ABSORPTION AND LUMINESCENCE OF Nd3+ :YaA15012

V. GAgPARfK, M. O~.VOLDOVX

Department of General Physics, Komensk~ University, Bratislava*)

In this paper there are briefly summarized the results of our experimental measurements of the fluorescence spectra, originating in levels of the 4F3/2 term and terminating on the first two terms of the '~I multipet, and the results of the absorption spectra at room temperature (RT) and at liquid-nitrogen temperature (NT) from which Nd 3+ : Y3A15012 energy-level diagram was determined. Further, using the splitting of the 4F3/2, 4:F5/2 and 4F7/2 terms the crystal-field parameters were calculated.

Yttrium aluminum garnets doped with neodymium (Nd3+ : Y3A15012 ) attract considerable attention as materials suitable for quantum equipments. Our attention was devoted to physical characteristics including first of all the energy levels of Nd 3 + ions which isomorphously substitute Y-sites of orthorhombic symmetry in Ya AlsO 12 (YAG).

The energy levels of the trivalent rare-earth ions in YAG host crystals were studied by KONINGSTEIN and GEusIc [1 -- 4]. They determined the energy levels of Nd 3 + : YAG and assigned to them quantum states on the basis of absorption measurements up to 26 000 cm- 1 and on the basis of fluorescence corresponding to the. transitions from the 4F3/z to the first two terms of the 4I multiplet (419/2 and "111/2) at RT and at liquid-helium temperature (HeT). The fluorescence which terminated on levels of the 41 multiplet was observed by FEOFILOV et. al. [5]. WATTS [6] also measured the fluorescence, however, he did not record transitions to the *I15/z term.

Synthetic transparent garnet used in our measurements was grown by the Czochralski method in Monokrystaly Turnov. The concentration of Nd 3 + ions built in crystal is evaluated to be 0.5 at ~.

The absorption spectra studies in the region 12 300 to 30 500 cm-1 were carried out with the aid of registration spectrometers Perkin-Elmer and UVVIS Specord with resolution better than 30 cm-1. The fluorescence at RT and at NT was detected in the region of 8 800 to 11 500 cm-1 by means of a spectrometer arranged from apparatuses of the Carl Zeiss firm, The fluorescence spectrum was excited with a filtered high-pressure NARVA HBO 500 light source and detected by cooled photo- multiplier M 12 F 35.

The results of the measurements of the Nd 3+ : YAG energy levels up to 30 500 era- 1 are given in Table 2. The obtained wave-number values of levels are in a good agreement with the published values. The 2S+lLj symbols were assigned to the quantum states of the Nd 3 + ions in YAG on the basis of knowledge of WVBOtmN's

*) ~meralova 2, 885 06 Bratislava, Czechoslovakia.

Czech. I. Phys. B 24 I197g) 699

V, Gagparik, M. O~voldovd

Table I

The energy-level diagram of Nd 3 + : Y A G at NT

Assignment

419/2

4Ili/2

4F3/2

4F5/2

2H9/2

~F7/2

453/2

4F9] 2

Energy cm- 1

0 133 201 311 880

2 032 2 049 2 133 2 165 2 467 2 521

11 422 11 509

" 12 355 12 430 12 521

I

12 567 12 697 12 890

13 385 13 455 13 605 13 635

14 620 I 14 680

14 790 14 810 14 920

Assignment

2Hll/2

4G5/2

2G7/2

4G7/2

2G9/2

+

2K13/2

4G9/2

4G:t 1/2

I Energy era- 1

15 745 15 835 15 875 15 965 16 105

16 848 16 978 17 043

17 248 17 308 17 558

18 713 18 813 18 973

19 140 19 280 19 460 19 558 19 600 19 635 19 797 20 028

20 707 20 750 20 775

20 957 21 027 21 077 21 115

Assignment

2K15/2

2D3/2

2p1/2

2D5/2

2P3/2

'~D3/2

2I l l /2

+

4D5/2

4D1/2

2L15/2

Energy e m - i

21 152

21 545 21 595 21 660 21 760 21 865

22 025

23 145

23 660 23 755 23 840

25 980

27 490 27 585

27 730 27 900 28 000 28 090 28 175 28 260 28 520 28 660 28 840

29 150

29 660 29 800 29 900 30 140 30 340

700 Czech. J. Phys. B 24 [lgr

Absorption and luminescence of Nd 3+ : Y3AlsO I 2

[7] calculated (see also CARLSON and DIEI~E [8])levels of the free Nd 3 + ions in agree- ment with the papers [1, 4].

The splitting of the individual terms, as it can be seen from the Table I indicates that the Nd a + ions occupy sites of the symmetry which is lower or equal to the tetra- gonal one. Such symmetry completely removes (J + 1/2)-fold degeneracy. In the calculations of the energy level splitting of ions with configuration f3 (this is the case of Nd 3 +), which we have also performed, we considered the tetragonal field symmetry.

The energy levels of 4F3/2, 419/2 and 4Ii1/2 were determined from the fluorescence measurements where the transitions from the both levels of 4Fa/2 to the split levels 419/z and 411~12 were observed. Although all number (2 x 11) of the emission lines were not observed, it was possible to determine all 13 levels owing to two series of transitions from which we have obtained more values of the *F3/2 term splitting with an average value 83 ___ 5 cm -~. The splitting of this term reported by FEOnLOV et al. [5] at NT is 84 cm- 1 and the one reported by KONINGSTEIN, GEUSIC [1] is 88 cm- 1 at RT and 92 cm- a at NT.

The quantum states assignment in the region above 26 000 cm- 1 were carried out only preliminarily on the basis of the theoretically computed levels of the free Nd a + ion and the number of lines in separated groups. We consider them to be the transi- tions to the split 4D3/2, 2Ill/2 , 4D5/2, 4D1/2 and 2L15/2 terms. This can be also compared with Nd a+ :LaC13 [10]. For the correct assignment of the absorption transitions to the individual terms it would be necessary to carry out the measurements at HeT when the absorption from the thermally populated levels should disapear. It would be also useful to measure the absorption spectrum of the Nd a § ions in other garnets when the difference in splitting of the individual terms could indicate the line group which belongs to the splitting of the same term. With these problems, however, we intend to deal henceforth.

Further we have computed crystal-field parameters A~, which were obtained from the splitting of the 4F multiplet (A ~ = 274 cm -a, A ~ = -300 cm -1, A** = = -1017 cm -1, A ~ = 114 era -1 and A~ = -811 cm-1). For operator equivalent constants of these terms we have used the values determined by KONINGSTEIN, and GEoslc [1] who quote: A ~ = 2 7 0 c m -1, A ~ = - 2 5 0 c m -1, A ~ = 1250cm -1, A ~ = 92 cm- 1 and A6* = - 965 cm- 1 as the good values. The values of the crystal field parameters calculated from the higher terms remarkably differ from the values above. From this fact and from the existence of the nonradiafive transitions between 2Gv/z (this state corresponds to the excitation radiation frequency) and 4F3/2 we infer that the crystal-field theory cannot be well applied to these levels. Since the displacement of lines and their width were not strongly temperature dependent, obviously nonradiative transitions are assisted with higher energy phonons which Were not yet excited at RT, so that the energy of these phonons is expected to be higher than 400 cm-1. ZV~REV et al. [9] investigated the one-frequency model and showed that in the nonradiative processes the dominant role was played by phonons about 700 cm- ~.

CZ~tl. ~. Phys. B 24 [1974} 7 0 1

V. Gagparik, M. O~voldov6: Absorption and luminescence of Nd a+ : Y3Als012

The authors are indebted to Monokrystaly Turnov for the granted sample and to M. LIVAf~ for enabling us to perform the measurements on the registration photometer Perkin-Elmer.

Received 9. 11. 1973.

References

[1] KONINGSTEIN J. A., GEUSlC J. E., Phys. Rev. A 136 (1964), 711. [2] KONINGSTEIN J. A., Phys. Rev. A 136 (1964), 717. [3] KONINGSTEIN J. A., GEUSlC J. E., Phys. Rev. A 136 (1964), 726. [4] KONINGSTEIN J. A., J. Chem. Phys. 44 (1966), 3 957. [5] FEOFmOV P. P., TIMOFEEVA V. A., TOLSTOI M. N., BELAEV L. M., Optika i spektroskopia 19

(1965), 817. [6] WATTS R. K., J. Opt. Soc. Am. 61 (1971), 123. [7] Wybourn B. G., J. Chem. Phys. 32 (1960), 639. [8] CARLSON E. H., DIEKE G. H., J. Chem. Phys. 34 (1961), 1 602. [9] ZVEREV G. M., K.OLOBNYI G. J,, ONUSHTCHENKO A. M., ZHETF 60 (1971), 920.

[10] DIEKE G. H., Spectra and Energy Levels of Rare Earth Ions in Crystals, Interscience Publishers, New York 1968.

702 czech. J. Phys, B 24 {1974}