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Subpicosecond structure in mode-locked Nd: Glass lasers
Volume 30A, number 2
PHYSICS
found for o s c i l l a t i o n s f r o m the p r e d i c t e d " s u p e r e g g " e l e c t r o n s u r f a c e ; this m a y be b e c a u s e of high c y c l o t r o n m a s s and b e c a u s e of the r e l a t i v e l a c k of a n i s o t r o p y of this s u r f a c e , which m a k e s t he t o r q u e m e t h o d i n s e n s i t i v e . It is hoped that e x p e r i m e n t s at l o w e r t e m p e r a t u r e s u s in g the f i e l d m o d u l a t i o n m e t h o d m a y r e v e a l the m i s s i n g oscillations. I w i s h to thank Dr. D. S h o e n b e r g and Dr. J. Van-
SUBPICOSECOND
STRUCTURE
22 September 1969
LETTERS
d er k o o y f o r v a l u a b l e d i s c u s s i o n , Dr. G. J o h a n s e n and P r o f e s s o r A. R. Mackintosh f o r i n f o r m i n g m e of t h e i r c a l c u l a t i o n s b e f o r e publication, and Dr. P. H. Schmidt f o r a d v i c e on the p u r i f i c a t i o n of barium.
References 1. J. Vanderkooy, J. Phys. (E) 2 (1969) 719. 2. T. E. Brown and K. A. MeEwen, to be published. 3. G. Johansen, Solid State Commun. 7 (1969) 731.
IN M O D E - L O C K E D
Nd:GLASS
LASERS
D. J. BRADLEY, G . H . C . NEW and S. J. CAUGHEY Department of Pure and Applied Physics, The Queen's Unive~'sity of Belfast Belfast BT7 INN, Northern Ireland, UK Received 14 August 1969
Sub-picosecond structure (s x 10 -13 sec), with a photographic peak contrast of ~ 2.6, has been recorded in the two photon fluorescence patterns produced by mode-locked Nd:glass lasers. Simultaneous third harmonic measurements do not indicate any systematic variation of the fundamental pulse durations.
O b s e r v a t i o n s of s u b p i c o s e c o n d s t r u c t u r e in N d : g l a s s l a s e r r e l a x a t i o n o s c i l l a t i o n s [1] and in the m o d e - l o c k e d N d : g l a s s l a s e r [2] have r e c e n t l y b e e n r e p o r t e d . In both c a s e s photon f l u o r e s c e n c e m e a s u r e m e n t s [3] w e r e e m p lo y e d , but the e x p e r i m e n t a l a r r a n g e m e n t s w e r e quite different. The p u l s e s t r u c t u r e of the r e l a x a t i o n o s c i l l a t i o n s was r e c o r d e d by i n s e r t i n g a 2 c m long c e l l containing a Rho d am i n e 6G solution i n s i d e the l a s e r cavity and p h o t o g r a p h i n g the two photon f l u o r e s c e n c e t r a c k s . S u b p i c o s e c o n d s t r u c t u r e in the m o d e l o c k e d l a s e r of ref. 2 was o b s e r v e d by e m p l o y in g a v e r y thin (28bL) c e l l outside the c a v it y , with p h o t o m u l t i p l i e r d e t e c t i o n of the f l u o r e s c e n c e . T h i s technique has the d r a w b a c k that plotting of a c o m p l e t e two photon f l u o r e s c e n c e pulse p r o f i l e r e q u i r e s s o m e 20 shots of the l a s e r . T h e s e phot o e l e c t r i c m e a s u r e m e n t s e s s e n t i a l l y showed a v e r y n a r r o w spike, of a f r a c t i o n of a p i c o s e c o n d in width and of peak c o n t r a s t 3, on top of a b r o a d b a s e of s e v e r a l p i c o s e c o n d s duration. We wish to r e p o r t the o b s e r v a t i o n of two photon f l u o r e s c e n c e p r o f i l e s of s i m i l a r c h a r a c t e r f r o m a m o d e l o c k e d l a s e r , p h o t o g r a p h e d in a 2 c m f l u o r e s c e n c e dye c e l l outside the cavity. The m o d e - l o c k e d l a s e r c o n s i s t e d of a 6" × ~" 78
B r e w s t e r angled N d : g l a s s r o d (Owens-Illinois) in a one m e t r e cavity. Q - s w i t c h i n g and m o d e locking was a c h i e v e d by i m m e r s i n g a 70% r e f l e c t i v i t y d i e l e c t r i c 1 ° wedged m i r r o r in a 1 m m l a y e r of Kodak 9860 dye. The output b e a m through this m i r r o r was em p l o y ed , with b e a m s p l i t t e r s , f o r s i m u l t a n e o u s r e c o r d i n g of (i) the two photon f l u o r e s c e n c e p l u se p r o f i l e using a t r i a n g u l a r conf i g u r a t i o n [2], (ii) the l a s e r output s p e c t r u m with a 3.4 m e t r e e c h e l l e s p e c t r o g r a p h and (iii) the int e n s i t y of the m o d e - l o c k e d pulse t r a i n with a b i p l a n a r photodiode and T e k t r o n i x 519 o s c i l l o s c o p e . The l a s e r b e a m f r o m the second 60% r e f l e c t i v i t y c a v i t y m i r r o r was e m p l o y e d for t h i r d h a r m o n i c g e n e r a t i o n f r o m a fused q u a r t z disc, also s i m u l t an eo u sl y with the other t h r e e m e a s u r e m e n t s . Fig. 1 shows a m i c r o d e n s i t o m e t e r t r a c e of a two photon f l u o r e s c e n c e t r a c k (photographed on I l f o r d F P 4 f i l m , with 75 m m lens at f / 8 ) , t y p i cal of s e v e r a l hundreds r e c o r d e d . The step on the r i g h t - h a n d side g i v e s a m e a s u r e of peak c o n t r a s t , si n ce it r e s u l t s f r o m c o v e r i n g m o s t of the c a m e r a f i e l d with a n e u t r a l density f i l t e r , which p r o duced an i n t en si t y r e d u c t i o n of 2.6 at the f l u o r e s c e n c e wavelength. The peak two-photon f l u o r e s c e n c e c o n t r a s t is t h e r e f o r e s e e n to be about 2.5,
Volume 30A, number 2
PHYSICS LETTERS
22 September 1969
!
Fig. 1. Microdensitometer trace of two-photon-fluorescence display outside cavity of mode-locked Nd:glass l a s e r . (Ordinate: arbitrary linear density scale. Intensity step of × 2.6 on right-hand-side. ) c o m p a r e d with the t h e o r e t i c a l l y e x p e c t e d v a l u e of 3 [4,5]. H o w e v e r the c e n t r a l spike is v e r y s h a r p , being only 50/z wide, and this width is a p p r o a c h i ng the l i m i t of the s p a t i a l r e s o l u t i o n of the c a m e r a . The width of the fine spike v a r i e d f r o m shot to shot within the r a n g e of 0.2 p s e c to 1 p s e c but it was always c o r r e l a t e d with the s i m u l t a n e o u s l y r e c o r d e d s p e c t r a l widths of 150 /~ to 30 A. L i k e w i s e the s h o u l d e r of e a c h t r a c e o c c u r s at a cont r a s t of ~ 1.9. As a l r e a d y pointed out [I,2], the shape of a two photon f l u o r e s c e n c e t r a c k does not uniquely define the duration of the l a s e r puls e s , n o r d e t e r m i n e w h e t h e r the fine s t r u c t u r e o r i g i n a t e s f r o m i s o l a t e d s u b p i c o s e c o n d p u l s e s or f r o m n o i s e fluctuations. S i m u l t a n e o u s o s c i l l o g r a m s of the fundamental p u l s e t r a i n and the c o r r e s p o n d i n g t h i r d h a r m o n i c output a r e shown in fig. 2. The t h i r d h a r m o n i c p u l s e t r a i n e n v e l o p e , which is a m o r e s e n s i t i v e i n d i c a t o r of v a r i a t i o n s in f u n d a m e n t a l p u l s e d u r a ti ons than is the c o r r e s p o n d i n g s e c o n d h a r m o n i c e n v e l o p e [6], u s u al l y c l o s e l y followed that of the f unda m en t al p u l s e t r a i n indicating that t h e r e was no s y s t e m a t i c v a r i a t i o n of p u l s e d u r a t i o n t h r o u g h out the t r a i n , in c o n t r a s t to e a r l i e r r e p o r t s [7]. H o w e v e r o c c a s i o n a l r a n d o m f l u c t u a t i o n s w e r e obs e r v e d , as in fig. 2. The to ta l amount of t h i r d
Fig. 2. Simultaneous oscillograms of the fundamental mode-locked laser pulse train, and the corresponding third harmonic output. Horizontal scale 20 nsec cm -~. h a r m o n i c light was c o n s i s t e n t with a t r a i n of puls e s , each of d u r a t i o n 10 p s e c s . The two photon f l u o r e s c e n c e m e a s u r e m e n t s w e r e r e p e a t e d with an A m e r i c a n Optical r o d of the s a m e d i m e n s i o n s and e n t i r e l y s i m i l a r r e s u l t s w e r e obtained. We would like to thank Mr. R. J. F r a m e for making the m i c r o d e n s i t o m e t e r t r a c e s and f o r s u g g e s t i o n s on photography. The s p e c i a l l a s e r m i r r o r s a r e m a n u f a c t u r e d by Mr. D. Blanc.
References 1.. D. J. Bradley, G.H.C. Hew, B. Sutherland and S.J. Caughey, Phys. Letters 28A (1969) 532. 2. S. L. Shapiro and M. A. Duguay, Phys. Letters 28A (1969) 69S. 3. J. A. Giordmaine, P.M. Rentzepis, S. L. Shapiro and K.W.Weeht, Appl. Phys. Letters 11 (1967) 216. 4. J.R. Klauder, M.A. Duguay, J. A. Giordmaine and S. L. Shapiro, Appl. Phys. Letters 13 (1968) 174. 5. H.P.Weber, Phys. Letters 27A (1968) 321. 6. E.B. Treacy, Appl. Phys. Letters 14 (1969) 112. 7. W.H. Glenn and M. J. Brienza, Appl. Phys. Letters 10 (1967) 221.
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PHYSICS
found for o s c i l l a t i o n s f r o m the p r e d i c t e d " s u p e r e g g " e l e c t r o n s u r f a c e ; this m a y be b e c a u s e of high c y c l o t r o n m a s s and b e c a u s e of the r e l a t i v e l a c k of a n i s o t r o p y of this s u r f a c e , which m a k e s t he t o r q u e m e t h o d i n s e n s i t i v e . It is hoped that e x p e r i m e n t s at l o w e r t e m p e r a t u r e s u s in g the f i e l d m o d u l a t i o n m e t h o d m a y r e v e a l the m i s s i n g oscillations. I w i s h to thank Dr. D. S h o e n b e r g and Dr. J. Van-
SUBPICOSECOND
STRUCTURE
22 September 1969
LETTERS
d er k o o y f o r v a l u a b l e d i s c u s s i o n , Dr. G. J o h a n s e n and P r o f e s s o r A. R. Mackintosh f o r i n f o r m i n g m e of t h e i r c a l c u l a t i o n s b e f o r e publication, and Dr. P. H. Schmidt f o r a d v i c e on the p u r i f i c a t i o n of barium.
References 1. J. Vanderkooy, J. Phys. (E) 2 (1969) 719. 2. T. E. Brown and K. A. MeEwen, to be published. 3. G. Johansen, Solid State Commun. 7 (1969) 731.
IN M O D E - L O C K E D
Nd:GLASS
LASERS
D. J. BRADLEY, G . H . C . NEW and S. J. CAUGHEY Department of Pure and Applied Physics, The Queen's Unive~'sity of Belfast Belfast BT7 INN, Northern Ireland, UK Received 14 August 1969
Sub-picosecond structure (s x 10 -13 sec), with a photographic peak contrast of ~ 2.6, has been recorded in the two photon fluorescence patterns produced by mode-locked Nd:glass lasers. Simultaneous third harmonic measurements do not indicate any systematic variation of the fundamental pulse durations.
O b s e r v a t i o n s of s u b p i c o s e c o n d s t r u c t u r e in N d : g l a s s l a s e r r e l a x a t i o n o s c i l l a t i o n s [1] and in the m o d e - l o c k e d N d : g l a s s l a s e r [2] have r e c e n t l y b e e n r e p o r t e d . In both c a s e s photon f l u o r e s c e n c e m e a s u r e m e n t s [3] w e r e e m p lo y e d , but the e x p e r i m e n t a l a r r a n g e m e n t s w e r e quite different. The p u l s e s t r u c t u r e of the r e l a x a t i o n o s c i l l a t i o n s was r e c o r d e d by i n s e r t i n g a 2 c m long c e l l containing a Rho d am i n e 6G solution i n s i d e the l a s e r cavity and p h o t o g r a p h i n g the two photon f l u o r e s c e n c e t r a c k s . S u b p i c o s e c o n d s t r u c t u r e in the m o d e l o c k e d l a s e r of ref. 2 was o b s e r v e d by e m p l o y in g a v e r y thin (28bL) c e l l outside the c a v it y , with p h o t o m u l t i p l i e r d e t e c t i o n of the f l u o r e s c e n c e . T h i s technique has the d r a w b a c k that plotting of a c o m p l e t e two photon f l u o r e s c e n c e pulse p r o f i l e r e q u i r e s s o m e 20 shots of the l a s e r . T h e s e phot o e l e c t r i c m e a s u r e m e n t s e s s e n t i a l l y showed a v e r y n a r r o w spike, of a f r a c t i o n of a p i c o s e c o n d in width and of peak c o n t r a s t 3, on top of a b r o a d b a s e of s e v e r a l p i c o s e c o n d s duration. We wish to r e p o r t the o b s e r v a t i o n of two photon f l u o r e s c e n c e p r o f i l e s of s i m i l a r c h a r a c t e r f r o m a m o d e l o c k e d l a s e r , p h o t o g r a p h e d in a 2 c m f l u o r e s c e n c e dye c e l l outside the cavity. The m o d e - l o c k e d l a s e r c o n s i s t e d of a 6" × ~" 78
B r e w s t e r angled N d : g l a s s r o d (Owens-Illinois) in a one m e t r e cavity. Q - s w i t c h i n g and m o d e locking was a c h i e v e d by i m m e r s i n g a 70% r e f l e c t i v i t y d i e l e c t r i c 1 ° wedged m i r r o r in a 1 m m l a y e r of Kodak 9860 dye. The output b e a m through this m i r r o r was em p l o y ed , with b e a m s p l i t t e r s , f o r s i m u l t a n e o u s r e c o r d i n g of (i) the two photon f l u o r e s c e n c e p l u se p r o f i l e using a t r i a n g u l a r conf i g u r a t i o n [2], (ii) the l a s e r output s p e c t r u m with a 3.4 m e t r e e c h e l l e s p e c t r o g r a p h and (iii) the int e n s i t y of the m o d e - l o c k e d pulse t r a i n with a b i p l a n a r photodiode and T e k t r o n i x 519 o s c i l l o s c o p e . The l a s e r b e a m f r o m the second 60% r e f l e c t i v i t y c a v i t y m i r r o r was e m p l o y e d for t h i r d h a r m o n i c g e n e r a t i o n f r o m a fused q u a r t z disc, also s i m u l t an eo u sl y with the other t h r e e m e a s u r e m e n t s . Fig. 1 shows a m i c r o d e n s i t o m e t e r t r a c e of a two photon f l u o r e s c e n c e t r a c k (photographed on I l f o r d F P 4 f i l m , with 75 m m lens at f / 8 ) , t y p i cal of s e v e r a l hundreds r e c o r d e d . The step on the r i g h t - h a n d side g i v e s a m e a s u r e of peak c o n t r a s t , si n ce it r e s u l t s f r o m c o v e r i n g m o s t of the c a m e r a f i e l d with a n e u t r a l density f i l t e r , which p r o duced an i n t en si t y r e d u c t i o n of 2.6 at the f l u o r e s c e n c e wavelength. The peak two-photon f l u o r e s c e n c e c o n t r a s t is t h e r e f o r e s e e n to be about 2.5,
Volume 30A, number 2
PHYSICS LETTERS
22 September 1969
!
Fig. 1. Microdensitometer trace of two-photon-fluorescence display outside cavity of mode-locked Nd:glass l a s e r . (Ordinate: arbitrary linear density scale. Intensity step of × 2.6 on right-hand-side. ) c o m p a r e d with the t h e o r e t i c a l l y e x p e c t e d v a l u e of 3 [4,5]. H o w e v e r the c e n t r a l spike is v e r y s h a r p , being only 50/z wide, and this width is a p p r o a c h i ng the l i m i t of the s p a t i a l r e s o l u t i o n of the c a m e r a . The width of the fine spike v a r i e d f r o m shot to shot within the r a n g e of 0.2 p s e c to 1 p s e c but it was always c o r r e l a t e d with the s i m u l t a n e o u s l y r e c o r d e d s p e c t r a l widths of 150 /~ to 30 A. L i k e w i s e the s h o u l d e r of e a c h t r a c e o c c u r s at a cont r a s t of ~ 1.9. As a l r e a d y pointed out [I,2], the shape of a two photon f l u o r e s c e n c e t r a c k does not uniquely define the duration of the l a s e r puls e s , n o r d e t e r m i n e w h e t h e r the fine s t r u c t u r e o r i g i n a t e s f r o m i s o l a t e d s u b p i c o s e c o n d p u l s e s or f r o m n o i s e fluctuations. S i m u l t a n e o u s o s c i l l o g r a m s of the fundamental p u l s e t r a i n and the c o r r e s p o n d i n g t h i r d h a r m o n i c output a r e shown in fig. 2. The t h i r d h a r m o n i c p u l s e t r a i n e n v e l o p e , which is a m o r e s e n s i t i v e i n d i c a t o r of v a r i a t i o n s in f u n d a m e n t a l p u l s e d u r a ti ons than is the c o r r e s p o n d i n g s e c o n d h a r m o n i c e n v e l o p e [6], u s u al l y c l o s e l y followed that of the f unda m en t al p u l s e t r a i n indicating that t h e r e was no s y s t e m a t i c v a r i a t i o n of p u l s e d u r a t i o n t h r o u g h out the t r a i n , in c o n t r a s t to e a r l i e r r e p o r t s [7]. H o w e v e r o c c a s i o n a l r a n d o m f l u c t u a t i o n s w e r e obs e r v e d , as in fig. 2. The to ta l amount of t h i r d
Fig. 2. Simultaneous oscillograms of the fundamental mode-locked laser pulse train, and the corresponding third harmonic output. Horizontal scale 20 nsec cm -~. h a r m o n i c light was c o n s i s t e n t with a t r a i n of puls e s , each of d u r a t i o n 10 p s e c s . The two photon f l u o r e s c e n c e m e a s u r e m e n t s w e r e r e p e a t e d with an A m e r i c a n Optical r o d of the s a m e d i m e n s i o n s and e n t i r e l y s i m i l a r r e s u l t s w e r e obtained. We would like to thank Mr. R. J. F r a m e for making the m i c r o d e n s i t o m e t e r t r a c e s and f o r s u g g e s t i o n s on photography. The s p e c i a l l a s e r m i r r o r s a r e m a n u f a c t u r e d by Mr. D. Blanc.
References 1.. D. J. Bradley, G.H.C. Hew, B. Sutherland and S.J. Caughey, Phys. Letters 28A (1969) 532. 2. S. L. Shapiro and M. A. Duguay, Phys. Letters 28A (1969) 69S. 3. J. A. Giordmaine, P.M. Rentzepis, S. L. Shapiro and K.W.Weeht, Appl. Phys. Letters 11 (1967) 216. 4. J.R. Klauder, M.A. Duguay, J. A. Giordmaine and S. L. Shapiro, Appl. Phys. Letters 13 (1968) 174. 5. H.P.Weber, Phys. Letters 27A (1968) 321. 6. E.B. Treacy, Appl. Phys. Letters 14 (1969) 112. 7. W.H. Glenn and M. J. Brienza, Appl. Phys. Letters 10 (1967) 221.
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