Laser Physics, Vol. 11, No. 1, 2001, pp. 116–119.
PHYSICS OF LASERS
Original Text Copyright © 2001 by Astro, Ltd. Copyright © 2001 by MAIK “Nauka /Interperiodica” (Russia).
Crystal Growth and Spectroscopic Investigation of Yb2+-Doped Fluoride Crystals S. Kück, M. Henke, and K. Rademaker Institut für Laser-Physik, Universität Hamburg, Jungiusstraße 9a, 20355 Hamburg, Germany e-mail:
[email protected] Received July 16, 2000
Abstract—In this presentation we report on the preparation and spectroscopic properties of Yb2+-doped MgF2 , KMgF3 , LiCaAlF6 , and LiSrAlF6 with respect to the realization of a tunable solid state laser based on a interconfigurational transition of Yb2+. All materials exhibit broadband emission in the short-wavelength region due to a 5d–4f transition. The peak emission wavelengths and bandwidths at room temperature are in particular 485 nm and 4510 cm–1 for Yb2+ : MgF2, 400 nm and 3890 cm–1 for Yb2+ : KMgF3, 393 nm and 3260 cm–1 for Yb2+ : LiCaAlF6, 440 nm and 5180 cm–1 for Yb2+ : LiCaAlF6, respectively. The emission can be excited in the energy level of the 4f 135d configuration. An energy level scheme for Yb2+ : MgF2 in the strong field assignment is proposed. The room temperature decay time of the Yb2+ emission is 52 µs in MgF2, 80 µs in KMgF3, 5.4 µs in LiCaAlF6 and 9.9 µs in LiSrAlF6, respectively.
INTRODUCTION Lasers in the UV and visible spectral range can be used in a wide field of applications such as biology, medicine, display technology, data storage, printing industry, scientific research, and entertainment. One possible way to obtain laser oscillation in this spectral range is the exploitation of the parity allowed (and thus electric-dipole allowed) 4f –5d transitions of divalent and trivalent rare earth ions, such as, e.g., Ce3+ [1]. In this presentation, we report on the preparation and spectroscopic investigation of the Yb2+ ion. Yb2+ has a completely filled 4f shell, therefore there are no innershell 4f –4f transitions. The observed transitions are thus assigned to interconfigurational transitions. First investigations of Yb2+ were performed in alkaline earth halides, alkali halides, and recently in fluorides and oxides [2]. Here, preparational aspects and laser relevant spectroscopic data on Yb2+-doped MgF2 , KMgF3 , LiCaAlF6 (LiCAF), and LiSrAlF6 (LiSAF) will be presented. Detailed spectroscopic data on Yb2+ : MgF2 and Yb2+ : KMgF3 can be found in [2, 3]. CRYSTAL GROWTH All crystals were grown from stoichiometric melts by the Czochralski method using RF-heating, carbon crucibles and automatic diameter control. Single crystals were grown with a pulling rate of 1 mm/h in good optical quality and diameters up to 14 mm. The starting materials were premelted under HF atmosphere to avoid traces of oxygen and water. LiSAF, LiCAF, and MgF2 contain a suitable divalent cation site, so a reduc-
ing atmosphere (N2 + 5% H2) during the growth was sufficient to stabilize the divalent state of Yb. In KMgF3 the Yb2+ ion occupies the monovalent K+ site. Therefore charge compensation was utilized by codoping with lithium. Some properties of the crystals investigated are summarized in Table 1. EXPERIMENTAL SETUP The absorption spectra are measured with a commercial Cary 2400 photospectrometer. The excitation and emission spectra for the wavelength region above 250 nm were obtained with a Spex Fluorolog 3 system. For the excitation measurements between 130 and 250 nm the SUPERLUMI setup at the Hamburger Synchrotronstrahlungslabor (HASYLAB) was used. For details of this VUV setup see [4]. For the lifetime measurements, a Q-switched frequency-tripled flash lamp pumped Nd : YAG laser (Spectra-Physics Quanta-Ray GCR-130) with a pulse duration of 6ns was used. SPECTROSCOPIC RESULTS AND DISCUSSION The room temperature absorption, excitation, and emission spectra of Yb2+-doped MgF2, KMgF3, LiCAF, and LiSAF are shown in Fig. 1. The excitation bands are in good agreement with the absorption bands. All observed absorption and excitation bands are assigned to 4f –5d transitions of the Yb2+-ion. The absorption cross section of the lowest absorption band is in the order of 1 × 10–18 cm2, which is a typical value for a parity allowed transition. The room temperature lifetimes
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CRYSTAL GROWTH AND SPECTROSCOPIC INVESTIGATION
LiCaAlF6
20 10 0 20 10
LiSrAlF6
0 20
α, cm –1
Intensity, photon flux, a.u.
KMgF3 Excitation Absorption Emission
117
10 7 10 9 8
6
5 4
2
MgF2
1
3
0 20 10
200
300
400 Wavelength, nm
500
600
0
Fig. 1. Room temperature absorption, emission, and excitation spectra of Yb2+-doped MgF2 , LiSrAlF6 , LiCaAlF6 , and KMgF3 . The excitation spectra were measured in different setups and combined.
are 52 µs for MgF2, 80 µs for KMgF3 [3], 9.9 µs for LiSAF, and 5.4 µs for LiCAF. Using these lifetimes and assuming a quantum efficiency of 100% at room temperature, an upper limit for the emission cross section can be determined with the formula of McCumber [5]: λ ln 2 1 - ------- , σ em = η -------- ----------------2 π 4πcn τ ∆λ 4
(1)
where η is the quantum efficiency, c is the speed of light, n is the refractive index, τ is the emission lifetime, λ is the peak emission wavelength, and ∆λ is the emission bandwidth. For all crystals investigated σem is determined to be between 0.3 × 10–20 cm2 and 5.6 × 10−20 cm2, which is a rather low value for a parity allowed transition and much smaller than the absorption cross section. The spectroscopic data are summarized in Table 2.
DISCUSSION The ground configuration of the Yb2+-ion is 4f 14. This completely filled shell leads to a 1S0 ground state of the free ion. In an octahedral crystal field, this state transforms like the 1A1 irreducible representation. No other 4f levels exists. The excited 4f 135d configuration consists in total of 140 energy levels. For the interpretation of the observed excitation spectra, it is instructive to use the strong-field-assignment by Loh [6, 7]. The 5d electron splits in a crystal field into a threefold degenerated t2 and a twofold degenerated e level. In an octahedral coordination of the Yb2+-ion, the t2 level is lower, in a tetrahedral or cubic coordination the e level is lower. The splitting between the t2 and e level is per definition ∆ = 10 Dq. In lower symmetry crystal fields, these levels split further. The 4f 13-core is considered in the same way as for an Yb3+-ion, with the two manifolds 2F7/2 and 2F5/2 separated by about ∆E4f ≈
Table 1. Material parameters of the crystals investigated KMgF3
MgF2
LiSrAlF6
LiCaAlF6
Symmetry
Cubic
Tetragonal
Trigonal
Trigonal
Ionic radius
K(12): 1.74 Å
Mg(6): 0.86 Å
Sr(6): 1.27 Å
Ca(6): 1.14 Å
Band gap
120 nm (10.2 eV)
115 nm (10.6 eV)
115 nm (10.6 eV)
110 nm (11.1 eV)
1070°C
1260°C
750°C
825°C
Melting temperature LASER PHYSICS
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Table 2. Spectroscopic data of Yb2+-doped KMgF3 , MgF2 , LiSrAlF6 , and LiCaAlF6 Yb2+ concentration, 1018 cm–3 Absorption coefficient, cm–1 σabs, 10–18 cm2 τ1.5 K, ms τRT, µs λem, max, nm ∆λ, nm
KMgF3
MgF2
LiSrAlF6
LiCaAlF6
9.5 5.8 (@291 nm) 0.6 11* 80* 400 62
3.7 6.0 (@378 nm) 1.6 23* 52 485 113
6.0 7.1 (@325 nm) 1.2 – 10 440 108
4.3 6.9 (@330 nm) 1.6 – 5.4 393 51
0.3
0.6
2.2
5.6
–20 cm2 σ** em , 10
* Data from [2]. ** Upper limit under the assumption η = 100%.
Table 3. Energy levels of the 4f 135d configuration of Yb2+: MgF2 . 1, 2, 3, …, 10 denote the observed excitation bands. ECF , crystal field splitting due to the D2h-symmetry; ∆E4f , splitting due to the LS-coupling of the Yb3+-core; ∆, splitting of the 5d electron levels in an Oh crystal field; ∆conf , energy difference between the 4f 14 and 4f 135d configuration. All values are given in wavenumbers Measured 1A 1 2F 7/2
2F 5/2
∆ECF
Mean
0 ∗ t2g
∗ t2g
∆E4f
Mean
10825
34337
∆
Mean = ∆conf
22192
43214
0
1 2 3
26455 29851 32787
3396 2936
4 5 6
37313 41152 43103
3839 1951
40523
29698
2F 7/2
∗ eg
7 8
51151 54054
2903
52602
2F 5/2
∗ eg
9 10
60241 63291
3050
61766
9164
10000 cm–1. The spin of the 5d electron can be parallel or antiparallel to that of the 4f 13-core; thus, the whole energy level scheme exists for singlet and for triplet states, from which the triplet states are energetically lower. The transitions to the triplet states are spin-forbidden, thus in the excitation and absorption spectra mainly the transitions to the singlet levels are observed. In Fig. 2 the energy level scheme obtained from the strong field assignment of Yb2+ in MgF2 (D2h site symmetry) is shown. In Table 3 the measured values for the observed excitation bands are listed. From these data, the splittings due to the different configurations (∆conf = 43214 cm–1), due to the 5d electron in an Oh crystal field (∆ = 22192 cm–1), due to the LS-coupling of the 4f 13-core of Yb3+ (∆E4f ≈ 9000–11000 cm–1) and due to the lowering of the crystal field symmetry to D2h (∆ECF ≈ 1950–3850 cm–1) are determined. The obtained
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results are in reasonable agreement with the expected values for the corresponding energy level splittings. However, for a detailed calculation of the energy levels, the Coulomb interaction between the 5d electron and the 4f 13-core has to be taken into account. The lifetimes are rather long for a parity allowed transition, especially at low temperatures, where they are in the order of several ms, see Table 2. The possible reason is, that the lowest 4f 135d level is a triplet state, thus the emission is spin-forbidden. Lizzo et al. [2, 3] explained the long lifetime also with a small overlap between the wavefunctions of the hole in the contracted 4f-orbital and of the delocalized 5d electron. Kaplyanski et al. [8] also considered the possibility, that the metastable level could be a 4f 136s state. In this case the emission would require ∆l = 3 and would therefore be strongly forbidden. LASER PHYSICS
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..... ..... ..... ..... ...
4f 13 ∗
4f 135d
4f 14
eg.... ... ..
∆ = 10Dq
4f 13 ∗
∗ eg ........
∆E4f
....
.. .... .... .... ....
∆conf
2F 5/2
... ....
... 2
F7/2 ∗
2F 5/2 . ....
... ...... .... t2g ..... 2
are assigned to 4f–5d transitions of the Yb2+-ion. An energy level scheme for Yb2+ : MgF2 in the strong field assignment was developed. The observed long lifetime of the emission is thus far not understood. Laser oscillation was not obtained yet, probably due to excited state absorption.
∆ECF
........ .
...
... ............. .... eg
∗ t2g ......
∆E4f
F7/2 ∗
.................. ..... ....
.. ........ .... ........... ...... t2g .
. ....................... .......... LS . .............................. . HS
1 1S 1S S0(1A1) 0 0 ....................................................................
5d electron in LS-splitting Oh crystal field of 4f 13
D2h site symmetry
Spin
LS
Fig. 2. Energy level scheme of the Yb2+-ion in an octahedral crystal field of D2h-symmetry in the strong field assignment. Only the two lowest high spin states are shown for simplicity.
First laser tests with Yb2+-doped MgF2 were not successful. In the experiments, there were hints for excited state absorption at the pump wavelength of 355 nm. Further measurements of the excited state absorption are in progress. SUMMARY AND CONCLUSION Yb2+-doped MgF2, KMgF3, LiCaAlF6, LiSrAlF6 crystals were successfully grown by the Czochralski method. The observed absorption and excitation bands
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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft within the frame of the Graduierten-Kolleg no. 463. Parts of the measurements were performed at the SUPERLUMI station of the Hamburger Synchrotronstrahlungslabor (HASYLAB) within the frame of project II-99-065. REFERENCES 1. Bayramian, A.J., Marshall, C.D., Wu, J.H., et al., 1996, J. Luminesc., 69, 85. 2. Lizzo, S., 1995, Luminescence of Yb2+, Eu2+ and Cu+ in solids, PhD Thesis, Universiteit Utrecht. 3. Lizzo, S., Meijerink, A., Dirksen, G.J., and Blasse, G., 1995, J. Luminesc., 63, 223. 4. Gürtler, P., Roick, E., Zimmerer, G., and Pouey, M., 1983, Nuclear Instrum. Methods, 208, 835. 5. McCumber, D.E., 1964, Phys. Rev., 134, 299; McCumber, D.E., 1964, Phys. Rev., 136, 954. 6. Loh, E., 1969, Phys. Rev., 184, 348. 7. Loh, E., 1972, Phys. Rev. B, 7, 1846. 8. Kaplyanskii, A.A. and Feofilov, P.P., 1962, Opt. Spectrosc., 13, 129.
