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Submitted on 1 Jan 1983
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LASER MODULATION INDUCED BY
INTRACAVITY PHOTOTHERMAL DEFLECTION
C. Johnson, R. Brundage, T. Glynn, W. Yen
To cite this version:
C. Johnson, R. Brundage, T. Glynn, W. Yen. LASER MODULATION INDUCED BY INTRACAV- ITY PHOTOTHERMAL DEFLECTION. Journal de Physique Colloques, 1983, 44 (C6), pp.C6-253- C6-258. �10.1051/jphyscol:1983640�. �jpa-00223199�
Colloque C6, supplement au nO1O, Tome 44, octobre 1 9 8 3 page C 6 - 2 5 3
LASER MODULATION INDUCED BY INTRACAVITY PHOTOTHERMAL DEFLECTION
C . J o h n s o n , R.T. Brundage, T . J . Glynn a n d W.M. Yen
D e p a r t m e n t of P h y s i c s , U n i v e r s - i t y of Wisconsin, M d i s o n , V i s c o n o i n 53706, U.S.A.
~ g s u m e ' : C e t a r t i c l e d e r e v u e p r e ' s e n t e u n e n o u v e l l e t e c h n i q u e p o u r l a spectroscopic d e l ' a b s o r p t i o n - i n t r a c a v i t e ' p a r l ' e f f e t p h o t o t h e r m i q u e . L f e x c i t a t i o n o p t i q u e e x t e r n e i n d u i t une d e ' f l e c t i o n d u f a i s c e a u l u m i n e u x du l a s e r l ' i n t e r i e u r d e l a c a v i t g . C e t t e d g f l e c t i o n r g s u l t e d a n s une m o d u l a t i o n d e l f i n t e n s i t e ' d u l a s e r and p e r m e t d ' o b t e n i r l ' a b s o r p t i o n s p e c t r a l e d e s m a t e ' r i a u x d e l a p h a s e c o n d e n s & .
On p r & e n t e l e s c a r a c t & i s t i q u e s d e l f a b s o r p t i o n s p e c t r a l c o b s e r v e h . On d o n n e e n s u i t e un rgsurne/ d e s a v a n t a e e s v a r i g s a i n s i que d e q u e l q u e s a p p l i c a t i o n s de'jg r g a l i s f ' e s d e c e t e f f e t p h o t o t h e r m i q u e . A b s t r a c t : We r e p o r t a new t e c h n i q u e f o r i n t r a c a v i t y a b s o r p t i o n s p e c - t r o s c o p y v i a a p h o t o t h e r m a l e f f e c t . D e f l e c t i o n o f t h e l a s e r beam w i t h i n t h e c a v i t y i s i n d u c e d by e x t e r n a l o p t i c a l e x c i t a t i o n r e s u l t i n g i n
m o d u l a t i o n o f t h e l a s e r o u t p u t a n d e n a b l i n g a b s o r p t i o n s p e c t r a o f c o n d e n s e d p h a s e m a t e r i a l s t o b e o b t a i n e d . We d e s c r i b e t h e c h a r a c t e r i s t i c s o f t h e a b s o r p t i o n s p e c t r a we h a v e o b s e r v e d and d i s c u s s v a r i o u s a d v a n t a g e s and a p p l i c a t i o n s o f t h i s e f f e c t .
I n t r o d u c t i o n . - R e c e n t l y , t h e r e h a s b e e n r a p i d p r o g r e s s i n t h e d e v e l o p m e n t o f s p e c t r o s c o p i c t e c h n i q u e s b a s e d o n t h e g e n e r a t i o n o f h e a t by t h e a b s o r p t i o n o f l i g h t , e . g . i n t e r f e r o m e t r i c , 1 p h o t o a c o u s t i c , 2 t h e r m a l l e n s i n g 3 1 4 and p h o t o - t h e r m a l d e f l e c t i o n (PTD) s p e c t r o s c o p y . 5 The PTD t e c h n i q u e h a s b e e n shown t o b e a h i g h l y s e n s i t i v e method o f a b s o r p t i o n s p e c t r o s c o p y . I n t h i s t e c h n i q u e , o p t i c a l a b s o r p t i o n and s u b s e q u e n t n o n r a d i a t i v e d e c a y c a u s e s a r i s e i n temper- a t u r e i n t h e s a m p l e and a c o r r e s p o n d i n g r e f r a c t i v e - i n d e x g r a d i e n t i n t h e s a m p l e a n d i t s s u r r o u n d i n g s . The d e f l e c t i o n of a l i g h t beam a s i t t r a v e r s e s t h e i n d e x d i s t r i b u t i o n i s m e a s u r e d u s i n g p o s i t i o n s e n s o r s i n c o n v e n t i o n a l PTD. I f , i n s t e a d , t h e s a m p l e i s p l a c e d w i t h i n a l a s e r c a v i t y s o t h a t t h e i n d e x d i s t r i b u t i o n i s p r o b e d by t h e i n t r a c a v i t y beam, i t s p e r i o d i c d e f l e c t i o n w i l l p r o d u c e a mod- u l a t i o n i n t h e l a s e r o u t p u t power. I f t h e p r o b e l a s e r i s o p e r a t i n g n e a r t h r e s h o l d , t h e e f f e c t of t h e r e f r a c t i v e - i n d e x g r a d i e n t c a n b e q u i t e l a r g e , r e s u l t i n g i n 100% m o d u l a t i o n o f t h e l a s e r o u t p u t . We d e s c r i b e b e l o w o u r p r e l i m i n a r y r e s u l t s w h i c h d e m o n s t r a t e and h e l p t o c h a r a c t e r i z e t h e s i g n a l g e n e r a t i o n p r o c e s s .
E x p e r i m e n t a l R e s u l t s . A s c h e m a t i c o f t h e e x p e r i m e n t a l s y s t e m i s shown i n F i g . 1. The s a m p l e u n d e r s t u d y i s p l a c e d b e n e a t h , and c l o s e t o , t h e beam w i t h i n a low power a r g o n l a s e r . A C o h e r e n t Model 590 s t a n d i n g wave d y e l a s e r , w i t h DCM a s t h e l a s i n g d y e was u s e d f o r e x c i t a t i o n . T y p i c a l pump l a s e r powers w e r e i n t h e r a n g e 20-100 mli. A b s o r p t i o n by t h e chopped pump beam c a u s e s a p e r i o d i c r e f r a c t i v e - i n d e x g r a d i e n t i n t h e a i r c l o s e t o t h e s a m p l e s u r f a c e and t h e r e s u l t i n g m o d u l a t i o n of t h e a r g o n l a s e r o u t p u t i s d e t e c t e d b y p h a s e - s e n s i t i v e t e c h n i q u e s a n d , a f t e r n o r m a l i z a t i o n t o t h e d y e l a s e r power, is d f s p l a y e d o n a n X-Y r e c o r d e r . The p o s i t i o n o f t h e s a m p l e w i t h i n t h e c a v i t y was a d j u s t a b l e by means o f m i c r o m e t e r s t a g e s . T y p i c a l s p e c t r a o b t a i n e d i n t h i s way w i t h s i n g l e - c r y s t a l s a m p l e s a r e shown i n F i g . 2 . I n F i g . 2 ( a ) t h e e l e c t r i c f i e l d o f t h e
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Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983640
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F i g . 1. S c h e m a t i c o f 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 i n t r a - c a v i t y p h o t o t h e r m a l d e f l e c t i o n .
e x c i t a t i o n beam was p a r a l l e l t o t h e o p t i c a x i s o f t h e c r y s t a l ( i . e . n - p o l a r i z - a t i o n ) and c o m p a r i s o n w i t h a c o n v e n t i o n a l a b s o r p t i o n s p e c t r u m i n t h e same p o l a r i z a t i o n showed t h a t t h e s i g n a l was p r o p o r t i o n a l t o t h e a b s o r p t i o n c o e f f i c i e n t ( a ) t o w i t h i n e x p e r i m e n t a l e r r o r . S i n c e t h e l a s e r m o d u l a t i o n s i g n a l o b s e r v e d i n o u r e x p e r i m e n t i s s e n s i t i v e t o t h a t f r a c t i o n of t h e a b s o r b e d o p t i c a l e n e r g y w h i c h i s c o n v e r t e d i n t o h e a t , t h i s r e s u l t s u g g e s t s t h a t t h e l u n i n e s c e n c e quantum e f f i c i e n c y d o e s n o t v a r y s i g n i f i c a n t l y i n t h e v a r i o u s l e v e l s o f t h e e x c i . t c d s t a t e . A s i m i l a r r e s u l t was o b t a i n e d f o r t h e o - p o l a r i z a t i o n . The s p e c t r a f o r b o t h p o l a r i z a t i o n s a r e shown i n more d e t a i l i n F i g . 3 . I n a d d i t i o n , F i g . 3 shows t h e s i g n a l o b t a i n e d from t h e same m a t e r i a l i n powdered form i n w h i c h t h e s p e c t r u m i s t h e w e i g h t e d a v e r a g e o v e r t h e d i f f e r e n t p o l a r i z a t i o n s . F o r t h i s e x p e r i m e n t t h e powder was p r e s s e d i n t o a 5 mm d e e p x 8 mm d i a m e t e r h o l e i n a T e f l o n b l o c k . N o t e t h a t t h e s p e c t r a l r e s o l u t i o n i s l i m i t e d o n l y by t h e l i n e w i d t h o f t h e d y e l a s e r (-1 8 ) .
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F i g . 3 . S p e c t r a of 7 . 5 % Er:LiYF4 o b t a i n e d w i t h s i n g l e c r y s t a l s and powder s a m p l e s .
F i g . 4 . V a r i a t i o n o f l a s e r m o d u l a t i o n s i g n a l w i t h v e r t i c a l o f f s e t of b e t w e e n t h e s a m p l e and t h e p r o b e beam f o r s a m p l e s of d i f f e r e n t o p t i c a l d e n s i t y .
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JOURNAL DE PHYSIQUE
I n a more d e t a i l e d s t u d y of t h e s i g n a l g e n e r a t i o n p r o c e s s we i n v e s t i g a t e d t h e s i g n a l d e p e n d e n c e on t h e pump-probe o f f s e t x , t h e s a m p l e - p r o b e o f f s e t z a n d t h e c h o p p i n g f r e q u e n c y o f t h e pump l a s e r . H e a t p r o d u c e d by o p t i c a l a b s o r p t i o n i n t h e s a m p l e p r o p a g a t e s a s a t h e r m a l wave away from t h e p o i n t of g e n e r a t i o n . 6 T h e a m p l i t u d e o f t h e s e waves d e c r e a s e s e x p o n e n t i a l l y a s e x p ( - z / p ) w h e r e z i s t h e d i s t a n c e from t h e h e a t s o u r c e and u = ( 2 k / p c w ) l f 2 i s t h e t h e r m a l d i f f u s i o n l e n g t h f o r t h e medium ( t h e r m a l c o n d u c t i v i t y k , s p e c i f i c h e a t c , d e n s i t y p) a t t h e c h o p p i n g f r e q u e n c y f = w/2n. T h u s f o r a s t r o n g l y a b s o r b i n g s a m p l e ( e . g . GaAs a b o v e i t s band-gap) t h e o p t i c a l pump e n e r g y w i l l b e d e - p o s i t e d c l o s e t o t h e s a m p l e s u r f a c e , g i v i n g r i s e t o a t e m p e r a t u r e g r a d i e n t i n t h e a i r w h i c h d e c r e a s e s e x p o n e n t i a l l y w i t h d i s t a n c e z from t h e s u r f a c e . The a n g u l a r d e f l e c t i o n o f t h e p r o b e beam i s g i v e n by7
w h e r e X i s t h e d i s t a n c e o v e r w h i c h t h e p r o b e beam i n t e r a c t s w i t h t h e t e m p e r a t u r e g r a d i e n t d T / d z and n i s t h e r e f r a c t i v e i n d e x o f t h e a i r . I f we a s s u m e t h a t t h e l a s e r m o d u l a t i o n a m p l i t u d e i s p r o p o r t i o n a l t o @ t h e n we would e x p e c t t h e s i g n a l t o d e c r e a s e e x p o n e n t i a l l y w i t h v e r t i c a l o f f s e t z b e t w e e n s a m p l e and p r o b e . T h i s d e p e n d e n c e i s shown i n F i g . 4 f o r s a m p l e s of d i f f e r e n t o p t i c a l d e n s i t y . F o r GaAs a-l < < d , w h e r e d i s t h e s a m p l e t h i c k n e s s , t h e e x p o n e n t i a l d e p e n d e n c e i s c l e a r a n d t h e v a l u e o f uair o b t a i n e d from t h e s l o p e was w i t h i n 1 0 % o f t h e c a l - c u l a t e d v a l u e f o r t h i s p a r a m e t e r . uair may a l s o b e d e t e r m i n e d f r o m t h e m e a s u r e d v a r i a t i o n o f t h e p h a s e s h i f t b e t w e e n t h e l a s e r m o d u l a t i o n s i g n a l and
t h e chopped e x c i t a t i o n beam. T h e p h a s e s h i f t i s d u e t o t h e f i n i t e d e l a y r e - q u i r e d f o r t h e p r o p a g a t i o n o f t h e h e a t wave f r o m t h e p o i n t o f g e n e r a t i o n w i t h i n t h e s a m p l e t o t h e p o s i t i o n o f t h e p r o b e beam. S i n c e t h e t h e r m a l wave s o l u t i o n s a r e o f t h e f o r m e x p j ( w t - z / u ) , w h e r e z i s measured v e r t i c a l l y , t h e p h a s e v a r i e s l i n e a r l y w i t h z a s shown i n F i g . 5 , a n d t h e t h e r m a l d i f f u s i o n l e n g t h may b e o b t a i n e d f r o m t h e s l o p e .
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F o r s a m p l e s o f low o p t i c a l d e n s i t y ( a < d ) t h e h e a t i s d e p o s i t e d t h r o u g h - o u t t h e s a m o l e and t h e t e m p e r a t u r e a t t h e p o s ? t i o n o f t h e p r o b e beam i s
d e t e r m i n e d by t h e f l o w o f h e a t f r o m d i f f e r e n t d e p t h s i n t h e s a m p l e . The a m p l i t u d e s a n d p h a s e s of t h e s e c o n t r i b u t i o n s a r e d e t e r m i n e d b y t h e t h e r m a l d i f f u s i o n l e n g t h s i n t h e a i r a n d i n t h e s a m p l e . (We s h o u l d m e n t i o n t h a t t h e d a t a i n F i g . 4 w e r e o b t a i n e d u s i n g t h e v e c t o r o p t i o n on t h e l o c k - i n d e t e c t o r , i n w h i c h i t r e s p o n d s t o t h e o v e r a l l m a g n i t u d e and p h a s e o f t h e s i g n a l r a t h e r t h a n t h e component t h a t i s i n - p h a s e o r o u t - o f - p h a s e w i t h t h e r e f e r e n c e . ) T h u s , when a - I +, d , a s i n t h e c a s e of NdF3 and NdP5014, t h e s i g n a l n o l o n g e r shows a s i m p l e e x p o n e n t i a l d e p e n d e n c e . Even i n t h e s e s a m p l e s , s i n c e t h e t h e r m a l d i f f u s i o n l e n g t h s i n s o l i d s a r e t y p i c a l l y much l e s s t h a n hir, we e x p e c t t h a t c o n t r i b u t i o n s f r o m w i t h i n t h e s a m p l e w i l l b e l e s s i m p o r t a n t a t l a r g e o f f s e t s , and t h e l i m i t i n g s l o p e s h o u l d c o r r e s p o n d a g a i n t o (pair)-1. T h i s t r e n d i s c l e a r from F i g . 4 .
T h e f r e q u e n c y d e p e n d e n c e o f t h e m o d u l a t i o n s i g n a l was m e a s u r e d a t s e v e r a l f r e q u e n c i e s u s i n g f i x e d s p e e d c h o p p e r s . I t s h o u l d b e n o t e d t h a t t h e d i a m e t e r o f t h e beam was f i x e d i n o u r e x p e r i m e n t and was l e s s t h a n p a i r f o r t h e l o w e s t c h o p p i n g f r e q u e n c y ( 2 . 5 H z ) . A s t h e f r e q u e n c l w a s i n c r e a s e d t h e s i g n a l
d e c r e a s e d a s w - l i n i t i a l l y b u t f a s t e r t h a n w- a b o v e 20 Hz. The r a p i d d e c r e a s e atly?.gher f r e q u e n c i e s o c c u r s b e c a u s e t h e t h e r m a l d i f f u s i o n l e n g t h d e c r e a s e s a s w- , t h e r e b y p r e v e n t i n g h e a t from r e a c h i n g t h e r e g i o n o f t h e p r o b e beam, w h i l e t h e h e a t d e p o s i t e d i n t h e s a m p l e d e c r e a s e s as w-I.
The t e m p e r a t u r e p r o f i l e i n t h e r e g i o n a b o v e t h e s a m p l e was i n v e s t i g a t e d b y d i s p l a c i n g t h e pump beam h o r i z o n t a l l y r e l a t i v e t o t h e p r o b e beam. I n g e n e r a l , t h e s i g n a l f a l l s o f f r a p i d l y w i t h o f f s e t x a t a r a t e w h i c h i s d e t e r m i n e d b y t h e pump beam s p o t s i z e and t h e r a t i o u /uai . T h e s i g n a l d e p e n d e n c e i s v e r y s i m i l a r t o t h a t o b s e r v e d f o r t h e beam R e F l e c t f o n i n t r a n s v e r s e PTD s p e c t r o s c o p y , 5 i n c l u d i n g t h e s e c o n d a r y maximum a n d t h e 180" p h a s e s h i f t w h i c h r e s u l t s from h e a t f l o w i n g b a c k t o t h e s a m p l e a t c e r t a i n d i s p l a c e m e n t s . T h e s e m e a s u r e m e n t s a l s o p r o v i d e a method f o r e s t i m a t i n g t h e e f f e c t i v e i n t e r a c t i o n l e n g t h L i n v o l v e d i n t h e d e f l e c t i o n p r o c e s s .
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F i g . 5 . V a r i a t i o n o f s i g n a l p h a s e w i t h s a m p l e - p r o b e o f f s e t z .
E x a m i n a t i o n of t h e f a r - f i e l d p r o f i l e r e v e a l e d n o v a r i a t i o n i n t h e mode s t r u c t u r e o f t h e a r g o n l a s e r , w h i c h r e m a i n e d i n t h e T E G 0 a t a l l t i m e s .
( T h i s o b s e r v a t i o n may b e c o n t r a s t e d w i t h t h e r e s u l t s of o u r i n i t i a l e x p e r i m e n t s i n w h i c h a s t a n d i n g - w a v e d y e l a s e r was u s e d as t h e p r o b e . I n t h a t a r r a n g e m e n t , t h e p r o b e beam was f o r c e d i n t o t h e TEMIO mode when t h e pump l a s e r was f o c u s s e d t o a d i a m e t e r much l e s s t h a n t h a t o f t h e p r o b e beam. However, t h e d y e l a s e r c a v i t y i s d e s i g n e d t o r e s i s t s m a l l c h a n g e s a r i s i n g from m i s a l i g n m e n t and was much l e s s s e n s i t i v e t h a n t h e p r e s e n t a r r a n g e m e n t f o r i n t r a c a v i t y d e f l e c t i o n s p e c t r o s c o p y . ) By o p e r a t i n g c l o s e t o t h r e s h o l d , t h e s i g n a l c o u l d b e i n c r e a s e d 4 o r 5 t i m e s b u t t h e n o i s e was a l s o i n c r e a s e d . Optimum s i g n a l - t o - n o i s e was o b t a i n e d when t h e a r g o n l a s e r was o p e r a t i n g i n t h e r a n g e 30-50 mw. I n o r d e r t o compare o u r method w i t h o t h e r p h o t o t h e r m a l l y - b a s e d t e c h n i q u e s we e s t i m a t e t h e s e n s i t i v i t y
S = (crL) . x Pump Power ( W a t t s ) . mln
w h e r e a i s t h e a b s o r p t i o n c o e f f i c i e n t a n d !L i s t h e i n t e r a c t i o n l e n g t h a s d e - f i n e d e a r l i e r . We e s t i m a t e S t o b e i n o u r s e t u p , w h i c h i s c o m p a r a b l e t o t h a t o b t a i n a b l e i n t r a n s v e r s e PDT s p e c t r o s c o p y , and b e t t e r t h a n t h a t t y p i c a l l y o b t a i n a b l e i n m i c r o p h o n e PAS of s o l i d s .
C o n c l u s i o n . - We h a v e shown t h a t t h e i n t r a c a v i t y p h o t o t h e r m a l e f f e c t p r o v i d e s t h e b a s i s f o r a h i g h l y s e n s i t i v e form o f s p e c t r o s c o p y . By i n v e s t i g a t i o n o f t h e s i g n a l d e p e n d e n c e on v a r i o u s p a r a m e t e r s , we h a v e o p t i m i z e d t h e e x p e r i m e n t a l s e t u p and d e t e r m i n e d t h e l i m i t s on t h e s e n s i t i v i t y of t h e t e c h n i q u e . Note t h a t t h e method d o e s n o t r e q u i r e s p e c i a l s e n s o r s a s d o e s c o n v e n t i o n a l PDT s p e c t r o s c o p y . A l s o , it s h o u l d b e p o s s i b l e t o u s e i n c o h e r e n t s o u r c e s f o r e x c i t a t i o n a t t h e e x p e n s e o f r e d u c e d s p e c t r a l r e s o l u t i o n . Powder, l i q u i d , o p a q u e s a m p l e s a n d s a m p l e s o f p o o r o p t i c a l q u a l i t y may b e r e a d i l y i n v e s t i g a t e d u s i n g t h i s t e c h n i q u e . The s e n s i t i v i t y c o u l d b e improved by a t l e a s t a n o r d e r o f m a g n i t u d e b y u s i n g a l i q u i d , s u c h a s CCl4, a s t h e d e f l e c t i n g medium b e c a u s e o f t h e h i g h e r v a l u e o f dn/dT f o r l i q u i d s compared t o a i r . IIowever, i t i s d i f f i c u l t t o i n c l u d e s u c h a n a r r a n g e m e n t i n t h e a r g o n l a s e r c a v i t y b e c a u s e o f t h e l a r g e i n s e r t i o n l o s s . C e r t a i n a p p l i c a t i o n s o f t h e e f f e c t t o d e v i c e s a l s o become a p p a r e n t . F o r e x a m p l e , t h e e f f e c t p r o v i d e s a way t o m o d u l a t e i n t e n s e l a s e r beams w i t h i n t h e c a v i t y w i t h o u t n e c e s s i t a t i n g t h e i r p a s s a g e t h r o u g h a d a m a g e a b l e s o l i d medium.
T h i s work was s u p p o r t e d by t h e U.S.-Army R e s e a r c h O f f i c e (DAAG-29-794- 0040) and b y t h e U.W. G r a d u a t e R e s e a r c h Committee. T. 3 . Glynn a c k n o w l e d g e s t h e s u p p o r t of t h e I r i s h N a t i o n a l Board f o r S c i e n c e and T e c h n o l o g y . ( G r a n t No. 1 6 7 1 8 2 ) .
JOURNAL DE PHYSIQUE
R e f e r e n c e s
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