopic measurements were X-ray oriented, cut by diamond saw, and polished. To perform the optical measurements along three different crystallographic directions, the samples were polished on surfaces parallel to the (0 0 1) and (0 1 0) planes and cleaved along the (1 0 0) ones. The samples were parallelepipeds with edge dimensions in the 4–10 mm range. The optical homogeneity was tested by optical and microscopic methods. The dopant concentration was evaluated by means of atomic absorption spectroscopy. The high resolution absorption spectra were measured by means of a Fourier Transform BOMEM DA8 spectrometer, capable of a resolution as fine as 0.02 cm1. The spectra were acquired in the 4500–25,000 cm1 range at a resolution between 0.1 and 0.5 cm1, which was fine enough to resolve the Ho-induced lines in Bi2TeO5 even at 9 K. The sample temperature was varied between 9 and 300 K by assembling the sample in a 21SC Model Cryodine Cryocooler of CTI Cryogenics equipped with KRS5 and quartz windows. The standard resolution polarized absorption spectra in the visible and near infrared

689

ranges were measured at room temperature (RT) by JASCO V-550 and BRUKER IFS 66 V/S spectrometers. The luminescence excitation and emission spectra were monitored in the 400–800 nm range at resolution ranging between 1 and 5 nm by a Hitachi F4500 spectrofluorimeter at RT. 3. Judd–Ofelt model The absorption lines of trivalent rare-earth ions (RE) in a crystalline host are due to intra-configurational f–f transitions. The non-centro-symmetric ligand field, probed by RE in Bi2TeO5, makes the RE electric-dipole transitions partially allowed. Judd–Ofelt (J–O) model [13,14] has been employed to describe the absorption and photoluminescence properties of RE in many ion-host combinations [15]. Fitting procedure of the experimental absorption oscillator strengths to the theoretical ones allows evaluating the phenomenological J–O parameters X2, X4, and X6, which can be used to calculate other important parameters, as radiative transition probabilities, branching ratios, etc. The crystal field splitting of the RE levels is neglected, thus the absorption measurements are performed at temperatures, as RT, at which the sublevels may be considered equally populated within an acceptable error [13]. The experimental oscillator strength fexp for an absorption transition between the initial 4f state aJ and the final 0 one bJ 0 (i.e. for the transition between 2S+1LJ and 2S þ1 L0J 0 manifolds) can be evaluated from the absorption spectra as Z mc 0 aðmÞ dm ð1Þ fexp ðaJ ; bJ Þ ¼ 2 pe N where a(m) is the polarization dependent absorption coefficient at frequency m, m and e are the electron mass and charge, respectively, c is the light speed in vacuum, and N is the RE concentration. From theoretical point of view, the oscillator strength fcalc, for electric dipole transitions [13,14,16], can be written as fcalc ðaJ ; bJ 0 Þ ¼

X 8p2 mc 2 ðnÞ Xt jhaJ jU t jbJ 0 ij v ed 3hð2J þ 1Þn2 k t¼2;4;6

ð2Þ  where h is the Planck constant, k is the mean wavelength at which the transition occurs, ved(n) is the Lorentz field correction for the RE embedded in a given medium of refrac0 tive index n, haJjUtjbJ i is the matrix element of a unit operator Ut of rank t, and Xt are the phenomenological J–O parameters. Ut depends on the RE-crystal system, while the matrix elements for a given RE vary only slightly from medium to medium. Therefore their values are tabulated for many RE ions and transitions: those reported by Weber [16] and Carnall [17] were used in the present work. The anisotropy of Bi2TeO5 refractive index n(k) was considered by taking advantage of the polarization dependent values given in Ref. [18].

I. Fo¨ldva´ri et al. / Optical Materials 29 (2007) 688–696

The J–O parameters Xt (t = 2, 4, 6) are determined by fitting the measured oscillator strengths fexp (Eq. (1)) to the calculated ones fcalc (Eq. (2)). To evaluate the accuracy of the Judd–Ofelt parameters and the theoretical oscillator intensities, the root mean square value drms was calculated as rhffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi X 2 i. drms ¼ fcalc  fexp ð n  3Þ ð3Þ

4.5

5

G5

3

K8

4.0

5

3.5 5

Absorbance

690

where n is the number of considered transitions. The probability AbJ 0 ;aJ for radiative spontaneous emission from an excited state bJ 0 to a lower state aJ (i.e. for 0 the transition between 2S þ1 L0J 0 and 2S+1LJ manifolds) is given by X 64e2 p4 2 AbJ 0 ;aJ ¼ v ðnÞ Xt jhbJ 0 jU t jaJ ij ð4Þ ed 3  3hð2J þ 1Þk

3.0 2.5 2.0

S2+ F4

5 5

5

I7

5

I6

F1+5G6

F2

5

F3

F5

I4

5

I5

E || z

5

5

1.5 1.0

E || y

0.5 E || x

0.0 5000

10000

15000

20000

25000

Wave number (cm-1)

t¼2;4;6

The radiative lifetime srad of the excited manifold 2S0 þ1 0 LJ 0 (bJ 0 state) can be calculated as X 1 ¼ AbJ 0 ;aJ ð5Þ srad aJ Thus all possible transitions from a given excited manifold L0J 0 to all possible lower lying 2S+1LJ ones are considered. Non-radiative transitions may also occur, which reduce the measured srad values. For anisotropic crystals, as in the case of Bi2TeO5, AbJ 0 ;aJ are calculated for each basic polarization direction q. The radiative lifetime of a given manifold is the reciprocal of the total spontaneous emission probability Atot, evaluated as 1XX Atot ¼ Aq;bJ 0 ;aJ ð6Þ 3 q aJ

2S0 þ1

where the subscript q indicates the polarization direction. The fraction of photons emitted from an excited mani0 fold 2S þ1 L0J 0 to a single lower one 2S+1LJ (with respect to the total number of photons emitted from the given state to all the lower ones) is the luminescence branching ratio b and is expressed as

window (400–7000 nm) [8]. Fig. 1 gives an overview of the absorption spectra, measured at RT for three different polarizations, at a standard resolution. As a consequence of the host crystal anisotropy, the absorption peak intensity strongly depends on the orientation. This was considered in the Judd–Ofelt calculations. In Fig. 1 all the absorptions induced by the above mentioned transitions were monitored indeed, except for the 5I8 ! 5I4. The high resolution spectra measured at low temperature (i.e. 9 K) allowed to resolve the complex structure of all the absorptions displayed in Fig. 1 (included the weak 5I8 ! 5I4). Figs. 2 and 3 provide, as examples, details of the high resolution spectra in the regions of the 5I8 ! 5F5 and 5 I8 ! 5F3 transitions, respectively. The spectra are quite

0-5

1-0 2-1

8

5

2-0

I8

(b)

5

F5

(a)

0-6

6

ð7Þ

4. Results and discussion

15350 15360 15370 15380 15390 15400 15410

0-3

0-1

μ (cm -1)

bbJ 0 ;aJ ¼ AbJ 0 ;aJ srad

AbJ 0 ;aJ ¼P 0 aJ AbJ ;aJ

Fig. 1. Polarized absorption spectra of Bi2TeO5:Ho3+ (1%) single crystal for the basic crystallographic directions, measured at room temperature.

0-0

0-4 0-7

0-2

(a)

4

Ho3+ is a non-Kramers ion, since the electron number (10) in the 4f shell is even. As a result, the degeneracy of each 2S+1LJ manifold may be removed completely by the crystal-field potential splitting it into 2J + 1 sublevels, if the local symmetry is low enough [19] (as in the case of Bi2TeO5). The ground manifold is 5I8. Absorptions, due to transitions from 5I8 to the 5I7, 5I6, 5I5, 5I4, 5F5, 5 S2 + 5F4, 5F3, 5F2, 3K8, 5F1 + 5G6, and 5G5 excited manifolds, are expected to occur in the Bi2TeO5 transmission

0-9

1-5 1-3

4.1. Absorption spectra of Ho3+ in Bi2TeO5 crystals

0-10

0-8

1-1

2

(b)

1-0

(c)

9K 40K 100K

(d)

300K

0

15350 15400 15450 15500 15550 15600 15650 15700 15750

Wave number (cm-1) Fig. 2. Optical absorption spectra of Bi2TeO5:Ho3+ (1%) single crystal measured along the [1 0 0] direction at different temperatures in the 5 I8 ! 5F5 spectral range (res. 0.5 cm1). The insert is a magnification of 9 and 40 K spectra (curves a and b) on the low wave number side to show some hot lines.

I. Fo¨ldva´ri et al. / Optical Materials 29 (2007) 688–696 0-1

10

0-2 5

F3

1-1

(b)

1-0

(a)

8 20480

μ (cm-1)

0-0

2-1

5

I8

20490

20500

20510

20520

20530

0-3

6 0-4

0-5

0-0

4

0-6

1-0

(a) (b)

1-1

2

0

9K 40K 100K

(c)

2-1

20500

20550

20600

(d)

300K

20650

20700

Wave number (cm-1) Fig. 3. Optical absorption spectra of Bi2TeO5:Ho3+ (1%) single crystal measured along the [1 0 0] direction at different temperatures in the 5 I8 ! 5F3 spectral range (res. 0.5 cm1). The insert is a magnification of 9 and 40 K spectra (curves a and b) on the low wave number side to show some hot lines.

complex, as a consequence of the non-Kramers character of Ho3+ and the low symmetry of the Bi2TeO5 crystal sites. However, even in the high wave number range, where the excited manifolds are close one to each other (see Fig. 1), the sets of lines originated by the transitions from the ground to a given excited manifold, could be separated, except in the case of the 5S2 + 5F4 and 5F1 + 5G6 manifolds. The absorption spectra of the two samples with

different Ho3+ concentration (1% and 0.1%) did not show meaningful differences, except of course for the line intensity. The complete energy level scheme and the correct line attribution for the transitions identified can be obtained by (i) measuring absorption spectra as a function of temperature and (ii) fitting the experimental energy levels with a single-ion Hamiltonian model. By increasing the temperature the line intensity scales with the thermal population of the ground state sublevels, thus supplying a key for attributing all the transitions [20]. At high temperatures, the intensities of the lines associated to transitions starting from the zero sublevel of the ground manifold gradually decrease, while those arising from non-zero sublevels increase and become dominant (see Figs. 2 and 3). By considering the strongest lines in each set, it was possible to identify nearly all the transitions starting from the lowest sublevel of the ground manifold to the 2J + 1 sublevels of the excited 2S+1LJ manifold (see Table 1). The stark splitting of the 5I8 ground state was derived by comparing the 9 K spectra to the 40 and 100 K ones. Because of line broadening and overlapping displayed by the high temperature spectra, only seven sublevels could be identified, see Table 1 (the highest sublevels might be more easily identified by high resolution photoluminescence measurements performed at low temperature). Among these, the first two are separated by 30 ± 2 and 48 ± 3 cm1 from the lowest sublevel, respectively. The two values result from averaging 12 and 13 of such separations, respectively, related to different lines and different manifolds. These excited sublevels are thermally populated

Table 1 Energy levels (cm1) of Ho3+ in Bi2TeO5 crystal with the total splitting D of each

5 5

I8 I7

5

I6

5

I5

5

I4

5

F5

5

S2 + 5F4

5

F3 F2 3 K8 5

5

F1 + 5G6

5

G5

691

2S+1

LJ manifold

0 8

1 9

2 10

3 11

4 12

5 13

6 14

7 15

2J+1

Obs

0 5132 5275 8651 8775 11,220 11,362 (13,210) – 15,413 15,646 18,449 18,571 20,521 21,060 21,305 21,385 21,544 21,966 22,131 23,862 23,989

30 ± 2 5161 5290 – 8795 11,252 11,410 13,272

48 ± 3 5176 5304 8682 8800 11,259 – 13,309

99 ± 4 5189 5336 8703 8881 11,275

134 ± 1 5232 5352 8712 8898 11,285

228 ± 3 5250 5394 8719

286 ± 8 5256 5414 8734

304 ± 4 5267

17 15

8 15

282

8746

13

12

247

11,297

11,330

11,341

11

10

(190)

13,342

13,357

13,387

13,416

13,543

9

8

(332)

15,447 15,704 18,456 18,610 20,541 21,093 21,312 21,412

15,461 15,728 18,477 18,629 20,578 21,113 21,320 21,424

15,489

15,513

15,541

15,590

15,636

11

11

315

18,492 18,657 20,599 21,173 21,324 21,437

18,500 18,684 20,624 21,224 21,334 21,445

18,521 18,695 20,669

18,538

18,547

9

9

246

21,345 21,477

21,359 21,488

21,371 21,522

7 5 17

7 5 17

184 164 239

21,972 22,196 23,872 24,025

21,980 22,226 23,900 24,061

21,993 22,272 23,911

21,998 22,314 23,925

22,015 22,339 23,946

22,060 22,352 23,958

22,116 22,391 23,976

16

16

425

11

11

199

20,705

D

The number (2J + 1) of predicted sublevels is compared with that monitored by the spectra (Obs). The sublevel attribution (given in the table headings) is suggested by the comparison of the experimental values with the calculated ones (see Table 2).

I. Fo¨ldva´ri et al. / Optical Materials 29 (2007) 688–696

692

even at 9 K, as proved by the weak absorptions (hot lines), indicated in the inserts of Figs. 2 and 3. Their intensities increase by increasing the temperature (compare curves a and b in the inserts of Figs. 2 and 3). In Bi2TeO5 the ground state sublevel separations, which could be clearly identified, are larger than those recently reported for Ho3+ in BaY2F8 [21]. In that case a splitting between the first two sublevels as low as 0.6 ± 0.06 cm1 could be evaluated, due to the sharpness of the lines. Despite of resolution as fine as 0.1 cm1 used to measure the 9 K spectra, the Ho3+ absorption lines in Bi2TeO5 remained rather broad: the full width at half maximum (FWHM) was typically in the 5–10 cm1 range for samples doped with the higher Ho3+ concentration (1%). These FWHM values may be explained by the rather large Bi–O distances (Ho should substitute Bi in doped sample, see below), which might allow wide oscillations of the light oxygen ions, even at low temperatures [5]. In some cases, shoulders were overlapped to the main lines. The lines were slightly narrower and better resolved in the sample with lower Ho3+ concentration (0.1%), due to the reduced inhomogeneous broadening, caused by the random RE ion distribution. In the Bi2TeO5 lattice Ho3+ should substitute Bi3+ ions, ˚ due to the same charge and similar ionic radius (i.e. 1.2 A ˚ for Ho3+ in oxides, with coordination for Bi3+ and 1.02 A number 8 [8]). This hypothesis is supported by PIXE/channeling measurements performed on Er doped Bi2TeO5, which showed that Er3+ occupies the Bi lattice sites [22]. The existence of three different sites, Bi(1), Bi(2), and Bi(3) with coordination numbers 8, 7, and 7, respectively [5], suggests that the RE might probe different crystal fields,

according to the site occupied. This should originate three different sets of manifolds and, as a consequence, three different sets of absorption lines. In Er doped Bi2TeO5 more lines were detected with respect to the (2J+1)/2 expected for each absorption transition of the Er3+ Kramers ion from the ground state to the 2S+1LJ manifold. The large number of lines was attributed to Er3+ occupying different Bi sites, although a preferential occupation of the Bi(1) site was reported [9]. On the contrary, for Ho doped Bi2TeO5 only in a few manifolds the number of the main lines, detected in the spectra, exceeded that expected (2J+1) from the Ho3+ non-Kramers ion transitions. Furthermore the sequence of lines, due to transitions from the lowest seven identified sublevels in the ground state to a given sublevel of an excited manifold, could be detected for several sublevels of different excited manifolds. These observations suggest that the majority of Ho3+ ions occupy a single Bi site. The presence of additional weak lines and shoulders overlapping the main ones can be attributed to a minority of Ho3+ which either sit in different Bi sites or are affected by Ho–Ho interaction. The latter possibility should be taken into account since, for example, loose interaction among RE has been recently monitored by high resolution spectroscopy in Er-doped BaY2F8 [20]. The experimental energy levels were fitted with a singleion Hamiltonian that accounts for free-ion and crystal-field interactions. According to [23], the atomic part is written as X H FI ¼ Eav þ F k fk þ fH SO þ aLðL þ 1Þ þ bGðG2 Þ k

þ cGðR7 Þ þ

X

T i ti þ

i

Table 2 Calculated energy levels (cm1) of Ho3+ in Bi2TeO5 crystal with the total splitting D of each

5

I8

5

I7

5

I6

5

I5

5

I4

5

F5

5

S2 + 5F4

5

F3 F2 3 K8a 5

5

F1+5G6a

5

G5a a

0 8

1 9

2 10

3 11

4 12

1.7 305.4 5145 5279 8660 8771 11,217 11,357 13,160 13,589 15,442 15,666 18,450 18,580 20,499 21,073 21,675 21,777 22,017 22,266 23,987 24,157

29.9 329.5 5151 5295 8669 8817 11,242 11,421 13,261

46.0 357.6 5179 5300 8677 8819 11,252 11,430 13,298

113.7 381.7 5197 5340 8696 8882 11,269

127.0 446.8 5228 5340 8714 8885 11,284

11,287

13,351

13,373

15,442 15,692 18,472 18,601 20,527 21,089 21,677 21,782 22,019 22,308 23,994 24,187

15,502 15,705 18,480 18,620 20,584 21,120 21,721 21,794 22,064 22,338 24,059 24,209

15,511 18,498 18,647 20,604 21,164 21,724 21,815 22,082 22,339 24,062

Not included in the fitting procedure (see text for details).

2S + 1

5 13 223.0 466.2 5243 5402 8716

X

M j mj þ

X

j

k

7 15

16

P k pk

ð8Þ

LJ manifold 6 14 279.4 475.6 5259 5403 8733

304.1 560.3 5266

D 564.7 563.0 259

8753

225

11,317

11,342

213

13,391

13,423

13,568

429

15,516

15,540

15,602

15,633

263

18,503 18,668 20,642 21,215 21,729 21,847 22,168 22,381 24,090

18,512 18,675 20,662

18,525

18,564

225

21,756 21,848 22,192 22,409 24,120

21,759 21,874 22,218 22,411 24,123

20,725

226 21,760 21,878 22,256 22,430 24,132

239 21,888 413 222

I. Fo¨ldva´ri et al. / Optical Materials 29 (2007) 688–696

H CF ¼

X k¼2;4;6

( ðkÞ B0k C 0

þ

k X

Bqk

h i q ðkÞ C ðkÞ q þ ð1Þ C þq

) ð9Þ

q¼1

where we have fixed B12 ¼ 0, thus exploiting the approximate C2 symmetry of the Bi(1) site. The fitting results (Tables 2 and 3) are good, with a r.m.s. deviation r ffi 11 cm1. The experimental energy levels above 21,300 cm1 (i.e. those belonging to the 3K8, 5F1, 5G6, and 5G5 manifolds) were not included in the fitting procedure, since the complexity of the corresponding spectra did not allow an unambiguous attribution to a single rare-earth site; however, we have calculated their position with the fitted single-ion parameters. Although their barycenters do not perfectly match their experimental counterparts, the intra-manifold crystal-field splittings are satisfactorily reproduced, as also witnessed by the comparison of the experimental and theoretical total manifold splitting D. For this reason we did not attempt any further correction, since we are interested in the crystal-field details and a precise determination of the free-ion potential is out of the scope of this work. Our preliminary crystal-field results seem to indicate that: (i) major occupation of the Bi(1) site by Ho3+ ions is likely, as in the case of Er-doped Bi2TeO5 [9]; (ii) the sequence of the ground-manifold splittings proposed in Table 1 is essentially correct and does not show any quasi-degenerate structures. A more complete theoretical analysis is currently underway and will also include studies of other rare-earth dopants, in the hope to reach a complete understanding of the crystal-field potential by a unified model as for example in BaY2F8 [26]. 4.2. Luminescence spectra of Ho3+ in Bi2TeO5 crystals Former experiments showed that undoped Bi2TeO5 crystals do not exhibit photoluminescence, at least at RT [27]. The presence of impurities (i.e. Er3+) induces photoluminescence, if the excitation wavelength falls in Er-absorption ranges [10]. A similar role is played by Ho3+, as proved in the present case by means of excitation and emission spectra measured at RT in the visible range. The excitation spectra, displayed in Fig. 4, were monitored by selecting a few characteristic emission wavelengths

Table 3 Free-ion and crystal-field parameters for Ho3+ in Bi2TeO5 crystal Parameter

Value (cm1)

Eav F2 F4 F6 f a b c T2 T3 T4 T6 T7 T8 M0 M2 M4 P2 P4 P6 B02 B22 B04 B14 B24 B34 B44 B06 B16 B26 B36 B46 B56 B66

48,602 94,072 66,826 53,677 2120.8 [17.15] [607.9] [1800] [400] [37] [107] [264] [316] [336] [2.54] [1.42] [0.79] [605] [302.5] [60.5] 428 539 565 38 237 1009 269 160 600 160 104 627 318 43

The values reported in square brackets were kept fixed during the fitting procedure.

5

5

F4 , S2

300

Emission intensity (arb. units)

where k = 2, 4, 6; i = 2, 3, 4, 6, 7, 8; j = 0, 2, 4. This model free-ion Hamiltonian accounts for two-body electrostatic repulsion (Fk), two- and three-body configuration interactions (a, b, c, and Ti, respectively), spin–orbit coupling (f), spin-other-orbit interactions (Mj) and electrostatically correlated spin–orbit interactions (Pk). The spherically symmetric one-electron contribution is represented by a uniform energy shift of the 4fn configuration (Eav). Detailed descriptions of the various operators and parameters are available in the literature [24]. The crystal-field Hamiltonian is expressed in terms of the tensor operators C ðkÞ q defined in Ref. [25]

693

3

5

F1 , G6

5

F3

5

200

F2 5

5

F5

G5

5

100

(c)

3

K8

I4

(b) 0 400

(a) 500

600

700

Wavelength (nm)

Fig. 4. Luminescence excitation spectra of Bi2TeO5:Ho3+ (1%) single crystal measured at room temperature for different emission wavelengths kem. Curve a: kem = 490 nm (5F3 ! 5I8), curve b: kem = 660 nm (5F5 ! 5 I8), and curve c: kem = 752 nm (5I4 ! 5I8).

I. Fo¨ldva´ri et al. / Optical Materials 29 (2007) 688–696

(i.e. 490, 660, and 752 nm) in the ranges of Ho3+ transitions (i.e. 5F3 ! 5I8, 5F5 ! 5I8, and 5I4 ! 5I8, respectively). BTO:Ho3+ is efficient in emitting visible luminescence (i.e. at 490, 660, and 752 nm), not only as a consequence of excitation within the examined transition (i.e. 5F3 ! 5 I8, 5F5 ! 5I8, and 5I4 ! 5I8, respectively), but also if excitation light populates higher energy lying Ho3+ levels. A convincing example is provided by the emission at 752 nm (5I4 ! 5I8): it is excited only weakly by light stimulating the 5I8 ! 5I4 transition and much more efficiently by light inducing transitions to manifolds lying at energies higher than the 5I4 manifold (see curve c in Fig. 4). However, excitation in the tail of BTO absorption edge (400– 410 nm) does not induce emission in the above selected wavelengths (see curves a, b, and c in Fig. 4). This may suggest that energy transfer from the host matrix to Ho3+ is negligible. Similar conclusions were achieved in the case of Bi2TeO5:Er system [10]. By exciting at 452 nm (5I8 ! 5F1, 5G6 transition), the major emission peaks in the visible correspond to decay from 5I4, 5F5, and 5S2 + 5F4 manifolds to the ground state. Among them the emission from the 5 S2 + 5F4 levels dominates the visible luminescence spectra (see Fig. 5). As reported above for the emissions occurring at 490, 660, and 752 nm, the 5S2 + 5F4 ! 5I8 transitions can be induced by populating any higher energy Ho3+ levels. Emissions in the visible due to transitions between excited states were not observed at least at RT. 4.3. Judd–Ofelt calculation The results of the Judd–Ofelt calculation are collected in Table 4. Experimental and calculated oscillator strength values were close to each other for the three chosen orientations. The drms was reasonably low (from 5 · 107 to 1 · 106), falling in the range of values usually obtained from J–O fittings. Thus the J–O theory gives a framework

5

800

Emission intensity (arb. units)

694

5

F4 , S2

5

I8

600

400

200 5

I4

5

F5

5

I8

5

I8

0 550

600

650

700

750

Wavelength (nm)

Fig. 5. Luminescence emission spectrum of Bi2TeO5:Ho3+ (1%) single crystal measured at room temperature after excitation at 452 nm (5I8 ! 5F1, 5G6).

in which the intensities of the f–f transitions of Ho3+ in Bi2TeO5 can be adequately described. The calculated spontaneous emission probabilities A and the fluorescence branching ratios b for the three polarization directions are listed in Table 5. It is well established that the majority of potential laser transitions possesses large branching ratios and stimulated emission probabilities. In few exceptions, despite rather low A values, the branching ratios associated with the laser transition are particularly high and the upper level lifetime is long, allowing for the buildup of a large population inversion. Examination of Table 5 shows that the laser transitions 5I7 ! 5I8 and 5F4 ! 5I8 have high branching ratios and emission probabilities in the Ek[1 0 0] polarization, therefore, laser operation appears feasible. Possible contribution from the allowed magnetic dipole transitions should be expected in the former case.

Table 4 Experimental fexp and calculated fcalc oscillator strengths, related root mean square deviation drms (given in units of 107), and Judd–Ofelt parameters Xn (given in units of 1020 cm2) for Ho3+ in Bi2TeO5 crystal along the basic crystallographic polarization directions Wavenumber (cm1) 5

I7 I6 5 I5 5 F5 5 F4 + 5S2 3 K8 5 F1 + 5G6 5 G5 5

5149 8568 11,203 15,408 18,540 21,317 21,978 23,753

Ek[1 0 0]

Ek[0 1 0]

Ek[0 0 1]

fexp

fcalc

fexp

fcalc

fexp

fcalc

6.72 2.36 2.49 15.21 11.56 2.64 114.87 34.29

2.70 1.49 2.99 17.68 12.57 3.01 114.88 32.82

8.28 6.10 0.82 31.03 31.15 6.56 121.46 67.71

7.54 4.69 6.19 37.73 29.07 5.66 121.48 64.60

6.98 5.54 0.68 22.35 19.27 6.41 96.50 47.11

4.89 2.99 4.31 25.92 19.63 3.92 96.55 45.07

drms

5.15

9.95

6.87

X2 X4 X6

1.200 1.555 4.975

0.448 2.930 0.288

0.497 1.997 0.165

I. Fo¨ldva´ri et al. / Optical Materials 29 (2007) 688–696 Table 5 Polarized spontaneous emission probabilities and fluorescence branching ratios b for Transition

Wavelength (nm)

Ek[1 0 0] 0

5

5

I7 ! I8 5 I6 ! 5I8 5 I6 ! 5I7 5 I5 ! 5I8 5 I5 ! 5I7 5 I5 ! 5I6 5 F5 ! 5I8 5 F5 ! 5I6 5 S2 ! 5I8 5 S2 ! 5I7 5 S2 ! 5I6 5 S2 ! 5I5 5 F4 ! 5I8 5 F4 ! 5I7 5 F4 ! 5I5 5 F3 ! 5I8 5 F2 ! 5I8 3 K8 ! 5I8 5 G6 ! 5I8 5 G5 ! 5I8

1997 1187 2922 902 1644 3758 655 1461 551 761 1029 1417 544 748 1372 490 478 472 457 424

2Sþ1

695

LJ ! 5 I08 transitions of Ho3+:Bi2TeO5 crystal

Ek[0 1 0] 1

A(J–J ) (s )

b (%)

662.1 1640.3 139.0 780.4 937.7 58.7 11,456 1163.9 13,674 14,655

100 92 8 44 53 3 91 9 46 50

1229.3 27,203

4 94

1750.1 21,162 17,729 4904.2 31,318 10,002

6 100 100 100 100 100

5. Conclusion The first successful growth of Ho-doped Bi2TeO5 crystal is reported. The absorption and luminescence spectra were measured and interpreted. In the absorption spectra thirteen manifolds were identified with their Stark components. Preliminary crystal-field calculations indicate that Ho–Bi substitution mostly takes place at the Bi(1) site, although the presence of several smaller peaks in the 9K absorption spectra might reflect incorporation of the Ho3+ ions into the other Bi sites. The strong anisotropy observed in the polarized absorption spectra is consistent with the low symmetry of the lattice sites. Three characteristic emissions were observed in room temperature luminescence spectra corresponding to the 5F4, 5S2 ! 5I8, 5 F5 ! 5I8, and 5I4 ! 5I8 transitions, which were observed for any excitation to higher energy Ho3+ levels. Judd–Ofelt calculation was performed using the polarized absorption spectra. The deviation between the experimental and calculated oscillator strength values was reasonably low. Consequently the Judd–Ofelt parameters could be used for calculating the spontaneous emission probabilities. The 5I7 ! 5I8 and 5F4 ! 5I8 transitions have high branching ratios and emission probabilities in the Ek[1 0 0] polarization allowing the possibility of laser action in the Bi2TeO5:Ho single crystal. Acknowledgements The present work has been supported by CNR-HAS joint project, Hungarian-Italian intergovernmental R&T project, Hungarian Research Found N. OTKA T-046481 and T-046667, MIUR (Italy), and CONACYT (Mexico). The authors want to thank Prof. G. Amoretti

0

Ek[0 0 1] 1

A(J–J ) (s )

b (%)

A(J–J 0 ) (s1)

b (%)

77.9 160.9

100 100

51.1 103.0

100 100

436.6

100

310.2

100

5025.1 378.7 864.8 926.8 621.0 222.4 6875.1 3667.0 606.1 1338.4 1121.2 944.5 28,272 20,469

93 7 33 35 24 8 62 33 5 100 100 100 100 100

3547.9 257.7 504.9 541.1 480.3 150.5 4749.9 2669.6 428.9 781.3 654.5 669.2 23,118 14783

93 7 30 32 29 9 61 34 5 100 100 100 100 100

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