Luminescence study of Cr4+-doped silicates

March ! 995

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Optical Materials 4 ( 1995) 507-513

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Luminescence study of Cr4+-dOped silicates N . V . K u l e s h o v a, V . P . M i k h a i l o v a, V . G . S c h e r b i t s k y a, B.I. M i n k o v b, T . J . G l y n n c,., R . S h e r l o c k c a International Laser Center, Belarus State University, Minsk 220064, Belarus b Institute for Single Crystals, Ukraine Academy of Sciences, Kharkov 310141, Ukraine c University College Galway, Galway, Ireland

Received 30 July 1994; revised manuscript received 10 August 1994

Abstract The results of optical absorption, luminescence, and lifetime measurements on chromium-doped Y2SiO5and Gd2SiO5 crystals are reported. The dominant absorption and emission bands in both crystals are assigned to transitions on the Cr4+ ion in distorted tetrahedral sites. An additional near infrared emitting center was observed in YzSiOs.

I. Introduction Chromium-doped forsterite and YAG have been shown to be efficient laser materials for the near infrared [1,2]. The lasing centers were described as Cr 4÷ ions occupying tetrahedrally coordinated positions in crystal lattices. Significant efforts were made subsequently in the investigation of Cr 4+ in other hosts, in which laser action might occur in the near infrared. Recently, laser action in Cr4+ :YzSiO5 was demonstrated at temperatures up to 257 K [ 3 ], and at room temperature [4]. An interesting feature of this new laser material is the dependence of the output spectrum on the p u m p wavelength. An overview o f the optical spectroscopy o f Cr 4 ÷ : Y2SiO5 has been published recently [ 5 ]. In this paper we discuss the spectroscopic properties o f tetravalent c h r o m i u m in two oxy-orthosilicares Y2SiO5 (YSO) and Gd2SiO5 (GSO). Both yttrium and gadolinium silicates have a monoclinic cell. The space group of Y2SiO5 is C6h ( I 2 / c ) and the di* Corresponding author.

mensions of the cell are a = 10.410/~, b=6.721 A, c = 12.490 A and r = 102039 ' [6]. The y3+ ions occupy two different distorted octahedral sites, while the Si 4+ site has a slightly distorted tetrahedral symmetry. GdzSiO5 belongs to the space group P2 ~/c and the cell parameters are a = 9.12/~, b = 7.06 A, c = 6.73 and r = 107°35 ' [5]. The Gd 3+ ion occupies two sites; it is surrounded by nine oxygen ions in one site and by seven oxygen ions in the other.

2. Experimental details High quality Cr-doped YSO and GSO single crystals were grown by the Czochralski technique in the iridium crucible. The Cr-concentration in the raw materials was varied from 0.2 to 2.0 wt %. The polished samples were 15-20 m m in diameter and 3-10 m m in thickness. Absorption spectra were recorded with the SPEC O R D M40 spectrophotometer in the UV-visible and S P E C O R D 61-NIR in the near infrared. Emission spectra were excited with a Spectra Physics 2017 ar-

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N.V. Kuleshovet al. / Optical Materials 4 (1995)507-513

gon ion laser, a Spectra Physics 3900S Ti-sapphire laser, and a Coherent CR-599 dye laser with DCM as lasing dye, and were analysed using a 0.5 m SPEX monochromator. The fluorescence signal was detected with a North Coast model FO-817L cooled germanium detector and analyzed using a Stanford Research SR530 lock-in-amplifier. For temperaturedependent measurements, the samples were placed in closed cycle cryogenic refrigerator (Air Products type Air DE202). Decay time measurements were made using a 28ELU-F15K photomultiplier working in the photon-counting mode with spectral sensitivity range 0.38-1.3 gm, time-to-amplitude converter and IBM computer. The pump source in these measurements was a copper vapor laser with 10 kHz repetition rate giving 15 ns pulses at 510.6 nm and 578.2 nm.

3. E x p e r i m e n t a l results and d i s c u s s i o n

The room temperature polarized absorption spectra of as-grown 0.2% Cr:YSO are shown in Fig. 1. The dominant strong absorption bands at 16700 c m - 1 and 14000 c m - ~ and weak absorption band at about 10000 cm-1 were assigned to transitions on the C r 4+ ions substituting for Si 4+ ions in distorted tetrahedral sites. The strong broad band at about 25800 c m - 1 was decreased strongly after reduction annealing and was attributed to color centers. The assignment of a weak absorption band at 20500 c m - 1 and a shoulder

at 12400 c m - i observed in the spectrum of Cr: YSO will be discussed below. The energy level diagram for the d 2 configuration of the Cr 4+ in tetrahedral sites is shown in Fig. 2. It is identical to that for Ni 2+ ions in a crystal field of octahedral symmetry. Assuming Dq/B_< 2, the order of energy levels would be 3A2<3TE
50 0.5

~

E#a

1T2 1E 3T1 1T1 1T2 3T ~///'1

40 30

•

0.5

A

e//b

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-~

.

i~,

I

t

i

i

~"'-/

1A1

f

1E

113 10

3

3A2 I

30000

25000 20000 15000 10000 ENERGY {crn -1 )

Fig. 1. Polarizedabsorption spectra of the 0.2% Cr:YSO at room temperature.

I

I

I

2

3

Dq/B

Fig. 2. Tanabe-Suganodiagram, showingthe energylevelsof the tetrahedrally coordinated d2 ions. This diagram is also valid for ds ions in an octahedral field.

509

N.V. Kuleshov et al. / Optical Materials 4 (1995) 507-513 1.0

z lit

o

0.5

i-.o_ O I

30000

25000

20000

15000

10000

p h o n o n lines (Fig. 5) indicated that the 33.5 c m - 1 splitting is in the excited state o f the emitting center. The emission spectrum o f C r : Y S O after 488 n m Ar-ion laser excitation was significantly different from that o f Fig. 4. A b r o a d b a n d was observed with a m a x i m u m at 1.2 p.m and without any sharp structure, even at helium temperatures (Fig. 6 ). The same emission b a n d with a trace o f tetrahedral C # ÷ emis-

(cm-1 } Fig. 3. Absorption spectrum of the 0.3% Cr:GSO at room temperature. ENERGY

_c

>~

21K

21K

1.140

,

1.145

1.150

1.155

1.160

Wavelength (#m)

1.0

1.2

1.4

1.6

Fig. 5. Temperature dependence of the intensity of the zerophonon lines in Cr:YSO (excitation wavelength 750 nm).

W a v e l e n g t h (t-tin)

Fig. 4. Fluorescence spectra at different temperatures of 0.2% Cr:YSO, excited by a Ti-sapphire laser tuned to 750 nm. 1.0

Cr 4+ ion environment. A b r o a d b a n d at 25000 c m d e p e n d s strongly on the annealing conditions, as in C r : Y S O , and is attributed to color centers. Fluorescence spectra o f as-grown 0.2% C r : Y S O , after excitation at 750 n m (13333 c m - ' ) , are depicted in Fig. 4. The same emission spectra were observed after excitation in the d o m i n a n t absorption b a n d s (16700 cm -~ a n d 14000 cm - ~ ) assigned to Cr 4+ ion in distorted tetrahedral sites. The features o f these spectra are a b r o a d b a n d with peak at app r o x i m a t e l y 1250 n m a n d a strong sharp line at 1148.7 nm. The linewidth o f this zero-phonon line was measured to be 3.0 + 0.6 c m - t at 5 K. Between 30 K a n d 60 K another sharp line was observed in the spectrum at 1144.3 nm. The t e m p e r a t u r e dependence o f the relative intensity o f these two zero-

21K

.8

.6

2 .4

.2

•9

1.1

1.3

1.5

1.7

W a v l e n g t h (~m)

Fig. 6. Emission spectrum of the 0.2% Cr: YSO, excited by the 488 nm line from an argon ion laser.

510

N. V. Kuleshov et aL /Optical Materials 4 (I 995) 507-513

sion (a small sharp line at 1148.7 nm) was observed under excitation in the long wavelength edge of the absorption shoulder observed at 12400 c m - 1. We assigned this emission band at 1.2 ~tm to another type of emitting center associated with chromium ions. Under constant excitation conditions, the emission intensity for both centers decreases as the temperature increases. Fig. 7a shows the total intensity of 0.2% Cr:YSO as a function of temperature under 750 nm and 488 nm pumping, respectively. An extremely rapid temperature quenching of the fluorescence was observed for the center emitting at shorter wavelength (with peak at 1200 nm). The emission intensity of this center was about 500 times weaker at room temperature than that at 21 K, while for the long wavelength center at 1250 nm it was only approximately 20 times weaker at room temperature. To investigate fluorescence quenching we measured the temperature dependence of emission decay time. The decay curves for both emitting centers were 1.0 ~..-l:--k.

' ~,'-,.,

Cr:YSO

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¥,

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z

".... ".,

"' 0.4 l.Z

0.2

-

, .%.

............... 10 6o (o)

$.. , ,

7: ,~.,~::,-..~

single exponential for all Cr concentrations. The fluorescence lifetime of 0.2% Cr:YSO excited by 578 nm pumping varied from 11.5 txs at 77 K to 0.65 las at 300 K; when pumping at 510 nm, the lifetime varied from 2.7 las at 77 K to 25 ns at 255 K (Fig. 7b). At higher temperatures, the emission for 510 nm pumping was too weak to be detected. The temperature dependences of the decay times are consistent with the intensity data and indicate that the fluorescence quenching is due to intra-ionic nonradiative phonon assisted relaxation processes [ 5 ]. The emission spectrum of the 0.3% Cr: GSO at low temperature is shown in Fig. 8. A broad band with peak at 1320 nm and sharp structure at about 1200 nm was observed. A doublet ofzero-phonon lines with peaks at 1192.3 nm and 1200.5 nm was observed (Fig. 9) at temperatures up to 50 K. The linewidth of the 1192.3 nm zero-phonon line was measured to be 15 cm-1 at 5 K and it increased rapidly with increasing temperature. The emission spectrum did not depend on the excitation wavelength. The intensity of the Cr-GSO emission decreased with increasing temperature, starting at about 70 K (Fig. 10a). At room temperature, the total intensity was less than 1% of that at 30 K. The emission decay curves were single exponential. The fluorescence lifetime ofCr: GSO was the same after excitation at 578 and 510 nm and varied from 1.68 ~ts at 77 K to 36 ns at 250 K (Fig. 10b) in quantitative agreement with

110 160 no 260 310 TEMPERATURE (K) 5K

1,0

10 8 ---=6 UJ

:1,,,

Cr:YSO

.8

4-,

~-,

\,

I.-LI.I

u.. _J 2 o . . . . . . . ":¥.,..~ ...... ~,, ;,+, 70 120 170 220 270 (b) TEMPERATURE (K)

Fig. 7. (a) Temperature dependence of the emission intensity of the 0.2% Cr:YSO under excitation at 750 nm (dashed curve) and 488 nm (dotted curve). (b) Temperature dependence of the emission lifetime of long-wavelength (dashed curve) and shortwavelength centers (dotted curve) in Cr :YSO.

.~

.6

@

.4

.2

0

10

1.1

1'2

1.3

114

15

1'6

1.7

Wavlength (p.m)

Fig. 8. Luminescencespectrum of GSO:0.3% Cr at 5 K.

N.F.. Kuleshov et al. I Optical Materials 4 (1995) 507-513

1.0 5K .8

>,

2 _c

.2

o

1.20

1.18

1.22

t.24

Wavelength (p.rn) Fig. 9. The structure of the luminescence spectrum near the zerophonon transition in GSO: Cr at 5 K.

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the intensity data. The intra-ionic nonradiative relaxation was assumed to be responsible for the fluorescence quenching. The weak absorption band at 20500 c m - ' and a shoulder at about 12400 c m - ~ in YSO: Cr is associated with the short wavelength emitting center with peak at 1200 nm. This center was observed in YSO: Cr, Mg crystals annealed in an oxidising atmosphere [ 10]. In these crystals the absorption (and emission) of tetrahedrally coordinated Cr 4+ (with luminescence band at 1250 nm) was strongly suppressed, and the short wavelength center was dominant. The emission spectrum, decay time, and temperature dependence of the emission intensity of this center were very close to that observed in our measurements. The presence of two types of emitting centers in Cr: YSO would explain the dependence of the laser output spectrum on the excitation wavelength observed in Ref. [ 3 ]. Lasing at 1225 nm under 532 nm and 840 nm pumping was associated most probably with the short wavelength center, the absorption bands of which were observed in the spectrum of Y2SiO5 used in those experiments, while the laser emission at 1270 nm under 1.06 ~tm pumping belongs to the dominant tetrahedrally coordinated Er a+ center.

°'81 ~. o.6~

0.2

4. Discussion

"~

I L '~

0 ,, . . . . . . . . . . . . . . . ":~,',-..-~ ..... r,, 10 60 110 160 210 260 310 (a) TEMPERATURE (K) 2.0

Cr:GSO

%7

1.5

%

::L Iii

1.0

",,

:r

LU 0.5 LL .d

0

(b)

70

'

'

130 160 190 TEMPERATURE (K)

I00

Fig. 10. Temperature dependence of (a) the emission intensity and (b) the lifetime in GSO:0.3% Cr.

The results reported here for YSO:Cr 4+ are similar to those reported recently by other workers [ 5,11 ]. There is some disagreement, however, about the interpretation of the data. In Ref. [ 5 ], the sharp feature on the low-energy side of the luminescence band in YSO: Cr 4÷ is assigned to emission from the ~E level of the Cr 4+ ion to the ground state; in Ref. [ 11 ], it is argued that the stress dependence of this sharp feature indicates that it is the zero-phonon line of the emission from 3T2 to the ground state. The latter argument is based on the similarity of the crystal-field dependence of the (~E, 3T2) levels in Cr 4÷ systems and of the (2E, 4T2) levels in Cr 3+ systems. It should be noted, however, that there is considerable mixing of the 2E and 4T2 levels in CP + systems when these levels cross [ 12 ]; consequently, both the stress and temperature dependence of features arising from both levels would be similar in materials where the energy

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N.V. Kuleshov et al. / Optical Materials 4 (1995) 507-513

separation between these levels is small. This mixing makes it difficult to assign the identity of levels based on the shift of the corresponding features with temperature or applied stress. Similar arguments should apply to the 1E and 3T2 levels of Cr 4+ and it may be argued that, when these levels are close, stress measurements alone are not sufficient to identify the emitting levels. As already mentioned, the Cr 4 ion in a tetrahedral field has the same Tanabe-Sugano diagram (Fig. 2 ) as the Ni 2+ ion (d 8) in an octahedral field; the latter has been much more extensively studied [ 13,14 ]. The ratio Dq/B in most of the N ? + systems studied is lower than that proposed for YSO:Cr 4+ so that there are no Ni-doped systems in which the ~E and 3T2 levels are close, and the 3T2 level is usually significantly lower in energy than ~E. With reference to the discussion in the preceding paragraph, it is worth noting that where the ~E level is observed in excitation it consists of a broad feature, even at low temperature. In Ref. [ 14], it is suggested that the ~E level parabola is offset from the ground state (3A2) parabola on the configuration coordinate diagram, although this would not be expected from the Tanabe-Sugano diagram (Fig. 2) in which the ~E and 3A2 levels have the same dependence on crystal field. We are not aware of any Ni-doped system in which the ~E-, 3A2 transition is as narrow as the sharp feature in Fig. 4, as suggested in Ref. [ 5 ]. The Ni2+ion is unusual in that it often exhibits luminescence also from the ~T2 state to several lower states including the ground state; no corresponding emission has been reported from Cr4+-doped systems. As mentioned in Ref. [ 13 ], in some Ni2+-doped systems, the relative weakness of the 1E,--~T2 transition compared to the 3T2,--~T2 and 3T~.--IT2 transitions is puzzling, since it seems to defy the spin selection rule. Although both the ~T2 and the ~E states will almost certainly have a considerable amount of triplet state admixed (since both are close to triplet states) it is difficult to explain the reversal of spin selection rules on this basis alone; absorption transitions in this system adhere well to the spin selection rule, with spin-forbidden transitions being approximately an order of magnitude weaker than those which are spin-allowed. These results highlight the crudeness of our methods of characterising various impurity ion states and indicate how difficult it is to

identify levels from absorption or excitation spectra alone, even when the site symmetry is purely tetrahedral. Additional complications arise when there are large deviations from tetrahedral symmetry, as in the present case. (The C3v splitting of the 3T 2 s t a t e is estimated at ~ 3000 cm- 1 [ 5 ]. )

5. Conclusion Spectroscopic measurements of chromium-doped Y2SiO5 and Gd2SiO5 are reported. Two types of the near infrared emitting centers were observed in Y2SiOs. The dominant absorption and broadband emission with peak at 1250 nm are assigned to transitions on the Cr 4+ ions substituting for the SP + ions in distorted tetrahedral sites. The temperature dependence of two zero-phonon lines observed at 1148.7 nm and 1144.3 nm indicated that the 33.5 cm -~ splitting is in the lowest energy component of the 3T2 excited state of this center. The weak absorption band at 20500 cm-1 and a shoulder at 12400 cm-1 are associated with another center with emission peak at 1200 nm. Strong fluorescence quenching due to intraionic nonradiative relaxation was observed for both centers. The absorption and near infrared emission in Cr:Cd2SiO5 were assigned to the Cr 4+ in a distorted tetrahedral site.

Acknowledgments This work was carried out with partial support from FORBAIRT, the Irish Science and Technology Agency, under the Optronics Programme in Advanced Technology. R. Sherlock wishes to acknowledge the award of a maintenance grant from FORBAIRT and a postgraduate fellowship from University College Galway. V. Mikhailov and N. Kuleshov are grateful to the spectroscopy group for support of a research visit to UCG, where most of this work was carried out.

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