LUMINESCENCE AND FLUORESCENCE LINE NARROWING STUDIES OF CHROMIUM DOPED GLASS

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LUMINESCENCE AND FLUORESCENCE LINE

NARROWING STUDIES OF CHROMIUM DOPED

GLASS

F. Bergin, J. Donegan, T. Glynn, G. Imbusch

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C7, suppl6ment au nOIO, Tome 46, octobre 1985 page C7-337

LUMINESCENCE AND FLUORESCENCE L I N E N A R R O W I N G STUDIES O F CHROMIUM DOPED GLASS

F . J . Bergin, J . F . Donegan, T . J . Glynn and G.F. Imbusch

Department o f Physics, University College, GaZway, Ireland

Abstract

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The technique of fluorescence l i n e narrowing i s applied t o the TE

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4A2 transition on Cr3+ ions i n ED-2 glass. The resultant sharp

zero-phonon l i n e is accompanied by a vibrational sideband which carries information about the range of vibrational modes of the glass. The homogeneous broadening of the zero-phonon l i n e is measured as a function of temperature.

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INTRODUCTION

Dopant ions have been employed successfully as probes of the internal ionic

arrangement of crystals using the techniques of EPR and optical spectroscopy. This i s made possible by the fact that i n general a l l the ions occupy identical s i t e s and, particularly a t low temperatures, sharp signals are obtained. A d i f f i c u l t y with applying these techniques to amorphous materials i s that the ions occupy a range of s i t e s with different energy level structures and the resultant signal from these materials is broad, obscuring the fine structure of the individual lines. The technique of fluorescence l i n e narrowing (FLN)

,

however, should i n principle

overcome some of these d i f f i c u l t i e s . In FLN a narrowband laser i s used to excite a small subset of ions whose absorptions are i n resonance with the laser and the resultant emission from these ions can be very narrow. There have been many studies of FLN of rare earth-doped glasses /1/ which aimed t o probe the symmetry and range of s i t e s encountered i n glasses. Because of their stronger coupling to

neighbouring ions transition metal ions are much more sensitive probes of the ionic environment. This greater sensitivity, a l l i e d t o the range of crystal fields encountered i n a glass, results i n exceedingly broad emission from transition metal-doped glasses. When doped i n crystalline hosts the optical transitions on such ions appear as phonon-assisted bands which may be accompanied by sharp zero- phonon lines. In t h i s study we report on FW experiments carried out on the zero- phonon 2~

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4 ~ 2 transition of ~ r 3 + i n a s i l i c a t e glass.

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EXPERIMENTAL DETAILS

We chose ED-2 glass, a s i l i c a t e glass with Li20, CaO and other oxides acting as

C7-338 JOURNAL DE PHYSIQUE

network modifiers, and doped with 0.05% Cr2O3. Luminescence spectra were

recorded using an argon ion laser for excitation, a SPEX 14018 double monochromator, and either an EMI 9863B or an RCA 31034 photomultiplier with photon counting

electronics. For phase-sensitive detection the pumping l i g h t was chopped and a Brookdeal 9501 lock-in amplifier was used to analyse the signal. For resonant FLN measurements a single wheel chopping system was used t o chop the output from a Coherent 590 cw dye laser and t o block the spectrometer from scattered l a s e r l i g h t while the sample was being excited. The system was designed such that the luminescence was detected approximately one millisecond a f t e r the laser was fired. A variable temperature cryostat was used t o obtain data between 4 K and 300 K.

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SPECTROSCOPIC MEASLllUWNTS

The luminescence from chromium doped ED-2 glass a t low temperatures consists of a peak, whose half width i s approximately 180 cm-l, and a very broad band stretching from 14500 cm-1 t o below 10,000 cm-l. The sharp feature a t 14600 cm-l, shown in d e t a i l i n Fig. l ( a ) , i s due t o emission from ions i n high-field s i t e s ; ions i n these s i t e s have 2~ levels below 4 ~ 2 levels and a t low temperatures the luminescence i s emitted from the 2~ levels. The emission consists of zero-phonon R lines and t h e i r accompanying sidebands. This feature, a t 14600 cm-1, i s the composite of a l l the zero-phonon R lines from chromium ions i n different crystal fields. The decay

is not a single exponential and i s of millisecond duration. The broader

luminescence a t longer wav~length is due t o emission from ions i n low f i e l d s i t e s

Fig. 1. (a) The p a r t of the luminescence spectrum of &'+-doped ED-2 glass due t o emission from 2~ levels.

(b) The FLN signal a t 20 K.

14500 14000 13500

whose 4 ~ 2 levels l i e lowest. The decay i s again not a single exponential and i t s duration i s of the order of tens of microseconds. This difference i n lifetimes allows the overlapping 2~ and 4~ emissions t o be distinguished using phase-

sensitive techniques /2/. In t i e separated 2E emission one can now see vibrational sidebands accompanying the composite zero-phonon emission. Since the shape of the sideband i s a weighted spectrum of vibrational modes t h i s sideband i s of particular interest. However, we cannot observe the sideband due t o low frequency vibrations

as it i s masked by the inhomogeneously broadened emission of the zero-phonon lines.

We s e t out t o cbtain a narrow zero-phonon l i n e by employing the technique of FLN. The FLN signal a t 20 K i s shown i n Fig. l ( b ) . We expect t h a t cr3+ should occupy a somewhat distorted octahedral environment and so the 2E level should be s p l i t i n t o an upper R2 and a lower R 1 level. A s the l a s e r i s tuned into the 2~ band we excite some ions whose R1 l i n e s a r e resonant with the l a s e r and other ions whose R2 l i n e s are resonant with the laser and hence the sharp resonant FLN signal contains both R1 and R2 l i n e s , a l b e i t from d i f f e r e n t ions. This narrow l i n e is accompanied by two broader transitions. The broad t r a n s i t i o n a t lower energy covers the range of R1 l i n e s from ions whose R2 l i n e s a r e i n resonance with the l a s e r , and the weaker t r a n s i t i o n a t higher energy covers the range of R2 l i n e s from ions whose R1

l i n e s are i n resonance with the laser.

g.

tuning the l a s e r t o the lower energy side of the composite 2E band the R 1 l i n e s can be eliminated and a clear sideband spectrum can be observed. Fi 2(a) shows the shape of the sideband which we could observe t o within 15 cm-f'of the zero-phonon l i n e , the remainder being masked by the wing of the resonant R line. We note the high density of low frequency modes. For co arison, i n Fig. 2(b) we show the sideband of the corresponding t r a n s i t i o n of C 3 + i n crystalline MgO. Such low frequency modes i n glasses have been observed using other techniques e.g. F?aman scattering, infra-red spectroscopy.

(a) R l i n e sideband spectrum of R l i n e of

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cr3+-doped E D 2 glass

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(b) sideband spectrum of R l i n e of cr3+ i n

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octahedral s i t e in MgO

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I I I I , 400 800 energy s h i f t (aw1) 2

JOURNAL DE PHYSIQUE

Fig. 3. Measured values of the temperature-de endent broadening (in cXP) of the R l i n e of cr3+-doped ED-2 glass. The broken curve shows the observed broadening of the R1 l i n e of ruby /5,6/. R l i n e of ~r3+-doped ED-2 glass

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Rl l i n e ,;'

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of ruby ; temperature (K)

Because of the width of our laser (A" = 0.8 cm-1) we were unable t o observe the ground s t a t e s p l i t t i n g . Despite t h i s we measured the width of the FLN signal as a function of temperature from 5 K to 150 K, above which the signal became too weak for accurate measurement. A t 100 K the linewidth has a d i s t i n c t Lorentzian form i n contrast to the almost Gaussian lineshape a t lower temperatures. As the

temperature i s raised one expects relaxation mechanisms t o cause broadening of the zero-phonon line and these mechanisms result i n a Lorentzian lineshape. We separated the Lorentzian component from the residual low temperature width using standard techniques /3/, and the extracted Lorentzian linewidth, divided by two t o allow for the resonant excitation /4/, yields the broadening due t o relaxation mechanisms. The measured l i n e broadening i s shown i n Fig. 3. For comparison we also show the analogous broadening of the R 1 l i n e of

cr3+

i n A1203 /5,6/ which is typical of that found in Cr-doped crystals. We note that a t low temperatures the broadening in the glass is much larger

than

that found i n crystals; being almost two orders of magnitude greater a t 30 K. In addition, the temperature dependence of the broadening is different i n the two cases. In crystals one expects the temperature dependent broadeningatlowtemperaturestovaryas ahigh power of temperature /5/, much more so than the linear dependence found i n the glass up t o 70

K.

Above 70 K the broadening in the glass tends towards a quadratic dependence

on temperature. The larger broadening observed i n Cr-doped glass i s consistent with measurements made on rare earth-doped glasses, where the broadening i s mch in

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CONCLUSIONS

We have applied the technique of FLN t o ~ r 3 ' transitions i n glass. The r e s u l t s presented above show t h a t the 2~ emission from ~ r 3 + , in those glasses where it can be observed, carries useful information about the coupling of the

cr3+

ion t o the vibrating glass host.

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ACKNOWLEDGEMENTS

We are grateful t o M.J. Weber for the sample of ED-2 glass used in these

experiments. We wish t o acknowledge the assistance of the I r i s h National Board f o r Science and Technology under grants no. ST 45/84 and SRG 167/82. F.J. Bergin and J.F. Donegan wish t o acknowledge the receipt of Maintenance Grants from the I r i s h Department of Education during the period of t h i s work.

T . J . Glynn and G.F. Imbusch acknowledge the support of the U.S. D.O.E. during the course of a sabbatical stay a t the Univ. of Wisconsin.

REFERENCES

/1/ Weber, M.J., i n Topics i n Applied Physics (eds. W.M. Yen and P.M. Selzer) Vol. 49, Springer-Verlag 1981 and references therein.

/2/ Henry, M.O., Larkin, J.P., and Imbusch, G.F., Phys. Rev.

E,

(1976), 1983. /3/ Wertheim, G.K., Butler, M.A., West, K.W., and Buchanan, D.N., Rev. Sci.

Instrum.

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(1974), 1369.

/4/ Kushida, T., and Takushi, E., Phys. Rev.

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(1975), 824.

/5/ Muromota, T., Fukuda, Y., and Hashi, T., Physics Letters

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(1974), 181.

/6/ McCumber, D.E., and Sturge, M.D., J. Appl. Phys.

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(1963), 1682.

/7/ Selzer, P.M., Huber, D.L., Hamilton, D.S., Yen, W.M., and Weber, M.J., Phys. Rev. Letters

36,

(1976), 813.