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Laser induced gratings in CdS
~
Selid State Communications, Printed in Great Britain.
LASER
INDUCED
Vol.51,No.9, pp.675-677, 10:,
GRATINGS
IN
$3.00 + .00 Pergamon Press Ltd.
0038-I098/84
CdS
H. Kalt, V.G. L y s s e n k o , R. R e n n e r and C. K l i n g s h i r n P h y s i k a l i s c h e s I n s t i t u t der Universit~t, R o b e r t - M a y e r - S t r . 2-4, D - 6 o o o F r a n k f u r t am Main, G e r m a n y (Received
24 May,
FR
1984 by M. Cardona)
The d i f f r a c t i o n from l a s e r - i n d u c e d g r a t i n g s is obs e r v e d in CdS at low t e m p e r a t u r e in the s p e c t r a l region of the a b s o r p t i o n edge. The e x c i t a t i o n i n t e n s i t y is v a r i e d from about too W / c m 2 to I M W / c m 2 . The reasons for the f o r m a t i o n of gratings are a b r o a d e n i n g of the e x c i t o n i c a b s o r p t i o n band, a n e w r e s o n a n c e due to two p o l a r i t o n t r a n s i t i o n s to the b i e x c i t o n and the formation of an e l e c t r o n - h o l e p l a s m a with i n c r e a s i n g e x c i t a t i o n intensities.
I Introduction
7K, is p r o d u c e d by the spatial and temporal c o i n c i d e n c e of two beams, called "a" and "b" in the following, of almost equal intensities. They are c r e a t e d by s p l i t t i n g the b e a m of an e x c i m e r - l a s e r pumped, p u l s e d dye-laser. The spectral and t e m p o r a l h a l f w i d t h s are o,o3 m e V and 3 nsec, respectively. The d i a m e t e r of the e x c i t a t i o n spot on the sample is o,7 mm. The two beams form an angle of 0 = 1,6 ° thus p r o d u c i n g a grating with a s p a c i n g of about A l =17,9~m (at a w a v e l e n g t h 2sin@/2 I = 486 nm). The crystal for w h i c h data are given here is a p l a n e p a r a l l e l platelet with a thickness of about d = 7 ~m thus f u l f i l l i n g the r e l a t i o n for a 'thin grating': d << 2A~2 1
L a s e r - i n d u c e d gratings (LIG) are a u s e f u l tool to study o p t i c a l n o n l i n e a r i ties c o n n e c t e d w i t h e l e c t r o n i c excitations in s e m i c o n d u c t o r s . These gratings are c r e a t e d by the i n t e r f e r e n c e of coh e r e n t laser beams in a crystal c o m b i n e d w i t h a n o n l i n e a r response of the semic o n d u c t o r to the s p a t i a l l y m o d u l a t e d field a m p l i t u d e or e x c i t a t i o n intensity. Due to this a m o d u l a t i o n of the absorption c o e f f i c i e n t ~ or of the r e f r a c t i v e index n (or of both together) occurs l e a d i n g to an a m p l i t u d e or a p h a s e grating, r e s p e c t i v e l y I. C o n c e r n i n g the direct gap semicond u c t o r CdS most e x p e r i m e n t s on LIG's have been p e r f o r m e d w i t h an e x c i t a t i o n high into the c o n d u c t i o n - b a n d . Thus the results have been m a i n l y i n t e r p r e t e d by free c a r r i e r s or thermal gratings 2 Only 3 tried to e x p l a i n the o b s e r v e d d i f f r a c t i o n by the c r e a t i o n of e x c i t o n ic m o l e c u l e s in analogy to CuCI 4 however at e x c i t a t i o n i n t e n s i t i e s a r o u n d 3 M W / c m 2 w h e r e g e n e r a l l y p l a s m a phenomena d o m i n a t e in CdS. E x p e r i m e n t a l and t h e o r e t i c a l results show a rich v a r i e t y of n o n l i n e a r effects just in the vicinity of the e x c i t o n i c resonances, see e.g. 5-Io. The i n t e n t i o n of our experiments was to find out w h i c h p r o c e s s e s are in this spectral region responsible for the c r e a t i o n of LIG's depending on the e x c i t a t i o n intensity Iex c and energy h ~ e x c.
The p o l a r i z a t i o n of the e l e c t r i c field is chosen to be o ~ t h o g o n a l to the c r y s t a l l o g r a p h i c c-axis. The lowest intensities at w h i c h d i f f r a c t e d orders can be o b s e r v e d are I a = I b = 1ooW/cm 2 or I a = 5 k W / c m 2 and I b = 1oW/cm 2. At h i g h e r i n t e n s i t i e s several d i f f r a c t e d orders appear (see insert in Fig. la) e i t h e r due to m u l t i p l e d i f f r a c t i o n or to a nons i n u s o i d a l m o d u l a t i o n of the optical properties. The i n t e n s i t i e s of the t r a n s m i t t e d and d i f f r a c t e d orders are d e t e c t e d by f o c u s s i n g the diffraction p a t t e r n onto the vidicon target of an optical m u l t i c h a n n e l - a n a l y z e r system. 3 Results
and D i s c u s s i o n
The figure la-c shows the spectral d e p e n d e n c e of the i n t e n s i t y of the first d i f f r a c t e d o r d e r at three different e x c i t a t i o n levels. The spectral shape of the d i f f r a c t i o n e f f i c i e n c y varies strongly w i t h the e x c i t a t i o n intensity. At low e x c i t a t i o n (fig. la) there is an e f f i c i e n t grating only just b e l o w the p o s i t i o n of the A F ~ - e x c i t o n (2.5521 eV) on the flank of ~he absorp-
2 Experiment The i n t e n s i t y m o d u l a t i o n in the CdS crystals, w h i c h are c o o l e d down to * P e r m a n e n t address: I n s t i t u t e of Problems of M i c r o e l e c t r o n i c Technology, A c a d e m y of S c i e n c e s of the USSR, M o s c o w district, 142432 C h e r n o g o l o v k a , USSR 675
676
LASER INDUCED GRATINGS IN CdS
tion edge. In this s p e c t r a l r e g i o n one has an d i s t i n c t c h a n g e of the d i e l e c t r i c f u n c t i o n b e c a u s e of a b r o a d e n i n g of the excitonic resonances with increasing e x c i t a t i o n , see e.g. 7,9. This e f f e c t p r o d u c e s thus a l a s e r - i n d u c e d a m p l i t u d e grating. For v a l u e s of Iex c b e t w e e n 1o and 5 o k W / c m 2 one o b s e r v e s a r e s o n a n c e like s t r u c t u r e in the s p e c t r a l e f f i c i e n -
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the w a v e - p i c t u r e is s t r e s s e d h e r e the p h e n o m e n o n can e q u a l l y w e l l be d e s c r i b e d in the p a r t i c l e p i c t u r e as v i r t u a l e x c i t a t i o n of a b i e x c i t o n by two q u a n t a of the b e a m s "a" a n d / o r "b" and s u b s e q u e n t decay s t i m u l a t e d e i t h e r by b e a m "a" or "b". For a m o r e d e t a i l e d d i s c u s s i o n of this a s p e c t see e.g. 4,11, for t h e o r y e.g. 12 and the l i t e r a t u r e c i t e d therein. At the h i g h e s t p u m p i n g power, (fig. Ic) the g r a t i n g is m o s t e f f i c i e n t in the region just above the c h e m i c a l p o t e n tial p of the e l e c t r o n - h o l e p l a s m a (EHP) w h i c h is s i t u a t e d a r o u n d 2.54 eV. The dip in the p l a t e a u of the s c a t t e r e d i n t e n s i t y a p p e a r s b e c a u s e of the P e r o t Fabry m o d u l a t e d t r a n s m i t t i v i t y of the c r y s t a l p l a t e l e t . The f o r m a t i o n of an EHP s t r o n g l y a f f e c t s the a b s o r p t i v e and d i s p e r s i v e p r o p e r t i e s of the s e m i c o n d u c t o r 5-6. In these e f f e c t s are inv e s t i g a t e d u n d e r the same c o n d i t i o n s as in o u r e x p e r i m e n t , i.e. the s e l f - i n d u c e d c h a n g e of the d i e l e c t r i c f u n c t i o n at the s p e c t r a l p o s i t i o n of the laser is m e a s u r e d , h o w e v e r w i t h an e x c i t e 9 and p r o b e technique. A c o m p a r i s o n to shows that the m a x i m u m of the d i f f r a c ted e f f i c i e n c y is d i r e c t l y r e l a t e d to the p o s i t i o n of the m a x i m u m c h a n g e of ~. A c c o r d i n g to 9 the e x c i t a t i o n ind u c e d i n c r e a s e of ~ (Iexc, ~ e x c ) tends to zero for ~ e x c z 2.54 eV, but a v a r i a t i o n of n can still be found to lower e n e r g i e s . Thus the LIG can be i n t e r p r e t e d p r e d o m i n a n t l y as an a m p l i tude g r a t i n g for Iex c ~ IM W / c m 2 above 2.54 eV a n d as a p h a s e - g r a t i n g below.
4 Conclusion
i
2.535 Fig.
l
a) lpurnp= 4 0 0 W I cm 2
Vol. 51, No. 9
2.545 ellergy
2.550 [eV]
The i n t e n s i t y of the d i f f r a c t e d o r d e r +I of a l a s e r - i n d u c e d g r a t i n g as a f u n c t i o n of the p h o t o n e n e r g y of the laser for three d i f f e r e n t v a l u e s of the e x c i t a t i o n intensity. The i n s e r t in Fig. la shows s c h e m a t i c a l l y the e x p e r i m e n t a l a r r a n g e m e n t .
cy of L I G w i t h a dip s i t u a t e d i n d e p e n d e n t l y of Iex c at 2 . 5 4 9 7 eV. This value c o i n c i d e s e x a c t l y w i t h h a l f the b i e x c i ton e n e r g y at 5.0994 eV as d e d u c e d from LATS or T P R S - e x c i t a t i o n s p e c t r o s c o p y 1o So the dip c o r r e s p o n d s to the t w o - p o l a r iton t r a n s i t i o n to the b i e x c i t o n . T h e r e s o n a n c e - l i k e i n c r e a s e on both sides is due to e x c i t a t i o n i n d u c e d r e n o r m a l i zation of b o t h ~ and n c a u s e d by twop o l a r i t o n t r a n s i t i o n s to the b i e x c i t o n . This i n t e r p r e t a t i o n is in a g r e e m e n t w i t h the one given in 4 for CuCI. W h i l e
Our e x p e r l m e n t s on LIG in CdS at low t e m p e r a t u r e s h o w clearly, that this t e c h n i q u e a l l o w s to d e t e c t and to interpret excitation induced optical n o n l i n e a r i t i e s in the s p e c t r a l r e g i o n of the e x c i t o n i c resonances. The effic i e n c y ~ is c l o s e l y r e l a t e d to the change of the o p t i c p r o p e r t i e s due to these effects. For i n t e n s i t i e s Ib<< I a the i n v e s t i g a t i o n of LIG c o r r e s p o n d s to a k i n d of m o d u l a t i o n s p e c t r o s c o p y since the e f f i c i e n c y then d e p e n d s on d~/dI and dn/dI. F u r t h e r m e a s u r e m e n t s like d e p e n d e n c e of the e f f i c i e n c y on I a a n d I b at a fixed laser e n e r g y w i l l be p u b l i s h e d e l s e w h e r e 11. A c k n o w l e d g e m e n t s : T h i s w o r k is a project of the S o n d e r f o r s c h u n g s b e r e i c h " F e s t k ~ r p e r s p e k t r o s k o p i e " f i n a n c e d by the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t . The h i g h q u a l i t y CdS p l a t e l e t s h a v e b e e n grown in the K r i s t a l l - und M a t e r i a l l a b o r der U n i v e r s i t ~ t K a r l s r u h e .
Vol. 51, No. 9
LASER INDUCED GRATINGS IN cdS REFERENCES
I H.J. Eichler, Festk6rperprobleme XVIII, 241 (1978) 2 A.A. Borshch, M.S. Brodin, V.V. Ovchar, S.G. Odulov, M.S. Soskin, ZhETF Pis. Red. 18, No. 11, 679 (1973) K. Jarasiunas, H~-J. Gerritsen, Appl. Phys. Lett. 33(2), 19o (1978) H.J. Eichler, C. Hartig, J. Knof, Phys. Stat. Sol. (a)45, 433 (1978) 3 A. Maruani, D. Chemla, E. Batitol, Sol. State Comm. 33, 805 (198o) 4 A. Maruani, D. Chemla, Phys. Rev. B23, 841 (1981) Y.Masumoto, S. Shionoya, J. Phys. Soc. Jpn. 4_~9, 2236 (198o) 5 A. Kreissl, K. Bohnert, V.G. Lyssenko, C. Klingshirn, Phys. Stat. Sol. (b) 114, 537 (1982) 6 S. Schmitt-Rink, J.P. L6wenau, H.Haug, K. Bohnert, A. Kreissl, K. Kempf, C. Klingshirn, Physica 117B + 118B, 339 (1983)
677
7 V.G. Lyssenko, V.I. Revenko, Soy. Phys. Sol. Stat. 2__oo, 1238
(1978)
8 K. Kempf, G. Schmieder, G. Kurtze, C. Klingshirn, Phys. Stat. Sol. (b) 10___7, 297 (1981) 9 K. Bohnert, lished
C. Klingshirn,
to be pub-
Io V.G. Lyssenko, K. Kempf, K. Bohnert, G. Schmieder, C. Klingshirn and S. Schmitt-Rink, Sol. State Commun. 42, 4oi (1982) 11 H. Kalt, Ph.D. Thesis Frankfurt (1985), to be p u b l i s h e d 12 V. May, K. Henneberger, F. Henneberger, Phys. Stat. Sol. (b) 94, 611 (1979) R. M~rz, S. Schmitt-Rink, H. Haug, Z. Phys. B4_oo, 9 (198o) S. Schmitt-Rink, H. Haug, Phys. Stat. Sol. (b)1o8, 377 (1981) H. Haug, S.W. Koch, R. M~rz, S.SchmittRink, J. Lum. 24/25, 621 (1981) S. Schmitt-Rink, H. Haug, Phys. Stat. Sol. (b)113, K143 (1982)
Selid State Communications, Printed in Great Britain.
LASER
INDUCED
Vol.51,No.9, pp.675-677, 10:,
GRATINGS
IN
$3.00 + .00 Pergamon Press Ltd.
0038-I098/84
CdS
H. Kalt, V.G. L y s s e n k o , R. R e n n e r and C. K l i n g s h i r n P h y s i k a l i s c h e s I n s t i t u t der Universit~t, R o b e r t - M a y e r - S t r . 2-4, D - 6 o o o F r a n k f u r t am Main, G e r m a n y (Received
24 May,
FR
1984 by M. Cardona)
The d i f f r a c t i o n from l a s e r - i n d u c e d g r a t i n g s is obs e r v e d in CdS at low t e m p e r a t u r e in the s p e c t r a l region of the a b s o r p t i o n edge. The e x c i t a t i o n i n t e n s i t y is v a r i e d from about too W / c m 2 to I M W / c m 2 . The reasons for the f o r m a t i o n of gratings are a b r o a d e n i n g of the e x c i t o n i c a b s o r p t i o n band, a n e w r e s o n a n c e due to two p o l a r i t o n t r a n s i t i o n s to the b i e x c i t o n and the formation of an e l e c t r o n - h o l e p l a s m a with i n c r e a s i n g e x c i t a t i o n intensities.
I Introduction
7K, is p r o d u c e d by the spatial and temporal c o i n c i d e n c e of two beams, called "a" and "b" in the following, of almost equal intensities. They are c r e a t e d by s p l i t t i n g the b e a m of an e x c i m e r - l a s e r pumped, p u l s e d dye-laser. The spectral and t e m p o r a l h a l f w i d t h s are o,o3 m e V and 3 nsec, respectively. The d i a m e t e r of the e x c i t a t i o n spot on the sample is o,7 mm. The two beams form an angle of 0 = 1,6 ° thus p r o d u c i n g a grating with a s p a c i n g of about A l =17,9~m (at a w a v e l e n g t h 2sin@/2 I = 486 nm). The crystal for w h i c h data are given here is a p l a n e p a r a l l e l platelet with a thickness of about d = 7 ~m thus f u l f i l l i n g the r e l a t i o n for a 'thin grating': d << 2A~2 1
L a s e r - i n d u c e d gratings (LIG) are a u s e f u l tool to study o p t i c a l n o n l i n e a r i ties c o n n e c t e d w i t h e l e c t r o n i c excitations in s e m i c o n d u c t o r s . These gratings are c r e a t e d by the i n t e r f e r e n c e of coh e r e n t laser beams in a crystal c o m b i n e d w i t h a n o n l i n e a r response of the semic o n d u c t o r to the s p a t i a l l y m o d u l a t e d field a m p l i t u d e or e x c i t a t i o n intensity. Due to this a m o d u l a t i o n of the absorption c o e f f i c i e n t ~ or of the r e f r a c t i v e index n (or of both together) occurs l e a d i n g to an a m p l i t u d e or a p h a s e grating, r e s p e c t i v e l y I. C o n c e r n i n g the direct gap semicond u c t o r CdS most e x p e r i m e n t s on LIG's have been p e r f o r m e d w i t h an e x c i t a t i o n high into the c o n d u c t i o n - b a n d . Thus the results have been m a i n l y i n t e r p r e t e d by free c a r r i e r s or thermal gratings 2 Only 3 tried to e x p l a i n the o b s e r v e d d i f f r a c t i o n by the c r e a t i o n of e x c i t o n ic m o l e c u l e s in analogy to CuCI 4 however at e x c i t a t i o n i n t e n s i t i e s a r o u n d 3 M W / c m 2 w h e r e g e n e r a l l y p l a s m a phenomena d o m i n a t e in CdS. E x p e r i m e n t a l and t h e o r e t i c a l results show a rich v a r i e t y of n o n l i n e a r effects just in the vicinity of the e x c i t o n i c resonances, see e.g. 5-Io. The i n t e n t i o n of our experiments was to find out w h i c h p r o c e s s e s are in this spectral region responsible for the c r e a t i o n of LIG's depending on the e x c i t a t i o n intensity Iex c and energy h ~ e x c.
The p o l a r i z a t i o n of the e l e c t r i c field is chosen to be o ~ t h o g o n a l to the c r y s t a l l o g r a p h i c c-axis. The lowest intensities at w h i c h d i f f r a c t e d orders can be o b s e r v e d are I a = I b = 1ooW/cm 2 or I a = 5 k W / c m 2 and I b = 1oW/cm 2. At h i g h e r i n t e n s i t i e s several d i f f r a c t e d orders appear (see insert in Fig. la) e i t h e r due to m u l t i p l e d i f f r a c t i o n or to a nons i n u s o i d a l m o d u l a t i o n of the optical properties. The i n t e n s i t i e s of the t r a n s m i t t e d and d i f f r a c t e d orders are d e t e c t e d by f o c u s s i n g the diffraction p a t t e r n onto the vidicon target of an optical m u l t i c h a n n e l - a n a l y z e r system. 3 Results
and D i s c u s s i o n
The figure la-c shows the spectral d e p e n d e n c e of the i n t e n s i t y of the first d i f f r a c t e d o r d e r at three different e x c i t a t i o n levels. The spectral shape of the d i f f r a c t i o n e f f i c i e n c y varies strongly w i t h the e x c i t a t i o n intensity. At low e x c i t a t i o n (fig. la) there is an e f f i c i e n t grating only just b e l o w the p o s i t i o n of the A F ~ - e x c i t o n (2.5521 eV) on the flank of ~he absorp-
2 Experiment The i n t e n s i t y m o d u l a t i o n in the CdS crystals, w h i c h are c o o l e d down to * P e r m a n e n t address: I n s t i t u t e of Problems of M i c r o e l e c t r o n i c Technology, A c a d e m y of S c i e n c e s of the USSR, M o s c o w district, 142432 C h e r n o g o l o v k a , USSR 675
676
LASER INDUCED GRATINGS IN CdS
tion edge. In this s p e c t r a l r e g i o n one has an d i s t i n c t c h a n g e of the d i e l e c t r i c f u n c t i o n b e c a u s e of a b r o a d e n i n g of the excitonic resonances with increasing e x c i t a t i o n , see e.g. 7,9. This e f f e c t p r o d u c e s thus a l a s e r - i n d u c e d a m p l i t u d e grating. For v a l u e s of Iex c b e t w e e n 1o and 5 o k W / c m 2 one o b s e r v e s a r e s o n a n c e like s t r u c t u r e in the s p e c t r a l e f f i c i e n -
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b) Ipump=50 kWlcm2
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"~ C
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0 10
0
2.540 Photon
I
the w a v e - p i c t u r e is s t r e s s e d h e r e the p h e n o m e n o n can e q u a l l y w e l l be d e s c r i b e d in the p a r t i c l e p i c t u r e as v i r t u a l e x c i t a t i o n of a b i e x c i t o n by two q u a n t a of the b e a m s "a" a n d / o r "b" and s u b s e q u e n t decay s t i m u l a t e d e i t h e r by b e a m "a" or "b". For a m o r e d e t a i l e d d i s c u s s i o n of this a s p e c t see e.g. 4,11, for t h e o r y e.g. 12 and the l i t e r a t u r e c i t e d therein. At the h i g h e s t p u m p i n g power, (fig. Ic) the g r a t i n g is m o s t e f f i c i e n t in the region just above the c h e m i c a l p o t e n tial p of the e l e c t r o n - h o l e p l a s m a (EHP) w h i c h is s i t u a t e d a r o u n d 2.54 eV. The dip in the p l a t e a u of the s c a t t e r e d i n t e n s i t y a p p e a r s b e c a u s e of the P e r o t Fabry m o d u l a t e d t r a n s m i t t i v i t y of the c r y s t a l p l a t e l e t . The f o r m a t i o n of an EHP s t r o n g l y a f f e c t s the a b s o r p t i v e and d i s p e r s i v e p r o p e r t i e s of the s e m i c o n d u c t o r 5-6. In these e f f e c t s are inv e s t i g a t e d u n d e r the same c o n d i t i o n s as in o u r e x p e r i m e n t , i.e. the s e l f - i n d u c e d c h a n g e of the d i e l e c t r i c f u n c t i o n at the s p e c t r a l p o s i t i o n of the laser is m e a s u r e d , h o w e v e r w i t h an e x c i t e 9 and p r o b e technique. A c o m p a r i s o n to shows that the m a x i m u m of the d i f f r a c ted e f f i c i e n c y is d i r e c t l y r e l a t e d to the p o s i t i o n of the m a x i m u m c h a n g e of ~. A c c o r d i n g to 9 the e x c i t a t i o n ind u c e d i n c r e a s e of ~ (Iexc, ~ e x c ) tends to zero for ~ e x c z 2.54 eV, but a v a r i a t i o n of n can still be found to lower e n e r g i e s . Thus the LIG can be i n t e r p r e t e d p r e d o m i n a n t l y as an a m p l i tude g r a t i n g for Iex c ~ IM W / c m 2 above 2.54 eV a n d as a p h a s e - g r a t i n g below.
4 Conclusion
i
2.535 Fig.
l
a) lpurnp= 4 0 0 W I cm 2
Vol. 51, No. 9
2.545 ellergy
2.550 [eV]
The i n t e n s i t y of the d i f f r a c t e d o r d e r +I of a l a s e r - i n d u c e d g r a t i n g as a f u n c t i o n of the p h o t o n e n e r g y of the laser for three d i f f e r e n t v a l u e s of the e x c i t a t i o n intensity. The i n s e r t in Fig. la shows s c h e m a t i c a l l y the e x p e r i m e n t a l a r r a n g e m e n t .
cy of L I G w i t h a dip s i t u a t e d i n d e p e n d e n t l y of Iex c at 2 . 5 4 9 7 eV. This value c o i n c i d e s e x a c t l y w i t h h a l f the b i e x c i ton e n e r g y at 5.0994 eV as d e d u c e d from LATS or T P R S - e x c i t a t i o n s p e c t r o s c o p y 1o So the dip c o r r e s p o n d s to the t w o - p o l a r iton t r a n s i t i o n to the b i e x c i t o n . T h e r e s o n a n c e - l i k e i n c r e a s e on both sides is due to e x c i t a t i o n i n d u c e d r e n o r m a l i zation of b o t h ~ and n c a u s e d by twop o l a r i t o n t r a n s i t i o n s to the b i e x c i t o n . This i n t e r p r e t a t i o n is in a g r e e m e n t w i t h the one given in 4 for CuCI. W h i l e
Our e x p e r l m e n t s on LIG in CdS at low t e m p e r a t u r e s h o w clearly, that this t e c h n i q u e a l l o w s to d e t e c t and to interpret excitation induced optical n o n l i n e a r i t i e s in the s p e c t r a l r e g i o n of the e x c i t o n i c resonances. The effic i e n c y ~ is c l o s e l y r e l a t e d to the change of the o p t i c p r o p e r t i e s due to these effects. For i n t e n s i t i e s Ib<< I a the i n v e s t i g a t i o n of LIG c o r r e s p o n d s to a k i n d of m o d u l a t i o n s p e c t r o s c o p y since the e f f i c i e n c y then d e p e n d s on d~/dI and dn/dI. F u r t h e r m e a s u r e m e n t s like d e p e n d e n c e of the e f f i c i e n c y on I a a n d I b at a fixed laser e n e r g y w i l l be p u b l i s h e d e l s e w h e r e 11. A c k n o w l e d g e m e n t s : T h i s w o r k is a project of the S o n d e r f o r s c h u n g s b e r e i c h " F e s t k ~ r p e r s p e k t r o s k o p i e " f i n a n c e d by the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t . The h i g h q u a l i t y CdS p l a t e l e t s h a v e b e e n grown in the K r i s t a l l - und M a t e r i a l l a b o r der U n i v e r s i t ~ t K a r l s r u h e .
Vol. 51, No. 9
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