AN OPTICAL PUMPING SCHEME FOR EUV LASERS IN TWO COUPLED MEDIA USING A QUASI-CONTINUUM EMITTED BY RARE-EARTH ELEMENTS

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AN OPTICAL PUMPING SCHEME FOR EUV LASERS IN TWO COUPLED MEDIA USING A QUASI-CONTINUUM EMITTED BY RARE-EARTH

ELEMENTS

M. Finkenthal, J. Schwob, P. Mandelbaum

To cite this version:

M. Finkenthal, J. Schwob, P. Mandelbaum. AN OPTICAL PUMPING SCHEME FOR EUV LASERS IN TWO COUPLED MEDIA USING A QUASI-CONTINUUM EMITTED BY RARE-EARTH ELEMENTS. Journal de Physique Colloques, 1986, 47 (C6), pp.C6-129-C6-133.

�10.1051/jphyscol:1986617�. �jpa-00225860�

AN OPTICAL PUMPING SCHEME FOR EUV LASERS IN TWO COUPLED MEDIA USING A QUASI-CONTINUUM EMITTED BY RARE-EARTH ELEMENTS

M. FINKENTHAL, J.L. SCHWOB and P. MANDELBAUM

Racah I n s t i t u t e of P h y s i c s , The Hebrew U n i v e r s i t y , J e r u s a l e m 9 1 0 0 0 , I s r a e l

Dans l a p r k s e n t e k t u d e on propose l ' u t i l i s a t i o n d e bandes C t r o i t e s d e rayonnement quasi-continu k m i s p a r l e s Clkments de t e r r e s r a r e s dans l e domaine 60- 100W, a f i n d ' e f f e c t u e r l e pompage o p t i q u e d'ions chargCs p r o d u i t s indkpendamment a u s e i n d'un m i l i e u coupl6. Les t r a n s i t i o n s l a s e r s e s i t u e r a i e n t d a u s l e domaine XUV ( X > 3001). Le m i l i e u d e pompage p r o p o s d c o n s i s t e e n u n e Q t i n c e l l e s o u s v i d e d f a i b l e i n d u c t a n c e e t & c o u r a n t &lev&. Les i o n s d e s k l k m e n t s du Lanthane a u D y s p r o s i u m p o s s k d a n t une s o u s - c o u c h e 4dk o u v e r t e , C m e t t e n t d a n s l ' d t i n c e l l e un t r d s g r a n d n o m b r e d e r a i e s c o r r e s p o n d a n t a u x t r a n s i t i o n s 4p64dk - 4p54dk+1 e t 4 d k - 4 d k - l 4 f , d l ' i n t e r i e u r d'une b a n d e d e 2 a 4 d e l a r g e . P a r a i l l e u r s , l e s d e g r d s d ' i o n i s a t i o n r e l a t i v e m e n t b a s d'616ments de f a i b l e Z du m i l i e u l a s e r s e r o n t o b t e n u s a l ' a i d e d e f i l s e x p l o s a n t s e n r o b k s . Ce d e r n i e r t y p e d e s o u r c e d e v r a i t p e r m e t t r e l ' o b t e n t i o n d e plasmas p r k s e n t a n t un v a s t e domaine d e d e n s i t 6 5 d e s tempk- r a t u r e s r e l a t i v e m e n t b a s s e s , a f i n de pouvoir m i n i m i s e r l e s p r o c e s s u s d e t r a n s f e r t c o l l i s i o n n e l a i n s i que l ' d l a r g i s s e m e n t Doppler d e s r a i e s considCrCes. On propose e n p a r t i c u l i e r un schkma f a i s a n t a p p e l d l a bande Cmise p a r l e C4rium, c e n t r 6 e d 891, a f i n de r k a l i s e r l e pompage v e r s l e s n i v e a u x zp5hp de S i V. L e s t r a n s i t i o n s l a s e r 4 p + 3 s p r C d i t e s d o i v e n t a p p a r a l t r e d a n s la r d g i o n 300-4001.

A b s t r a c t

The p r e s e n t work proposes t h e u s e of n a r r r o w bands of quasi-continuum r a d i a t i o n e m i t t e d by r a r e e a r t h e l e m e n t s i n t h e 60 - 100 1 range, i n o r d e r t o o p t i c a l l y pump charged i o n s o b t a i n e d s e p a r a t e l y i n a coupled medium. The l a s i n g t r a n s i t i o n s would b e i n t h e XUV r a n g e ( A > ^{3 0 0} 1). The p r o p o s e d pumping medium i s a l o w - i n d u c t a n c e h i g h - c u r r e n t vacuum s p a r k . I o n s o f t h e e l e m e n t s Lanthanum t o D y s p r o s i u m h a v i n g a n o p e n 4dk s u b - s h e l l e m i t a v e r y l a r g e number o f l i n e s b e l o n g i n g t o t h e 4p64dk - 4p54dk'1 and 4dk-4dL-l4f t r a n s i t i o n s , w i t h i n a 2 t o 4 1 w i d t h band. R e l a t i v e l y low i o n i z a t i o n s t a t e s of low Z e l e m e n t s i n t h e l a s i n g medium a r e produced by means of a n embedded e x p l o d i n g w i r e (EEW). The EEW s o u r c e w i l l e n a b l e t o o b t a i n plasmas having a wide r a n g e o f d e n s i t i e s and r e l a t i v e l y low t e m p e r a t u r e s - t h u s m i n i m i z i n g b o t h c o l l i s i o n a l t r a n s f e r p r o c e s s e s and Doppler broadening of t h e l i n e s o f i n t e r e s t . A s c h e m e , u s i n g a C e r i u m band c e n t e r e d a t 89R t o pump t h e 2 ~ l e v e l s o f SiV i s ~ 4 ~ proposed. The l a s i n g 4p -+ 3 s t r a n s i t i o n s w i l l be e m i t t e d i n t h e 300 - 400 11 region.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986617

JOURNAL D E PHYSIQUE

I n t r o d u c t i o n

O p t i c a l p u m p i n g h a s b e e n p r o p o s e d by s e v e r a l a u t h o r s a s a p o s s i b l e way t o a c h i e v e p o p u l a t i o n i n v e r s i o n s i n t h e XUV region(1-3). A d i f f i c u l t y s p e c i f i c t o t h i s a p p r o a c h i s t h e need t o f i n d e x t r e m e l y c l o s e l i n e c o i n c i d e n c e s b e t w e e n pumping a n d pumped t r a n s i t i o n s . A n o t h e r m a j o r p r o b l e m i s r e l a t e d t o t h e v e r y h i g h pump power r e q u i r e m e n t s f o r i n v e r s i o n i n t h e X-ray r e g i o n ( t h e pump power n e e d e d s c a l e s as

A - 4 ) . H i g h p o w e r p u l s e d l a s e r s f o c u s e d o n m u l t i c o m p o n e n t t a r g e t s , a r e u s e d t o p r o d u c e b o t h t h e b r i g h t pump s o u r c e a n d t h e pumped medium. B u t t h e t o o h i g h d e n s i t y - h i g h t e m p e r a t u r e p l a s m a s t h u s c r e a t e d h a v e d e l e t e r i o u s e f f e c t s o n t h e p o p u l a t i o n i n v e r s i o n .

T h e p r e s e n t p a p e r p r o p o s e s a n o p t i c a l pumping XUV laser e x p e r i m e n t i n w h i c h some o f t h e a b o v e m e n t i o n e d d i f f i c u l t i e s c o u l d b e r e d u c e d s i g n i f i c a n t l y . A n a r r o w b a n d o f q u a s i - c o n t i n u u m e m i t t e d by i n t e r m e d i a t e i o n i z a t i o n s t a t e s o f r a r e e a r t h e l e m e n t s i s p r o p o s e d a s a pump s o u r c e ; t h e band is l a r g e enough t o o v e r c o m e t h e l i n e m a t c h i n g p r o b l e m , b u t n o t t o o l a r g e , as t h a t o t h e r l e v e l s o f t h e pumped a t o m s w i l l n o t b e o p t i c a l l y pumped. A l s o , t h e peak b r i g h t n e s s o f t h e band is much l a r g e r t h a n t h a t o f a n y i n d i v i d u a l l i n e i n t h e s p e c t r u m ( & ) . S i n c e t h e b a n d ' m o v e s ' t o w a r d s l o w e r w a v e l e n g t h s as t h e Z o f t h e p r o d u c i n g e l e m e n t i n c r e a s e s , v a r i o u s i o n i c s t a g e s c a n b e c h o s e n as l a s i n g media. The m a j o r q u e s t i o n , w h e t h e r t h e vacuum s p a r k s o u r c e i s b r i g h t enough t o b e u s e d a s a pump i n s u c h e x p e r i m e n t s is d i s c u s s e d i n t h i s p a p e r i n a g e n e r a l way; f u r t h e r e x p e r i m e n t s w i l l b e p e r f o r m e d i n o r d e r t o g i v e a d e f i n i t i v e a n s w e r t o t h i s q u e s t i o n .

Ordinary spark na = 10'6cm-3 T ' 10 US I

F i g . 1. The vacuum s p a r k d i s c h a r g e a n d m a j o r p l a s m a p a r a m e t e r s .

The pumped i o n s i n t h e p r o p o s e d scheme, w i l l b e o b t a i n e d i n a n i n d e p e n d e n t l y p r o d u c e d p l a s m a , t h u s e n a b l i n g t o t a y l o r t h e o p t i m a l d e n s i t y a n d t e m p e r a t u r e c o n d i - t i o n s f o r p o p u l a t i o n i n v e r s i o n . T h e d r a w b a c k o f t h e c o u p l e d m e d i a s c h e m e c o m e s f r o m t h e l o w e f f i c i e n c y o f t h e c o u p l i n g . However, as shown i n t h e p a p e r s p r e s e n t e d by Dhez a n d V i n o g r a d o v a t t h i s c o n f e r e n c e , new p o s s i b i l i t i e s t o t r a n s f e r s o f t X-ray may i m p r o v e t h e q u a l i t y o f t h e c o u p l i n g i n f u t u r e e x p e r i m e n t s .

from a low - i n d u c t a n c e h i g h - c u r r e n t vacuum spark(5). It h a s been shown t h a t a p o i n t plasma i s o b t a i n e d i n s u c h a d i s c h a r g e ; t h e p a r a m e t e r s o f t h e main plasma column and t h e p o i n t s o u r c e a r e p r e s e n t e d i n f i g u r e 1.

The r a d i a t i o n d e n s i t y r e q u i r e d t o pump t h e upper l e v e l i n a scheme such a s t h e o n e p r e s e n t e d h e r e , c a n b e e v a l u a t e d i n t h e f o l l o w i n g way: t h e p h o t o e x c i t a t i o n c r o s s s e c t i o n a t 1008 assuming t h a t t h e t r a n s i t i o n p r o b a b i l i t i e s i n v o l v e d a r e of t h e o r d e r o f 1012 sec-i and t h e major broadening mechanism i s Doppler broadening, i s g i v e n by:

5 r e p r e s e n t i n g t h e l i n e shape f a c t o r .

I n o r d e r t o a c h i e v e umpin rates o f t h e o r d e r o f 1012 s e c - l , r a d i a t i o n d e n s i - t i e s o f p h o t o n s cm-' sec-' a r e r e q u i r e d . A b l a c k b o d y r a d i a t o r e m i t t i n g a t 50eV, would r a d i a t e p e r u n i t wavelength and s o l i d angle:

B = 1.2 X lo1' ( k ~ ) ' ~ ~ = photons cm-28-1sec-1sr-1 ( 2 )

The p o i n t s o u r c e o b t a i n e d i n t h e vacuum s p a r k c o u l d indeed be o p t i c a l l y t h i c k f o r t h e t r a n s i t i o n s o f i n t e r e s t i n t h e 6 0 - 1 0 0 8 r a n g e , a n d t h u s c l o s e t o t h e c o n d i t i o n s of a blackbody r a d i a t o r :

Noreover, i t i s p o s s i b l e t h a t a n MHD i n s t a b i l i t y of t h e plasma column c r e a t e s f o r a p e r i o d of t i m e l o n g r e l a t i v e t o t h e p o i n t s o u r c e l i f e t i m e , s e v e r a l s m a l l r e g i o n s of h i g h d e n s i t y plasma which could t h u s r e p r e s e n t a n extended, o p t i c a l l y t h i c k source.

T h i s i s a p o i n t w h i c h w i l l b e s t u d i e d i n a f u t u r e e x p e r i m e n t . I n a n y c a s e , t h e comparison of t h e d a t a shown i n f i g u r e 1 w i t h t h e e s t i m a t e s g i v e n above, i n d i c a t e s t h a t t h e vacuum s p a r k a s a pump s o u r c e i s a p l a u s i b l e s t a r t i n g p o i n t .

The band of quasi-continuum r a d i a t i o n i s o b t a i n e d u s i n g a vacuum s p a r k d i s c h a r g e ( V = 15kV, I = 260kA, L = 80nH) w i t h a n o d e s made o f p u r e , r a r e e a r t h e l e m e n t s . The s p e c t r a have been r e c o r d e d by means of a 2m g r a z i n g i n c i d e n c e , Schwob

& F r a e n k e l s p e c t r o g r a p h ( 6 ) i n t h e 1 0 - 350 1 r a n e. The q u a s i - c o n t i n u u m band i s e m i t t e d by a v e r y l a r g e number of 4p64dk - 4p54dk+q and 4dk - 4dk-l4f t r a n s i t i o n s of s e v e r a l i o n i z a t i o n s t a t e s , bunched t o g e t h e r i n a v e r y n a r r o w s p e c t r a l r e g i o n . (A d e t a i l e d d e s c r i p t i o n and e x p l a n a t i o n o f t h e s e s p e c t r a i s under p u b l i c a t i o n ) . What i s t h e s p a t i a l e x t e n t o f t h e e m i t t i n g r e g i o n i n t h e p l a s m a a n d how c l o s e t o a blackbody r a d i a t o r i s t h i s e m i t t i n g r e g i o n a r e q u e s t i o n s t o be s t i l l answered.

The l a s i n g medium

The p r o p e r t i e s o f t h e l a s i n g medium a r e d e f i n e d i n terms o f t h e p h o t o a b s o r p t i o n c r o s s s e c t i o n , t h e number d e n s i t y of t h e photoabsorbing l e v e l - t h e ground l e v e l i n t h e c a s e d i s c u s s e d h e r e , and t h e c o l l i s i o n a l e x c i t a t i o n and d e e x c i t a t i o n r a t e s a t t h e e l e c t r o n d e n s i t i e s o f t h e l a s i n g plasma.

A s shown i n e q u a t i o n ( I ) , i n o r d e r t o o p t i m i z e t h e p h o t o - a b s o r p t i o n c r o s s s e c t i o n a t a given wavelength, h i g h t r a n s i t i o n p r o b a b i l i t i e s and narrow l i n e pro- f i l e s a r e r e q u i r e d . T h i s l a t e r r e q u i r e m e n t i m p l i e s a low t e m p e r a t u r e l a s i n g medium i n o r d e r t o minimize t h e Doppler broadening (assuming t h i s t o be t h e major broad- e n i n g mechanism). Concerning t h e p a r t i c l e d e n s i t i e s , two r a t h e r c o n t r a d i c t o r y demands a r e i m p o s e d upon t h e l a s i n g medium. I n o r d e r t o o b t a i n h i g h g a i n , o n o n e hand, l a r g e i o n number d e n s i t i e s a r e r e q u i r e d :

JOURNAL DE PHYSIQUE

U = N i Uab (cm -1 ) (4)

Thus, i n o r d e r t o o b t a i n a g a i n a of t h e o r d e r of 10, w i t h a p h o t o a b s o r p t i o n c r o s s s e c t i o n uqb of t h e o r d e r of t h a t q u o t e d above, and a s s u m i n g a pumping e f f i c i e n c i n t o t h e l a s l n g l e v e l o f one n e e d s p a r t i c l e d e n s i t i e s of t h e o r d e r of 1 0 14

(Those a r e q u i t e s t r o n g r e q u i r e m e n t s ; i n f a c t t h e m a j o r p a r a m e t e r i s t h e a.1

( g a i n - l e n g t h ) p r o d u c t a n d a may b e s m a l l e r t h a n 1 0 i n a s u c c c e s s f u l e x p e r i m e n t . A l s o , much b e t t e r p u m p i n g e f f i c i e n c i e s c a n b e a c h i e v e d ) . On t h e o t h e r h a n d , t h e e l e c t r o n d e n s i t y s h o u l d b e k e p t l o w , o t h e r w i s e c o l l i s i o n a l m i x i n g o f t h e l e v e l p o p u l a t i o n w i l l c a n c e l q u i c k l y t h e i n v e r s i o n . The l a s i n g medium s h o u l d t h e r e f o r e b e a plasma of r e l a t i v e l y low t e m p e r a t u r e ( s e v e r a l t e n s t o h u n d r e d s of eV, depending on t h e i o n i z a t i o n p o t e n t i a l of t h e l a s i n g i o n ) and i n t e r m e d i a t e d e n s i t y , 1017 - 1018 c ~ n - ~ . T h e f i r s t c o n d i t i o n i s r e a l i z e d i n a P e n n i n g I o n i z a t i o n Gauge (PIG) t y p e d i s c h a r g e ( 7 ) ; t h i s t y p e o f s o u r c e i s o p e r a t e d a t l o w p r e s s u r e s a n d t h u s t h e d e n s i t i e s r e q u i r e d t o a c h i e v e s i g n i f i c a n t g a i n c a n n o t b e o b t a i n e d u s i n g t h i s p l a s m a . H o w e v e r , b e c a u s e o f i t s s i m p l i c i t y we i n t e n d t o u s e a PIG d i s c h a r g e i n s o m e p r e l i m i n a r y e x p e r i m e n t s , i n o r d e r t o t e s t a f e w p r o b l e m s r e l a t e d t o t h e c o u p l i n g between t h e pump and pumped media and even t o o b s e r v e enhanced e m i s s i o n from t h e l a s i n g l e v e l . I n a second phase of t h e e x p e r i m e n t s , a n embedded e x p l o d i n g w i r e (EEW) w i l l b e u s e d i n o r d e r t o p r o d u c e a n e x t e n d e d , h o m o g e n e o u s , a n d i n t e r m e d i a t e - d e n s i t y p l a s m a . s h o r t l y , t h e EEW c o n s i s t s o f a c o n d u c t i n g w i r e i n t r o d u c e d i n a t h i n - w a l l e d c y l i n d e r made of t h e working m a t e r i a l o r a w i r e c o a t e d w i t h a t h i n f i l m o f t h e working m a t e r i a l . The plasma i s o b t a i n e d i n t h e p r o c e s s o f t r a n s f e r o f e n e r g y f r o m t h e e x p l o d i n g w i r e t o t h e s u r r o u n d i n g m a t e r i a l . I t s c o n d i t i o n s a r e d e t e r m i n e d b y t h e e x p l o d i n g w i r e d i s c h a r g e c o n d i t i o n s a n d t h e c o m p o s i t i o n and t h i c k n e s s of t h e embedding c y l i n d e r .

The Cerium - S i l i c o n scheme

F i g . 2. The Cerium - S i l i c o n scheme.

A r t r u (8), i s 1 1 2 9 9 4 3 . 5 ~ m - ~ , w h i c h c o r r e s onds t o 88.58. The e n e r g y gap b e t w e e n t h e 2p54p and t h e n e i g h b o u r i n g 2p54s a n d 2p 4d c o n f i g u r a t i o n s

5

C o n c l u s i o n s

A new o p t i c a l pumping scheme,based on t h e u s e of a n a r r o w q u a s i - c o n t i n u u m band e m i t t e d by i o n s o f r a r e e a r t h e l e m e n t s f r o m a v a c u u m s p a r k d i s c h a r g e h a s b e e n proposed. The pumped i o n s c a n be l o w i o n i z a t i o n states of l o w Z e l e m e n t s , which c a n b e e a s i l y o b t a i n e d i n r e l a t i v e l y l o w t e m p e r a t u r e plasmas. S p e c i f i c a l l y , t h e case o f SiV zp5hp l e v e l s , pumped by a Cerium band h a s been p r o p o s e d .

F u r t h e r e x p e r i m e n t s w i l l be p e r f o r m e d i n o r d e r t o s t u d y t h e p r o p e r t i e s o f t h e pump a n d pumped s o u r c e s , a l o w - i n d u c t a n c e h i g h - c u r r e n t vacuum s p a r k a n d t h e embedded e x p l o d i n g w i r e , r e s p e c t i v e l y . A l s o t h e e f f i c i e n c y o f t h e t w o c o u p l e d media w i l l b e s t u d i e d .

T h e a d v a n t a g e o f t h e p r o p o s e d e x p e r i m e n t c o n s i s t s i n f e a s i b i l i t y

-

-

R e f e r e n c e s

1 ) A.V. Vinogradov, 1.1. Sobelman and E.A. Yukov, Sov. J. Q u a n t . E l e c t r o n . 5 , ( 1 9 7 5 ) , 59.

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2 ) B.A. Norton and N.J.Peacock, J, Phys. B 3, (1975), 6.

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