OPTICAL PROPERTIES OF Nd3+ ION PAIRS IN LiYF4 CRYSTAL

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OPTICAL PROPERTIES OF Nd3+ ION PAIRS IN

LiYF4 CRYSTAL

R. Barthem, R. Buisson, J. Vial

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C7, supplément au nolO, Tome 46, octobre 1985 page C7-483

O P T I C A L P R O P E R T I E S

OF

~ d ION ~ +

PAIRS

I N

L i Y F 4 CRYSTAL* R.B. ~arthem+, R. Buisson and J.C. Via1

++

Laboratoire de Spectrométrie Physique

,

U n i v e r s i t é S c i e n t i f i q u e e t MédicaZe de Grenoble, B . P. 8 7 , 38402 Saint-Martin d 'Hères Cedex, France

Résumir Les p a i r e s d'ions Nd3+ ont 4tb btudi4es par des techniques optiques sur un c r i s t a l de LiYF,. Les t r a n s f e r t s non r a d i a t i f s ont b t b analysbs grace B l a fluorescence de up-conversion e t au quenching du niveau 4F,,,

.

Les spectres d8exci ta t i o n A haute-rbsolution e t de fluorescence permettent d ' a t t r i b u e r l e s r a i e s s a t e l l i t e s $ 6 c a t i g o r i e s de p a i r e s constitubes d'ions l e s p l u s proches. Le mecanisme responsable du t r a n s f e r t est

A

courte port4e e t l e super-dchange e s t invoquC pour l'expliquer.

Abstract; The Nd3' ion p a i r s are s t u d i e d by o p t i c a l techniques i n a weakly doped LiYF, c r y s t a l . Non r a d i a t i v e energy t r a n s f e r s are analysed thanks t o up-conversion fluorescence and t o fluorescence quenching of the 4F3,, l e v e l . High r e s o l u t i o n s e l e c t i v e e x c i t a t i o n and f l u o r ~ s c e n c e spectra allow an assignment o f s a t e l l i t e l i n e s w i t h 6 classes of p a i r s made o f ions i n the nearest neighbour p o s i t i o n s . The r n e c h a n i ~ responsible f o r the t r a n s f e r i s short range and the superexchange ir suggested t o explain i t .

LiYF, i s Knwn f o r a l o n g time as a laser material. I n a review piper, Weber has drscribed sane o f the more i n t e r e s t i n g oaiong the various l a s e r s which have been operated wi t h t h i s c r y s t a l u r l n g various r a r e e a r t h dopings / l / . The i n t e r e s t of laser manufacturing o f t h i s c r y s t a l has r e c e n t l y increased. Due t o t h a t r o l e i n laser applications, the various energy t r a n s f e r processes must be very w e l l understood, as w e l l those which favour than those which I i m i t the laser performances. h o n g the l a s t ones, the quenching i s known as an important obstacle which p r o h i b i t the increase o f the ion concentration. I t has r e c e n t l y been shown by experiments on LaF, /2,3,4,W, t h a t d i r e c t r t u d y o f ion p a i r s i s a f r u i t f u l l approach t o anal yse the ion-ion i n t e r a c t ion which i s responsible of t h i s quenching. The main advantapr of t h i s d i r e c t study l i e s i n the f a c t that, f o r a very weaKly doped c r y s t a l i n which ions are randanly d i s t r i b u t e d , p a i r s of ions are s p a t i a l l y w e l l i s o l a t e d frm others species ( i s o l a t e d ions, other p a i r s , etc...) and can be studled as e n t i t i e s . I n t h i s work, a t t e n t i o n w i l l be c a l l e d t o the s a t e l l i t e assignment and t o the nature of the ion-ion i n t e r a c t i o n . The simple s t r u c t u r e o f LiYF, ought t o make possible such an assignment.

LiYF, i s isanorphous w i t h the minerai s c h e e l i t e CaWO,. The Nd3+ in pu rit^ ions s u b s t i t u t e f o r V+ and t h i s s u b s t i t u t i o n appears t-O be s p a t i a l l ~ randm. With the very weakly doped c r y s t a l (0.1%) /6/ a Nd3'pair i s w e l l i s o l a t e d frm other ions *work partly supported by D.R.E.T.

+supported by CNPq. (Brazil)

i+

Laboratoire associé au C.N.R.S.

C7-484 JOURNAL DE PHYSIQUE

or o t h e r p a i r s and c o n s t i t u t e s t h u s a w e l l d e f i n e d system. Fig.1 s h w e the LiYF, s t r u c t u r e . A s i t e l o c a t e d a t the p t h neighbor p o s i t i o n w i t h respect t o the s i t e O has been numbered b y p. Indexes 3 and 3' d i s t i n g u i s h e s i t e s which a r e a t the sane d i s t a n c e f r a n O b u t a r e n e v e r t h e l e s s n o t e q u i v a l e n t by a srnrnetry operation. I n t a b l e 1 are l i s t e d , f o r each c l a s s the d i s t a n c e between the two ions, the number of e q u i v a l e n t p a i r s , t h e s m e t r y o p t r a t i o n s which interchange o r r e l a t e the two ions, the l e n g t h s o f the Y-F-Y a r c h b r i d g e s and the number o f these b r i d g e s u s i n g the c r y s t a l c h a r a c t e r i s t i c s given b y Blanchf i e d e t a l / 7 / .

Table 1 i Y t t r i u m s i t e p a i r c h a r a c t e r i s t i c s f r a n the c r y s t a l p a r m e t e p s given b y Blanchf i e l d a t a l . / 7 / .

. - ~

F i g u r e I r <a) LiYF, s t r u c t u r e . The s i x nearest neighbor s i t e s o f an Y t t r i u m i o n (O) are l a b e l l e d b y index i=i t o 6. (b> Energy l e v e l s o f Nds'single i o n d W ( I e f t p a r t ) and Nd3*-Nd3' p a i r s ( r i g h t p a r t ) . Double arrows correspond t o the l a s e r e x c i t a t i o n s , s i n g l e arrows t o the observed fluorescences, wavy l i n e s t o the non r a d i a t i v e r e l a x a t i o n ( l o f t p a r t ) o r a s s i s t e d energy t r a n s f e r ( r i g h t p a r t ) .

III

-

MPERlliaJTCILRESULTF

111.1

-

Experimental p r o c r d u r e

Nd3+ -Nd 3+ a s w e l l the var i o u s t r a n s i t i o n s i n v o l v d i n the work r e p o r t e d here. The

transmission and fluorescence s p e c t r a were o b t a i n e d w i t h a HR-1000 Jobin-Yvon s p e c t r c m t e r . The e x c i t a t i o n s p e c t r a have bern o b t a i n r d w i t h the h e l p o f a p p r o p r i a t e wide band f i l te r s i n order t o s e l e c t many 1 in e s corresponding t o the emission fram a g i v e n l e v e l : the s i g n a l i s t h u s v e r y l a r g e r than through a monochromator. The e x c i t a t i o n s p e c t r a have been produced e i t h e r w i t h a p u l s e d 1 aser o r wi t h a cw l a s e r . The former, a dye 1 aser pumped wi t h a doubled Nd-YAG l a s e r , g i v e s 10 n s p u l s e s h a v i n g a s p e c t r a l w i d t h o f 3 GHz. The l a t t e r i s a monunode r i n g dye l a s e r pumped b y an A F Q O ~ l a s e r and whose wavelength i s m o n i t o r e d by a lambdameter 1 i t s frequency s t a b i l i t y d u r i n g the scanning tirne for a spectrum i s a f r w MHz. For the l i f e t i m e measurements, an ATNE s i g n a l averager w i t h 0.1 p s p e r canal was employed.

T-1.5K

17145 17150

ENERGY 1

oc')

F i g u r e 2: I n (a) i s s h w n the transmission spectrum around the 41912+4G,i, t r a n s i t i o n . (b) and (d) are, r e s p e c t i v e l y , the 1 .R. and U.V. e x c i t a t i o n s p e c t r a o b t a i n e d w i t h a p u l w d dye l a s e r e x c i t i n g the 419f2+46,, t r a n s i t i o n and (c) the 1 .R. normal i z e d e x c i t a t i o n spectrum o b t a i n e d w i t h

d

cw monanode l a s e r . The s p e c t r a l w i d t h s o f the spectraneter (a> and o f the pulsed l a s e r (O.lcR1) a r e shaun.

111.2

-

E x c i t a t i o n and Fluorescence Spectra

F i g u r e 2 p r e s e n t s the 1 .R. ( b and c) and U.V.(d) LiYF, ZN&' e x c i t a t i o n spectra, a t l.SK, obsorved i n the v i c i n i t y o f the 4191~+4G1/2 t r a n s i t i o n by a p u l s o d l a s e r < b and d> and b y a cw monomode l a s e r < c > . Broad band f i l t e r s u t i l i z e d t o observe those s p e c t r a were s e l e c t e d i n order t o d e t o c t o n l y the fluorescences n i t e d f r a n the o r 4D3 s t a t e s . I n <a> i s shclwn the transmission spectrum around do '19,

+4z:12

t r a n s i ( i o n . F i v e u t e 1 1 i t e s l o c a t e d i n the v i c i n i t y o f the s i n g l e i o n l i n e (S.1.) have been l a b e l l e d i n the I.R. e x c i t a t i o n spectrum r A

,

CI,, BI, B,

,

C. The spectrum l c shows t h a t C, A, and A, 1 i n e r a r e s p l i t t e d afthough BI and 6, a r e n o t . The U.V. s a t e l l i t e s a r e produced by the double e x c i t a t i o n o f the Nd3'-Nd3' p a i r s d u r i n g the l a s e r p u l s e / 9 / . The l i n e o f the U.V. e x c i t a t i o n spectrum l o c a t e d a t the S.I. l i n e p o s i t i o n i s a t t r i b u t e d t o p a i r s made o f s p a t i a l l y d i s t a n t i o n s producing s a t e l l i t e s w a l i l y s h i f t e d from the c e n t e r o f the spectrum. L i f e t i m e s o f the 4F,1, and 4D3p f l u o r e s c e n t l e v o l s have beeen i n r a w r e d f o r the S.I. l i n e and f o r s a t e l l i t e s . Except f o r l i n e C, f o r which t h r s e l i f e t i m e s are r o o p e c t i v e l y 88 ps: 5% and l e s s than 150 ns, the l i f e t i m e s o f the s a t e l l i t e s a r e the rame than those o f the S.I. l i n e , namely 600 p s i 1% and 1.5 y s 2 15% r e s p e c t i v e l y .

JOURNAL DE PHYSIQUE

ENERGY

[cm-')

Figure 3 i I n (a) i s shown the transmission spectrum around the 419/2+4&/2 t r a n s i t i o n . I n ( b ) , (c), (d), (e) and ( f ) are shown the 9F3/,~I,/, fluorescence spectra observed a f t e r e x c i t a t i o n a t A,, A,, BI, B2and C r e s p e c t i v e l y i n the41q2+

40rJ2 t r a n s i t i o n . The instrumental w i d t h s used t o r e c o r d the v a r i o u s spectra i r e represen ted.

The quenching r a t e W can be obtained f o r each fluorescent t r a n s i t i o n byr W

=

1/r0

-

1/r

,

where ro and 7 are the 1 if e t i m e s f o r r e s p e c t i v e l y the S.I. 1 ine and the

p a i r l i n e . For t b 4Fy2 l e v e l o f s a t e l l i t e C, the quenching r a t e i s found t o be equal t o ix104 O1 a l though i t i s ertiiaated smaller than 20 Tl f o r other s a t e l l i t e s since an upper l i m i t of 600-6

=

594 p s can be deduced from the p r e c i s i o n o f the measurements.

Due t o i t s s t r o n g s h i f t from the S.I. l i n e , t o the l a r g e s p l i t t i n g , of i t s canponents (about 0 . 5 ~ 6 '

>,

and t o i t s s t r o n g quenching, s a t e l l i t e C i s associated w i t h p a i r s made o f two Nds'in f i r s t neighbor p o s i t i o n . Quenching r e s u l t s shcu t h a t the 4F3/2 quenching r a t e i s a t l e a s t 500 t i n e s smaller f o r other p a i r s . Taking t h i s low r a t e f o r t h a t o f second neighbor p o s i t i o n p a i r s , one can t r y t o determine the nature o f the i n t e r a c t i o n . I f t h i s i n t e r a c t i o n was m u l t i p o l a r e l e c t r i c , i t s dependence w i t h respect t o the separation between the ions ought t o be (r, /r2 > K , r, and r, being the separations f o r r e s p e c t i v e l y

f i r s t and second neighbor p a i r s . With the above quenching r a t e s and the r values o f t a b l e 1, W 4 Ad,

=

104/20

=

(3.72/5.16)K frœn which I(

=

-19 i s deduced. A so s t r o n g separation dependence cannot be explained by a m u l t i p o l a r i n t e r a c t i o n . One can thus conclude t h a t superexchange p l a y s a major r o l e i n the i n t e r a c t i o n f o r nearby ions. Indeed, i t can be seen f r a n table 1 that two ions i n f i r s t neighbor p o s i t i o n are l i n k e d by two Y-F-Y b r i d g e s having v e r r s h o r t 'arches' as cocnpared t o the arches of the o t h e r b r i d g e s . I t ir thus n o t s u r p r i s i n g , due t o the w e l l k n w n s t r o n g separation dependence o f the superexchange i n t e r a c t i o n , t h a t o n l y f i r s t neighbor p a i r s s h w t h i s i n t e r a c t i o n . I t can be r e c a l l e d t h a t s i m i l a r r e s u l t s were obtained f o r P r 3 + p a i r s i n LaF, /3/ i n which the arch b r i d g e s F-La-F have n e a r l y the same l e n g t h s and t h a t the i n t e r a c t i o n induced s p l i t t i n p values are of the s m e order o f magnitude than t h a t estimated by Birgeneau / I O / . For the other classes o f p a i r s , m u l t i p o l a r i n t e r a c t i o n can be considerrd but, our r e s u l t s do not g i v e i t s nature, n e i t h e r i t s order o f mrgn i tude.

e x c i t a t i o n o f t h i s s a t e l l i t e produces two 1 ines, separated by 2 cm-', i n the fluorescence spectrum. F r m t h i s f i g u r e and fram the h i p h r e s o l u t i o n e x p e r i n e n t s o f f i g u r e 2c, the f o l l o w i n g assignment o f s a t e l l i t e s t o the c l a s s e s o f p a i r s can be deduced. S a t e l l i t e B,, the more s h i f t e d f r u n the c e n t r a l 1 in e a f t e r s a t e l l i t e C, i s a s s o c i a t e d w i t h the second neighbor p a i r s . S a t e l l i t e s A,, and A

,,

are associated w i t h the two species o f the t h i r d neighbor p a i r s : i f the Y-Y s e p a r a t i o n s a r e the saine f o r these two species, the Y-La-Y-bridges are s l i g h t l y d i f f e r e n t making these p a i r s n o t t o o much d i f f e r e n t . S a t e l l i t e A,,? whow area i n the I.R. e x c i t a t i o n spectrum i s t w i c e t h a t o f the o t h e r s i n d i c a t i n g t h a t the r s s o c i a t e d p a i r c l a s s i s t w i c e more numerous, i s r r s o c i a t e d w i t h f o u r t h neighbor p a i r s . S a t e l l i t e s BI and Azl are a s s o c i a t e d w i t h e i t h e r f i f t h o r s i x t h neighbor p a i r s , b u t i t i s n o t p o s s i b l e , f r a n the present r e s u l t s , t o p r e c i s e t h i s assignment.

D i r e c t s t u d y o f Nd-Nd p a i r s i n weakly doped LiYF, c r y s t a l s h a r s h w n the daninant r o l e o f superexchange i n t e r a c t i o n i n t r a n s f e r process. A l t h w g h a s i m i l a r o b s e r v a t i o n had been p r e v i o u s l y done i n LaF,, the present r e s u l t shows t h a t LaF, i s n o t a pecul i a r c r y s t a l

.

Indeed, a c m p a r i r o n o f the RE-F-RE arch b r i d g e l e n g t h s c l e r r l y e s t a b l i r h e o t h a t superexchange i s the r u l e as soon as these l e n g t h s a r e m a l l . I t can be r e c a l l e d t h a t the LaF, e x p e r i n e n t s u c r e the f i r s t t o prove t h i s dominant r o l e , the m u l t i p o l a r i n t e r a c t i o n s b e i n g g e n e r a l l y considared b e f o r e . I t i s nou c l e a r t h a t t h i s s h o r t range i n t e r a c t i o n i n f l u o r cmpounds must be t r k e n i n t o r c c o u n t i n the naw m a t e r i r l r e s e r r c h f i e l d .

For p a i r s which are n o t i n f i r s t neighbor p o s i t i o n , the quenching r a t e s of the observed f l u o r e r c e n c e s are t o o m a l 1 t o g i v e i n f o r m a t i o n on the ion-ion i n t e r a c t i o n mechanism. Nevertheless, arguments based on s p e c t r a l s h i f t s of s a t e l l i t e s and h i * r e s o l u t i o n experiments have p e r m i t t e d an arsignment o f S i x s a t e l l i t e s t o t h e d i f f e r e n t c l a s s e s o f p a i r s . Such assignment had n o t been p o s s i b l e i n LaF3 due t o the more camplex s t r u c t u r e o f t h i s m a t r i x .

I t i s a p l e a s u r e f o r the a u t h o r s t o thank F.HadPore f o r b i s t e c h n i c a l a s s i r t a n c e and a l s o 0r.J.Y.Henry and H.Harmand f o r the c r y s t a l growth and c h a r a c t e r i z a t i o n .

/1/ H.J.Weber i n 'Methods o f E x p c r i n e n t a l P h y s i c r * (C.L.Tang, ed.2 Vo1.15-h, 167 Acadonic Press

-

New York 1979.

121 J.C.Via1 and R.Buisson, J.Physique

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L m R E S (1982) L339. /3/ J.C.Vial and R.Buisson, J.Physique

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/6/ C r y s t a l s have been groun b y J.Y.H.nry and H.Harmand, LET1

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CM. Centre d8Etuden N u c l 4 a i r e s de Grenoble CNEG

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Av. d e r M a r t y r s 85 X

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38041 Grenoble Cedex F M C E .

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a

,

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