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LASER SPECTROSCOPY OF Bi4Ge3O12 SINGLE CRYSTALS : EMISSION MECHANISM AND

SATURATION EFFECTS

F. Rogemond, C. Pedrini, B. Moine, G. Boulon

To cite this version:

F. Rogemond, C. Pedrini, B. Moine, G. Boulon. LASER SPECTROSCOPY OF Bi4Ge3O12 SIN-

GLE CRYSTALS : EMISSION MECHANISM AND SATURATION EFFECTS. Journal de Physique

Colloques, 1985, 46 (C7), pp.C7-459-C7-462. �10.1051/jphyscol:1985781�. �jpa-00225111�

JOURNAL DE PHYSIQUE

Colloque C7, supplément au nolO, Tome 46, octobre 1985 page C7-459

LASER SPECTROSCOPY OF B i 4 G e 3 0 1 2 SINGLE CRYSTALS

:

EMISSION MECHANISM AND SATURATION EFFECTS

F. Rogemond, C . P e d r i n i , B. Moine and G . Boulon

Laboratoire de Physico-Chimie des Matériaux ~urninescents', Université Lyon I, 43 Bd du 11 novembre 1918, 6 9 6 2 2 ViZZeurbanne Cedex, France

Résumé

-

Sous e x c i t a t i o n l a s e r de f o r t e puissance, l a bande l a r g e de f l u o r e s - cence du germanate de bismuth présente des t r o u s dont l a formation e s t r e l i é e à un processus d ' a b s o r p t i o n saturée des d i v e r s centres émetteurs q u i c o n t r i - buent à l a fluorescence. On montre qu'une c o r r é l a t i o n e x i s t e e n t r e ce phéno- mène e t l e s mécanismes de t r a n s f e r t d ' e x c i t a t i o n e t un modèle e s t proposé pour e x p l i q u e r 1 es r é s u l t a t s expérimentaux.

A b s t r a c t

-

Under powerful l a s e r e x c i t a t i o n , t h e wide fluorescence band o f bismuth germanate shows holes t h e formation o f which i s r e l a t e d t o a saturated

absorption process o f v a r i o u s e m i t t i n g centers which c o n t r i b u t e t o t h e o v e r a l l fluorescence. I t i s shown t h a t a c o r r e l a t i o n e x i s t s between t h i s phenornenon and t h e e x c i t a t i o n t r a n s f e r mecanism and a mode1 i s proposed t o i n t e r p r e t t h e experimental r e s u l t s .

I n a r e c e n t paper, we have presented numerous new r e s u l t s concerning t h e o p t i c a l p r o p e r t i e s o f bismuth germanate c r y s t a l s by u s i n g l a s e r - e x c i t e d techniques

(ROEEMOND, PEDRINI, MOINE and BOULON, t o be p u b l i s h e d ) . The a b s o r p t i o n was shown t o occur i n bismuth and germanate centers w h i l e b o t h i n t r i n s i c and perturbed ~ iions ~ + together w i t h some i m p u r i t i e s c o n t r i b u t e t o t h e o v e r a l l fluorescence. We have repor- t e d f o r t h e f i r s t time f o r m a t i o n o f deep holes i n t h e wide emission band. This phe- nomenon was found t o be s t r o n g l y temperature and l a s e r e x c i t a t i o n pump power depen- dent and was a t t r i b u t e d t o a saturated a b s o r p t i o n process o f v a r i o u s centers, l e a d i n g as a r e s u l t , t o a slowing down o f t h e growing o f t h e i r own emission i n t e n s i t i e s as the pump energy increases. The s a t u r a t i o n e f f e c t s were s t u d i e d by e x c i t i n g more e s p e c i a l l y i n t h e low energy side o f t h e s o - c a l l e d A e x c i t a t i o n peak, and were found t o be v e r y s t r o n g a t room temperature f o r a l 1 t h e e m i t t i n g c e n t e r s w h i l e a t v e r y low temperature, o n l y t h e r e d emission, assigned t o i m p u r i t y centers, was a f f e c t e d by t h e phenomenon. Thermally-activated energy m i g r a t i o n , which was found t o occur i n t h i s m a t e r i a l /1/, probably promotes t h e s a t u r a t i o n process. I n o r d e r t o e s t a b l i s h a c o r r e l a t i o n between energy t r a n s f e r and s a t u r a t i o n e f f e c t , new experiments were performed and i t i s t h e purpose o f t h i s paper t o r e p o r t new r e s u l t s and t o discuss p o s s i b l e models e x p l a i n i n g t h e a b s o r p t i o n and emission mechanisms occuring a t low and room temperature.

Under strong l a s e r e x c i t a t i o n i n t h e A peak ( 3 = 2775

A

o r 36036 cm-'), a satura- t i o n e f f e c t on t h e fluorescence a t low temperature i s c l e a r l y observed as i n d i c a t e d i n F i g . 1. For weak l a s e r pulses (0.09 mJ), t h e emission band has i t s usual shape and i s represented by curve 1. Curves 2, 3, 4, obtained by e x c i t i n g w i t h l a s e r p u l ~ ses o f higher energy, a r e represented as i f they were obtained under t h e same low e x c i t a t i o n energy (0.09 mJ) which g i v e s curve 1 and by supposing t h a t t h e i r i n t e n - s i t i e s Vary l i n e a r l y w i t h t h i s energy. Such a r e p r e s e n t a t i o n p e r m i t s t o compare the p r o f i l e s o f saturated emission bands w i t h t h a t o f non-saturated one. One observes a strong v a r i a t i o n o f t h e emission band p r o f i l e s w i t h f o r m a t i o n o f holes. As already seen i n Our previous work by e x c i t i n g w i t h photons o f lower energy, t h e r e l a t i v e decrease o f t h e fluorescence i s more pronounced i n t h e lower energy p a r t o f t h e wide fluorescence band. The r e a l v a r i a t i o n o f t h e emission i n t e n s i t y versus l a s e r p u l s e energy i s represented i n t h e i n s e r t o f F i g . 1. Curve 1 shows a l i n e a r dependence o f the p a r t o f the fluorescence taken i n t h e h i g h energy wing o f t h e band, i n d i c a t i n g + u n i t é a s s o c i é e a u C.N.R.S.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985781

C7-460 JOURNAL

DE

PHYSIQUE

t h a t no saturation occurs in t h i s region. On the other hand, the intensity of the fluorescence corresponding t o the maximum of the emission band (curve 2) presents f i r s t a l i n e a r dependence f o r weak excitation energy l e s s than around 1 0 0 ~ 5 , and then a strong slowing down of the increase of the signal which becomes almost cons- t a n t f o r energy more than 5 0 0 ~ 5 . Fig.

2

shows the temperature dependence of t h e saturation e f f e c t s . Curves

2

t o 7 were obtained with constant l a s e r excitation energy (0.9 mJ) and compared in t h e same way than in Fig.

1

t o the non-saturated band 1 weakly excited (0.13 mJ) a t very low temperature

( T =

4.4 K).

A

weak increase of the temperature induces a strong increase of saturation. Since f o r T<100

K,

no temperature dependence of the integrated fluorescence was detected previously under lamp excitation /2,3/, i t therefore e x i s t s a phenomenon responsible of t h e saturation promotion and occuring

w i t h

a very weak activation energy.

Laser excitation in the

C

peak region

( 3 =

2257 A or 44307 cm-') leads t o d i f f e r e n t r e s u l t s . Saturation e f f e c t s a r e not observed a t very low temperature but begin t o occur r e a l l y a t temperature greater than few tens K (see Fig. 3 ) . The thermal a c t i - vation energy of the process promoting the saturation i s therefore l a r g e r than in the previous case.

W e have shown in the e a r l i e r paper previously mentionned t h a t severall emitting centers contribute t o the overall fluorescence appearing a s a very wide band

:

i n t r i n s i c bismuth centers, perturbed bismuth centers so-called traps, and impurity centers, the most important of which giving r i s e t o a strong red contribution a t low temperature. If saturated absorption process occurs f o r one or some of these centers, formation of holes in the emission band i s expected and indeed observed.

The emission of traps and impurity centers which a r e present in weak concentration

in the material can be saturated even a t low temperature i f they a r e d i r e c t l y

excited in t h e i r absorption bands. Most of them probably a r e present i n the absorp-

tion t a i l below the band-edge energy

b u t

some may al so l i e a t higher energy cl ose

t o the B and

C

bands. However the most e f f i c i e n t way to excite these centers i s

indirect excitation in bismuth and germanium i n t r i n s i c absorbing centers, which a r e

in l a r g e r concentration, followed by a multistep energy migration process. Then

thermally-activated exciton migration can occur explaining the temperature dependence

of saturation e f f e c t s . In order t o i n t e r p r e t the experimental r e s u l t s and to describe

the fluorescence dynamics, we use the mode1 represented in Fig. 4. The d'ffusion of

excitation i s supposed t o occur along two channels

:

exciton band ( ~ e 0 ~ ) ~ - (peak

A )

and exciton band ( ~ i 0 ~ ) ~ - (peaks

B

and C), with an interaction between them. Because

the weak activation energy AE1 of the self-trapped exciton, excitation in the peak

A i s followed, even a t low temperature, by a f a s t diffusion among Ge04 tetrahedrons

and therefore induces an e f f i c i e n t i n d i r e c t excitation of traps and impurities lea-

ding t o saturation e f f e c t s . The same kind of excitonic process occurs among Bi06

octahedrons. However owing to saturation e f f e c t s a r e much l e s s e f f i c i e n t and begin

t o occur a t higher temperature when bismuth germanate i s excited in the

C

band, the

phenomenon involves a weaker excitation t r a n s f e r probability and a l a r g e r thermal

absorption energy AE2 of the self-trapped exciton. Saturated absorption process in

i n t r i n s i c ~ i 3 + centers i s much more d i f f i c u l t t o obtain since they a r e present in

large concentration i n the c r y s t a l . Such a phenomenon i s not observed in

O u r

experi-

ments since no decrease or quenching of the overall fluorescence occurs. Instead,

under strong e x c i t a t i o n , the i n t e n s i t y of the fluorescence a t the center of the

band c o n t i n u e s t o s l i g h l y increase with l a s e r power as shown by curve 2 of i n s e r t of'

Fig.

1.

These experimental data a r e interpreted as a r e s u l t of a balance between

l i n e a r increase of i n t r i n s i c bismuth center emission and strong decrease of fluores-

cences due to t r a p s and impurities.

Fig. 1 - Laser i n t e n s i t y dependence of the fluorescence excited in the

A

excitation peak ( A = 2775

A

or 36036 cm-1) a t

T =

12.5

K.

Laser pulse energy

:

( 1 ) 0.09 mJ

;

( 2 ) 0;225 mJ

; ( 3 )

0;550 mJ

;

(4) 1 . 1 mJ.

Insert

:

variation of the fluorescence i n t e n s i t y versus l a s e r pulse energy

:

( 1 ) in the high energy side of emission band (23810 cm-1)

;

( 2 ) a t the maximum of the emission band (20000 cm-l).

photon energy (lo4crn-')

Fig. 2 - Temperature dependence of the fluorescence excited in the

A

excitation peak ( h = 2775 a or 36036 cm-l) and obtained

(1) w j t h

weak l a s e r pulse energy (0.13 mJ) a t

T = 4 . 4

K and with strong l a s e r pulse energy (0.9 mJ) a t (2)

T =

4.4

K;

(3)

T =

10

K ;

(4)

T =

12.5

K ;

( 5 )

T =

15

K ;

( 6 )

T =

20

K ; ( 7 ) T =

25

K.

JOURNAL DE PHYSIQUE

Fig. 3

-

Temper t u r e dependen e o f t h e fluorescence e x c i t e d i n t h e C e x c i t a t i o n peak

( 2 ;

2257

1

^{o r} 44307 cm-

i

) obtained w i t h strong l a s e r pulse energy (0.6 mJ) a t : ( 1 ) T = 4.4 K ; ( 2 ) T = 20 K ; ( 3 ) T = 50 K ; ( 4 ) T = 8 0 K ; (5 ) T = 120 K ;

(6) T = 160 K ; ( 7 ) T = 200 K.

exci ton band

!83J9:

exciton bond

-

- - P E T = -

_{- _}

e O 4 , ;

tAEi

C

STEi

. . . -. . . . . . . . . ...

. . . ... - -- --- . -. . _{. . .} . .-- ^~ ..-

. . . .. . . .

. . . . .

- -. ...

-. - - - . . .

. . . . . . ... ..

impuri lies

F i g . 4

-

Simple mode1 e x p l a i n i n g fluorescence dynamics.

REFERENCES

/1/ REIKIRK, D.P. and POWELL, R.C., J. Luminescence 20 (1979) 261.

/2/ WEBER, L . J . and MONCHAMP, R.R., J. Appl

.

^Phys. 4 T ( 1 9 7 3 ) 5495.

/3/ MONCORGE, R., JACQUIER, B. and BOULON, G.

,

J. L%inescence

2

(1976) 337