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EXCITON DYNAMICS FOLLOWING TWO PHOTON
ABSORPTION IN GdCl3
R. Mahiou, B. Jacquier, R. Cone
To cite this version:
30URNAL DE PHYSIQUE
Colloque C7, supplément au n°10, Tome 46, octobre 1985 page C7-119
EXCITON DYNAMICS FOLLOWING TWO PHOTON ABSORPTION IN G d C l3
R. Mahiou, B. J a c q u i e r and R.L. Cone
Laboratoire de Chimie des Solides, Université Clermond II, 63170 Aubière, France
^Laboratoire de Physico-Chimie des Matériaux Luminescents, VA 442 du CNRS, Université Lyon I, 69622 Villeurbanne, France
++Department of Physics, Montana State University, Bozeman, Montana, U.S.A. Résumé - Nous reportons l'observation de la fluorescence ultraviolette de GdCl3 excitée par deux photons rouges dont la somme des fréquences corres-pond à la transition d'absorption 8S7/2—* P7/2• Sn l e s spectres de
fluo-rescence obtenus sont identiques à ceux de 1 absorption à un photon, on observe un rétrécissement de la raie d'absorption à deux photons et un dé-placement notable lorsqu'on diminue la température de 4.4 K à 1.5 K, tempé-rature- à laquelle le composé est ordonné ferromagnétiquement. L'analyse des déclins de la fluorescence intrinsèque fait apparaître deux composantes en profond désaccord avec les mesures d'absorption à un photon.
Abstract - Observation by fluorescence of direct two photon absorption within the ^7/2.—* P7/2 transition of the GcP+ ion following excitation
with a pulse dye laser is reported. The whole fluorescence spectra correlate well with the ones obtained from the one UV photon absorption. However excitation spectra exhibits a linenarrowing compare to the OPA and an obser-vable shift when decreasing the temperature from 4.4 K down to the ferroma-gnetic order below T = 2.2 K. At these temperatures, exciton decay exhibits a non exponential behaviour which can be only treated as a sum of two expo-entials in disagreament with the OPA measurements.
INTRODUCTION
In an earlier paper, we reported the first observation of the UV fluorescence of the magnetic insulator GdCl3 /l/. Both one and two photon absorption were used to characterize the transition energies arising from the ground state ®S7/2 to the first excited state 6
P7/2-More recently, we analyse the exciton dynamics following one photon absorption since R.S. Meltzer et al have shown that the Frenkel exciton picture using a two sublat-tice model yields of the homogeneous broadening of the line corresponding to the transition °S-//2-^"P7/2 which involves both exciton and magnon dispersions / 2 / . A similar result was also obtained by looking at the lines shape analysis of the excitation spectra ofgthe UV fluorescences either trap fluorescence or antistokes fluorescence ^>lj /p-*" S7/2 this latter arises from a biextonic process involving
two times the electronic transition °S7/2-»6P7/2 /3/. Recording the fluorescence decays of the intrinsic exciton 6p7y2 — ^ ^7/0 following selective pulsed laser
excitation into the highest stark component or the 6P7/2 manifold we have clearly
confirm the one dimensional behaviour of the excitation energy transfer within the 8s7/2->-6P7/2 transition / 4 / .
In this paper we present a detailed analysis of the UV fluorescence spectra follo-wing two photon absorption and we examine the exciton dynamics when exciting in the absorption peak.
JOURNAL DE PHYSIQUE
EXPERIMENTAL RESULTS
The GdC13 c r y s t a l was grown by bridgman method a t t h e l a b o r a t o i r e de croissance c r i s t a l l i n e du CNRS i n Orsay. Small p l a t e s were e a s i l y obtained by cleavage, t h e samples were packaged i n d r y atmosphere e i t h e r i n a q u a r t z ampoule f i l l e d w i t h h a l f atmosphere o f helium o f d i r e c t l y i n t h e c r y o s t a t . The present measurements were performed w i t h a pulsed neodymium YAG l a s e r pumped dye l a s e r w i t h one a m p l i f y i n g stage p r o v i d i n g a t l e a s t 1 mJ p u l s e energy. Temperature c o n t r o l o f t h e sample was achieved by l o o k i n g a t t h e r e l a t i v e i n t e n s i t i e s o f the fluorescence t r a p l i n e s s i n c e the populations o f the t r a p s a r e v e r y s e n t i t i v e t o t h e temperature m a i n l y i n t h e low temperature domain (4K
-
20K). E x c i t a t i o n s p e c t r a were recorded by m o n i t o r i n g t h e whole UV fluorescence of t h e sample ( t r a p and i n s t r i n s i c fluorescences) w i t h t h e lowest pumping power needing t o achieve a good i i g n a l t o norse r a t i o .Fig. 1
-
Two photon e x c i t a t i o n s p e c t r a monitored by t h e t o t a l UV fluorescence a t two d i f f e r e n t temperatures i n GdC13.6
o v e r a l l a b s o r p t i o n i n t h e P remains approximately t h e same. As a m a t t e r o f f a c t , the a p p l i c a t i o n o f a magnetiz/2 f i e l d along t h e C a x i s r e i n f o r c e s the e f f e c t o f t h e decrease o f temperature. These f e a t u r e s d i f f e r s t r o n g l y from t h e lineshape a n a l y s i s o f t h e one photon absorption.
F i g . 2
-
Q u a d r a t i c dependence o f t h e o v e r a l l UV fluorescence w i t h t h e pump i n t e n s i t y . I n F i g . 2, we check the q u a d r a t i c behavior o f t h e o v e r a l l fluorescence i n t e n s i t y versus t h e square o f t h e pulsed energy a t t h e temperature 1.5 K.By i n c r e a s i n g t h e pump i n t e n s i t y we have been a b l e t o anqlyse t h e UV f l u o r e s - cence u s i n g a 1 m spex monochromator w i t h a r a t h e r good r e s o l u t i o n . A t 4.4 K
( f i g u r e 3), t h e fluorescence spectrum i s q u i t e i d e n t i c a l t o t h e one obtained under one photon a b s o r p t i o n /4/. Indeed we f i n d again t h e broadening o f t h e i n t r i n s i c l i n e
Iq
and t h e main t r a p l i n e s (Pi) l o c a t e d on t h e lower. energy s i d e of 14. The i n t e n s i t i e s o f t h e l i n e s 14 and PI a r e smaller than expected from one photon ab- s o r p t i o n measurements.C7-122
JOURNAL
DEPHYSIQUE
Fig. 3
-
Fluorescence spectrum f llowing two photon absorption in the lowest s t a r k8
component 14 ( i n t r i n s i c ) of the
s
~
~
t r a n s i t i o n . Pi notes the d i f f e r e n t~
~
~
P
~
,
~
trap l i n e s using the notations described i n Ref. 4.t
. . . . . -. .0.2 0.4 0.6 0.8 1 t (ms)
I
0.2 0.4 0.6 0.8 1 t (ms)
As a consequence, b o t h i n t r i n s i c and t r a p fluorescence decays a r e independent o f t h e magnetic o r d e r . A p o s s i b l e e x p l a n a t i o n i n v o l v i n g the h e a t i n g o f t h e sample by t h e l a s e r i s r u l e d o u t by l o o k i n g a t t h e t r a p fluorescence i n t e n s i t i e s compare t o the OPA measurements and by v a r y i n g t h e e x c i t a t i o n d e n s i t y u n t i l the fluorescence i s j u s t detected i n t h e w e l l s u i t e d time s c a l e o f a n a l y s i s .
DISCUSSION
I n t h f o l l o w i n g s we w i l l o n l y discuss t h e e l e c t r o n i c n a t u r e o f t h e e x c i t o n t r a n s i -
fi
t i o n S7 2 + 6 ~ 7 a t k = 0 and i t s relevance w i t h respect t o energy m i g r a t i o n .I t
i s weilknown h i a t t h e l i n e s t r e n g t h o f TPA t r a n s i t i o n w i t h i n the c o n f i g u r a t i o n 4 f n r e q u i r e s second order terms i n t h e p e r t u r b a t i o n s e r i e s developped from t h e e l e c t r i c d i p o l e ap r o x i m a t i o n where t h e intermediate s t a t e s belong m a i n l y t o t h eP
c o n f i g u r a t i o n 4fn- 5d. Moreover h i g h order terms i n t h e p e r t u r b a t i o n s e r i e s i n v o l - v i n g s p i n o r b i t c o u p l i n g a d/or c r s t a l f i e l d p o t e n t i a l a r e needed t o reproduce t h eg
r i g h t s t r e n g t h o f 8 ~ ~ / ~ + P712 GdY+ two photon t r a n s i t i o n / 5 / . Downer and Bivas have shown t h a t t h i r d o r d e r terms w i t h s p i n o r b i t h a m i l t o n i a n breaking down t h e s p i n s e l e c t i o n r u l e e x p l a i n n i c e l y t h e enormous i s o t r o p i c enhancement o f8 ~ 7 / 2 6 6 ~ 7 / 2
f o r l i n e a r p o l a r i z a t i o n , i n c o n t r a s t one photon a b s o r p t i o n i n t h e s t a t ePij2
(MJ = 7/2) i s s t r o n g l y -f p o l a r i z e d i n t h e magnetic d i p o l e approxi- mation. a consequence o f the d i f f e r e n t e l e c t r o n i c n a t u r e o f t h e t r a n s i t i o n t o 6 ~ (MJ ~= 7/2) i n OPA and TPA and t h e a n i s o t r o p y and i s o t r o p y r e s p e c t i v e l y / ~ s t r o n g l y suggest t h e d i f f e r e n t behavior of energy m i g r a t i o n above and below the ferromagnetic order.REFERENCES
/1/ Mahiou, R., Jacquier, B. and Linares, C., J.Q.S.A. 73 (1983) 1383. /2/ Meltzer, R.S. and MOOS, H.W., Phys. Rev. (1972) x 4 .
/3/ Mahiou, R., Jacquier, B. and Moncorge, R., Proceedings o f t h e symposium by World S c i e n t i f i c P u b l i s h i n g CO. P.T.E. LTS.
