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PHOTOTHERMAL DETECTION OF PICOSECOND PHOTOINDUCED DICHROISM
C. Ironside, R. Taylor, J. Ryan
To cite this version:
C. Ironside, R. Taylor, J. Ryan. PHOTOTHERMAL DETECTION OF PICOSECOND PHO- TOINDUCED DICHROISM. Journal de Physique Colloques, 1983, 44 (C6), pp.C6-579-C6-585.
�10.1051/jphyscol:1983696�. �jpa-00223255�
JOURNAL DE PHYSIQUE
Colloque C6, supplement au n°10, Tome 44, octobre 1983 page C6- 579
PHOTOTHERMAL DETECTION OF PICOSECOND PHOTOINDUCED DICHROISM
C.N. I r o n s i d e , R.A. T a y l o r and J . Ryan
Clarendon Laboratory, Parks Road, Oxford 0X1 3PU, U.K.
Résume.- On décrit une technique qui utilise la spectroscopie par déflex- ion photothermique pour la mesure du dichroïsme photo induit. Elle possède l'avantage d'éliminer tous les effets non linéaires dus aux mélanges des faisceaux sonde et pompe dans les expériences picoseconde.
Abstract.- A technique which uses photothermai deflection spectroscopy to detect photoinduced dichroism is described. It has the advantage that ef- fects due to nonlinear mixing of pulse and probe in picosecond experiments are eliminated.
In picosecond spectroscopy a standard technique for obtaining temporal infor- mation about light sample interaction is the so-called pulse probe method. The sample is excited by the bleaching pulse and the induced absorption characteristics are monitored, as a function of time, by the delayed probe pulse. This method has been used widely to study various picosecond processes, however, in some cases it is difficult to separate effects due to nonlinear interactions between the pulse and probe (such as four wave mixing) and changes in absorption of the sample \_] J .
In 1975, Ippen and Shank [^2j showed how the pulse-probe method could be applied to the study of reorientation of dye molecules in various solvents. The bleaching pulse creates a dichroism in the dye solution by saturating the absorp-
tion of those molecules with their interaction dipole aligned parallel with the polarisation of the pump beam. The dichroism rotates the probe polarisation through a small angle, of the order of 1.5 degrees. The recovery from the induced dichro- ism is recorded by measuring the transmission of the probe pulse between crossed polarisers as a function of delay. At zero delay there is nonlinear mixing between the pulse and probe resulting in an anomalously large signal called the "coherence spike". This signal can be as much as three orders of magnitude larger than that due to the induced dichroism and obscures the dichroism signal close to zero delay.
Another reported difficulty [_3J with this experiment is the effect of any small birefringence of the optical components this can cause the results to be difficult to interpret.
These experimental artifacts may be overcome if, instead of detecting the probe beam after it has propagated through the sample, we observe directly the energy deposited in the sample by the prftbe beam. Photoacoustic and photothermai
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983696
C6-580 JOURNAL DE PHYSIQUE
t e c h n i q u e s o f f e r a method o f a c h i e v i n g t h i s . I n p a r t i c u l a r , i n t h i s p a p e r we i n v e s t i g a t e t h e d e t e c t i o n of i n d u c e d d i c h r o i s m by p h o t o t h e r m a l d e f l e c t i o n s p e c t r o - scopy (PDS) [ 4 ] . I n t h i s method t h e p r o p o r t i o n o f t h e e n e r g y d e p o s i t e d b y t h e probe which i s c o n v e r t e d i n t o h e a t i s d e t e c t e d by t h e change i n t h e r e f r a c t i v e i n d e x t h a t i t c a u s e s . The r e f r a c t i v e i n d e x change d e f l e c t s a PDS beam ( u s u a l l y a He Ne l a s e r ) . The d e f l e c t i o n i s measured u s i n g a p o s i t i o n s e n s i t i v e d e t e c t o r . To m o n i t o r t h e i n d u c e d d i c h r o i s m , t h e p o l a r i s a t i o n o f t h e p r o b e beam i s modulated and t h e d e f l e c t i o n o f t h e PDS beam a t t h e m o d u l a t i o n f r e q u e n c y i s r e c o r d e d u s i n g s t a n d a r d p h a s e s e n s i t i v e d e t e c t i o n e l e c t r o n i c s t o p r o c e s s t h e s i g n a l .
The t h e o r y o f t h e PDS d e t e c t i o n of d i c h r o i s m i s s i m i l a r t o t h a t o f conven- t i o n a l PDS. The d i c h r o i c a b s o r p t i o n c o e f f i c i e n t i s d e f i n e d a s
a D = - El- ( 1 )
where aA and a , , a r e t h e a b s o r p t i o n c o e f f i c i e n t s w i t h t h e p r o b e p u l s e p o l a r i s a t i o n p e r p e n d i c u l a r and p a r a l l e l t o t h e b l e a c h i n g p u l s e p o l a r i s a t i o n . F o r t h e c a s e of a sample o f low t h e r m a l c o n d u c t a n c e t h e n t h e d e f l e c t i o n a n g l e i s g i v e n by [5].
dN P L
8 = -
----
X2 2 ( 1 - e ~ ~ ( a $ ) ) ( - 2
( 5 )
e x p ( - - 2 ) ) dT wpcn a( 2 )
a a
dN
.
where - dT 1 s t h e change o f r e f r a c t i v e i n d e x w i t h t e m p e r a t u r e , P i s t h e l a s e r p r o b e i n c i d e n t power, w i s t h e m o d u l a t i o n f r e q u e n c y o f t h e l a s e r probe p o l a r i s a t i o n , p c i s t h e h e a t c a p a c i t y p e r u n i t volume, a i s t h e r a d i u s o f t h e i n t e r a c t i o n r e g i o n o f t h e l a s e r pump w i t h t h e l a s e r p r o b e . x i s t h e d i s t a n c e between t h i s i n t e r a c t i o n r e g i o n and t h e p h o t o t h e r m a l p r o b e . L i s t h e l e n g t h o f t h e i n t e r a c t i o n between t h e PDS p r o b e and t h e b l e a c h - p r o b e o v e r l a p r e g i o n .
The t h e o r y o f how t h e d i c h r o i s m r e c o v e r y i s r e l a t e d t o t h e c h a r a c t e r i s t i c r e o r i e n t a t i o n a l time can be s i m p l y d e r i v e d from t h e t h e o r y o f t h e I p p e n and Shank c r o s s p o l a r i s e r e x p e r i m e n t .
Our e x p e r i m e n t i s s e n s i t i v e t o
where CI i s t h e a b s o r p t i o n c r o s s s e c t i o n f o r t h e ground s t a t e t o t h e f i r s t e x c i t e d s t a t e N , , , , ( t ) i s t h e e f f e c t i v e c o n c e n t r a t i o n f o r B e e r ' s law a b s o r p t i o n o f l i g h t p o l a r i s e d i n t h e p a r a l l e l o r p e r p e n d i c u l a r d i r e c t i o n w i t h r e s p e c t t o t h e b l e a c h i n g p u l s e p o l a r i s a t i o n . The r e o r i e n t a t i o n a l i n f o r m a t i o n i s c o n t a i n e d w i t h i n , N ( t ) and N ( t )
.
To f a c i l i t a t e comparison w i t h p r e v i o u s work [ 6 ] , t h e f o l l o w i n g d e f i n i t i o n o f t h e p o l a r i s a t i o n a n i s o t r o p y i s u s e dT h e e x c i t e d s t a t e decay h a s a l s o t o b e c o n s i d e r e d k ( t ) = N , , ( t ) + 2NL(t)
Combining e q u a t i o n s ( 3 ) , (4) and (5) a D ( t ) can b e w r i t t e n as
I n t h e c a s e where r ( t ) and k ( t ) a r e s i n g l e e x p o n e n t i a l s w i t h time c o n s t a n t s t and t r e s p e c t i v e l y t h e e q u a t i o n ( 6 ) d e c a y s a s a s i n g l e e x p o n e n t i a l w i t h a measured t i m e c o n s t a n t
OPTICAL MODE LOCKED
I I
A' I
I
\
f = 12
0 . 5 m W HF-NE LASER
F i g , ( 1 ) ~ x p e r i m e n t a l la y o u t f o r P h o t o t h e r m a l d e t e c t i o n of i n d u c e d d i c h r o i s m i n Dyes
E x p e r i m e n t a l D e t a i l s
The e x p e r i m e n t a l a r r a n g e m e n t f o r p h o t o t h e r m a l d e t e c t i o n o f i n d u c e d d i c h r o i s m i s i l l u s t r a t e d i n F i g ( 1 ) . A s y n c h r o n o u s l y pumped mode-locked dye l a s e r (Cr 599.04) was o p e r a t e d w i t h Rhodamine 6G and p r o d u c e d p u l s e s of a b o u t 5 p s d u r a t i o n and
1.5 n J e n e r g y a t a r e p e t i t i o n r a t e of 2 2 8 MHz t h a t i s e q u i v a l e n t t o 4.2 n s between p u l s e s . The a v e r a g e power was 300 mw. The b l e a c h i n g p u l s e t r a v e l s t h r o u g h a v a r i a b l e o p t i c a l d e l a y l i n e t h a t c o u l d s c a n 1000 p s . The p r o b e beam i s d i r e c t e d t h r o u g h a P o c k e l s c e l l t h a t modulated i t s p o l a r i s a t i o n t h r o u g h 90 d e g r e e s a t a v a r i a b l e f r e q u e n c y . The c o u n t e r - p r o p a g a t i n g b l e a c h and p r o b e beams were a l i g n e d
t h r o u g h a 100 m i c r o n p i n h o l e and c r o s s e d i n t h e sample where t h e a v e r a g e power of b l e a c h and p r o b e beams were around 140 mw and 10 mw r e s p e c t i v e l y . The sample c e l l was 1 mm t h i c k and c o n t a i n e d dyes i n v a r i o u s s o l v e n t s i n c o n c e n t r a t i o n s o f t o
-
510 m o l a r . P h o t o t h e r m a l d e f l e c t i o n was o b s e r v e d u s i n g a 0 . 5 mW He Ne l a s e r beam whose p o s i t i o n was i n i t i a l l y f i x e d by a l i g n i n g t h r o u g h t h e same p i n h o l e a s t h e dye l a s e r b l e a c h and p r o b e beams and s u b s e q u e n t l y r e p o s i t i o n e d t o o b t a i n optimum PDS s i g n a l . The d e f l e c t i o n o f t h e PDS beam was r e c o r d e d w i t h a q u a d r a n t s i l i c o n photo- d i o d e . The p r o c e s s i n g e l e c t r o n i c s t o o b t a i n a s i g n a l p r o p o r t i o n a l t o d i s p l a c e m e n t o n t h e q u a d r a n t p h o t o d i o d e were made from a s t a n d a r d d e s i g n . P h a s e s e n s i t i v e e l e c t r o n i c p r o c e s s i n g produced a s i g n a l p r o p o r t i o n a l t o t h e PDS p r o b e d e f l e c t i o n a t m o d u l a t i o n f r e q u e n c y . T h i s s i g n a l i s d i r e c t l y p r o p o r t i o n a l t o t h e i n d u c e d
JOURNAL DE PHYSIQUE
d i c h r o i s m . The d i c h r o i s m r e c o v e r y a s a f u n c t i o n o f p r o b e d e l a y was o b s e r v e d by s c a n n i n g t h e o p t i c a l d e l a y l i n e .
TIME (pd
F i g . (2) P h o t o i n d u c e d d i c h r o i s m r e c o v e r y f o r D O D C I i n Methanol
R e s u l t s and Assessment
F i g u r e ( 2 ) and F i g u r e ( 3 ) show t h e i n d u c e d d i c h r o i s m decay c u r v e s f o r t h e d y e s D O D C I and D Q O C I i n mol. s o l u t i o n s o f m e t h a n o l . The w a v e l e n g t h o f t h e dye l a s e r was a p p r o x i m a t e l y 630 nm. The f i r s t o b s e r v a t i o n t o make a b o u t t h e two f i g u r e s i s t h a t t h e r e i s no c o h e r e n c e s p i k e a t z e r o d e l a y and i n d e e d i n a l l o u r r e s u l t s s o f a r we h a v e s e e n no e v i d e n c e of a c o h e r e n c e s p i k e .
The t i m e c o n s t a n t , t m o f D O D C I t a k e n from f i g ( 2 ) i s 389
+
5 0 p s . I t i s t a k e n from t h e f i r s t p a r t (0-220 p s ) o f t h e d e c a y ( t h e sudden d r o p i n s i g n a l i s p r o b a b l y due t o m i s a l i g n m e n t of t h e d e l a y l i n e ) . F o r a f l u o r e s c e n c e l i f e t i m e , t , of 1.5 n s t h e n u s i n g e q u a t i o n ( 7 ) t h e r e o r i e n t a t i o n a l l i f e t i m e , t i s c a l c u l a t e d t o b e 266 2 5 0 p s . T h i s compares w i t h t h e I o p e n and Shank measurement o f t r = 9 3 p s f o r t h e same dye i n t h e same s o l v e n t . However Fleming e t a 1 have d e m o n s t r a t e d t h a t a s i g n i f i c a n t b u i l d up o f p h o t o i s o m e r can a ~ p r o x i m a t e l y d o u b l e t h e measured v a l u e o f t r . T h i s may a c c o u n t f o r t h e d i s c r e p a n a y between o u r v a l u e o f t r and t h a t o f I p p e n and Shank a s o u r a v e r a g e power i s p r o b a b l y h i g h e r and t h e w a v e l e n g t h a t which t h e measurement was t a k e n i s n e a r t o t h e p h o t o i s o m e r peak a b s o r p t i o n .O R I E N T A T I O N A L R E L A X A T I O N
B 50 103 150
.
I I - L L U I I I 1 I I I v I
T I M E C p S >
F i g . (3) Induced d i c h r o i s m r e c o v e r y f o r DQOCI i n Methanol
The DQOCI tm can be found from f i g u r e (3) and i s 83 t 10 p s . The e x c i t e d s t a t e l i f e t i m e i s 3 p s t h e r e f o r e o u r measurement must b e depended on t h e c r e a t i o n o f a l o n g e r l i v e d photoisomer which w i l l a l t e r t h e ground s t a t e r e c o v e r y time t o around 4 n s ; t h i s would make t 81 2 10 ps. The r e o r i e n t a t i o n a l l i f e t i m e i n a c e t o n e was found t o be 52 t 10 p s . The r e l a t i o n s h i p between t r i n methanol, v i s c o s i t y 0.6 cp and t h a t i n a c e t o n e , v i s c o s i t y 0 . 4 c p , i s i n good agreement w i t h t h e s i m p l e hydrodynamic model o f m o l e c u l a r r e o r i e n t a t i o n which p r e d i c t s t h a t t r w i l l s c a l e l i n e a r l y w i t h v i s c o s i t y .
From t h e s i g n a l t o n o i s e r a t i o i n t h e e x p e r i m e n t we e s t i m a t e t h a t t h e minimum d e t e c t e d d e f l e c t i o n a n g l e was 3 . 3 x 10 -6 r a d , t a k i n g t h e f o l l o w i n g t y p i c a l and
dN -4
-
3approximate v a l u e s
-
= 4.1 x 10,
p c = 2 x I O - ~ J M - ~ , p = 10 x 10 V , w = 10 Hz and x = a = 100 x 10 d?6 m, t h e n from e q u a t i o n (2) i t i s c a l c u l a t e d t h a t a minimum aD o f 2 x 10-'m-'could be measured. I n c o n v e n t i o n a l PDS, u s i n g s i m i l a r absorbed powers, t h e minimum d e f l e c t i o n a n g l e r e p o r t e d i s 1 0 - ~ r a d t h a t i s t h r e e o r d e r s o f magnitude more s e n s i t i v e t h a n t h i s e x p e r i m e n t . The d i f f e r e n c e i s accounted f o r by t h e t u r b u l e n t c o n v e c t i o n c u r r e n t p r e s e n t i n t h e sample b e c a u s e o f a b s o r p t i o n from t h e b l e a c h i n g beam which i n d u c e s t h e d i c h r o i s m . The c o n v e c t i o n c u r r e n t n o i s e i s a t low f r e q u e n c i e s around 2 Hz b u t i n c r e a s i n g t h e m o d u l a t i o n f r e q u e n c y t o a v o i d t h i s n o i s e was l i m i t e d by t h e I / w r o l l o f f i n t h e s i g n a l e x p r e s s e d i n e q u a t i o n ( 2 ) .JOURNAL DE PHYSIQUE
-PDS probe
samp1 e
E s F ;on t
a ' e t e c t o ~F i g . ( 4 ) Experimental arrangement f o r s o l i d s
S o l i d s
The r e c e n t o b s e r v a t i o n of o r i e n t a t i o n a l g r a t i n g s i n semiconductors [8,9]
s u g g e s t s t h a t i t should be p o s s i b l e t o c r e a t e induced dichroism i n s o l i d s which would r e l a x on t h e picosecond time s c a l e . The o r i e n t a t i o n a l g r a t i n g h a s been produced i n germanium wafers and due t o a n i s o t r o p i c s t a t e f i l l i n g .
I n s o l i d s t h e r e i s no t u r b u l e n t convection c u r r e n t t o contend with and t h e r e - f o r e t h e photothermal d e f l e c t i o n d e t e c t i o n may be c o n s i d e r a b l y more s e n s i t i v e than i n l i q u i d s . F i g ( 4 ) shows a scheme f o r o b s e r v i n g photoinduced dichroism which we have t r i e d o u t i n GaSe, which has a convenient band gap f o r Rhodamine 6G o p e r a t i o n of t h e mode-locked dye l a s e r . Howewer, induced dichroism was n o t observed a t thesame peak power as t h a t i n t h e dye s o l u t i o n s s u g g e s t i n g t h a t t h e dye l a s e r p u l s e s have t o be a m p l i f i e d b e f o r e induced dichroism c a n be observed.
Conclusion
The use of photothermal d e f l e c t i o n t o d e t e c t induced dichroism has been demon- s t r a t e d . The major advantages of t h e technique a r e t h e absence of a coherence s p i k e and t h a t t h e r e appear t o be no s p u r i o u s e f f e c t s due t o b i r e f r i n g e n c e of t h e o p t i c a l components. The s i g n a l t o n o i s e r a t i o i n the experiment i n l i q u i d s was l i m i t e d by t h e t u r b u l e n t convection c u r r e n t from the b l e a c h i n g beam.
I n semiconductors a g r e a t e r peak power of t h e b l e a c h i n g beam i s r e q u i r e d b e f o r e induced dichroism can be ohserved.
Acknowledgements
This work was financed by t h e SERC.
R e f e r e n c e s
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