UBC Theses and Dissertations

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UBC Theses and Dissertations

Tunable high order anti-Stokes radiation produced by stimulated Raman scattering Sinnott, Timothy Joseph 1977

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TUNABLE HIGH ORDER ANTI-STOKES RADIATION PRODUCED BY STIMULATED RAMAN SCATTERING by TIMOTHY JOSEPH SINNOTT B . S . , U n i v e r s i t y of Texas a t A u s t i n , 1975 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE THE FACULTY OF GRADUATE STUDIES THE DEPARTMENT OF PHYSICS We a c c e p t t h i s t h e s i s as c o n f o r m i n g to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA A p r i l , 1977 (fcT) Timothy Joseph S i n n o t t , 1977 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t o f the r e q u i r e m e n t s f o r an advanced deg ree at the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t ha t the L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by the Head o f my Depar tment o r by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g o r p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Depar tment o f I C / C$ The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2 0 7 5 W e s b r o o k P l a c e V a n c o u v e r , C a n a d a V 6 T 1W5 Date ABSTRACT Tunab le h i g h o r d e r a n t i - S t o k e s r a d i a t i o n produced by s t i m u l a t e d Raman s c a t t e r i n g i n l i q u i d n i t r o g e n i s s t u d i e d e x p e r i m e n t a l l y . The l i g h t beams of two l a s e r s of d i f f e r e n t f r e q u e n c i e s a ruby l a s e r and t u n a b l e dye l a s e r - - . a r e s i m u l t a n e o u s l y f o c u s s e d i n l i q u i d n i t r o g e n and two d i s t i n c t s e r i e s of a n t i - S t o k e s l i n e s a r e o b s e r v e d . The dye l a s e r a n t i - S t o k e s l i n e s can be changed i n f r e q u e n c y by t u n i n g the dye l a s e r . The i n t e n s i t i e s of the two s e r i e s of l i n e s a r e i n v e s t i g a t e d as a f u n c t i o n of both ruby and dye l a s e r power , The dye l a s e r a n t i - S t o k e s l i n e i n t e n s i t i e s a r e found to have a d i s t i n c t t h r e s h o l d f o r ruby l a s e r power but no t h r e s h o l d f o r dye l a s e r power . The e x p e r i m e n t a l r e s u l t s s u p p o r t . a two s t e p model i n wh ich the ruby l a s e r a c t s as a pump f o r m o l e c u -l a r v i b r a t i o n s wh ich i n t u r n modu la te the dye l a s e r f r e q u e n c y . TABLE OF-CONTENTS Page ABSTRACT i i LIST OF FIGURES . . . . . . . , . v ACKNOWLEDGEMENTS v i i C h a p t e r 1 INTRODUCTION 1 2 THEORY 4 A. G e n e r a l D e s c r i p t i o n 4 B. D e r i v a t i o n of Rate o f Energy Exchange 6 C. D i s c u s s i o n of R e s u l t s 14 3 EXPERIMENTAL APPARATUS 20 A. E x p e r i m e n t a l A r rangement 20 B. The L a s e r s 22 C. D e t e c t i o n System 23 D. Other Components 24 4 EXPERIMENTAL PROCEDURES , , 27 A. Tunab le Dye High Order A n t i -S t o k e s L i n e s 27 B. Ruby L a s e r A n t i - S t o k e s L i n e I n t e n s i t i e s , . . 29 C. Dye A n t i - S t o k e s L i n e I n t e n s i t i e s . . . . . . . 32 i i i Chapter Page 5 DISCUSSION AND CONCLUSIONS 43 A. Discussion of Results 43 B. Conclusions 45 C. F e a s i b i l i t y Study of a Tunable U l t r a v i o l e t Laser . 46 BIBLIOGRAPHY 51 i v LIST OF FIGURES F i g u r e Page 1 Photon d iag ram of two s t e p dye a n t i -S t o k e s s t i m u l a t e d Raman s c a t t e r i n g p r o c e s s 7 2a Wave v e c t o r m a t c h i n g c o n d i t i o n f o r f i r s t o r d e r dye a n t i - S t o k e s g e n e r a t i o n 17 2b M a t c h i n g c o n d i t i o n f o r h i g h e r o r d e r p r o c e s s e s 17 3 E x p e r i m e n t a l a r rangement used to o b t a i n a n t i - S t o k e s s p e c t r a 21 4 S p e c t r a l r e s p o n s e of monochromator -pho tomu 1 t i p i i er 25 5 Observed w a v e l e n g t h s of S t o k e s and a n t i - S t o k e s l i n e s produced by ruby 1 a s e r . . . . 28 6 S p e c t r a d e m o n s t r a t i n g t u n a b i l i t y o f h i g h o r d e r dye a n t i - S t o k e s l i g h t . . . . , « . . . , . . . , 30 7 E x p e r i m e n t a l a r rangement used to measure i n t e n s i t i e s o f ruby a n t i -8 I n t e n s i t y o f f i r s t t h r e e ruby a n t i -S t o k e s l i n e s as a f u n c t i o n of ruby l a s e r power 33 9 E x p e r i m e n t a l a r rangement used to measure i n t e n s i t i e s of dye a n t i -S t o k e s l i n e s , , , 34 10 Dye l a s e r p h o t o d i o d e p u l s e a rea v . s . energy c a l i b r a t i o n c u r v e 36 11 T y p i c a l o s c i l l o g r a m s of p h o t o d i o d e and photomu 1 t i p i i er s i g n a l s 38 12 A n t i - S t o k e s i n t e n s i t y v . s . ruby l a s e r power a t a c o n s t a n t 1 MW dye l a s e r V Figure Page 13 Above threshold i n t e n s i t i e s of ruby and dye ant i -Stokes l ines v . s . ruby laser power 40 14 Dye ant i -Stokes in tens i ty v . s . dye laser power at a constant 3 . 7 ± . 7 MW ruby laser power 42 15 Possible arrangement for tunable u l t r a v i o l e t ant i -Stokes Raman laser experiment 48 vi ACKNOWLEDGEMENTS I w i sh to thank Dr . J . Meyer f o r h i s s u p e r v i s i o n of t h i s p r o j e c t . I am e s p e c i a l l y g r a t e f u l to D r . H. Menn icke f o r h i s he lp and h i s s u g g e s t i o n t h a t I c o n t i n u e t h i s e x p e r i m e n t wh ich he i n i t i a t e d d u r i n g h i s s a b b a t i c a l l e a v e here a t the U n i v e r s i t y of B r i t i s h C o l u m b i a . Thanks a re a l s o due to D r . M. B a c o n , L. G o d f r e y , and o t h e r members of the Plasma P h y s i c s Group f o r t h e i r h e l p f u l d i s c u s s i o n s . I would a l s o l i k e to thank N. S i n n o t t f o r her a s s i s t a n c e i n d a t a t a k i n g . I w i s h to thank Mar j McDouga l l f o r her e x c e l l e n t work i n t y p i n g t h i s t h e s i s . T h i s work was s u p p o r t e d by the N a t i o n a l R e s e a r c h C o u n c i l of Canada . v i i C h a p t e r 1 INTRODUCTION S t i m u l a t e d Raman s c a t t e r i n g was f i r s t o b s e r v e d i n 1962 by Woodbury and Ng (1) when i t was d i s c o v e r e d , w h i l e i n v e s t i -g a t i n g the r e l a t i v e l y new ruby l a s e r , t h a t an a p p r e c i a b l e o o f r a c t i o n of the 6943A ruby l i g h t was c o n v e r t e d i n t o 7670A l i g h t upon p a s s i n g t h r o u g h the n i t r o b e n z e n e of the l a s e r c a v i t y ' s K e r r c e l l . Spontaneous Raman s c a t t e r i n g , d i s c o v e r e d by S i r C h a n d r a s e k h a r a Raman i n the t w e n t i e s , c o u l d not e x p l a i n the i n t e n s i t y nor the c o h e r e n c e of the s c a t t e r e d l i g h t . T h i s f i r s t Raman l a s e r was indeed an e x e m p l i f i c a t i o n of a new phenomenon t h a t would be e x t e n s i v e l y i n v e s t i g a t e d by v a r i o u s a u t h o r s ( 2 - 1 3 ) i n the nex t few y e a r s . I t was found t h a t s t i m u l a t e d Raman s c a t t e r i n g c o u l d not be e a s i l y e x p l a i n e d , p a r t i a l l y due to c o m p l i c a t i o n s r e s u l t i n g from o t h e r n o n l i n e a r p r o c e s s e s such as s e l f - f o c u s s i n g and s t i m u l a t e d B r i l l o u i n and R a y l e i g h s c a t t e r i n g . The unusua l p r o p e r t i e s o f t h i s new e f -f e c t , such as the c o h e r e n c e and i n t e n s i t y o f the s c a t t e r e d l i g h t , were an i n c e n t i v e to f u r t h e r i n v e s t i g a t o r s ( 1 4 - 2 0 ) i n the l a t e s i x t i e s and s e v e n t i e s who expanded the scope of s t i m u l a t e d Raman s c a t t e r i n g and c o n t r i b u t e d new a p p l i c a t i o n s . One p o w e r f u l a p p l i c a t i o n of s t i m u l a t e d Raman s c a t t e r i n g i s the g e n e r a t i o n of new l i g h t f r e q u e n c i e s . S t a n s f i e l d (21) a t the U n i v e r s i t y of B r i t i s h C o l u m b i a , f o r e x a m p l e , used the d i f f e r e n c e f r e q u e n c y of the S t o k e s e m i s s i o n f rom two o r g a n i c 1 2 l i q u i d s i n an a t t e m p t to s t u d y the enhancement of s c a t t e r e d -l i g h t due to o p t i c a l m i x i n g i n a p l a s m a . S c a t t e r e d r a d i a t i o n produced by i l l u m i n a t i n g the many Raman a c t i v e l i q u i d s by h i g h power s o l i d s t a t e l a s e r s i n c r e a s e s d r a s t i c a l l y the number of p o w e r f u l p u l s e d narrow l i n e l i g h t f r e q u e n c i e s . F requency d o u b l i n g the i n c i d e n t l a s e r l i g h t f u r t h e r i n c r e a s e s t h i s f r e -quency range f rom the i n f r a r e d to the u l t r a v i o l e t . In t h i s e x p e r i m e n t t u n a b l e r a d i a t i o n i s produced (22) by f o c u s s i n g s i m u l t a n e o u s l y both ruby l a s e r l i g h t and t u n a b l e i n f r a r e d dye l a s e r l i g h t i n l i q u i d n i t r o g e n . The dye l a s e r , u n l i k e the h i g h e r power ruby l a s e r , i s o p e r a t e d below the power t h r e s h o l d f o r p r o d u c i n g , by i t s e l f , h igh o r d e r a n t i - S t o k e s r a d i a t i o n . T h i s t h r e s h o l d , however , i s l o w e r e d when both , l a s e r s s i m u l t a n e o u s l y i l l u m i n a t e the n i t r o g e n s a m p l e . The ruby l a s e r a c t s as the g e n e r a t o r of i n t e n s e c o h e r e n t m o l e c u l a r v i b r a t i o n s which cause the dye l a s e r r a d i a t i o n to s c a t t e r i ne l a s t i c a l l y i n t o many o r d e r s o f - t u n a b l e v i s i bl e and u l t r a * , v i o l e t a n t i - S t o k e s e m i s s i o n . A g e n e r a l d e s c r i p t i o n of s t i m u l a t e d R a m a n . s c a t t e r i n g i s p r e s e n t e d i n the second c h a p t e r . The t e c h n i q u e s of G a r m i r e , P a n d a r e s e and Townes (23) and Menn icke (24) a r e used to ex tend the t h e o r e t i c a l t r e a t m e n t to the case of s i x wave m i x i n g . T h i s t r e a t m e n t shows t h a t the S t o k e s and a n t i - S t o k e s waves c o r r e s p o n d i n g to the two l a s e r f r e q u e n c i e s have p o s i t i v e g a i n and thus growth f o r d i f f e r e n t momentum m a t c h i n g c o n d i t i o n s . The e x p e r i m e n t was o r i g i n a l l y s e t up i n 1 975 by H. ". Menn icke d u r i n g h i s s a b b a t i c a l l e a v e from M a x - P l a n c k - I n s t i t u t 3 f u r E x t r a t e r r e s t i s c h e P h y s i k . The e x p e r i m e n t a l a r rangement i s an a d a p t i o n of the o r i g i n a l s e t - u p and i s d e s c r i b e d i n C h a p t e r t h r e e . Some d e t a i l s of the more i m p o r t a n t components of the e x p e r i m e n t a r e g i v e n . The f i r s t r e s u l t o b t a i n e d was the e x p e r i m e n t a l e s t a b l i s h -ment of t u n a b l e a n t i - S t o k e s e m i s s i o n by two wave l i g h t m i x i n g . F u r t h e r s t u d i e s , r e s u l t i n g i n q u a n t i t a t i v e measurements of the ruby and dye a n t i - S t o k e s e m i s s i o n as a f u n c t i o n of ruby and dye power , a r e d e s c r i b e d i n C h a p t e r f o u r . The l a s t c h a p t e r c o n t a i n s some c o n c l u d i n g remarks and a s h o r t f e a s i b i l i t y s t u d y c o n c e r n i n g an e x p e r i m e n t a l d e s i g n of a Tunab le U l t r a v i o l e t A n t i - S t o k e s Raman (TUASR) l a s e r . A l t h o u g h t u n a b l e e m i s s i o n has been o b s e r v e d i n the 3500$ range i t i s f e l t t h a t more p o w e r f u l u l t r a v i o l e t r a d i a t i o n can be o b -t a i n e d u s i n g a d i f f e r e n t d y e . C h a p t e r 2 THEORY A. G e n e r a l D e s c r i p t i o n As has been ment ioned e a r l i e r , s t i m u l a t e d Raman s c a t t e r -ing produced by a s i n g l e h i g h power l a s e r has been e x t e n s i v e l y t r e a t e d i n the l i t e r a t u r e . Two t h e o r e t i c a l t r e a t m e n t s , c o n -s i d e r a b l y d i f f e r e n t i n a p p r o a c h , a re t h a t f i r s t p r e s e n t e d by G a r m i r e , Pandarese and Townes (23) i n 1963 and t h a t championed by B loembergen i n the l i t e r a t u r e (25) and i n h i s book , Non-T i n e a r O p t i c s ( 2 6 ) . B l o e m b e r g e n 1 s t h e o r y i s more g e n e r a l , but the a p p r o a c h by Townes e t . a l . w i l l be f o l l o w e d as i t i s more r e a d i l y a d a p t a b l e to s i x wave m i x i n g . Other a c c o u n t s of s t i -m u l a t e d Raman s c a t t e r i n g i n p a r t i c u l a r and n o n l i n e a r o p t i c s i n g e n e r a l a r e by B a l d w i n ( 2 7 ) , . K a i s e r and Ma i er ( 2 8 ) . Recent advances i n t h e - s u b j e c t a r e . ' p r e s e n t e d by Shen ( 2 9 ) . The i n t e n s i t y , I , of s t i m u l a t e d s c a t t e r e d l i g h t i s P e x p o n e n t i a l l y a m p l i f i e d a c c o r d i n g to the e x p r e s s i o n : I = I Q e x p ( g I L £ ) where I i s the i n t e n s i t y of spontaneous s c a t t e r e d l i g h t , g i s the Raman g a i n f a c t o r , 1^ i s the i n t e n s i t y of the l a s e r , and I i s the l e n g t h over which a m p l i f i c a t i o n t a k e s p l a c e . T h i s e x p r e s s i o n i n d i c a t e s the p o s s i b i l i t y o f p r o d u c i n g s c a t t e r e d r a d i a t i o n of h i g h i n t e n s i t y i n the f o c u s of a h i g h power p u l s e d l a s e r . 4 5 The b a s i c s i n g l e l a s e r s t i m u l a t e d Raman s c a t t e r i n g p r o -c e s s o c c u r s when the e x c i t i n g l a s e r f r e q u e n c y i s f o c u s s e d i n t o a Raman a c t i v e medium. Such a m a t e r i a l i s n e c e s s a r i l y c h a r a c t -e r i z e d by a n o n l i n e a r s u s c e p t i b i l i t y . The m o l e c u l e s b e g i n to o s c i l l a t e i n a s y n c h r o n i z e d f a s h i o n i n r e a c t i o n to the c e x t r e m e l y h i g h e l e c t r i c - f i e l d of the f o c u s s e d l a s e r l i g h t . The l a s e r r a d i a t i o n i s then Raman s c a t t e r e d by the m o l e c u l a r o s c i l l a t i o n s i n t o S t o k e s l i g h t of lower photon energy (Eg = F_L ~ E Q ) and a n t i - S t o k e s l i g h t of h i g h e r photon energy ( E ^ s = E^ + ^ 0 ) . ^ e P r o c e s s becomes s t i m u l a t e d Raman s c a t t e r i n g when the l a s e r f i e l d i s s u f f i c i e n t l y l a r g e to cause e x p o n e n t i a l g a i n of the s c a t t e r e d l i g h t . The ( n + l ) t h o r d e r a n t i - S t o k e s e m i s s i o n i s p roduced when the nth o r d e r a n t i - S t o k e s l i g h t i s o f s u f f i -c i e n t i n t e n s i t y to i n t e r a c t a g a i n w i t h the m o l e c u l a r o s c i l l a -t i o n s to y i e l d l i g h t of energy E^" + 1 ? = + E . On ly E , the energy of the f i r s t v i b r a t i o n l e v e l , i s i n v o l v e d i n p r o d u c -ing a s e r i e s of h i g h o r d e r S t o k e s or a n t i - S t o k e s l i g h t . However , as i t w i l l be seen i n C h a p t e r f o u r , c u r r e n t p u l s e d t u n a b l e dye l a s e r s of powers i n the one megawatt range are not c a p a b l e of p r o d u c i n g h i g h o r d e r a n t i - S t o k e s e m i s s i o n . T h i s i s because the dye l a s e r power per u n i t w a v e l e n g t h ( s p e c -t r a l power) i s i n s u f f i c i e n t , even when f o c u s s e d , to s u r p a s s the n e c e s s a r y t h r e s h o l d . Thus the b a s i c , approach of t h i s e x p e r i m e n t i s to p roduce the d e s i r e d t u n a b l e a n t i - S t o k e s l i n e s by m i x i n g a dye l a s e r beam w i t h a h i g h e r power ruby l a s e r beam. The ruby l a s e r a c t s as a pump by p r o d u c i n g the m o l e c u l a r v i b r a -t i o n s which i n t u r n modu la te the dye l a s e r f r e q u e n c y . 6 The photon p i c t u r e of the two l a s e r p r o c e s s i s shown i n F i g u r e 1 . A ruby photon p roduces a s c a t t e r e d S t o k e s photon and l e a v e s the n i t r o g e n m o l e c u l e i n i t s f i r s t v i b r a t i o n a l s t a t e . A dye photon can then i n t e r a c t w i t h the e x c i t e d m o l e -c u l e and produce a dye a n t i - S t o k e s p h o t o n , r e t u r n i n g the m o l e -c u l e to i t s z e r o energy v i b r a t i o n a l s t a t e . Of c o u r s e a second ruby photon c o u l d be s i m i l a r l y u p c o n v e r t e d ; both p r o c e s s e s do i n " f a c t o c c u r . A second o r d e r dye a n t i - S t o k e s photon i s p r o -duced when one of the f i r s t o r d e r dye a n t i - S t o k e s photons i n t e r a c t s w i t h an e x c i t e d m o l e c u l e . S e v e r a l o r d e r s of dye a n t i - S t o k e s e m i s s i o n can be p roduced i n t h i s f a s h i o n . B. D e r i v a t i o n of Rate of Energy Exchange The q u a n t i t y t h a t w i l l be d e r i v e d i n t h i s s e c t i o n i s P p , the r a t e of energy exchange between the m o l e c u l a r o s c i l l a t i o n s and a p a r t i c u l a r component Ep of the e l e c t r i c f i e l d . " dP PP = -<dT : E P> ^ The b r a c k e t s s t a n d f o r the t ime average and P i s the i n d u c e d charge p o l a r i z a t i o n . I f P p rs n e g a t i v e , then the p a r t i c u l a r f i e l d component E g i v e s up i t s energy to the m o l e c u l a r v i b r a -t i o n s . On the o t h e r h a n d , i f P i s p o s i t i v e then the e l e c t r o -P m a g n e t i c wave a s s o c i a t e d w i t h E i s a m p l i f i e d . The r e q u i r e m e n t t h a t Pp be p o s i t i v e w i l l i n d i c a t e the momentum m a t c h i n g c o n d i -t i o n s f o r p o s i t i v e growth of both dye and ruby S t o k e s and a n t i -S t o k e s waves . 7 v=.\ v-O Figure 1. Photon diagram of two step dye ant i -S tokes st imu-lated Raman scat te r ing process. 8 C o n s i d e r , t h e n , the f o l l o w i n g e l e c t r i c f i e l d s a s s o c i a t e d w i t h the s i x l i g h t waves of i n t e r e s t . The r u b y , E , and d y e , E^, f i e l d s a r e p r o v i d e d by the l a s e r s whereas the r e m a i n i n g f o u r s c a t t e r e d wave f i e l d s a r i s e from spontaneous Raman s c a t -t e r i n g and are i n i t i a l l y much s m a l l e r than the l a s e r f i e l d s i n m a g n i t u d e . H i g h e r o r d e r S t o k e s and a n t i - S t o k e s f i e l d s a r e n e g l e c t e d . E r " -- %A r {exp i - w r t ) + c . c . } E d " = %A d {exp i - codt + c(,d) + c . c . } - r s = % A r s { e x p r " m „ t + r s d> . ) + c . c .} E r a = = %A r a{exp ^ r a * r " w r a t + Y r a ) + c . c .} E d s 5 = % A d s { e x p ^ - k d s - r " ^ + *ds ) + c . c .} - Eda -= % A d a { e x p 1 < k d a " r " ^ d a * + *da ) c . c .} (2) The c o o r d i n a t e system i s such t h a t the ruby l a s e r beam i s p r o -p a g a t i n g i n the z - d i r e c t i o n w i t h x p o l a r i z a t i o n . The ty's a r e p o s s i b l e phase f a c t o r s , the k ' s a r e p r o p a g a t i o n v e c t o r s , the co's a r e the f r e q u e n c i e s of the waves , and c . c . w i l l a l w a y s s t a n d f o r the complex c o n j u g a t e of the p r e c e d i n g t e r m . The m o l e c u l a r p o l a r i z a b i 1 i t y , a , o f a Raman a c t i v e m a t e r i a l i s c h a r a c t e r i z e d by a dependence upon a v i b r a t i o n c o o r d i n a t e , x , of the m o l e c u l e . That i s , a = a + x|^ . Thus P i s g i v e n O a X -by P = NaE = Na E- + N x f % (3) - - O - o X — 9 where N i s the number of m o l e c u l e s per u n i t v o l u m e , a Q i s the l i n e a r pol a r i za b i 1 i t y , and E i s the t o t a l e l e c t r i c f i e l d . The second term on the r i g h t hand s i d e i s the n o n l i n e a r p o r t i o n of the i n d u c e d charge p o l a r i z a t i o n and i s d e s i g n a t e d by The r e l a t i o n g o v e r n i n g the i n t e r a c t i o n of the f i e l d s w i t h the Raman medium i s g i v e n by E q u a t i o n 5 , a f o r c e d harmonic o s -c i l l a t o r e q u a t i o n w i t h F and E d e f i n e d by E q u a t i o n s 6 and 7 . d 2 x I T , d x , 2 F / , - N F - §ff E.E (6) E + E . + E , + E , + E + E ( 7 ) - r - d - d a - d s - r a - r s ' F i s the f o r c e due to a l l e l e c t r o m a g n e t i c w a v e s , x i s the v i b r a t i o n c o o r d i n a t e , r i s a p h e n o m e n o l o g i c a l damping c o n s t a n t , u) Q i s the f i r s t v i b r a t i o n a l f r e q u e n c y of l i q u i d n i t r o g e n , and M i s the reduced mass o f the two o s c i l l a t i n g a t o m s . L a r g e c o u p l i n g i s e x p e c t e d o n l y f o r t h o s e d r i v i n g terms whose f r e q u e n c y sums to ±^>Q> the n a t u r a l f r e q u e n c y of o s c i l l a -t i o n . In the dot p r o d u c t E «E o n l y r e s o n a t e terms c o n t a i n i n g U~ - ~ U>A > - to . , or to - 03 w i l l be r e t a i n e d , r r s da d d ds ra r These terms are 10 where 2 E r - E r s = % A r - A r s { e x p i ( . 6 r - e r s ) + c c . } 2 E d * E d a - %A d .A d a {exp 1 ( 8 d - e d a ) + c c } 2E -E = JgA -A {exp i (0 - 0 J + c . c . } - r - r a - r - r a r r r a ' 2 E d - § d s = ^ d . A d s { e x p i ( 6 d - e d s ) + c c . } (8) (9) ) = k • r - co t r - r - r . d = k d - r - o,dt + 4>d da - d a - da T d a i = k • r - co t + cb rs - r s - r s r s i = k 'r-oo t + <j> ra - r a - ra ra i . = k , *-r - co , t + <b , da - d s - ds T d s The t ime dependence of t h e s e e x p r e s s i o n s can be shown e x p l i -c i t l y by d e f i n i n g the f o l l o w i n g a n g l e s 6 l . = ( - k r ^ r s } ' C ' *rs 6 2 = ( - k d ^ d a } * - + *d " *da (10) 8 3 = < V W - * I " *ra 8 4 = ( - k d - k d s } *• C + *d " *ds The f o l l o w i n g e q u a t i o n must then be s o l v e d x + Tx + ooo2x = L^-H ^ r - A r s exp i ( e r w 0 t ) + c . c + - d ' - d a e x p i^Q2 + 0iot^ + c , c - + - r ' - r a e x p 1 ^ 9 3 + w o t ^ + c . c . + A . *A. exp i (6 . - OJ t ) + c . c . } - d - d s 4 o The s o l u t i o n i s of the form X = x l + X 2 + X 3 + X 4 ^ 2 ^ where x k , k = 1 , 4 , s a t i s f i e s \ + ri k + <#k • w f f {Fk> (13> i n wh ich s t a n d s f o r the k th d r i v i n g t e r m . In t h i s manner x i s found to be N 1 3 a x = vnH-r^-^HiA -A exp i(6,-a) t ) + c . c . 4M rto ax - r - r s r 1 o o 1 A d ' - d a e x p i ^ e 2 + C J o t ^ + c > c - " " " - r ' - r a e x p i ^ 9 3 + a 3 o t ^ ^ 1 4 ^ + c . c . + i A . ' A . exp ife.-co t ) + c . c . } - d - d s 4 o Knowing x a l l o w s c a l c u l a t i o n of P n £ , the n o n l i n e a r p a r t of the i n d u c e d charge p o l a r i z a t i o n . P i s the d r i v i n g term f o r the wave e q u a t i o n d e s c r i b i n g the l i g h t waves V x ( V x E ) + -zr^p- = - p - j T T v 1 5 ) Waves a t co , co , , co , , and co w i l l grow o n l y a t r e s o n a n c e , r s d a d s ra 1 2 t h a t i s , f o r P a exp ±'i (w r±a>0)t or Ca)d±to0)t (16) Thus , c e r t a i n n o n r e s o n a n t terms of P 0 can be n e g l e c t e d . The c o u p l i n g of the s c a t t e r e d E - f i e l d among t h e m s e l v e s i s n e g l e c t e d because m u l t i p l i n g x w i t h (E + E + E , + E , ) g i v e s terms K 3 v - r s - r a - d s - d a 3 f o r P . w i t h t ime dependence not o f the form of E q u a t i o n 16. A d d i n g i n t h e s e terms would d e s c r i b e the growth of h i g h e r o r d e r s c a t t e r i n g p r o c e s s e s . P n £ i s thus t a k e n to be P „ = N x ~ (E + E .) - n £ 3x - r - d ' (17) a n d , upon c a l c u l a t i o n , y i e l d s 16 terms p l u s t h e i r complex c o n -j u g a t e s . Four of t h e s e terms are r e s o n a n t w i t h each s c a t t e r e d wave. That i s , f o u r a re p r o p o r t i o n a l to e x p ( i t O p t ) where i s the a n g u l a r f r e q u e n c y of a g i v e n wave. The t ime d e r i v a t i v e of each term i s found and the q u a n t i t y 3 E.. DP. L • F 4. 0 8t n£ Cis) i s c a l c u l a t e d f o r each s c a t t e r e d wave. The l i n e a r terms g i v e a z e r o c o n t r i b u t i o n to P p because d E n i K dt - p ' T ico A2 (exp 2 i ( e -to t ) - c . c ' . J d t (19) p p p p where T i s the p e r i o d 1 3 S i m i l a r l y i t can be shown t h a t a l l t ime dependent terms of the n o n l i n e a r c o n t r i b u t i o n t ime ave rage to z e r o . Four n o n - z e r o t ime i n d e p e n d e n t terms remain f o r each Pp . Most i n t e r e s t i n g f o r the purposes o f t h i s e x p e r i m e n t i s P d a as t h i s g i v e s the p o s s i b i l i t i e s f o r growth of the dye a n t i -S t o k e s wave. to 3a\ 2 da da (A -A A , -A,. , cos 3 x ' 8Mroo "2r - r s - d - d a o (k - k +k , - k , ) • r - r - r s - d - d a ) + <J>. -. * . r s d da : + ( A , - A . ) 2 + A , - A . A , - A , v-d -da7 -d rda -d -ds (2 0;) C O S C 2 k d - k d s - k d a ) - r + 2 * d . * d s . * d a ' t A,-A . A -A - d -da, - r - r a cos (k - k - k ,+ k . ) • r - <J> - <j> , + ch - r - r a - d - d a ' - - ra d da The growth of the o t h e r s c a t t e r e d waves i s governed by the f o l l o w i n g e x p r e s s i o n s : Pv.» = " ( N | ^ ) 2 •^rL- { A 'A A -A cos ra v 3 x ' 8Mrco - r - r a - r - r s o (2k - k - k ) • r v - r - r s - r a ' r s ra + (A *A ) + A 'A A *A, cos - r - r a ' - r - r a - d - d a ( k . - k . - k +k ) - d - d a - r - r a ' r + * . - <f> +•'-(> . d da ra + A *A A , -A . cos - r - r a - d - d s (k ,+ k - k , - k ) - d - r - d s - r a (21 ) r- + *d " *ds " *r.a 14 r s r s 3x-' 8Mr'co_ { ^ r ^ r s V * r a C 0 S (2k - k - k ) • r v - r - r a - r s d> - <b Y r a T r s + (A -A ) 2 + A -A A . »A . cos - r - r s ' - r - r s - d - d a < k d + k r - k d a - k r s > i" + <t> J - <t> A - <b y d Y d a y r s + A -A A , - A , cos - r - r s - d - d s (22) ( k . - k - k , +k ) - d - r - d s - r s r + d> , - d> , + d) Y d T d s Y r s ds U N 3 x ; 8M To) { A d ' A d s A d - A d a cos ( 2 k d - k d a - k d s ) *r + ^d ~ *da " *ds + (A , -A , J 2 + A , • A , A -A - d - d s ' - d - d s - r - r a (23) cos (k +k , - k - k , ) • r - d> + d> . - d> . - r - d - r a - d s ' - y r a y d v d s + A . - A . A «A - d - d s - r - r s cos (k - k - k , + k , ) • r - d> - <b . + d> , - r - r s - d - d s ' - v r s T d y d s C. D i s c u s s i o n of R e s u l t s The r e s u l t s o f t h i s s i x - w a v e c o n s i d e r a t i o n can be compared w i t h the t h r e e - w a v e m i x i n g r e s u l t s by G a r m i r e , P a n d a r e s e , and Townes. These a u t h o r s c o n s i d e r o n l y the i n t e r a c t i o n between the r u b y , ruby S t o k e s , and ruby a n t i - S t o k e s waves . T h e i r d e r i v e d e x p r e s s i o n s f o r P ^ s and P ^ a each c o n t a i n two t e r m s . An e x a m i n a t i o n of the e x p r e s s i o n s i n E q u a t i o n s 21 and 22 shows t h a t t h e s e terms a re a l s o p r e s e n t i n the s i x - w a v e c a s e s . M o r e -o v e r , both P r a i n E q u a t i o n 21 and P r s i n E q u a t i o n 22 c o n t a i n two a d d i t i o n a l t e r m s . 1 5 Each- of t h e s e - t e r m s r e p r e s e n t s a p o s s i b l e g a i n or l o s s due to a p a r t i c u l a r two s t e p i n t e r a c t i o n not p r e s e n t i n the t h r e e - w a v e c a s e . The new term i n ' P p r o p o r t i o n a l to the A ^ ' A ^ i n t e r a c t i o n i s n e g l i g i b l e s i n c e the dye l a s e r i s below the s t i m u l a t e d Raman s c a t t e r i n g t h r e s h o l d . The o t h e r new term i s a l s o s m a l l s i n c e i t i s p r o p o r t i o n a l to the A ^ - A ^  a i n t e r -a c t i o n which i s a second o r d e r e f f e c t because the A . „ wave da must be g e n e r a t e d by a n o t h e r two s t e p p r o c e s s . S i m i l a r l y , the new term i n P r s p r o p o r t i o n a l to A d * A d s i s s m a l l w h i l e the ^ d ' ^ d a t e r m " i n d i c a t e s s l i g h t l y l e s s g a i n f o r the ruby S t o k e s wave i n the d i r e c t i o n s a t i s f y i n g k^+k^, * k<H,a + ! S r S ' T n e e x p r e s -sion's f o r P ^ s and P r a i n the s i x - w a v e c a s e a r e e q u i v a l e n t , t h e n , to those i n the t h r e e - w a v e case e x c e p t f o r the a d d i t i o n of s m a l l c o r r e c t i o n t e r m s . T h i s i n d i c a t e s t h a t the ruby S t o k e s and a n t i -S t o k e s e m i s s i o n s h o u l d be q u i t e s i m i l a r i n the case a t h a n d , i n which i n c i d e n t ruby and moderate power dye l a s e r s a r e u s e d , to the u s u a l case o f s i n g l e ruby l a s e r s t i m u l a t e d Raman s c a t -t e r i n g . The main new c o n t r i b u t i o n of t h i s t r e a t m e n t i s the e x p r e s -s i o n f o r P^g) the r a t e of energy t r a n s f e r f rom the m o l e c u l a r o s c i l l a t i o n s to the dye a n t i - S t o k e s wave. I t i s seen t h a t t h i s e x p r e s s i o n , shown i n E q u a t i o n 2 0 , i s o f the same form as the e x p r e s s i o n f o r P r Q i n E q u a t i o n 2 1 . The t h i r d term i s n e g l i g i b l e because i t i s p r o p o r t i o n a l to the s m a l l A d « A d s i n t e r a c t i o n . The f o u r t h term •. r e p r e s e n t s a second o r d e r e f -f e c t s i n c e i t i n v o l v e s A a wh ich must be g e n e r a t e d by a p r e v -r a i o u s i n t e r a c t i o n . T h i s l a s t term i s p a r t i c u l a r l y i n t e r e s t i n g :1 6 s i n c e i t s u g g e s t s the p o s s i b i l i t y of dye a n t i - S t o k e s g a i n f o r the c o n d i t i o n k - k - k . + k . = 0 ; however , s i n c e i t i s s m a l l - r - r a - d - d a compared to the f i r s t two t e r m s , i t w i l l be n e g l e c t e d a l o n g w i t h the t h i r d term i n the c o n s i d e r a t i o n s to f o l l o w . Thus i s g i v e n by the a p p r o x i m a t e e x p r e s s i o n : p ~ _fwA9L)2 A . A / A . A C O S ^da U N 9 x ; 8MrtoQ - d - d a ^ - r - r s c o s (k - k + k , - k . ) • r - r - r s - d - d a ' *rs + ^d " *da + V A d a } (24) i n which i t i s s e e n , because of the n e g a t i v e s i g n , t h a t the dye a n t i - S t o k e s wave i s i n g e n e r a l not a m p l i f i e d . However , p o s i t i v e g a i n can o c c u r i f the f i r s t term i s n e g a t i v e and l a r -ger i n a b s o l u t e v a l u e than the s e c o n d . T h i s i s i n f a c t poss s i b l e , f o r A - A ^ c > A . - A . , i f k - k +k , - k . = 0 and cos(<j). -- r - r s - d - d a - r - r s - d - d a r d ^da ~ ^ r s ^ < ®' Maximum g a i n o c c u r s f o r tjj^ - t f j ^ - cj>rs = IT. Thus the momentum m a t c h i n g c o n d i t i o n f o r dye a n t i - S t o k e s g a i n i s k + k, = k + k , (25) - r - d - r s - d a T h i s c o n d i t i o n , due to the d i s p e r s i o n of the r e f r a c t i v e index of l i q u i d n i t r o g e n , c a n n o t be s a t i s f i e d w i t h f o u r c o l l i near wave v e c t o r s . The v e c t o r s must then be o r i e n t e d as shown i n F i g u r e 2a f o r c o l l i n e a r i n c i d e n t ruby and dye l a s e r s . Shown i n F i g u r e 2b i s the w a v e v e c t o r d i a g r a m c o r r e s p o n d i n g to the momentum m a t c h i n g c o n d i t i o n s f o r h i g h e r o r d e r a n t i - S t o k e s e m i s s i o n . T h e r e f o r e dye a n t i - S t o k e s e m i s s i o n i s p roduced i n (a) Figure 2a. Wave vector matching condit ion for f i r s t order dye ant i -Stokes generat ion. (b) igure 2b. .Matching condit ion for high order processes. 1 8 narrow cones i n the f o r w a r d d i r e c t i o n about the x a x i s . The a n g l e of the f i r s t o r d e r dye a n t i - S t o k e s e m i s s i o n s h o u l d be c o n s i d e r a b l y s m a l l e r than t h a t of the f i r s t o r d e r ruby a n t i -S t o k e s l i g h t s i n c e the f r e q u e n c y range of the dye p r o c e s s i s f rom about 6900 to 8281K as opposed to 5977 to 8281K f o r the ruby p r o c e s s . However , i n t h i s e x p e r i m e n t no a t t e m p t was made to i n v e s t i g a t e the s p a t i a l s t r u c t u r e of the a n t i - S t o k e s l i g h t . Of i n t e r e s t i n t h i s e x p e r i m e n t i s the r e l a t i v e b e h a v i o u r of the i n t e n s i t y o f the dye and ruby a n t i - S t o k e s l i g h t . N e g l e c t i n g the s m a l l terms and s u b s t i t u t i n g i n the momentum m a t c h i n g c o n d i t i o n s f o r maximum g a i n y i e l d s the f o l l o w i n g e x p r e s s i o n s f o r and P, ra da 3a \ 2 r a A «A 8MTco. - r - r a V Ar s " A r * A r a (26) 9ou 2 CO da 8Mrco. 0 - d - d a A -A - A , - A , - r - r s - d - d a (27) P d a ' i t i s s e e n , c o n t a i n s a g a i n term p r o p o r t i o n a l to A r « A ^ s and a l o s s term p r o p o r t i o n a l to A ^ ' A ^ . The g a i n term i n d i -c a t e s p h y s i c a l l y t h a t the a m p l i f i c a t i o n of the dye a n t i - S t o k e s wave depends on the g e n e r a t i o n of a ruby S t o k e s wave i n the p a r t i c u l a r d i r e c t i o n g i v e n by t h e momentum m a t c h i n g c o n d i t i o n . However , the l o s s term r e p r e s e n t s an o m n i d i r e c t i o n a l S t o k e s p r o c e s s i n which the newly c r e a t e d dye a n t i - S t o k e s wave l o s e s energy by a m p l i f y i n g the dye wave. The e x p r e s s i o n f o r P has r a s i m i l a r terms and thus one would e x p e c t t h e s e two a n t i - S t o k e s 1 9 waves to behave s i m i l a r l y i n some s i t u a t i o n s . One m i g h t a l s o e x p e c t d i f f e r e n c e s to o c c u r because the ruby and dye l a s e r s p l a y d i f f e r e n t r o l e s and because o n l y the dye a n t i - S t o k e s e m i s -s i o n depends on both t h e s e l a s e r s . C h a p t e r 3 EXPERIMENTAL APPARATUS A. E x p e r i m e n t a l Ar rangement Three d i f f e r e n t e x p e r i m e n t a l a r r a n g e m e n t s were u s e d : one to p h o t o g r a p h i c a l l y r e c o r d the Raman s p e c t r a , a second to l o o k a t the r e l a t i v e i n t e n s i t i e s o f the ruby a n t i - S t o k e s l i n e s , and a t h i r d to q u a n t i t a t i v e l y s t u d y the dye a n t i - S t o k e s e m i s s i o n . The f i r s t a r rangement i s shown i n F i g u r e 3 , the l a t t e r s e t - u p s w i l l be d e s c r i b e d i n the f o l l o w i n g c h a p t e r . The h i g h power ruby l a s e r pumps a moderate power t u n a b l e dye l a s e r . The two l a s e r beams a r e then made c o l l i n e a r by a p r i s m and d i e l e c t r i c m i r r o r and are f o c u s s e d by an f / 2 l e n s i n a c o n t a i n e r of l i q u i d n i t r o g e n . L i q u i d n i t r o g e n i s used because o f . i t s ' ; h i g h g a i n f a c t o r and l a r g e f r e q u e n c y s h i f t of 2327 cm" 1 . The f o r w a r d s c a t t e r e d Raman e m i s s i o n i s c o l l i m a t e d by a second l e n s and p r o j e c t e d onto the e n t r a n c e s l i t of a g r a t i n g s p e c t r o m e t e r . A p o r t i o n of the spec t rum i s r e c o r d e d on 35 mm f i l m . In the second and t h i r d a r r a n g e m e n t s a p e r c e n t a g e of each l a s e r beam i s s p l i t o f f and d e t e c t e d by l i g h t s e n s i t i v e d i o d e s i n o r d e r to m o n i t o r the l a s e r power . In t h e s e q u a n t i t a t i v e e x p e r i m e n t s the s p e c t r o s c o p i c p l a t e i s r e p l a c e d by a monochro -mator s l i t and p h o t o m u 1 t i p i i e r . 20 22 B. The L a s e r s The ruby l a s e r i s an o s c i l l a t o r - a m p l i f i e r c o m b i n a t i o n , q - s w i t c h e d by a P o c k e l s c e l l , c a p a b l e of 250 MW maximum power . T y p i c a l o p e r a t i n g power i s 50 MW w i t h a p u l s e w i d t h of 2 0 - 2 5 nsec FWHM. The o s c i 1 1 a t o r c o n s i s t s of a 3 . 5 X . 37 5 i n c h ruby rod e x c i t e d by two w a t e r - c o o l e d f l a s h t u b e s , each of wh ich can d i s c h a r g e up to 2 KJ of e n e r g y . The a m p l i f i e r employs a 6 X . 5 i n c h rod w i t h f l a s h t u b e s of 4 KJ maximum e n e r g y . The c a p a c i t o r bank c u r r e n t p u l s e l e n g t h s , measured by a Rogowski c o i l and i n t e g r a t e d , a re 1 .05 ms f o r the o s c i l l a t o r and . 9 0 ms f o r the a m p l i f i e r . The a m p l i f i e r b a n k a n d P o c k e l s c e l l t r i g g e r p u l s e s a r e s u i t a b l y d e l a y e d such t h a t the v o l t a g e on the P o c k e l s c e l l i s dropped j u s t as the c a p a c i t o r banks f i n i s h d i s c h a r g i n g . The a l i g n m e n t of the P o c k e l s c e l l i s c r i t i c a l f o r p r o d u c i n g a t e m p o r a l l y c l e a n , 2 0 - 2 5 nsec p u l s e . The l a s e r i s r e a s o n a b l y r e p r o d u c i b l e ( w i t h i n 20% of a v e r a g e power) i f the t e m p e r a t u r e of the ruby rod i s c o n t r o l l e d and i f t h e l a s e r i s f i r e d a t r e g u l a r i n t e r v a l s . The t u n a b l e dye l a s e r i s l o n g i t u d i n a l l y pumped s l i g h t l y o f f a x i s by the ruby l a s e r . The dye l a s e r o p t i c a l c a v i t y c o n -s i s t s of a 40%R f r o n t m i r r o r and a 1200 1ines/mm g r a t i n g b l a z e d o at 7500A i n L i t t r o w c o n f i g u r a t i o n . The dye used i s DTTC I o d i d e o d i s s o l v e d i n DMSO which l a s e s between 8100 -830OA. The g r a t i n g o reduces the g a i n bandwidth to 5 -1 OA and the t u n a b i l i t y i s a c c o m p l i s h e d by a d j u s t i n g the g r a t i n g a n g l e by a m i c r o m e t e r . A c o n v e n i e n t w o r k i n g range of dye c o n c e n t r a t i o n i s between 2X.I0T.5M and. .5XT0" 5 M . The dye i s c o n t i n u o u s l y c i r c u l a t e d 23 between the dye c e l l ' a n d a 300 ml r e s e r v o i r . DTTC i n DMSO i s an e f f i c i e n t l a s e r dye as powers of 1 MW a r e r e a d i l y o b t a i n -a b l e ( h i g h e r powers w i l l damage the g r a t i n g ) , but i t s d i s a d v v a n t a g e s a r e extreme i n s t a b i l i t y and r a p i d d e c o m p o s i t i o n when exposed to ruby l i g h t . C. D e t e c t i o n System The p r i n c i p a l component of the d e t e c t i o n system i s a Spex 1800 3/4 meter Czerny Tu rner s p e c t r o m e t e r . The g r a t i n g i s o b l a z e d a t 3500A and used i n the f i r s t o r d e r . D i s p e r s i o n i s o about 1 OA per mm so t h a t a 23 cm l e n g t h of f i l m can sample o about 2300A of the s p e c t r u m . The w a v e l e n g t h s of the a n t i - S t o k e s l i n e s were d e t e r m i n e d by the f o l l o w i n g p r o c e d u r e : 1 . P l a c e a p i e c e of f i l m i n the s p e c t r o m e t e r p l a t e and expose i t to the s p e c t r a l l i n e s of one or more G e i s s l e r tube s o u r c e s (He, A r , Ne) . 2 . I n c r e a s e the s p e c t r o m e t e r s l i t h e i g h t and expose the f i l m to the a n t i - S t o k e s r a d i a t i o n . 3 . Deve lop the n e g a t i v e and measure a l l s p e c t r a l l i n e s w i t h G r a n t c o m p a r a t o r . 4 . C a l c u l a t e the d i s p e r s i o n of the s p e c t r o m e t e r w i t h the a i d of the known w a v e l e n g t h s and measured p o s i t i o n s of G e i s s l e r tube 1 i nes . 5 . D e t e r m i n e a n t i - S t o k e s w a v e l e n g t h s to w i t h i n ± . 5 $ . The p h o t o m u l t i p i i e r used i s an RCA 31034 w i t h a Ga -As p h o t o c a t h o d e h a v i n g a r i s e t i m e < 2 . 5 nsec a t the o p e r a t i n g 24 v o l t a g e of 1500 v o l t s . W r a t t e n n e u t r a l d e n s i t y f i l t e r s a r e used to a t t e n u a t e the Raman e m i s s i o n b e f o r e e n t e r i n g the mono-c h r o m a t o r . The s p e c t r a l r e s p o n s e of the system was d e t e r m i n e d by m e a s u r i n g the b l a c k b o d y e m i s s i o n of a t u n g s t e n lamp and i s shown i n F i g u r e 4 . Two Monsanto d i o d e s w i t h r i s e t i m e s < . 5 nsec a r e used i n the l a t e r e x p e r i m e n t s to m o n i t o r the ruby and dye l a s e r i n t e n -s i t y . The o s c i l l o s c o p e used i s a T e k t r o n i x 7704 w i t h 7A16 and dua l t r a c e 7A24 v e r t i c a l p l u g i n a m p l i f i e r s w i t h r i s e t i m e s of 2 . 4 nsec and 1 . 5 nsec r e s p e c t i v e l y . The t h r e e i n d e p e n d e n t c h a n n e l s a r e used to add the ruby d i o d e s i g n a l and e l e c t r i c a l l y d e l a y e d (100 nsec and 300 nsec ) dye d i o d e and p h o t o m u l t i p i i e r s i g n a l s on a s i n g l e t r a c e of 50 n s e c / d i v i s i o n . D. Other Components The l i q u i d n i t r o g e n c e l l i s a d o u b l e d w a l l e d c o n t a i n e r w i t h the space between the i n n e r and o u t e r w a l l s e v a c u a t e d to i n h i b i t heat t r a n s f e r . Q u a r t z windows w i t h i n d i u m vacuum s e a l s a l l o w the l a s e r beams to e n t e r and Raman e m i s s i o n to e x i t . The 25 mm d i a m e t e r f o c u s s i n g and c o l l i m a t i n g l e n s e s a r e immersed i n the l i q u i d n i t r o g e n , wh ich i n c r e a s e s the e f f e c t i v e f o c a l l e n g t h of the l e n s e s to j u s t over 50 mm. The vacuum system i s c o m p r i s e d of a r o t a r y and d i f f u s i o n pump. Any f r o s t i n g of the windows t h a t may o c c u r can be e f f e c t i v e l y removed by a hot a i r b l a s t f rom a heat gun p r i o r to f i r i n g the l a s e r s . The d i e l e c t r i c m i r r o r used to combine the l a s e r beams i s a f u s e d s i l i c a f l a t wh ich was a n t i r e f l e c t ion c o a t e d f o r 8200% 25 26 and c o a t e d f o r maximum r e f l e c t i o n a t 69.43$ f o r " s " p o l a r i z a -t i o n a t an i n c i d e n c e a n g l e of 45 d e g r e e s . I t s p e r f o r m a n c e c h a r a c t e r i s t i c s a t an i n c i d e n t a n g l e of 49 degrees were 98% r e f l e c t i o n a t 6943$ and around 80% t r a n s m i s s i o n between 8 1 0 0 -8300$ . C h a p t e r 4 EXPERIMENTAL PROCEDURES A. Tunab le Dye High Order A n t i - S t o k e s L i n e s P r e v i o u s work by Menn icke (30) showed t h a t i t was p o s s i b l e to p roduce h i g h o r d e r u l t r a v i o l e t a n t i - S t o k e s e m i s s i o n when l i g h t f rom a h i g h power (250 MW) ruby l a s e r was f o c u s s e d onto a t a r g e t of s o l i d n i t r o g e n . An e x p e r i m e n t , t h e n , was s e t up to ensure t h a t the same e f f e c t o c c u r r e d f o r l i q u i d as w e l l as s o l i d n i t r o g e n . I n d e e d , h i g h o r d e r s o f a n t i - S t o k e s l i g h t were o b s e r v e d even f o r i n c i d e n t ruby l a s e r powers of j u s t a few mega-w a t t s . A d iag ram of the o b s e r v e d ruby l a s e r Raman s p e c t r u m of l i q u i d n i t r o g e n i s shown i n F i g u r e 5 . I n d i c a t e d i n the f i g u r e a r e the s p e c t r a l p o s i t i o n s o f - t h e ; f i r s t Sto kes 1 i ne , the ruby l a s e r l i n e , and seven o r d e r s o f a n t i - S t o k e s l i n e s . The a r rangement shown p r e v i o u s l y i n F i g u r e 3 , e m p l o y i n g c o l l i n e a r ruby and dye l a s e r beams, was used to t e s t the p r e -d i c t i o n s s e t f o r t h i n C h a p t e r 2 . Wi th the ruby l a s e r b l o c k e d o f f i t was found t h a t the dye l a s e r a l o n e had i n s u f f i c i e n t s p e c t r a l power to produce h i g h o r d e r a n t i - S t o k e s r a d i a t i o n . However , when both l a s e r s were employed the h i g h o r d e r a n t i -S t o k e s l i n e s c h a r a c t e r i s t i c o f each l a s e r beam i n c i d e n t on the Raman a c t i v e medium were p r o d u c e d . Seven o r d e r s o f each a n t i -S t o k e s s e r i e s can c l e a r i l y be o b s e r v e d on p h o t o g r a p h i c f i l m , even f o r moderate ruby l a s e r powers . As the dye l a s e r i s ' t u n e d , the dye a n t i - S t o k e s l i n e s a r e 27 8281 6943 5977 5247 4676 4217 3840 3525 3258 °~A 1st Stokes Ruby Anti-Stokes Figure 5. Observed wavelengths of Stokes and ant i -Stokes l ines produced by ruby laser;.-. ro OD 29 tuned a c c o r d i n g l y ; an example of t h i s t u n i n g i s shown i n F i g -ure 6. The f i g u r e c o n s i s t s o f t h r e e s e p a r a t e s i n g l e s h o t s p e c t r a c o v e r i n g the 3 7 0 0 - 4 8 0 0 $ w a v e l e n g t h r a n g e . The b r i g h t l i n e s a r e the t h i r d , f o u r t h and f i f t h o r d e r ruby a n t i - S t o k e s O O 0 l i n e s a t 4676A, 4217A and 3840A. The s e r i e s of l i n e s t h a t a r e d i s p l a c e d s l i g h t l y i n w a v e l e n g t h w i t h each s u c c e s s i v e s h o t a re the f o u r t h , f i f t h and s i x t h o r d e r dye a n t i - S t o k e s . The t h i r d s e t of f a i n t l i n e s i s due to g r a t i n g g h o s t s . The dye l a s e r o o was tuned f rom 8240A i n the top p l a t e to 8120A i n the bottom p l a t e , c a u s i n g the s i x t h dye a n t i - S t o k e s l i n e to s h i f t i n wave-l e n g t h f rom 3832A to 3805A. The s h o r t e s t w a v e l e n g t h dye a n t i -S t o k e s l i n e o b s e r v e d was t h e s e v e n t h o r d e r t u n a b l e i n the 3500$ r a n g e . B. Ruby L a s e r A n t i - S t o k e s L i n e I n t e n s i t i e s A second e x p e r i m e n t was s e t up to q u a n t i t a t i v e l y i n v e s t i -ga te the ruby a n t i - S t o k e s l i n e i n tens i t i es . Of s p e c i a l : . i n t e r e s t are the t h r e s h o l d powers f o r the d i f f e r e n t o r d e r a n t i - S t o k e s e m i s s i o n , t h e i r r e l a t i v e i n t e n s i t i e s , and the dependence of t h e i r i n t e n s i t y upon ruby l a s e r power . The p r e v i o u s e x p e r i m e n t a l a r rangement was m o d i f i e d a c c o r d -i n g l y ; a d i a g r a m of the s e t - u p i s shown i n F i g u r e 7 . F i f t e e n per c e n t of the ruby beam power i s s p l i t o f f by a g l a s s p l a t e and s e n t i n t o an energy meter c o n s i s t i n g of a TRG model 100 t h e r m o p i l e c o u p l e d to a model 102 energy m e t e r . A second g l a s s p l a t e sends a p e r c e n t a g e of the ruby beam t h r o u g h a c o r n i n g g l a s s f i l t e r and s e v e r a l n e u t r a l d e n s i t y f i l t e r s i n t o 4 6 7 6 A 4 2 !7A 3 8 4 0 A o Figure 6. Spectra demonstrating t u n a b i l i t y of high order dye ant i -Stokes l i g h t P H O T O M U L T I P L I E R R U B Y L A S E R E N E R G Y I M E T E R L _ _» D E L A Y L I N E T O O S C I L L O S C O P E gure 7. Experimental arrangement used to measure i n t e n s i t i e s of ruby ant i -Stokes l i n e s . 32 a Monsanto MD2 p h o t o d i o d e i n o r d e r to r e c o r d the p u l s e shape of the l a s e r beam. The power of the i n c i d e n t ruby l a s e r beam can then be m o n i t o r e d f o r each s h o t . The f i l m p l a t e was r e p l a c e d by the monochromator s l i t and p h o t o m u l t i p i i e r . The c o l l i m a t e d f o r w a r d s c a t t e r e d Raman e m i s -s i o n was a t t e n u a t e d by n e u t r a l d e n s i t y f i l t e r s r a n g i n g i n v a l u e to 7 . 0 b e f o r e e n t e r i n g the monochromator s l i t . The ruby d i o d e s i g n a l was d e l a y e d by 100 nsec then added to the p h o t o m u l t i -p l i e r s i g n a l . Both s i g n a l s a re then d i s p l a y e d on a s i n g l e t r a c e and r e c o r d e d by P o l a r o i d 410 f i l m . The f i r s t t h r e e a n t i - S t o k e s l i n e s a t 5 9 7 7 $ , 5247$ and 4676$ were i n v e s t i g a t e d as a f u n c t i o n of i n c i d e n t f o c u s s e d .> ruby power . T y p i c a l l y 3 - 4 l a s e r s h o t s per a n t i - S t o k e s compon-ent were made f o r a g i v e n ruby l a s e r power . The h e i g h t of each p h o t o m u l t i p ! i e r t r a c e - - t a k e n to be p r o p o r t i o n a l to the i n t e n s i t y s i n c e the shapes were s i m i l a r - - was measured and c o r r e c t e d a c c o r d i n g to the s p e c t r a l r e s p o n s e c u r v e shown i n F i g u r e 8 . Most of the d a t a p o i n t s have e r r o r bars wh ich i n d i -c a t e the spread i n the d a t a about the ave rage v a l u e . Those p o i n t s w i t h o u t e r r o r bars r e p r e s e n t s i n g l e s h o t r e s u l t s . C. Dye A n t i - S t o k e s L i n e I n t e n s i t i e s In the t h i r d e x p e r i m e n t the r e l a t i v e i n t e n s i t i e s of the dye a n t i - S t o k e s components were measured as a f u n c t i o n of both ruby and dye l a s e r i n c i d e n t power . The e x p e r i m e n t a l a r rangement was r e f i n e d and m o d i f i e d as i l l u s t r a t e d i n F i g u r e 9 . The l i n e w i d t h of the dye l a s e r was 33 T 1 1 r - i 1 1 i t 4 8 12 16 RUBY LASER POWER (MW) Figure 8. Intensity of f i r s t three ruby ant i -S tokes l i nes as a funct ion of ruby laser power. 35 improved to l e s s than 5 angst roms w i t h the a d d i t i o n of i r i s d iaphragms i n the l a s e r c a v i t y . The s p a t i a l q u a l i t y of the ruby beam was somewhat improved by the i n s e r t i o n of an i r i s d iaphragm i n i t s path a f t e r the beam has t r a v e r s e d the dye c e l l . A v a r i a b l e c o n c e n t r a t i o n copper s u l p h a t e c e l l was used to a t t e n u a t e the ruby beam w h i l e n e u t r a l d e n s i t y f i l t e r s were used to reduce the i n t e n s i t y of the dye beam. L a s e r powers ranged f rom . 5 MW to 5 MW f o r the ruby l a s e r and .1 MW to 1 MW f o r the dye l a s e r . A c a l i b r a t e d p h o t o d i o d e was used to m o n i t o r the dye l a s e r power . The energy of the dye l a s e r was found to be propor-.", , t i o n a l to the a r e a under the c u r v e of the d i o d e p u l s e , as : . shown i n F i g u r e 1 0 , and the power was found by m e a s u r i n g the FWMH of the p u l s e and d i v i d i n g the energy by t h i s v a l u e . C o n -s i s t e n t r e s u l t s , however , were not a t t a i n e d when a s i m i l a r c a l i b r a t i o n p r o c e s s was a t t e m p t e d i n the case of the ruby l a s e r . Two f a c t o r s c o n t r i b u t e d to the i n c o n s i s t e n t r e s u l t s : i r r e p r o d u c a b l e tempora l p u l s e shape due to i n t e r f e r e n c e of d i f f e r e n t l a s e r modes, and s p a t i a l v a r i a t i o n s i n the p u l s e i n t e n s i t y caused by n o n u n i f o r m a b s o r p t i o n i n the dye c e l l . The s p a t i a l v a r i a t i o n s were ave raged out by i n s e r t i n g a l e n s i n f r o n t of the ruby d i o d e wh ich reduced the beam d i a m e t e r to t h a t of the p h o t o s e n s i t i v e s u r f a c e . The energy meter was employed i n c o n j u n c t i o n w i t h the p h o t o d i o d e i n o r d e r to g i v e s e p a r a t e v a l u e s f o r the energy and FWHM of the ruby l a s e r p u l s e . o o A 6943A i s o l a t i o n f i l t e r was p l a c e d i n f r o n t of the d i o d e i n o r d e r to b l o c k out r e f l e c t e d backward s t i m u l a t e d S t o k e s 36 DYE LASER ENERGY (mJ) Figure 10. Dye laser photodiode pu lse area v . s . energy cal i -brat ion curve. 37 s c a t t e r i n g a t 8 2 8 0 $ . The s i g n a l s f rom the l a s e r d i o d e s and p h o t o m u l t i p i i e r . were e l e c t r i c a l l y d e l a y e d then s e n t i n t o s e p a r a t e o s c i l l o -scope c h a n n e l s and added i n o r d e r to be d i s p l a y e d on a s i n g l e o s c i l l o g r a m . A p o l a r o i d f i l m of a t y p i c a l o s c i l l o g r a m i s shown i n F i g u r e 1 1 . The t i m e s c a l e i s 50 n s e c / d i v i s i o n w h i l e the t r a c e s , f rom l e f t to r i g h t , a r e the s i g n a l s f rom the dye d i o d e , ruby d i o d e and p h o t o m u l t i p i i e r . The h e i g h t and base of each t r a c e a r e measured to the n e a r e s t .1 cm; f rom t h e s e v a l u e s the a r e a of each t r a c e i s computed u s i n g the t r i a n g l e a p p r o x i m a t i o n t h a t A = %BH. The p u l s e w i d t h s of the ruby and dye l a s e r m o n i t o r s i g n a l s a r e measured to w i t h i n 2 n s e c . The ruby l a s e r power i s then known as i t s energy was m o n i t o r e d per shot w h i l e the power of the dye l a s e r must be computed w i t h r e f e r e n c e to i t s c a l i b r a t i o n c u r v e . The a r e a s of the p h o t o -m u l t i p l i e r s i g n a l s a re then c o r r e c t e d i n a c c o r d a n c e w i t h the monochromator s p e c t r a l r e s p o n s e c u r v e . In the f i r s t p a r t of t h i s e x p e r i m e n t t h e . r u b y l a s e r power was v a r i e d w h i l e the dye l a s e r was kept a t a c o n s t a n t 1 MW a t 8 2 2 0 $ . The r e l a t i v e i n t e n s i t i e s of the f i r s t ruby a n t i - S t o k e s at 5977$ and the f i r s t two dye a n t i - S t o k e s components a t 6901$ and 5945$ r e s p e c t i v e l y were i n v e s t i g a t e d . The r e s u l t s a r e shown i n the s e m i - l o g graph of F i g u r e 12 and i n the l i n e a r graph of F i g u r e 1 3 . Each p o i n t r e p r e s e n t s a s i n g l e l a s e r s h o t . The ruby a n t i - S t o k e s c u r v e was i n c l u d e d to compared i t s shape and t h r e s h o l d w i t h t h o s e of the dye l i n e s . Some c a u t i o n , how-e v e r , must be e x e r c i s e d i n compar ing r e l a t i v e i n t e n s i t i e s . 38 Figure 11. Typical osc i l lograms of photodiode and photo-m u l t i p l i e r s i g n a l s . 39 Figure 12. Ant i -Stokes in tens i t y v . s . ruby constant 1 MW dye laser power. laser power at a 40 Figure 13. Above threshold i n t e n s i t i e s of ruby and dye a n t i -Stokes l i n e s v . s . ruby laser power. 41 The f i r s t o r d e r ruby a n t i - S t o k e s l i n e has a v e r y nar row wave -l e n g t h d i s t r i b u t i o n w h i l e the dye a n t i - S t o k e s l i n e s e x t e n d over a few a n g s t r o m s . S e c o n d l y , the s p a t i a l d i s t r i b u t i o n s of the ruby and dye Raman e m i s s i o n a t the monochromator s l i t a r e not l i k e l y to be the same. In the f i n a l i n v e s t i g a t i o n the . i n t e n s i t i e s of the f i r s t o o t h r e e o r d e r s of dye a n t i - S t o k e s e m i s s i o n a t 6901A, 5945A and 5222$ were o b t a i n e d a s . a f u n c t i o n of dye l a s e r power a t a c o n s t a n t ruby l a s e r power . The p o i n t s f o r wh ich the ruby l a s e r power was equal to 3 . 7 ± .7 MW a r e graphed i n F i g u r e 1 4 . gure 14. Dye ant i -S tokes in tens i t y v.s. dye laser power at a constant 3 . 7 ± . 7 MW ruby laser power. C h a p t e r 5 DISCUSSION AND CONCLUSIONS A. D i s c u s s i o n of R e s u l t s The two s i g n i f i c a n t r e s u l t s o f t h e s e e x p e r i m e n t s a r e the o b s e r v a t i o n of t u n a b l e u l t r a v i o l e t a n t i - S t o k e s r a d i a t i o n and the measurements of the r e l a t i v e i n t e n s i t i e s o f the f i r s t few o r d e r s o f ruby and dye a n t i - S t o k e s e m i s s i o n . The f i r s t r e s u l t i s summarized s u c c i n c t l y i n the p h o t o g r a p h s shown i n F i g u r e 6 ; the u n d e r l y i n g p r i n c i p l e of t h i s p r o c e s s may l e a d to the deve lopment of a p r a c t i c a l l i g h t s o u r c e . The r e s u l t s o f the i n t e n s i t y m e a s u r e m e n t s , however , r e q u i r e f u r t h e r d i s c u s s i o n . As the graph of F i g u r e 8 i n d i c a t e s , the ruby a n t i - S t o k e s e m i s s i o n c l e a r l y e x h i b i t s a t h r e s h o l d b e h a v i o u r . The i n t e n s i t y of each of the f i r s t t h r e e o r d e r s o f a n t i - S t o k e s r a d i a t i o n r i s e s by s e v e r a l o r d e r s o f magn i tude as the ruby l a s e r power i s i n c r e a s e d from j u s t below ;5 MW to about 1 MW. Such b e h a v i o u r i s c h a r a c t e r i s t i c of n o n l i n e a r a m p l i f i c a t i o n systems and i s e v i d e n c e t h a t the p r o c e s s i n v o l v e d i s indeed s t i m u l a t e d Raman s c a t t e r i n g . The a n t i - S t o k e s c o n v e r s i o n e f f i c i e n c y , t h a t i s , the r a t i o the ( n + l ) t h o r d e r a n t i - S t o k e s i n t e n s i t y to the n t h ' . o r d e r a n t i - S t o k e s i n t e n s i t y X 1 0 0 , n = 1 , 2 , 3 i s a p p r o x i -m a t e l y c o n s t a n t f o r moderate ruby l a s e r powers and i s i n the 15%-20% r a n g e . I t i s e x p e c t e d t h a t measurements of even h i g h e r o r d e r components w i l l g i v e s i m i l a r r e s u l t s . These o b s e r v a t i o n s , u s i n g o n l y the ruby l a s e r , were made b a s i c a l l y f o r c o m p a r i s o n 43 • 44 purposes w i t h the two l a s e r p r o c e s s . Employ ing both the dye l a s e r and ruby l a s e r as r a d i a t i o n s o u r c e s produced the r e s u l t s graphed i n F i g u r e s 1 2 - 1 4 . I t i s c l e a r i n F i g u r e 12 t h a t the dependence of the dye a n t i - S t o k e s l i n e s on ruby l a s e r power i s r e m a r k a b l y s i m i l a r to t h a t of the ruby a n t i - S t o k e s components . Both s p e c i e s of a n t i - S t o k e s l i n e s e x h i b i t the same ruby l a s e r power t h r e s h o l d b e h a v i o u r . F i g u r e 13 i n d i c a t e s t h a t once over t h r e s h o l d the i n t e n s i t i e s of the f i r s t o r d e r ruby and dye a n t i - S t o k e s l i n e s i n c r e a s e a p p r o x i m a t e l y l i n e a r l y w i t h i n c r e a s i n g ruby power . However , the r a t i o of the second to f i r s t o r d e r dye l i n e s i s h i g h e r than t h a t of the c o r r e s p o n d i n g ruby a n t i - S t o k e s l i n e s - -about . 5 0 as compared to . 2 0 . In the graph of F i g u r e 14 - - D y e a n t i - S t o k e s i n t e n s i t y v e r s u s dye l a s e r power a t a c o n s t a n t ruby l a s e r power - - i t i s seen t h a t each o r d e r of dye e m i s s i o n r i s e s l i n e a r l y w i t h i n c r e a s i n g dye l a s e r power up to about .6 MW. At t h i s p o i n t the i n t e n s i t i e s beg in to l e v e l out and then d e c r e a s e as the dye l a s e r power i s f u r t h e r i n c r e a s e d to 1 MW. T h i s e f f e c t i s i n f a c t p r e d i c t e d by e q u a t i o n 27 of Chap -t e r 2 . The p h y s i c a l i n t e r p r e t a t i o n i s the f o l l o w i n g . Only a f r a c t i o n of the ruby photons a re c o n v e r t e d i n t o S t o k e s pho -tons i n the p a r t i c u l a r d i r e c t i o n n e c e s s a r y to f u l f i l l the momentum m a t c h i n g c o n d i t i o n s f o r dye a n t i - S t o k e s g e n e r a t i o n . One of t h e s e S t o k e s photons i s n e c e s s a r y to u p c o n v e r t each dye p h o t o n . In the graph of F i g u r e 1 4 , the ruby l a s e r power - -and thus the i n t e n s i t y of the ruby S t o k e s waves of i n t e r e s t - -45 i s c o n s t a n t . And s o , as the dye l a s e r power i s i n c r e a s e d , a p o i n t w i l l be reached - - a p p a r e n t l y a t .6 MW dye l a s e r power - -a t wh ich the maximum c o n v e r s i o n of dye l a s e r photons i n t o dye a n t i - S t o k e s photons i s a c h i e v e d . However , the S t o k e s p r o c e s s of d o w n c o n v e r t i n g the dye a n t i - S t o k e s wave i n t o the dye l a s e r wave w i l l c o n t i n u e to i n c r e a s e as the dye l a s e r power i s i n c r e a s e d . T h u s , once t h i s p o i n t i s r e a c h e d , s i n c e the g a i n i s c o n s t a n t and l o s s e s i n c r e a s e w i t h dye l a s e r power , the dye a n t i - S t o k e s i n t e n s i t y wi11 d e c r e a s e as the dye l a s e r power i s f u r t h e r i n c r e a s e d . I t i s e x p e c t e d , however , upon i n c r e a s i n g the ruby l a s e r power , t h a t t h e . s a t u r a t i o n e v i n c e d i n F i g u r e 14 w i l l t a k e p l a c e a t a g r e a t e r dye l a s e r power and e v e n t u a l l y d i s a p p e a r f o r a s u f f i c i e n t l y p o w e r f u l ruby l a s e r pump. There i s f u r t h e r e v i d e n c e t h a t the mechanism f o r the g e n e r a t i o n of dye a n t i - S t o k e s r a d i a t i o n i s i n f a c t the p r o c e s s d e s c r i b e d i n t h i s t h e s i s . That i s , the ruby l a s e r f i e l d - - i f i n t e n s e enough - - a c t s as the pump f o r the p r o c e s s , p r o d u c i n g s y n c h r o n i z e d m o l e c u l a r v i b r a t i o n s which i n t u r n i n e l a s t i c a l l y s c a t t e r the dye l a s e r beam. Thus no dye power t h r e s h o l d must be s u r p a s s e d f o r g e n e r a t i o n of dye a n t i - S t o k e s r a d i a t i o n . T h i s l a c k of t h r e s h o l d i s c l e a r l y shown i n the graph of F i g u r e 14 i n which the i n t e n s i t y of the dye a n t i - S t o k e s e m i s s i o n i s a p -p r e c i a b l e f o r dye l a s e r powers as low as .1 MW, f a r below the power t h r e s h o l d f o r ruby l a s e r s t i m u l a t e d Raman s c a t t e r i n g . B. C o n c l u s i o n s In c o n c l u s i o n , t u n a b l e a n t i - S t o k e s r a d i a t i o n has been 46 produced and s e v e r a l of i t s c h a r a c t e r i s t i c s have been i n v e s t i -g a t e d . The dye a n t i - S t o k e s components were found to l a c k a dye l a s e r power t h r e s h o l d but to p o s s e s s a ruby l a e r power t h r e s h o l d s i m i l a r to t h a t of the ruby a n t i - S t o k e s l i n e s . Such b e h a v i o u r c l e a r l y s u p p o r t s the v iew t h a t the mechanism r e s p o n -s i b l e f o r the dye a n t i - S t o k e s e m i s s i o n i s a two s t e p p r o c e s s i n wh ich o n l y the ruby l a s e r i s above the power t h r e s h o l d n e c e s s a r y f o r s t i m u l a t e d Raman s c a t t e r i n g . Tunab le w a v e l e n g t h s as low as 3500$ have been o b s e r v e d but even more e n e r g e t i c r a d i a t i o n i s e x p e c t e d f o r h i g h e r i n c i d e n t ruby l a s e r power . The powers n e c e s s a r y f o r g e n e r a t i o n o f t u n a b l e dye a n t i - S t o k e s e m i s s i o n a re s u r p r i s i n g l y low w h i l e c o n v e r s i o n e f f i c i e n c i e s a re r e l a t i v e l y h i g h . Such p r o m i s i n g c h a r a c t e r i s t i c s s u g g e s t the p o s s i b l e deve lopment o f a p r a c t i c a l s p e c t r o s c o p i c d e v i c e which w i l l be d i s c u s s e d i n the f o l l o w i n g s e c t i o n . C. F e a s i b i l i t y S tudy o f a Tunab le U l t r a v i o l e t L a s e r T h i s f i n a l s e c t i o n d e a l s w i t h some a s p e c t s of d e s i g n i n g an e x p e r i m e n t to t e s t the f e a s i b i l i t y of p r o d u c i n g a p r a c t i c a l Tunab le U l t r a v i o l e t A n t i - S t o k e s Raman (TUASR) l a s e r . The a r rangement used i n the p r e l i m i n a r y e x p e r i m e n t s does i n f a c t produce t u n a b l e u l t r a v i o l e t r a d i a t i o n . However , i t s uv power o u t p u t i s s m a l l ; assuming a 20% c o n v e r s i o n e f f i c i e n c y , a q u i c k c a l c u l a t i o n shows t h a t a p p r o x i m a t e l y 12 w a t t s of p u l s e d 3500$ r a d i a t i o n c o u l d be produced per 1 MW of dye l a s e r power . A p a r t i c u l a r means o f i m p r o v i n g upon t h i s power o u t p u t w i l l be d i s c u s s e d b e l o w . 47 -The most o b v i o u s r e a s o n beh ind the low uv power y i e l d s of the p r e l i m i n a r y e x p e r i m e n t i s the f a c t t h a t i n f r a r e d l i g h t must be u p c o n v e r t e d by a t l e a s t a s i x s t e p p r o c e s s a t v :'• r e l a t i v e l y low e f f i c i e n c y per s t e p . T h i s c o n v e r s i o n l o s s can be m i n i m i z e d ' by u s i n g a t u n a b l e v i o l e t dye l a s e r pumped by f r e q u e n c y d o u b l e d ruby r a d i a t i o n . The e x p e r i m e n t a l s e t - u p shown i n F i g u r e 15 employs', such an a r r a n g e m e n t . The ruby l a s e r r a d i a t i o n i s i m m e d i a t e l y s p l i t i n t o two beams. One i s u t i l i z e d to produce the f r e q u e n c y d o u b l e d beam which pumps the dye l a s e r , w h i l e the o t h e r i s used as b e f o r e to c r e a t e m o l e c u l a r o s c i l l a t i o n s i n the l i q u i d n i t r o g e n . T h i s system not o n l y a l l o w s f o r i n d e p e n d e n t o p t i m i z a t i o n o f ' b o t h ruby and dye l a s e r beams but a l s o f a c i l i t a t e s e x p e r i m e n t s wh ich i n v o l v e k e e p i n g one beam c o n s t a n t w h i l e v a r y i n g the o t h e r . An o p t i c a l m u l t i c h a n n e l a n a l y z e r (OMA) would be an i d e a l d e t e c t i o n system s i n c e s e v e r a l a n t i - S t o k e s l i n e s c o u l d be s i m u l t a n e o u s l y o b s e r v e d . An a l t e r n a t i v e to the e x p e n s i v e OMA i s a monochromator c o u p l e d to an u l t r a v i o l e t s e n s i t i v e p h o t o -m u l t i p l i e r or p h o t o d i o d e . The r e m a i n i n g components shown i n F i g u r e 16 a r e s i m i l a r to those d e s c r i b e d i n C h a p t e r 2 . A n y . e v a l u a t i o n of the m e r i t s of a l a s e r s h o u l d i n c l u d e a measurement of i t s o u t p u t power . In t h i s TUASR l a s e r many o r d e r s of a n t i - S t o k e s l i n e s a r e e m i t t e d a l o n g w i t h S t o k e s r a d i a t i o n and r e s i d u a l i n c i d e n t l a s e r l i g h t . T h i s m u l t i p l i -c i t y of l i n e r a d i a t i o n c o m p l i c a t e s the energy measurments of the d e s i r e d new f r e q u e n c i e s . However , the n e c e s s a r y d i s c r i m i -n a t i o n can be a c h i e v e d by p l a c i n g i n the path of the o u t p u t RUBY LASER BEAM SPUTTER — A CRYSTAL 0 VIOLET DYE LASER DIODE CuSQ4 CELL •P 7 TO OMA OR E-METER ti Figure 15. Poss ib le arrangment for tunable U l t r a v i o l e t ant i -Stokes Raman laser experiment. 49 beam s e l e c t e d C o r n i n g or S c h o t t g l a s s f i l t e r s or c o m b i n a t i o n s of such f i l t e r s . Two v e r y c l o s e l i n e s of moderate power c o u l d then be s e p a r a t e d by p l a c i n g a narrow bandwidth i s o l a t i o n f i l -t e r beh ind the g l a s s f i l t e r s . A s e n s i t i v e t h e r m o p i l e energy meter c o u l d then be used to r e c o r d the energy o f a g i v e n a n t i -S t o k e s component . An e s t i m a t e of the o u t p u t power of a p a r t i c u l a r u l t r a -v i o l e t t u n a b l e l i n e can be made by assuming t h a t the as y e t unknown c o n v e r s i o n e f f i c i e n c i e s w i l l be a p p r o x i m a t e l y the same as t h o s e found i n the p r e v i o u s e x p e r i m e n t . The s t a r t i n g p o i n t of t h i s e s t i m a t e i s a t y p i c a l l a b o r a t o r y o s c i l l a t o r - a m p l i f i e r ruby l a s e r c a p a b l e of 200 MW power . F i f t y MW i s d i v e r t e d by a beam s p l i t t e r and used to e x c i t e the m o l e c u l a r v i b r a t i o n s . The r e m a i n i n g 150 MW i s c o n v e r t e d to f r e q u e n c y d o u b l e d 3 4 7 1 . 5 $ l i g h t a t 10% e f f i c i e n c y , g i v i n g 15 MW of d o u b l e d ruby l a s e r l i g h t . Assuming a dye l a s e r e f f i c i e n c y of 10% y i e l d s 1 . 5 MW o of t u n a b l e dye l a s e r r a d i a t i o n a t w a v e l e n g t h s around 4000A depend ing upon the dye u s e d . S u p p o s i n g a 20% Raman a n t i -S t o k e s c o n v e r s i o n e f f i c i e n c y y i e l d s 300 KW of 3660$ l i g h t , 60 KW of 3370$ l i g h t , and 12 KW of 3130$ l i g h t . N a t u r a l l y , e x p e r i m e n t s have a tendency to.'dampen such n u m e r i c a l e n t h u -s i a m ; N e v e r t h e l e s s , i t i s c o n c e i v a b l e t h a t t h i s scheme c o u l d improve the power o u t p u t by a f a c t o r of more than 1 , 0 0 0 . A n o t h e r method of o b t a i n i n g t u n a b l e u l t r a v i o l e t l a s e r r a d i a t i o n i s to f r e q u e n c y d o u b l e the o u t p u t > o f a c o m m e r c i a l l y a v a i l a b l e f l a s h l a m p pumped t u n a b l e v i s i b l e dye l a s e r . The c o s t of t h i s s y s t e m , however , i s c u r r e n t l y about f o u r t i m e s 50 the p r o j e c t e d $3000 c o s t of a TUASR system e x c l u s i v e of the ruby and dye l a s e r s . M o r e o v e r , the TUASR l a s e r d e s c r i b e d above would have c e r t a i n a d v a n t a g e s f o r some a p p l i c a t i o n s over the p r e s e n t l y a v a i l a b l e u n i t s such as s h o r t tempora l p u l s e w i d t h and m u l t i p l e l i n e r a d i a t i o n . BIBLIOGRAPHY 1 . Woodbury, E . J . and W.K. Ng, P r o c . IRE 5JD, 2347 ( 1 962 ) . 2 . E c k h a r d t ? G . , R.W. H e l l w a r t h , F . J . M c C l u n g , S . E . S c h w a r z , D. W e i n t e r and E . J . Woodbury, P h y s . Rev . L e t t e r s 9_, 455 ( 1 9 6 2 ) . 3 . S t o i c h e f f , B . P . , P h y s . L e t t e r s ]_, 1 86 (1 9 6 3 ) . 4 . M i n c k , R . W . , R.W. Terhune and W.G. Rado , A p p l . P h y s . L e t t e r s 3, 181 ( 1 9 6 3 ) . 5 . H e l l w a r t h , R . W . , P h y s . Rev . 1_3_0 , 1 850 (1 9 6 3 ) . 6 . Z e i g e r , R . J . , P . E . T a n n e n w a l d , S . Kern and R. H e r e n d e e n , P h y s . Rev. L e t t e r s ' 1J_, 41 9 (1 9 6 3 ) . 7 . B l o e m b e r g e n , N. and Y . R . S h e n , P h y s . Rev . L e t t e r s 1 2 , 504 ( 1 9 6 4 ) . 8 . W e i n e r , D . , S . E . Schwarz and F . J . M c C l u n g , J o u r n a l of Appl . P h y s . 36 , 2395 ( 1 965) . 9 . Wang, C C , P h y s . Rev . L e t t e r s 1_6, 344 (1 9 6 6 ) . 10 . G i o r d m a i n e , J . A . and W. K a i s e r , P h y s . Rev . 1_44_, 676 (1 9 6 6 ) . 1 1 . M a i e r , M. and W. K a i s e r , P h y s . L e t t e r s 2J_, 529 ( 1 9 6 6 ) . 1 2 . G a r m i r e , E . , R .Y . Ch iao and C . H . Townes, P h y s . Rev . L e t t e r s 1_6 , 347 (1 9 6 6 ) . 1 3 . C h i a o , R. and B . P . S t o i c h e f f , P h y s . Rev . L e t t e r s VZ, 290 ( 1 9 6 4 ) . 14 . M a i e r , M . , W;. K a i s e r . a n d J . A . G i o r d m a i n e , P h y s . Rev . 1 7 7 , 580 ( 1 9 6 9 ) . 1 5 . G r u n , J . B . , A . K . M c Q u i l l a n and B . P . S t o i c h e f f , P h y s . Rev . 1 8 0 , 61 ( 1 9 6 9 ) . 1 6 . B a r a k , S . and S . Y a t s i v , P h y s . Rev . 3A, 382 ( 1 9 7 1 ) . 1 7 . C a r l s t e n , J . L . and T . J . M c I T r a t h , J . P h y s . B: Atom. M o l e c . P h y s . 6 , L80 ( 1 9 7 3 ) . 1 8 . B e g l e y , R . F . , A . B . Harvey and R . L . B y e r , A p p l . P h y s . L e t -t e r s 2J5 , 387 ( 1 974 ) . 51 1 9 . C a r l s t e n , J . L . and P . C . Dunn, O p t i c s Communicat ions 1 4 , 8 (1 975). . ~ 2 0 . M e n n i c k e , H . , P h y s . L e t t e r s 36A> 1 27 ( 1 971 ) . 2 1 . S t a n s f i e l d , B . , P h . D . T h e s i s , U n i v e r s i t y of B r i t i s h C o l u m b i a ( 1 9 7 1 ) . 2 2 . M e n n i c k e , H . , J . Meyer and T. S i n n o t t , P h y s . L e t t e r s 57A, 477 (1 9 7 6 1 . 2 3 . G a r m i r e , E . , F. P a n d a r e s e and C . H . Townes, P h y s . Rev . 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