Lanthanide and Actinide Lasers - ACS Publications - American

14 Lanthanide and Actinide Lasers

Downloaded by STANFORD UNIV GREEN LIBR on April 11, 2013 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0131.ch014

MARVIN J. WEBER LawrenceLivermoreLaboratory,University of California, Livermore, CA 94550

Lanthanides and actinides were among the very first elements used t o demonstrate l a s e r action. Although the first laser used an i r o n group element, Cr i n A I O ( r u b y ) , l a s e r a c t i o n from an actinide i o n (U ) was also reported (1) i n t h e same year, 1960. In the f o l l o w i n g year stimulated emission from both d i v a l e n t (Sm ) (2) and trivalent (Nd ) (3) lanthanide ions was observed. The f o l l o w i n g two decades witnessed an a s t o n i s h i n g proliferation o f lasing ions and media. Elements of the lanthanide s e r i e s c o n t r i b u t e d t o this proliferation and in one case, that o f solid-state l a s e r s , dominate the field. To date stimulated emission has been obtained from eleven trivalent and three d i v a l e n t lanthanide i o n s ; in hosts in cluding crystalline and amorphous solids, m e t a l l o - o r g a n i c and i n o r g a n i c aprotic liquids, and neutral and i o n i z e d gases and molecular vapors; a t wavelengths ranging from the infrared to the ultraviolet; from l a s e r s operating pulsed and c o n t i n u o u s l y ; and from l a s e r s ranging i n size from t h i n f i l m s and small fibers f o r i n t e g r a t e d o p t i c s applications t o large disks f o r high-power Nd:glass l a s e r s f o r inertial confinement fusion experiments. The v e r s a t i l i t y and wide a p p l i c a b i l i t y o f lanthanide ions f o r l a s e r s a r i s e s from several d e s i r a b l e s p e c t r o s c o p i c f e a t u r e s . The e l e c t r o n i c s t a t e s o f the ground 4 ί · c o n f i g u r a t i o n s provide complex and v a r i e d o p t i c a l energy l e v e l s t r u c t u r e s , thus many d i f f e r e n t l a s i n g schemes are p o s s i b l e . The large number o f e x c i t e d s t a t e s s u i t a b l e f o r o p t i c a l pumping and the subsequent decay t o metastable s t a t e s having high quantum e f f i c i e n c i e s and narrow f * f emission l i n e s are favorable f o r a c h i e v i n g l a s e r a c t i o n . Because the l o c a t i o n s of the energy l e v e l s do not change g r e a t l y with host, a given ion can be lased i n many d i f f e r e n t hosts. The host can t h e r e fore be s e l e c t e d t o optimize performance f o r a s p e c i f i c a p plication. The s p e c t r o s c o p i c p r o p e r t i e s o f the lanthanides and a c t i n i d e s as they r e l a t e t o l a s e r a c t i o n are the p r i n c i p a l 3+

3+

2

3

2+

3+

Ί

0-8412-0568-X/80/47-131-275$05.0O/0 © 1980 American Chemical Society In Lanthanide and Actinide Chemistry and Spectroscopy; Edelstein, N.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


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t o p i c of t h i s a r t i c l e . A f t e r a b r i e f review of l a s e r fundamentals ( 4 ) , the e x t e n t to which these elements have been employed f o r l a s e r s i n v a r i o u s media a r e surveyed. A c o m p r e h e n s i v e l i s t i n g o f a l l l a n t h a n i d e l a s e r i o n s and h o s t s i s beyond the scope o f t h i s paper, however r e f e r e n c e s are given to t a b u l a t i o n s containing references to the o r i g i n a l w o r k . T h e p a r t i c u l a r t r a n s i t i o n s u s e d f o r l a s i n g a r e shown a n d d i s c u s s e d . T h e s e i l l u s t r a t e how a n d why l a s e r a c t i o n i s o b t a i n e d and form t h e b a s i s f o r c o n s i d e r i n g p o s s i b l e s t i m u l a t e d e m i s s i o n i n v o l v i n g o t h e r i o n s and t r a n s i t i o n s . R e c e n t work, c u r r e n t a c t i v i t i e s , and f u t u r e d i r e c t i o n s a r e a l s o noted. Because of space l i m i t a t i o n s , engineering d e t a i l s and a p p l i c a t i o n s o f l a n t h a n i d e a n d a c t i n i d e l a s e r s a r e n o t d i s c u s s e d , but a r e w e l l c o v e r e d i n a book b y K o e c h n e r ( 5 J . Laser

Fundamentals

To o b t a i n s t i m u l a t e d e m i s s i o n b e t w e e n two e n e r g y l e v e l s , a population inversion i s necessary. This i s usually achieved by e x c i t a t i o n i n t o a t h i r d l e v e l o r l e v e l s w h i c h r a p i d l y a n d e f f i c i e n t l y t r a n s f e r energy to a metastable upper l a s e r l e v e l . A g e n e r a l i z e d e n e r g y l e v e l s c h e m e f o r l a s e r a c t i o n i s shown i n F i g . 1. I f t h e t e r m i n a l l a s e r l e v e l i s t h e g r o u n d s t a t e a n d t h e i n i t i a l and f i n a l l a s e r s t a t e s have equal degeneracies, then more than o n e - h a l f o f the i o n s must be e x c i t e d t o o b t a i n an i n v e r t e d p o p u l a t i o n a n d 3 - l e v e l l a s e r a c t i o n . I f , i n s t e a d , the terminal l e v e l 2 i s above the ground s t a t e , then only an e x c i t e d - s t a t e population i n l e v e l 3 s u f f i c i e n t to overcome the Boltzmann p o p u l a t i o n i n l e v e l 2 i s needed f o r p o p u l a t i o n i n v e r s i o n . T h i s d r a s t i c a l l y reduces the pumping requirements. Phonon-terminated or v i b r o n i c l a s e r s are those in which l e v e l 2 is a vibrational-electronic state. When i t i s d i f f i c u l t t o e x c i t e i o n s i n t o l e v e l 3 o r t h e l e v e l d e c a y s v e r y r a p i d l y , a p o p u l a t i o n i n v e r s i o n and o s c i l l a t i o n may b e o b t a i n e d u s i n g a c a s c a d e l a s e r s c h e m e i n v o l v i n g two c o n s e c u t i v e l a s i n g t r a n s i t i o n s . A n e x a m p l e i s shown a t t h e r i g h t o f F i g . 1. T o l a s e t h e 3+2 t r a n s i t i o n , i o n s a r e f i r s t pumped i n t o l e v e l 4 a n d t h e n s t i m u l a t e d t o e m i t t o l e v e l 3, t h e r e b y c r e a t i n g a p o p u l a t i o n i n v e r s i o n w i t h r e s p e c t t o l e v e l 2. H e r e one r e l i e s o n a s t i m u l a t e d r a t h e r t h a n a s p o n t a n e o u s e m i s s i o n r a t e f o r t h e 4-K3 t r a n s i t i o n . The r a t e c a n t h e r e f o r e b e made v e r y f a s t a n d c o n t r o l l e d v i a t h e beam i n t e n s i t y or the Q of the resonant o p t i c a l c a v i t y . Cascade l a s i n g s c h e m e s a r e a l s o u s e f u l when t h e t e r m i n a l l e v e l o f a lasing t r a n s i t i o n relaxes soslowly that o s c i l l a t i o n selft e r m i n a t e s b e c a u s e t h e p o p u l a t i o n i n v e r s i o n and a s s o c i a t e d gain decrease to a value i n s u f f i c i e n t to overcome the l o s s e s . If the terminal s t a t e i s s t i m u l a t e d to emit to a lower l e v e l , o s c i l l a t i o n can b e m a i n t a i n e d and the t o t a l e n e r g y s t o r e d i n the f i r s t upper l a s e r l e v e l e x t r a c t e d

In Lanthanide and Actinide Chemistry and Spectroscopy; Edelstein, N.; ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


14.

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t

4-level laser

Cascade laser

Figure 1. Representative energy-level diagram and transitions for four-level and cascade lasing schemes: v and v are the pump and laser frequencies; wavy lines denote nonradiative transitions. P

L


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Cascade l a s i n g r e q u i r e s that t h e host be transparent and t h e r e be n o d e l e t e r i o u s g r o u n d o r e x c i t e d - s t a t e a b s o r p t i o n s at e i t h e r l a s e r wavelength. T r a n s i t i o n s L ] a n d l_2 may b e a s s o c i a t e d w i t h two d i f f e r e n t i o n s . In t h i s c a s e t h e e n e r g y i n t h e t e r m i n a l l e v e l 3 o f t h e f i r s t l a s i n g i o n s must be q u i c k l y and e f f i c i e n t l y t r a n s f e r r e d t o t h e upper l a s e r l e v e l 3 of t h e second i o n . The t h r e s h o l d c o n d i t i o n f o r l a s e r o s c i l l a t i o n i s g i v e n b y R R exp(2G£) = 1 , Downloaded by STANFORD UNIV GREEN LIBR on April 11, 2013 | http://pubs.acs.org Publication Date: September 23, 1980 | doi: 10.1021/bk-1980-0131.ch014

]

(1)

2

w h e r e R] a n d R2 a r e t h e r e f l e c t i v i t i e s o f t h e m i r r o r s i n t h e o p t i c a l resonator c a v i t y , G i s t h e gain p e r u n i t l e n g t h , and £ i s t h e l e n g t h o f t h e a c t i v e l a s i n g medium. The g a i n i s d e termined by G =

a - a

s

- aj,

(2)

where a i s t h e g a i n c o e f f i c i e n t o f t h e l a s i n g medium, and a and c q a r e l o s s c o e f f i c i e n t s due t o s c a t t e r i n g and i m p u r i t y a b s o r p t i o n . I f N3 a n d N2 a r e t h e p o p u l a t i o n s i n t h e u p p e r a n d l o w e r l a s e r l e v e l s i n F i g . 1, t h e n e t g a i n c o e f f i c i e n t o f t h e l a s e r medium i s s

a

= N a 3

3 2

- N a 2

2 3

-

H ^ .

(3)

In E q . ( 3 ) , 030 a n d 0 2 3 a r e t h e c r o s s s e c t i o n s f o r s t i m u l a t e d emission and a b s o r p t i o n . F o r narrow-line a b s o r p t i o n and e m i s s i o n s p e c t r a , t h e s e two c r o s s s e c t i o n s a r e e q u a l . F o r broadband s p e c t r a with emission bandwidth g r e a t e r than kT, the cross s e c t i o n s a r e connected by a g e n e r a l i z e d E i n s t e i n r e l a t i o n ( 6 j . The f i n a l term i n Eq. (3) accounts f o r p o s s i b l e e x c i t e d - s t a t e a b s o r p t i o n from t h e upper l a s e r l e v e l t o higher e x c i t e d - s t a t e s i n d i c a t e d b y t h e d a s h e d l e v e l i n F i g . 1. I f e s a > 32> a b s o r p t i o n from l e v e l 3 dominates s t i m u l a t e d emission and l a s e r a c t i o n i s n o t p o s s i b l e . The g a i n , from Eq. ( 3 ) , i s g o v e r n e d b y a p r o d u c t o f t h e stimulated emission cross s e c t i o n and t h e population i n v e r s i o n (N3-N2). T h e l a t t e r i s dependent upon t h e a b s o r p t i o n spectrum a n d i t s s p e c t r a l m a t c h w i t h t h e pump s o u r c e , t h e l i f e t i m e o f the m e t a s t a b l e l e v e l 3 which determines t h e pumping r a t e r e q u i r e d , and t h e quantum e f f i c i e n c y . The l a s t q u a n t i t y i n c l u d e s t h e f l u o r e s c e n c e c o n v e r s i o n e f f i c i e n c y ( t h e number o f i o n s e x c i t e d t o t h e f l u o r e s c i n g l e v e l p e r i n c i d e n t pump p h o t o n ) and t h e q u a n t u m e f f i c i e n c y o f t h e f l u o r e s c i n g s t a t e ( t h e f r a c t i o n a l number o f p h o t o n s e m i t t e d p e r e x c i t e d i o n i n t h e upper l a s e r l e v e l ) . The peak c r o s s s e c t i o n i s d e t e r m i n e d b y t h e o s c i l l a t o r s t r e n g t h and t h e l i n e w i d t h o f t h e t r a n s i t i o n . The l i n e w i d t h may b e ( 1 ) t h e n a t u r a l o r h o m o g e n e o u s w i d t h , g o v e r n e d b y a

a



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r a d i a t i v e and/or n o n r a d i a t i v e t r a n s i t i o n s between S t a r k l e v e l s , or (2) inhomogeneously broadened i n t h e case o f d i s o r d e r e d media such as g l a s s e s o r mixed c r y s t a l s . In t h e l a t t e r c a s e the r a t e o f energy e x t r a c t i o n v a r i e s from i o n t o i o n and s p e c t r a l h o l e b u r n i n g i n t h e g a i n p r o f i l e may o c c u r . Both f - f and f - d t r a n s i t i o n s have been used f o r l a n t h a n i d e and a c t i n i d e l a s e r s . T h e s p e c t r o s c o p i c p r o p e r t i e s o f t h e s e t r a n s i t i o n s a r e compared i n T a b l e I. S i n c e t h e d s t a t e s have s h o r t e r l i f e t i m e s , f a s t e r pumping as w e l l a s h i g h e r e n e r g i e s a r e r e q u i r e d f o r e x c i t a t i o n . P o s s i b l e pumping s o u r c e s i n c l u d e u l t r a f a s t f l a s h ] a m p s , o t h e r l a s e r s , e l e c t r o n beams, o r s y n c h r o t r o n r a d i a t i o n , with one e x c e p t i o n , a l l l a n t h a n i d e and a c t i n i d e l a s e r s h a v e b e e n o p t i c a l l y pumped. Table I. Comparison o f s p e c t r o s c o p i c p r o p e r t i e s o f f - f and f - d t r a n s i t i o n s o f l a n t h a n i d e i o n s i n s o l i d s a t 300 K. 5d -> 4 f -1 -2 -10 -10 intermediate — strong

4 f -> 4 f Oscillator strength

6

~10" -10"

Ion-lattice coupling Fluorescence Fluorescence

8

L

weak

wavelength

- 2 0 0 - 5 0 0 0 nm

linewidth

-10

cm'

1

- 1 5 0 - 1 0 0 0 nm > 1000 cm" 8

- 5

1

Fluorescence l i f e t i m e

10" -10" s

10- -10

Excited-state absorption

f +f f + d

d -> d d -> h i g h e r configurations

5

2

s

Table II l i s t s a l l lanthanide and a c t i n i d e l a s e r ions and types o f t r a n s i t i o n s . Table I I .

E l e c t r o n i c t r a n s i t i o n s and ions used f o r l a n t h a n i d e and a c t i n i d e l a s e r s .

Transition 4 f -> 4 f

Ions n 3+ .3+ 2+ 3+ .3+ 3 + 2+ P r , Nd , Sm , E u , G d , T b , Dy 3+ „ 3+ _ 3+ 2+ _ 3+ 3 + Dy , Ho , E r , Tm , Tm , Y b M

c

n

r

P

T

5d •> 4 f

Ce ,

5f + 5 f

u

3 +

T k

n

v k

2 + S m

3 +


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Lasing Media S t i m u l a t e d e m i s s i o n has been o b s e r v e d from l a n t h a n i d e elements i n gases, l i q u i d s , and s o l i d s . The l a n t h a n i d e s used and t h e number o f i o n - h o s t c o m b i n a t i o n s l a s e d i n each medium a r e g i v e n i n T a b l e I I I . F i g u r e 2 shows t h e s p e c t r a l ranges covered by lanthanide l a s e r s i n t h e d i f f e r e n t media. Gas l a s e r s o p e r a t i n g from t h e f a r i n f r a r e d t o t h e vacuum u l t r a v i o l e t a r e known ( 7 j , t h u s t h e l a n t h a n i d e s c o v e r o n l y a m o d e s t range f o r t h i s medium. L i q u i d l a s e r a c t i o n from l a n t h a n i d e ions o r organic dye molecules i s l i m i t e d t o wavelengths between/the i n f r a r e d and u l t r a v i o l e t t r a n s m i s s i o n c u t - o f f s , t h e r e f o r e t h e s p e c t r a l coverages o f both a r e comparable. I n s o l i d s , l a n t h a n i d e s d o m i n a t e both t h e number o f l a s e r s a n d Table III.

L a s i n g m e d i a a n d number o f i o n - h o s t used f o r l a n t h a n i d e l a s e r s .

combinations

Lanthanides

Total

Gases Metal vapors

Sm, E u ( I , I I ) , Tm, Y b ( I , I I ) 3+

6

Nd

1

Molecular vapors Liquids Chelates

3 +

3 +

Nd , Eu , T b 3+

Aprotic solvents

3 +

28

Nd

8

Sol i d s Glasses Crystals

3 +

3 +

Nd , Ho , E r Divalent:

3 +

2 +

3 +

, Tm , Y b 2 +

Sm , Dy , Tm

3 +

>100

2 +

3 +

3 +

t h e w a v e l e n g t h r a n gTer i vc ao vl ee rn et d: . a Tlhle oe nx lc ye p to t hPemr ,i o nSsm u s e d f>200 or s o l i d - s t a t e l a s e r s a r e a few i r o n group ions (Cr3+, V + , N i 2 + , Co "*}; s e m i c o n d u c t o r a n d c o l o r - c e n t e r l a s e r s c o m p l e t e t h e category o f s o l i d - s t a t e lasers. 2

2

Gases. Gas l a s e r s a r e a t t r a c t i v e f o r high-power, h i g h e f f i c i e n c y systems and o f f e r advantages o f low m a t e r i a l s c o s t , a b i l i t y t o f l o w t h e l a s i n g medium t o remove h e a t , and l o w s u s c e p t i b i l i t y t o damage a n d d i s t o r t i o n d u e t o h i g h i n t e n s i t y o p t i c a l f i e l d s . Two a p p r o a c h e s t o o b t a i n i n g l a n t h a n i d e l a s e r a c t i o n i n a gaseous media a r e ( 1 ) l a n t h a n i d e metal vapors excited i n a gas discharge tube, and (2) lanthanide molecular v a p o r s e x c i t e d o p t i c a l l y o r w i t h a n e l e c t r o n beam.



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Lasers

281

S t i m u l a t e d e m i s s i o n i n t h e i n f r a r e d w a v e l e n g t h r e g i o n has been o b s e r v e d from n e u t r a l and/or s i n g l y - i o n i z e d atoms o f f o u r l a n t h a n i d e s : samarium, e u r o p i u m , t h u l i u m and y t t e r b i u m . A l i s t i n g o f r a r e - e a r t h v a p o r l a s e r s , w a v e l e n g t h s , and r e f e r e n c e s i s g i v e n i n R e f . 7. B e c a u s e t h e e n e r g y l e v e l s t r u c t u r e s f o r l a n t h a n i d e v a p o r s ' a r e c o m p l e x and c o m p r e h e n s i v e s p e c t r o s c o p i c d a t a i s n o t a l w a y s a v a i l a b l e , i d e n t i f i c a t i o n o f some o f t h e l a s e r t r a n s i t i o n s and d e t a i l s o f t h e m e c h a n i s m s f o r p o p u l a t i o n i n v e r s i o n are u n c e r t a i n . E x p e r i m e n t a l l y , the l a n t h a n i d e metal v a p o r t o g e t h e r w i t h a b u f f e r gas i s e x c i t e d i n a s t a n d a r d g a s d i s c h a r g e t u b e e q u i p p e d w i t h w i n d o w s a n d p l a c e d w i t h i n an optical resonator cavity. Population inversion i s obtained u s i n g c u r r e n t p u l s e s up t o s e v e r a l h u n d r e d a m p e r e s a n d d u r a t i o n s o f a few m i c r o s e c o n d s o r l o n g e r . The q u a n t u m e f f i c i e n c y of t r a n s i t i o n s used f o r s t i m u l a t e d e m i s s i o n to not e x c e e d 40 p e r c e n t . The o v e r a l l e l e c t r i c a l e f f i c i e n c y o f t h e l a s e r i s c o n s i d e r a b l y l e s s . R e c e n t l y an a v e r a g e p o w e r o f 2W was r e p o r t e d ( 8 ) f o r a H e - E u i o n l a s e r o p e r a t i n g on t h e 1.0019-ym Eu I I l i n e a t a p u l s e r e p e t i t i o n f r e q u e n c y o f 10 kHz. A n o t h e r a p p r o a c h t o gas l a s e r a c t i o n i s t o use f - f t r a n s i t i o n s of o p t i c a l l y - e x c i t e d lanthanide molecular vapors. The s p e c t r o s c o p i c p r o p e r t i e s o f s e v e r a l r a r e - e a r t h t r i h a l i d e a l u m i n u m c h l o r i d e c o m p l e x e s a n d v a r i o u s r a r e - e a r t h c h e l a t e s has b e e n s t u d i e d ( 9 j and o p t i c a l g a i n o b s e r v e d f o r a N d - A l - C l v a p o r c o m p l e x ( 1 0 ) . M e a s u r e m e n t s o f t h e f l u o r e s c e n c e k i n e t i c s show evidence of strong e x c i t e d - s t a t e e x c i t e d - s t a t e quenching. This p l u s t h e low m o l e c u l a r d e n s i t i e s a c h i e v a b l e r e d u c e t h e a t t r a c t i v e n e s s of these systems f o r p r a c t i c a l l a s e r a p p l i c a t i o n s . L i q u i d s . L a n t h a n i d e l a s e r a c t i o n has b e e n o b t a i n e d f o r two g r o u p s o f l i q u i d s : m e t a l l o - o r g a n i c and i n o r g a n i c a p r o t i c l i q u i d s . The f i r s t g r o u p i n c l u d e s c h e l a t e l a s e r s ( N d , E u , T b ) w h i c h a r e r e v i e w e d by L e m p i c k i and S a m e l s o n ( J J _ ) ; r e s e a r c h on a p r o t i c m a t e r i a l s and s y s t e m s f o r h i g h - p o w e r , p u l s e d l i q u i d l a s e r s a r e r e v i e w e d by S a m e l s o n a n d K o c h e r ( 1 2 ) . Stimulated e m i s s i o n i n both l i q u i d s o c c u r s between 4 f s t a t e s o f t r i v a l e n t l a n t h a n i d e s . The t u n i n g r a n g e s o f t h e s e l a s e r s a r e s m a l l compared t o t h a t o b t a i n a b l e from o r g a n i c dye l a s e r s ( 1 3 ) . The s p e c t r o s c o p i c p r o p e r t i e s o f l a n t h a n i d e s i n l i q u i d s a r e c h a r a c t e r i z e d by b r o a d a b s o r p t i o n a n d e m i s s i o n b a n d s w i t h l i n e widths t h a t approach those i n g l a s s e s . Lanthanide f l u o r e s c e n c e in l i q u i d s i s l e s s p r e v a l e n t than i n s o l i d s because high f r e q u e n c y v i b r a t i o n s a s s o c i a t e d w i t h t h e s o l v e n t cause nonr a d i a t i v e r e l a x a t i o n o f e x c i t e d e l e c t r o n i c s t a t e s . In c h e l a t e s , the lanthanide ion i s complexed to several organic groups or l i g a n d s . C h e l a t e s a r e s o l u b l e i n many o r g a n i c s o l v e n t s . L e m p i c k i (1_4) l i s t s s e v e r a l l i g a n d s , c a t i o n s , and s o l v e n t s c o m m o n l y u s e d f o r r a r e - e a r t h c h e l a t e l a s e r s . As i n g l a s s e s , the wavelengths of t r a n s i t i o n s e x h i b i t small s h i f t s with changing l i g a n d or c a t i o n (15).


282

LANTHANIDE

AND

ACTINIDE

CHEMISTRY

AND

SPECTROSCOPY

S t i m u l a t e d e m i s s i o n i s a c h i e v e d b y o p t i c a l pumping w i t h x e n o n - f i l l e d f l a s h l a m p s i n o p t i c a l c a v i t i e s and r e s o n a t o r s s i m i l a r t o t h o s e used i n s o l i d - s t a t e l a s e r s . The p r i n c i p a l pumping f o r Eu and T b c h e l a t e l a s e r s i s a s c r i b e d t o a b s o r p t i o n into the s i n g l e t s t a t e of the l i g a n d followed by intersystem c r o s s i n g t o t h e t r i p l e t s t a t e and s u b s e q u e n t i n t e r m o l e c u l a r t r a n s f e r t o a n e x c i t e d s t a t e o f t h e l a n t h a n i d e (TJ_). B e c a u s e the s i n g l e t absorption i s very strong at the c o n c e n t r a t i o n n e c e s s a r y f o r l a s i n g , o n l y small volumes o f a c t i v e m a t e r i a l can be pumped e f f e c t i v e l y . F o r t h e Nd c h e l a t e l a s e r , many a b s o r p t i o n b a n d s o f N d ^ a r e b e l o w t h e l i g a n d b a n d s and a r e utilized for optical excitation. The f l u o r e s c e n c e quantum e f f i c i e n c y o f e x c i t e d l a n t h a n i d e s i n most l i q u i d s i s v e r y low. T o r e d u c e f l u o r e s c e n c e q u e n c h i n g due t o i n t e r a c t i o n s w i t h h i g h - f r e q u e n c y v i b r a t i o n s i n l i q u i d s , s o l v e n t m o l e c u l e s s h o u l d have no t i g h t l y bonded atoms o f low a t o m i c m a s s (1_6). S o l v e n t s c o n t a i n i n g h y d r o g e n o r o t h e r l i g h t atoms a r e t h e r e f o r e u n d e s i r a b l e . A p r o t i c l i q u i d l a s e r m a t e r i a l s c o n s i s t o f s o l u t i o n s o f a r a r e - e a r t h s a l t and a n i n o r g a n i c aprotic solvent. The s p e c t r o s c o p i c p r o p e r t i e s and c h e m i s t r y o f a p r o t i c N d l a s e r l i q u i d s plus references to e a r l i e r s t u d i e s are discussed by B r e c h e r a n d F r e n c h (17_). The o s c i l l a t o r s t r e n g t h s and fluorescence l i f e t i m e s are comparable to those in s o l i d s with quantum e f f i c i e n c i e s near u n i t y . S i n c e f l u o r e s c e n c e l i n e widths are s m a l l e r than in g l a s s e s , the s t i m u l a t e d emission c r o s s s e c t i o n s a r e l a r g e r (18>), a l t h o u g h s t i l l l e s s t h a n i n c r y s t a l s . A p r o t i c l i q u i d l a s e r m a t e r i a l s and r e f e r e n c e s a r e l i s t e d i n R e f . 19. T h u s f a r o n l y Nd3+ has b e e n u s e d a s t h e l a s e r ion although other lanthanide ions could a l s o be used.


+

3 +

Sol i d s . S o l i d s are the most w i d e l y used host f o r l a n t h a n i d e and a c t i n i d e l a s e r a c t i o n . H o s t s i n c l u d e o v e r 200 d i f f e r e n t i o n - c r y s t a l c o m b i n a t i o n s a n d n u m e r o u s g l a s s e s . The n u m b e r o f i o n - c r y s t a l l a s e r c o m b i n a t i o n s f o r e a c h i o n i s shown i n F i g . 3. Lanthanide ions are g e n e r a l l y introduced into s o l i d s as a s u b s t i t u t i o n a l i m p u r i t y i n c o n c e n t r a t i o n s of -1%. O s c i l l a t i o n has a l s o b e e n o b t a i n e d w i t h t h e l a n t h a n i d e a s a s t o i c h i o m e t r i c c o m p o n e n t o f t h e h o s t . Among t h e d e s i r e d p r o p e r t i e s o f a l a s e r host, in a d d i t i o n to a b i l i t y to incorporate the lanthanide i o n w i t h a homogeneous doping d i s t r i b u t i o n , a r e high o p t i c a l q u a l i t y , t r a n s p a r e n c y t o t h e e x c i t a t i o n and l a s e r w a v e l e n g t h s , hardness s u f f i c i e n t f o r good o p t i c a l f i n i s h i n g , r e s i s t a n c e t o damage b y l a s e r - i n d u c e d e l e c t r i c b r e a k d o w n , a n d , i n t h e c a s e o f high r e p e t i t i o n r a t e o r c o n t i n u o u s o p e r a t i o n , good thermal c o n d u c t i v i t y and s m a l l s t r e s s - o p t i c c o e f f i c i e n t s . R e c e n t r e v i e w s o f s o l i d - s t a t e l a s e r s a r e g i v e n i n R e f s . 20, 2 1 , 22. M o r e t h a n 140 d i f f e r e n t c r y s t a l l i n e h o s t s h a v e b e e n u s e d f o r l a n t h a n i d e l a s e r s . T h e s e i n c l u d e s i m p l e and m i x e d o x i d e s a n d f l u o r i d e s , and m o r e c o m p l e x c o m p o s i t i o n s and s t r u c t u r e s ( 2 1 ) .


WEBER


14.

Lanthanide

Figure 2.

Figure 3.

and Actinide

Lasers

Spectral range of lanthanide lasers in various media

Number of different ion-crystal laser combinations grouped by lasing ion


283


284

LANTHANIDE

AND

ACTINIDE

CHEMISTRY

AND

SPECTROSCOPY

A l t h o u g h a l a r g e number o f c r y s t a l l i n e l a n t h a n i d e l a s e r s have b e e n e x p l o r e d , o n l y a v e r y few h a v e a c h i e v e d a n y p r a c t i c a l a c c e p t a n c e , the prime example b e i n g t h e g a r n e t Y3AI5O12 (YAG). T h i s m a t e r i a l has a p a r t i c u l a r l y f a v o r a b l e c o m b i n a t i o n o f being a very hard, o p t i c a l l y i s o t r o p i c c r y s t a l with s i t e s s u i t a b l e f o r t r i v a l e n t lanthanide s u b s t i t u t i o n without charge compensation. When t h e s u b s t i t u t i o n a l s i t e s f o r t r i v a l e n t l a n t h a n i d e o r a c t i n i d e ions are d i v a l e n t , such a s i n a l k a l i n e earth f l u o r i d e s , excessive f l u o r i n e or other charge-compensating ions are added to maintain charge n e u t r a l i t y . A thorough d i s c u s s i o n o f t h e c h e m i s t r y and g r o w t h o f l a s e r c r y s t a l s i s given by Nassau (23). In g l a s s , l a s e r a c t i o n has b e e n o b s e r v e d o n l y f r o m t r i v a l e n t l a n t h a n i d e s (24-, 2 5 ) . H o s t s i n c l u d e o x i d e g l a s s e s ( s i l i c a t e , p h o s p h a t e , b o r a t e , g e r m a n a t e , t e l l u r i t e ) , one f l u o r i d e g l a s s ( b e r y l l i u m ) , and m i x e d g l a s s e s such a s b o r o s i l i c a t e and f l u o r o p h o s p h a t e . For a given g l a s s network f o r m e r , c h a n g e s i n t h e number and t y p e o f n e t w o r k m o d i f i e r ions a f f e c t the s p e c t r o s c o p i c p r o p e r t i e s of the lanthanides. This feature i s applied to t a i l o r the glass compositions f o r s p e c i f i c l a s e r a p p l i c a t i o n s . I f each composition i s d e f i n e d a s a new g l a s s , t h e n t h e n u m b e r o f i o n - g l a s s c o m b i n a t i o n s l a s e d b e c o m e s v e r y l a r g e and t h e n u m b e r i n T a b l e I I I i s unknown a n d not very meaningful. G l a s s e s a r e i n h e r e n t l y a d i s o r d e r e d medium, t h e r e f o r e , the l o c a l environment at each l a n t h a n i d e s i t e i s s l i g h t l y d i f f e r e n t . T h i s appears a s s p e c t r a l b r o a d e n i n g and s i t e - d e p e n d e n t t r a n s i t i o n p r o b a b i l i t i e s . One m a n i f e s t a t i o n o f t h i s i n h o m o g e n e i t y i s s p e c t r a l h o l e b u r n i n g i n t h e g a i n p r o f i l e (2(5, 2 7 j . B e c a u s e the s t i m u l a t e d e m i s s i o n c r o s s s e c t i o n s are d i f f e r e n t at each s i t e , t h e e n e r g y e x t r a c t e d f r o m an i n h o m o g e n e o u s s y s t e m i s always l e s s than t h a t o b t a i n a b l e from a homogeneous system o f t h e same a v e r a g e c r o s s s e c t i o n ( 2 8 ) . The v i b r a t i o n a l s p e c t r u m o f t h e h o s t i s p a r t i c u l a r l y i m p o r t a n t f o r d e t e r m i n i n g t h e r a t e o f n o n r a d i a t i v e d e c a y and f l u o r e s c e n c e quantum e f f i c i e n c y o f l a n t h a n i d e s i o n s . S t u d i e s show t h a t i n b o t h c r y s t a l s and g l a s s e s , t h e r a t e o f m u l t i phonon e m i s s i o n i s determined p r i n c i p a l l y by the s i z e o f the e n e r g y gap t o t h e n e x t l o w e r l e v e l and t h e n u m b e r o f p h o n o n s r e q u i r e d to conserve energy (29). Therefore hosts in which t h e maximum p h o n o n s e n e r g i e s a r e r e l a t i v e l y s m a l l , e . g . , L a C l 3 , have more numerous and e f f i c i e n t f l u o r e s c i n g s t a t e s . U n f o r t u n a t e l y such m a t e r i a l s f r e q u e n t l y have poor p h y s i c a l p r o p e r t i e s f o r p r a c t i c a l l a s e r s . In g l a s s e s , t h e v i b r a t i o n a l f r e q u e n c i e s a s s o c i a t e d with the g l a s s network former, e.g., t h e S i O 4 t e t r a h e d r a , a r e c o m p a r a t i v e l a r g e and t h e n u m b e r o f f l u o r e s c i n g s t a t e s s m a l l . F o r t h i s r e a s o n t h e number o f l a n t h a n i d e l a s e r t r a n s i t i o n s i n g l a s s e s i s much l e s s t h a n i n crystals.



14.

WEBER

Lanthanide

and Actinide

285

Lasers

T h e o p t i c a l p u m p i n g e f f i c i e n c y and o u t p u t p o w e r o f many l a s e r s i s i n c r e a s e d by c o d o p i n g t h e m e d i u m w i t h o t h e r i o n s w h i c h a b s o r b pump r a d i a t i o n and e f f e c t i v e l y t r a n s f e r t h e e x c i t a t i o n t o t h e u p p e r l a s e r l e v e l . T h i s t r a n s f e r may be e i t h e r r a d i a t i v e o r n o n r a d i a t i v e . In g e n e r a l , s e n s i t i z a t i o n s c h e m e s u s e d f o r p h o s p h o r s and o t h e r l u m i n e s c e n c e p h e n o m e n a a r e a l s o a p p l i c a b l e t o l a s e r s (30_) • R e q u i r e m e n t s f o r t h e s e n s i t i z e r i o n i n c l u d e ( a ) no g r o u n d - o r e x c i t e d - s t a t e a b s o r p t i o n a t t h e l a s e r w a v e l e n g t h , (b) a b s o r p t i o n bands w h i c h complement r a t h e r than compete w i t h a b s o r p t i o n bands o f the l a s e r i o n (because the f l u o r e s c e n c e conversion e f f i c i e n c y u s u a l l y i s l e s s f o r the f o r m e r ) , ( c ) one o r more m e t a s t a b l e e n e r g y l e v e l s a b o v e t h e u p p e r l a s e r l e v e l , and ( d ) no o t h e r p a i r s o f l e v e l s w h i c h c a n q u e n c h t h e a c t i v a t o r f l u o r e s c e n c e . In a d d i t i o n , f o r e f f i c i e n t t r a n s f e r t h e c o n c e n t r a t i o n o f s e n s i t i z e r i o n s m u s t be s u f f i c i e n l y high to provide s i g n i f i c a n t t r a n s f e r w i t h i n the f l u o r e s c e n c e l i f e t i m e of the a c t i v a t o r . P o s s i b l e s e n s i t i z e r s f o r l a n t h a n i d e and a c t i n i d e i o n s i n c l u d e o t h e r l a n t h a n i d e and a c t i n i d e i o n s , o t h e r t r a n s i t i o n g r o u p i o n s , and m o l e c u l a r c o m p l e x e s . T h e s e may be p r e s e n t e i t h e r as a d d e d i m p u r i t i e s o r as a c o m p o n e n t o f t h e h o s t . O f t h e many s e n s i t i z a t i o n s c h e m e s r e p o r t e d , some o f f e r o n l y m a r g i n a l i m p r o v e m e n t . The m o s t e f f i c i e n t c r y s t a l l a s e r i s " a l p h a b e t " h o l m i u m : Ho3+ s e n s i t i z e d by E r , T m , a n d Y b ( 3 1 ) . The a b s o r p t i o n b a n d s o f t h e s e i o n s c o m b i n e t o f o r m a quasi-continuous spectrum. V i a a complex cascade, energy a b s o r b e d by t h e v a r i o u s i o n s i s e v e n t u a l l y t r a n s f e r r e d t o t h e l 7 l a s i n g level of Ho3 . The c o n c e p t o f u p c o n v e r s i o n (32) i n w h i c h h i g h e r - l y i n g s t a t e s o f an a c t i v a t o r a r e e x c i t e d by s u c c e s s i v e e n e r g y t r a n s f e r s f r o m a l e s s e n e r g e t i c s e n s i t i z e r has a l s o b e e n applied to lanthanide l a s e r s (33). A l i s t o f s e n s i t i z e d l a n t h a n i d e l a s e r s i s g i v e n i n T a b l e IV. The l a s e r t r a n s i t i o n s a r e shown i n t h e n e x t s e c t i o n ; f o r f i g u r e s o f t h e e n e r g y l e v e l s a n d t r a n s i t i o n o f t h e s e n s i t i z e r and a c t i v a t o r i o n s and t h e o r i g i n a l r e f e r e n c e s s e e R e f s . 21 and 34. O t h e r s e n s i t i z a t i o n s c h e m e s a r e known, b u t o n l y t h o s e a c t u a l l y used f o r l a s e r s a r e i n c l u d e d . These have most commonly used f - f t r a n s i t i o n s o f l a n t h a n i d e s . P o s s i b l e d-d s e n s i t i z a t i o n schemes have a l s o been noted ( 3 5 ) . 3 +

5

3 +

3 +

+

Survey o f Lanthanide

Ions

W i t h t h e e x c e p t i o n o f p r o m e t h i u m , s t i m u l a t e d e m i s s i o n has been r e p o r t e d f o r a l l o f the l a n t h a n i d e s . The t r a n s i t i o n s used and t h e l a s i n g c h a r a c t e r i s t i c s o f e a c h a r e r e v i e w e d b e l o w . More d e t a i l e d d i s c u s s i o n s o f t h e s p e c t r o s c o p i c f e a t u r e s o f t h e i o n and t h e p r o p e r t i e s o f t h e h o s t t h a t i n f l u e n c e t h e p o t e n t i a l f o r l a s e r a c t i o n a r e p r e s e n t e d i n a r e v i e w a r t i c l e (1_9) a n d a book (36) d e v o t e d t o r a r e - e a r t h l a s e r s .


286

L A N T H A N I D E A N D ACTINIDE CHEMISTRY A N D SPECTROSCOPY

T a b l e IV. Ions used as s e n s i t i z e r s f o r o p t i c a l l y - p u m p e d lanthanide lasers.


Laser Ion

Laser transition

Nd

3 +

4 p

Tb

3 +

5

Dy

3 +

Ho

3 +

4 l

3/2^ V

? F

I

5 r

I

C e 3 +

ll/2

'

C r 3 +

M n 2 +

'

U

' °

2 +

'

( V 0

4

) 3

"

G ( j 3 +

5

\ 3/A 5

Sensitizer ion(s)

5/2 Er

8

3 +

Er 3 +

3 +

3 +

3 +

3 +

, Tm , Yb , C r , Fe , Y b (*)

Ni

2 +

3 +

C Er

3

+

4

T

Vh

.ITQ/Q-^ 1 3 / 2 . I I1C /5O/ 2 T -> I 9/2 15/2 4

h

Tm

3 +

A

\

+

\

Y D

Er

3 +

Yb

3 +

2 f

Cr

F

N d 3 +

5/2^ 7/2

;

3 +

5

*Multistep upconversion

v

, Yb ,

J

%+ H

3 +

~Y b Y b (*)

'

Cr

3 +

J

C r 3 +

process

Energy l e v e l diagrams and l a s i n g t r a n s i t i o n s f o r a l l t r i v a l e n t l a n t h a n i d e i o n s a r e shown i n F i g . 4 a n d 5 ( t o s i m p l i f y the diagrams, t h e extent o f t h e c r y s t a l l i n e Stark s p l i t t i n g , which v a r i e s with host, i s n o t i n d i c a t e d ) . References t ot h e o r i g i n a l r e p o r t s a r e g i v e n i n R e f . 7 f o r g a s e s , i n R e f . 14 f o r l i q u i d s , a n d i n R e f . 21 f o r s o l i d s . n

T r i v a l e n t Ions. Energy l e v e l s a s s o c i a t e d with t h e 4 f ground e l e c t r o n i c c o n f i g u r a t i o n o f t h e t r i v a l e n t lanthanides a r e w e l l u n d e r s t o o d f o r s t a t e s u p t o - 3 0 , 0 0 0 - 4 0 , 0 0 0 cnr" b o t h e x p e r i m e n t a l l y (37) and t h e o r e t i c a l l y (38, 3 9 ) . The l i g a n d o r c r y s t a l f i e l d o f t h e host reduces t h e ( 2 J + 1 ) - f o l d degeneracy of t h e f r e e - i o n s t a t e s . Because t h e 4 f e l e c t r o n s a r e s h i e l d e d by t h e o u t e r 5 s a n d 5 p 6 e l e c t r o n s , t h e s h i f t i n t h e c e n t e r o f g r a v i t y o f t h e f r e e - i o n energy l e v e l s and t h e extent o f t h e c r y s t a l l i n e Stark s p l i t t i n g a r e s m a l l , on t h e order o f a few h u n d r e d cm-1, a n d v a r y w i t h t h e h o s t . L e v e l s o f 4 f 5d and o t h e r e x c i t e d c o n f i g u r a t i o n s a r e a t h i g h e r e n e r g i e s and have b e e n i n v e s t i g a t e d f o r w i d e b a n d g a p f l u o r i d e h o s t s (40, 41_, 4 2 ) . In many m a t e r i a l s , h o w e v e r , t h e l a t t e r l e v e l s a r e n e a r o r a b o v e the fundamental a b s o r p t i o n edge and t h e r e f o r e o f l i m i t e d usef u l n e s s f o r o p t i c a l pumping o r l a s i n g . 1

2

n - 1



35,-

3+

Figure 4.

Ce

d

CO

8

;5d

3+

Pr

s

3

3

3

H.

S

H

F

6

P»

"4

- H

-

3

3

d

O)

Nd

3+

3

Pm

3

Sm

6

H

C

S/2

J

CO

3

Eu

d

5

5

D„

D,

Energy levels and laser transitions for trivalent ions of the first half of the lanthanide series. Approximate wavelengths of transitions are given in micrometers.

d

2

3p

*3/2


CO

Gd

d

3+

3

w w w


re 5.

3

Dy

H„ 3

Ho

C\J

^ 1.67

I 1.40 |

« CO

5,

5,

\ %

5

S„

(D

—

in IT) o"

CO

3

Er

CO

r-

00 CN

4

S

'9/2

Tm

J

3

H

e

Energy levels and laser transitions for trivalent ions of the second half of the lanthanide series. Approximate wavelengths of transitions are given in micrometers.

6

0.75|

J.01

L

0.981

L _ 1.21

2.37 J.49 ^ 2.05

0.851

L

5

0.70


0.55 |

1.

0.67 ^

14.

WEBER

Lanthanide

and Actinide

289

Lasers

CeAlum. N e a r - u l t r a v i o l e t l a s i n g f r o m t h e l o w e s t 5d b a n d to s t a t e s of the 4 f ground c o n f i g u r a t i o n o f C e i n L i Y F 4 was r e p o r t e d r e c e n t l y (43). E x c i t a t i o n was a c h i e v e d by p u m p i n g i n t o h i g h e r l y i n g 5d b a n d s a t 248 o r 193 nm u s i n g e i t h e r a K r F o r an A r F e x c i m e r l a s e r , r e s p e c t i v e l y . A n e f f i c i e n t 4 - l e v e l l a s i n g s c h e m e i s f o r m e d by t r a n s i t i o n s t e r m i n a t i n g on the F7/2 s t a t e . O s c i l l a t i o n a l s o occurs to the ^5/2 ground s t a t e (44), t h e w a v e l e n g t h ( 3 0 8 nm) i s t h e s h o r t e s t o f a l l l a n t h a n i d e l a s e r s . Because the C e ^ f l u o r e s c e n c e band i s broad, t h e l a s e r a c t i o n i s t u n a b l e . The t u n i n g r a n g e a c h i e v e d t o d a t e f o r b o t h t r a n s i t i o n s i n C e : Y L F i s -500 cm"" (44). Although ckf l a s i n g of C e and o t h e r l a n t h a n i d e s h a v e b e e n d i s c u s s e d f o r s e v e r a l y e a r s ( 4 5 , 46), t h i s was t h e f i r s t s u c c e s s f u l d e m o n s t r a t i o n s i n c e t h e v e r y e a r l y d->f l a s i n g o f Sm^ i n C a F 2 - One d i f f i c u l t y i n o b t a i n i n g o s c i l l a t i o n i s p o s s i b l e e x c i t e d - s t a t e absorption to higher l y i n g s t a t e s . This was shown t o p r e v e n t o s c i l l a t i o n o f C e + : Y A G a t r o o m t e m p e r a t u r e ( 4 7 ) . S i n c e the n e p h e l a u x e t i c e f f e c t i s d i f f e r e n t i n o x i d e and f l u o r i d e hosts, the C e bands i n LiYF4 are s h i f t e d s u f f i c i e n t l y to reduce e x c i t e d - s t a t e a b s o r p t i o n . L a s i n g from C e in other h o s t c r y s t a l s a n d g l a s s e s s h o u l d be p o s s i b l e . I f t h e c k f l i n e w i d t h i s p r e d o m i n a n t l y homogeneous, h o l e b u r n i n g and r e d u c e d energy e x t r a c t i o n c h a r a c t e r i s t i c o f g l a s s e s should not occur. 1

3 +

2

+

1


3 +

+

3

3 +

3 +

PfKUzodymluun.The e n e r g y l e v e l s c h e m e o f P r includes s e v e r a l f l u o r e s c i n g and t e r m i n a l s t a t e s f o r 4 - l e v e l o p e r a t i o n . A b s o r p t i o n b a n d s a r e few i n n u m b e r , h e n c e t h r e s h o l d s a r e h i g h f o r b r o a d b a n d o p t i c a l pumps. P u l s e d l a s e r a c t i o n has b e e n o b s e r v e d f r o m s e v e r a l e x c i t e d s t a t e s a t 300 K a n d l o w e r t e m p e r a t u r e ( 2 1 , 3 4 ) . H o s t s i n c l u d e o x i d e and f l u o r i d e c r y s t a l s . L a s i n g f r o m t h e P s t a t e s h o u l d be p o s s i b l e f r o m Pr i n g l a s s e s and l i q u i d s g i v e n a d e q u a t e pumping. The 1SQ s t a t e l o c a t e d a t ^ 4 7 , 0 0 0 cm ! a l s o e x h i b i t s f l u o r e s c e n c e i n w i d e bandgap h o s t s , such as f l u o r i d e c r y s t a l s , and has b e e n c o n s i d e r e d f o r l a s e r a c t i o n ( 4 8 ) . E x c i t a t i o n i n t o t h e 4 f 5 d and h i g h e r l y i n g b a n d s r a p i d l y d e c a y s t o ^ S g . The "ISQ->- G4 t r a n s i t i o n a t ~271 nm a n d t h e F 4 transition a t * 2 5 0 nm a r e i n t e n s e and c o u l d p r o v i d e u l t r a v i o l e t l a s e r a c t i o n i f e x c i t e d - s t a t e absorption i s not dominant. In Y3AI5O-12 t h e l o w e s t 5d b a n d i s l o c a t e d b e l o w "ISo and f l u o r e s c e s w i t h h i g h q u a n t u m e f f i c i e n c y a t t e m p e r a t u r e s 4f t r a n s i t i o n s i n the nearu l t r a v i o l e t could provide tunable laser action barring strong e x c i t e d - s t a t e a b s o r p t i o n . Tunable l a s e r a c t i o n i n the 215-260 nm r a n g e i n L i Y F 4 may a l s o be p o s s i b l e ( 5 0 ) . 3 +

3

Q

3 +

-

1

2

3 +

As e v i d e n t f r o m T a b l e I I I and F i g . 3, N d i s t h e m o s t e x t e n s i v e l y e x p l o i t e d l a s e r i o n and i s t h e one t r i v a l e n t lanthanide ion lased in a l l states of matter. T h e many o p t i c a l a b s o r p t i o n b a n d s d i s t r i b u t e d t h r o u g h t h e v i s i b l e bl&odymium.


290

LANTHANIDE AND

ACTINIDE

CHEMISTRY

AND

SPECTROSCOPY

and n e a r - i n f r a r e d s p e c t r a l r e g i o n s c o m b i n e d w i t h r a p i d e n e r g y c a s c a d e t o t h e m e t a s t a b l e ^3/2 s t a t e p r o v i d e g o o d o p t i c a l p u m p i n g e f f i c i e n c y f o r b r o a d b a n d s o u r c e s . The F 3 / 2 ^ I n / 2 t r a n s i t i o n i s g e n e r a l l y t h e m o s t i n t e n s e t r a n s i t i o n and f o r m s a n e a r - i d e a l 4 - l e v e l l a s i n g scheme a t ambient t e m p e r a t u r e s . P u l s e d l a s e r a c t i o n has b e e n o b s e r v e d f r o m ^3/2 t o a l l I j s t a t e s and cw l a s e r a c t i o n t o t h e I n / 2 ^13/2 states ( 2 2 ) . L a s i n g i n v o l v i n g 5 d e m i s s i o n , w n i c h w o u l d De t u n a b l e i n t h e r e g i o n - 1 7 2 - 1 9 5 nm i n v a r i o u s f l u o r i d e h o s t s , has b e e n m e n t i o n e d (46J b u t n o t d e m o n s t r a t e d . S t i m u l a t e d e m i s s i o n has b e e n r e c o r d e d f o r N d i n more t h a n 100 d i f f e r e n t c r y s t a l s i n c l u d i n g d o p e d s i n g l e c r y s t a l s , m i x e d c r y s t a l s ( s o l i d s o l u t i o n s ) , and s e v e r a l c r y s t a l s i n w h i c h Nd i s a s t o i c h i o m e t r i c c o m p o n e n t o f t h e h o s t ( 2 1 ) . Because the s p e c t r o s c o p i c p r o p e r t i e s are host dependent, the s e l e c t i o n of materials provides v a r i a b i l i t y with respect to c r o s s s e c t i o n s and l i f e t i m e s as w e l l as o t h e r p h y s i c a l properties. The m o s t w i d e l y u s e d s o l i d - s t a t e l a s e r i s Nd:YAG. T h e p r o p e r t i e s and o p e r a t i n g c h a r a c t e r i s t i c s o f t h i s l a s e r h a v e b e e n t h o r o u g h l y r e v i e w e d b y D a r n e l m e y e r (5]_). A l l f l u o r e s c e n c e t r a n s i t i o n s from Ft o t h e I , s t a t e s h a v e l a s e d i n YAG. Cooling lowers the t h r e s h o l d f o r the F ~ -^1 transitions; the o t h e r t r a n s i t i o n s o p e r a t e at ambient' temperatures. Laser a c t i o n was a l s o o b s e r v e d f r o m e p i t a x i a l l y - g r o w n t h i n f i l m s ( 5 2 ) and s i n g l e - c r y s t a l f i b e r s ( 5 3 ) o f Nd:YAG. O t h e r c o m m e r c i a l l y a v a i l a b l e Nd-doped l a s e r c r y s t a l s i n c l u d e Y A 1 0 , L i Y F , L a B e 0 , and L a F ^ . Neodymium i s a f s o t h e most e x t e n s i v e l y d e v e l o p e d g l a s s l a s e r ( 2 5 ) . S y s t e m a t i c s t u d i e s h a v e shown t h a t spectroscopic p r o p e r t i e s can b e t a i l o r e d , w i t h i n l i m i t s , b y s e l e c t i n g the g l a s s n e t w o r k f o r m i n g and n e t w o r k m o d i f y i n g i o n s ( 5 4 ) . Many d i f f e r e n t o x i d e , f l u o r i d e , and m i x e d g l a s s f o r m e r s h a v e b e e n investigated (55). Thin f i l m waveguides of N d r s i l i c a t e glass have been f a b r i c a t e d (56). At the o t h e r extreme, l a s e r s f o r i n e r t i a l c o n f i n e m e n t f u s i o n e x p e r i m e n t s (57) e m p l o y l o n g c h a i n s o f Nd-doped g l a s s d i s k a m p l i f i e r s c o n t a i n i n g e l l i p t i c a l d i s k s i n s i z e s u p t o 5 0 x 600 nm. V a r i o u s b r o a d b a n d s o u r c e s e m p l o y e d t o o p t i c a l l y pump Nd i n c l u d e t u n g s t e n , m e r c u r y , x e n o n , and k r y p t o n l a m p s . The l a s t s o u r c e p r o v i d e s a n e s p e c i a l l y good s p e c t r a l match to the n e a r i n f r a r e d a b s o r p t i o n bands of N d i n YAG. To reduce l a t t i c e heating r e s u l t i n g from the multiphonon emission decay to the F3/2 s t a t e , s e m i c o n d u c t o r d i o d e s a n d l a s e r s o u r c e s a t 0 . 8 - 0 . 9 ym n a v e pumped Nd l a s e r s ( 5 8 ) . S u n - p u m p e d Nd and c h r o m i u m s e n s i t i z e d Nd l a s e r s h a v e b e e n d e m o n s t r a t e d and considered f o r space a p p l i c a t i o n s (59). Lasing of N d b y e l e c t r o n beam e x c i t a t i o n has a l s o b e e n r e p o r t e d (b0). N e o d y m i u m c h e l a t e l a s e r a c t i o n a t 300 K was o b t a i n e d , b u t t o r e d u c e n o n r a d i a t i v e d e c a y o f F3/2, a l i g a n d c o n t a i n i n g 4

4

a n c l


3 +

4

/ 2

4

/ 2

3

4

2

2

5

3 +

3 +

3 +


14.

Lanthanide

WEBER

and Actinide

291

Lasers


f l u o r i n e i n p l a c e o f h y d r o g e n w a s u s e d (61_). L a s e r a c t i o n h a s been observed from s e v e r a l a p r o t i c l i q u i d s c o n s i s t i n g o f a s o l u t i o n o f a Nd s a l t a n d a n i n o r g a n i c a p r o t i c s o l v e n t . A l i s t of lasers andsolvents i s given i n Ref. 19. G a i n was m e a s u r e d f o r t h e t r a n s i t i o n from one m o l e c u l a r vapor, a NdCl3-A1CI3 complex ( t u ) . Intense e x c i t e d s t a t e - e x c i t e d s t a t e quenching and low vapor pressures l i m i t t h e a t t r a c t i v e n e s s o f t h i s l a s i n g medium. T h e e x c i t e d - s t a t e k i n e t i c s f o r Nd(thd)3 c h e l a t e vapors have a l s o been i n v e s t i g a t e d a n d t h e prospects f o r laser action discussed (62). P/iomoXhiam. T h i s i o n h a s n o s t a b l e i s o t o p e s . The isotope Pm"l47 i s a b e t a e m i t t e r ( 0 . 2 2 MeV) w i t h a h a l f - l i f e o f 2.6 y e a r s . This r a d i o a c t i v i t y poses problems f o r t h e growth, f a b r i c a t i o n , operation, andl i f e t i m e o fa s o l i d - s t a t e laser. Stimulated e m i s s i o n has n o t been r e p o r t e d f o r a n y h o s t . ~ T h e e n e r g y l e v e l s c h e m e o f Pm3+ i s v e r y s i m i l a r t o t h a t o f Nd and hence i s a t t r a c t i v e f o r l a s e r a c t i o n . There a r e numerous a b s o r p t i o n bands f o r o p t i c a l pumping a n d f l u o r e s c e n c e from s t a t e t o l e v e l s o f $1 o c c u r s a t w a v e l e n g t h s r a n g i n g f r o m 0.81 t o 1.72 ym. T h e l a r g e e n e r g y g a p f r o m F-| t o 5 j g i n s u r e s high quantum e f f i c i e n c y i n most h o s t s . The most p r o m i s i n g t r a n s i t i o n f o r l a s i n g i s F]-> l5 w h i c h h a s a l a r g e branching r a t i o and no competing e x c i t e d - s t a t e a b s o r p t i o n . Krupke (63) c a l c u l a t e d f l u o r e s c e n c e i n t e n s i t i e s and t h e r a d i a t i v e l i f e t i m e o f F - j f o r Pm:YAG u s i n g J u d d - O f e l t i n t e n s i t y p a r a m e t e r s e x t r a p o l a t e d f r o m N d . T h e F]-> l5 t r a n s i t i o n a t 0.92 ym h a d a n o s c i l l a t o r s t r e n g t h w i t h i n 7 0 % o f t h e v a l u e f o r t h e F / 2 + I - | i / 2 t r a n s i t i o n o f Nd:YAG. T h e F ^ I o f t h e t r a n s i t i o n and t r a n s i t i o n s from t h e t h e r m a l l y populated F 2 state a r e also intense and l a s e r candidates. The p o s s i b i l i t y o f l a s i n g P m i n LiYF4 h a s a l s o b e e n considered (64). Thegreatest t r a n s i t i o n probability corresponds t o t h e ir-polarized e l e c t r i c - d i p o l e t r a n s i t i o n between wavelength o f 0.94 ym. E x p e r i m e n t s s u g g e s t t h a t t h e b e t a - r a y a c t i v i t y o f P m may n o t h a v e a s e v e r e e f f e c t o n t h e o p t i c a l p r o p e r t i e s o f LiYF4 i n t h e w a v e l e n g t h r e g i o n s o f i n t e r e s t ( 6 4 ) . 5

5

5

5

3 +

4

5

4

5

5

3

5

6

5

3 +

3 +

SamajLtum. S t i m u l a t e d e m i s s i o n has n o t been r e p o r t e d f o r t r i v a l e n t samarium i n a n y medium. T h e r e a r e numerous a b s o r p t i o n b a n d s a t w a v e l e n g t h s Hg/2 ( 6 5 ) . B e c a u s e o f t h e h i g h d e n s i t y o f h i g h l y i n g s t a t e s , t h e p r o b a b i l i t y f o r e x c i t e d - s t a t e a b s o r p t i o n from G5/2 may, i n many c a s e s , b e s t r o n g e r t h a n f o r s t i m u l a t e d emission. 4

6

4


292

LANTHANIDE

A N D ACTINIDE

CHEMISTRY

A N D SPECTROSCOPY

EuAopjjum. T h i s i s t h e most e x t e n s i v e l y s t u d i e d c h e l a t e l a s e r i o n . T r i v a l e n t Eu has l a s e d i n 24 o r g a n i c c h e l a t e s o l u t i o n s a t t e m p e r a t u r e s r a n g i n g f r o m - 1 5 0 t o 3 0 ° C . Some o f t h e l i g a n d s , c a t i o n s , and s o l v e n t s used a r e g i v e n i n R e f . 14. T h e p r i n c i p a l o p t i c a l p u m p i n g i s a s c r i b e d t o a b s o r p t i o n into t h e s i n g l e t state o f t h e ligand followed by intersystem c r o s s i n g t o t h e t r i p l e t s t a t e and subsequent i n t e r m o l e c u l a r t r a n s f e r t o an e x c i t e d s t a t e o f E u (11). In s o l i d s , o n l y p u l s e d D - > F 2 l a s e r a c t i o n h a s b e e n observed. Low t e m p e r a t u r e s w e r e u s e d t o n a r r o w t h e l i n e w i d t h s and r e d u c e t h e p o p u l a t i o n i n t h e F 2 t e r m i n a l s t a t e . S i n c e t h e r e a r e no i n t e n s e a b s o r p t i o n bands i n t h e v i s i b l e , l a s i n g t h r e s h o l d s were high. F o r e f f i c i e n t u t i l i z a t i o n o f t h e h i g h e r l y i n g pumps b a n d s , a r a p i d n o n r a d i a t i v e c a s c a d e t h r o u g h t h e D l e v e l s t o 5DQ i s n e c e s s a r y t o m i n i m i z e f l u o r e s c e n c e l o s s e s . A l t e r n a t i v e l y , l a s i n g c o u l d be obtained from metastable D ] and D 2 s t a t e s t o v a r i o u s l e v e l s o f F . T h e a b s o r p t i o n a n d e m i s s i o n c r o s s s e c t i o n s o f Eu3+ a r e r e l a t i v e l y s m a l l . T h e DQ-> F? l a s i n g t r a n s i t i o n i s a l s o a h y p e r s e n s i t i v e t r a n s i t i o n (66) and t h e r e f o r e very host dep e n d e n t . No s y s t e m a t i c e f f o r t a p p e a r s t o h a v e b e e n made t o e x p l o i t t h i s f e a t u r e t o improve l a s i n g performance. Because o f t h e a b s e n c e o f DQ q u e n c h i n g b y s i m p l e i o n p a i r i n t e r a c t i o n s , h i g h Eu c o n c e n t r a t i o n s a n d s t o i c h i o m e t r i c materials should beusable for l a s e r action. 3 +

5

7

Q


7

5

b

5

7

5

7

5

Gadolinium. E f f i c i e n t f l u o r e s c e n c e f r o m t h e l o w e s t e x c i t e d state P7/2 t o the g r o u n d s t a t e o c c u r s a t - 0 . 3 1 ym a n d forms a t h r e e - l e v e l l a s i n g scheme. The high t h r e s h o l d c h a r a c t e r i s t i c o f t h r e e - l e v e l operation and t h e requirements o f good host t r a n s p a r e n c y , low e x c i t e d - s t a t e a b s o r p t i o n , a n d a n u l t r a v i o l e t s o u r c e ( £ 0 . 3 ym) f o r o p t i c a l p u m p i n g a r e a l l o b s t a c l e s t o o b t a i n i n g s t i m u l a t e d e m i s s i o n . Only two cases o f l a s e r a c t i o n have been r e p o r t e d ; one i n a c r y s t a l 3 5 ° 1 2 (67) a n d one i n a s i l i c a t e g l a s s ( 6 8 ) . In both i n s t a n c e s p u m p i n g was v i a a x e n o n f l a s h l a m p a n d t h e t h r e s h o l d s were v e r y h i g h . 6

Y

A 1

ToAbXum. T h e ^Dd-Jf^ t r a n s i t i o n h a s t h e l a r g e s t f l u o r e s cence b r a n c h i n g from ^ 4 a n d forms a f o u r - l e v e l l a s e r scheme at ambient temperatures. However, s t i m u l a t e d e m i s s i o n has been o b s e r v e d i n o n l y o n e m a t e r i a l , c r y s t a l l i n e LiYF4 ( 6 9 ) . A s i n the case o f E u , t h e p r i n c i p a l a b s o r p t i o n bands f o r o p t i c a l pumping l i e i n t h e n e a r - u l t r a v i o l e t . I f t h e s e a r e used t o e x c i t e t h e D4 l e v e l , t h e 6 0 0 0 cm-" D3+ D4 e n e r g y g a p m u s t be e f f i c i e n t l y b r i d g e d . I n LiYF4 t h i s was d o n e b y u s i n g a h i g h T b c o n c e n t r a t i o n ( > 2 0 % ) s o t h a t t h e r e was r a p i d D3+ D4 d e c a y b y i o n - i o n i n t e r a c t i o n s a n d e n e r g y - c o n s e r v i n g F5-> FQ ] t r a n s i t i o n s . T h e 5 d bands o f Tb3+ a r e t h e l o w e s t - l y i n g o f ' the t r i v a l e n t l a n t h a n i d e s and, i f t o o l o w , they c a n prevent 3 +

5

1

5

5

5

7


5

7

14.

Lanthanide

WEBER

and Actinide

293

Lasers

l a s i n g d u e t o s t r o n g , D4 5 d e x c i t e d - s t a t e a b s o r p t i o n . I n UYF4, t h e n e p h e l a u x e t i c e f f e c t i s s m a l l a n d t h e 5d bands a r e s u f f i c i e n t l y high t o avoid this d i f f i c u l t y ; this i s not t h e c a s e i n many o t h e r c r y s t a l s , e s p e c i a l l y t h e o x i d e s . There i s one report o f optically-pumped T b chelate laser a c t i o n a t r o o m t e m p e r a t u r e ; t h e t h r e s h o l d was v e r y h i g h ( 7 0 ) . THe e x c i t e d - s t a t e k i n e t i c s o f T b 3 + i n v a p o r - p h a s e terbium c h e l a t e s ( 6 2 ) a n d t e r b i u m a l u m i n u m c h l o r i d e c o m p l e x e s (71_, 7 2 ) have been i n v e s t i g a t e d b u t no l a s e r a c t i o n r e p o r t e d . b

3 +

+


+

S t i m u l a t e d emission from Dy3 i n E r 3 s e n s i t i z e d B a Y z F g a t 3.02 ym i s t h e l o n g e s t w a v e l e n g t h n o n c a s c a d e l a s e r ( 7 3 ) . L a s e r a c t i o n was o b t a i n e d a t 7 7 K a n d involved a l3/2 ^' 15/2 t r a n s i t i o n . Fluorescence also occurs from t h e F g / 2 l e v e l l o c a t e d a t -21,000 cm" w i t h i n t e n s e e m i s s i o n t o H-|5/2 ^ ^ 1 3 / 2 ' b u t n o l a s e r a c t i o n h a s b e e n r e p o r t e d . T h e c o m m e n t s made e a r l i e r ? b s u t t h e a b s e n c e o f s t i m u l a t e d emission from S m a r e again apropos. Several p o s s i b l e c a s c a d e l a s e r s c h e m e s f o r Dy3+ h a v e b e e n d e s c r i b e d ( 7 4 ) . Vyt>ph,ot>AMm.

H

:

H

4

1

an
H5 l a s i n g h a s a l s o b e e n o b s e r v e d i n s i l i c a t e glass (82). Tm has o n l y a few a b s o r p t i o n bands i n t h e v i s i b l e and energy cascade i s i n e f f i c i e n t because o f t h e l a r g e energy gaps between J s t a t e s . The d e t r i m e n t a l e f f e c t s o f t h e s e c o n d i t i o n s o n o p t i c a l pumping e f f i c i e n t a r e a m e l i o r a t e d by c o - d o p i n g t h e m a t e r i a l s w i t h f l u o r e s c e n c e s e n s i t i z i n g i o n s (see Table I V ) . Recently ^ - ^ f y lasing o f Tm i n LiYF4 was o b t a i n e d b y d i r e c t e x c i t a t i o n i n t o t h e ID2 s t a t e u s i n g a X e F e x c i m e r l a s e r (83). Because t h e H 4 s t a t e decays r a d i a t i v e , t h i s l a s i n g scheme r e s u l t s i n minimal h e a t i n g o f t h e h o s t b y n o n radiative transitions. 1

:

4

4

1

3

3

3

3

3

3

3 +

3 +

3

yttvibhxm. T h e r e i s o n l y o n e a b s o r p t i o n b a n d , ^ 5 / 2 , f o r optical-pumped F5/2+ F7/2 l a s e r a c t i o n (the 5d bands begin a t e n e r g i e s >70,000 cm-';. T h e r e f o r e u n l e s s a n a r r o w b a n d resonant source such as a l i g h t - e m i t t i n g semiconductor diode or f l u o r e s c e n c e s e n s i t i z a t i o n a r e used, t h e thresholds f o r o s c i l l a t i o n a r e high. In a d d i t i o n , because t h e l a s e r t r a n s i t i o n terminates ona Stark level o f the ground-state manifold, low temperatures a r e r e q u i r e d f o r low-threshold o p e r a t i o n . In a s i l i c a t e g l a s s , l a s i n g has been o b t a i n e d a t 1.015 ym a t 7 7 K ( 8 4 ) a n d 1.06 ym a t 3 0 0 K ( 8 5 ) . L a s e r a c t i o n s h o u l d a l s o b e o b t a i n a b l e f o r Yb3+ i n s t o i c h i o m e t r i c m a t e r i a l s , 2

2


14.

WEBER

Lanthanide

and Actinide

295

Lasers

because s e l f - q u e n c h i n g i s absent, and i n a p r o t i c s o l v e n t s and c h e l a t e s s i m i l a r t o those used f o r N d lasers. 3 +


n

n _ 1

D i v a l e n t Ions. The 4 f and 4 f 5 d energy l e v e l s o f d i v a l e n t l a n t h a n i d e s have been s t u d i e d i n a l k a l i n e - e a r t h f l u o r i d e c r y s t a l s (86, 87). The 5d l e v e l s occur a t lower energies than f o r t h e i s o e l e c t r o n i c t r i v a l e n t s t a t e and i n most c a s e s e x t e n d i n t o t h e v i s i b l e . Because t h e s p i n - o r b i t parameters a r e smaller f o r the d i v a l e n t i o n s , t h e separations of the J s t a t e s o f the 4 f n c o n f i g u r a t i o n a r e reduced. A l k a l i n e - e a r t h f l u o r i d e s have been t h e p r i n c i p a l h o s t s f o r d i v a l e n t l a n t h a n i d e l a s e r s . These a r e r e l a t i v e l y s o f t , o p t i c a l l y isotropic materials. Lanthanides enter the alkaline earth s i t e s s u b s t i t u t i o n a l l y without charge compensation. Because t h e s e s i t e s have i n v e r s i o n symmetry, o n l y m a g n e t i c d i p o l e o r v i b r o n i c t r a n s i t i o n s a r e a l l o w e d between 4 f s t a t e s . T h e s e a r e weak a n d t h e r e s u l t i n g r a d i a t i v e l i f e t i m e s a r e l o n g . In c o m p a r i s o n , t h e r a d i a t i v e l i f e t i m e s o f 5d-»4f t r a n s i t i o n s , w h i c h a r e p a r i t y a l l o w e d , a r e - s h o r t . T h e 4f->-5d t r a n s i t i o n s a r e b r o a d a n d t h u s p r o v i d e good a b s o r p t i o n bands f o r o p t i c a l pumping. L a s e r a c t i o n has been r e p o r t e d f o r t h r e e d i v a l e n t l a n t h a n i d e s (21_, 3 4 ) . F i g u r e 6 s u m m a r i z e s t h e e n e r g y l e v e l s , t r a n s i t i o n s , and approximate wavelengths o f these l a s e r s . Only c r y s t a l s have been used a s h o s t s a n d r e d u c e d t e m p e r a t u r e s were used i n a l l cases. Of t h e l a n t h a n i d e s , Eu and Yb c a n be r e a d i l y r e d u c e d t o t h e d i v a l e n t s t a t e a n d r e m a i n s t a b l e i n many m a t e r i a l s . T h i s i s t r u e t o a l e s s e r d e g r e e f o r Sm a n d Tm. S p e c i a l m e t h o d s a r e u s u a l l y r e q u i r e d t o reduce t h e remaining t r i v a l e n t lanthanides to t h e d i v a l e n t s t a t e (23). These i n c l u d e i r r a d i a t i o n with x - r a y s , b e t a a n d gamma r a y s , m e t a l d i f f u s i o n , e l e c t r o l y s i s , and p h o t o c h e m i c a l r e a c t i o n . F r e q u e n t l y , t h e r e s u l t i n g m a t e r i a l s a r e n o t s t a b l e with r e s p e c t t o thermal and photochemical e f f e c t s and t h e i o n s r e v e r t back t o t h e t r i v a l e n t state. Smcuvuxm. D i v a l e n t Sm l a s e r a c t i o n h a s b e e n d e m o n s t r a t e d u s i n g b o t h d + f a n d f * f t r a n s i t i o n s . T h e f o r m e r was o b s e r v e d i n CaF2 ( 8 8 , 8 9 ) . A t o r below l i q u i d n i t r o g e n t e m p e r a t u r e s l a s i n g o c c u r s f r o m 7 0 8 t o 7 2 9 nm. F o r S m i n SrF2, t h e DQ s t a t e i s b e l o w t h e l o w e s t 5 d b a n d a n d DQ-> FI l a s i n g h a s t h e lowest t h r e s h o l d a t l i q u i d helium temperatures (90). Samarium l a s e r a c t i o n was p u l s e d u s i n g x e n o n f l a s h l a m p s o r a r u b y laser for excitation. 2 +

5

5

7

Eu/iopium. B r o a d b a n d , S t o k e s - s h i f t e d 5d->4f e m i s s i o n i s o b s e r v e d f r o m Eu^ i n many h o s t s . T h e f l u o r e s c e n c e o c c u r s i n t h e 4 0 0 - 5 0 0 nm r e g i o n a n d h a s a l i f e t i m e o f a b o u t 1-2 y s . Attempts t o observe l a s e r a c t i o n from E u i n a crystal +

2 +



296

LANTHANIDE

Figure 6.

A N D ACTINIDE

CHEMISTRY

AND

SPECTROSCOPY

Energy levels and laser transitions for divalent lanthanide ions. Approximate wavelengths of transitions are given in micrometers.


14.

WEBER

Lanthanide

and Actinide

297

Lasers

( C a F 2 ) (91_, 92) a n d a g l a s s ( 9 3 ) h a v e b e e n u n s u c c e s s f u l . T h i s i s due t o l o s s e s b y e i t h e r e x c i t e d - s t a t e a b s o r p t i o n o r c o l o r c e n t e r s b e c a u s e t r a n s i e n t a b s o r p t i o n has b e e n o b s e r v e d ( 9 2 ^ 9 4 ) . The e x c i t e d - s t a t e a b s o r p t i o n peak i n CaF2 i s t e m p e r a t u r e dependent. VyApioAium. B o t h p u l s e d and cw i n f r a r e d l a s e r a c t i o n a r e r e p o r t e d a t l i q u i d n i t r o g e n a n d h e l i u m t e m p e r a t u r e s (21_, 3 4 ) . Pump s o u r c e s i n c l u d e x e n o n , m e r c u r y , a n d t u n g s t e n l a m p s a n d s u n l i g h t . B r o a d a b s o r p t i o n bands t h r o u g h o u t t h e v i s i b l e and n e a r - i n f r a r e d plus the long l i f e t i m e of the magnetic-dipole 1 7 ~ ^ 1 t r a n s i t i o n i n C a F a n d S r F 2 (>10 ms) a r e f a v o r a b l e f o r good energy s t o r a g e .


5

5

8

2

ThixLuxm. T h i s i o n has l a s e d b o t h p u l s e d ( 9 5 ) a n d cw i n C a F 2 ( 9 6 ) , b u t t h e cw t h r e s h o l d a r e h i g h e v e n a t 4 K. E x c i t e d s t a t e a b s o r p t i o n from Fs/2 to 5d s t a t e s , w h i l e e n e r g e t i c a l l y c o m p a r a b l e t o t h e ^5/2*^7/? transition, obviously is not i n t e n s e enough t o prevent o s c i l l a t i o n . 2

l a s e r

Survey of A c t i n i d e

Ions

Q u a l i t a t i v e l y t h e r e a r e many s i m i l a r i t i e s b e t w e e n t h e e n e r g y l e v e l s and s p e c t r o s c o p i c f e a t u r e s o f l a n t h a n i d e a n d a c t i n i d e i o n s . H e n c e many o f t h e e a r l i e r comments a n d d i s cussions of lanthanide l a s e r s are a l s o apropos to p o s s i b l e a c t i n i d e l a s e r s . A s r e v i e w e d b y H e s s l e r and C a r n a l ! ( 9 7 ) , o u r k n o w l e d g e and u n d e r s t a n d i n g o f t h e e n e r g y l e v e l s a n d s p e c t r a l i n t e n s i t i e s o f t h e 5 f " c o n f i g u r a t i o n s has i m p r o v e d s i g n i f i c a n t l y i n r e c e n t y e a r s . Many o f t h e i n t e r a c t i o n s g o v e r n i n g the s p e c t r o s c o p i c p r o p e r t i e s have been s u c c e s s f u l l y parameterized. T h u s i t i s p o s s i b l e t o make s e m i - q u a n t i t a t i v e p r e d i c t i o n s about l a s i n g p r o s p e c t s . The p o s i t i o n s o f most o f t h e l o w e r J s t a t e s o f t h e g r o u n d 5 f c o n f i g u r a t i o n s o f t h e t r i v a l e n t a c t i n i d e s a r e known and a r e g i v e n f o r L a C l 3 i n Ref. 97. F i g u r e 7 shows s i m p l i f i e d e n e r g y l e v e l d i a g r a m s f o r t h e t r i v a l e n t a c t i n i d e s . The d e n s i t y of s t a t e s i n the v i s i b l e i s very high. Because o f the g r e a t e r degree o f intermediate c o u p l i n g , the J s t a t e s f o r the a c t i n i d e ion order d i f f e r e n t l y than f o r the corresponding 4 f l a n t h a n i d e i o n . L e v e l s are f r e q u e n t l y l a b e l e d by o n l y the J quantum n u m b e r b e c a u s e t h e e i g e n s t a t e s h a v e s u c h m i x e d S,L c h a r a c t e r t h a t t h e s e a r e n o l o n g e r m e a n i n g f u l q u a n t u m n u m b e r s . (We w i l l , however, sometimes l a b e l s t a t e s u s i n g Russel1-Saunders designations f o r purposes of comparison with lanthanide transitions.) We c o n s i d e r o n l y f - f t r a n s i t i o n s f o r l a s i n g . T h e a p p r o x i m a t e p o s i t i o n s o f t h e 6 d and c h a r g e t r a n s f e r bands t h r o u g h o u t t h e a c t i n i d e s e r i e s a r e known ( 9 8 , 9 9 ) . T h e 6 d bands are lower than are the c o r r e s p o n d i n g 5d bands o f the n

n



5 -

10

I 15 E

20

25

I I I I

I I

I

U

Np

3+

7/2

i-t.5/2

I I I I I I

19/2

0

5

Bk-

-^-7/2-^6

-7/2

Cf

3+

Es

3+

-^-15/2-^-8

11/2

-11/2-

-5/2

Fm

3+

Md

15/2

— 9/2

5/2

3+

6

4

2

5

-4

Figure 7. Energy levels of trivalent actinide ions: ( ), predicted levels; ( >) fluorescence and possible laser transitions, discussed in the text.

3+

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I!

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H

o

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to CD

00


14.

WEBER

Lanthanide

and Actinide

Lasers

299

l a n t h a n i d e s and t h e y u s u a l l y o v e r l a p t h e 5 f l e v e l s . Hence e x c i t a t i o n i n t o t h e s t r o n g 6d b a n d s w o u l d be f o l l o w e d by r a p i d n o n r a d i a t i v e d e c a y t o lower 5 f s t a t e s as o c c u r s f o r t h e a n a l o g o u s c a s e o f l a n t h a n i d e 5d->4f r e l a x a t i o n ( 4 9 ) . I am n o t a w a r e o f a n y r e p o r t s o f 6d->5f e m i s s i o n o f a c t i n i d e i o n s i n s o l i d s . The p r e s e n c e o f l o w - l y i n g 6d b a n d s s h o u l d p r e v e n t l a s i n g o f many v i s i b l e a n d s h o r t e r w a v e l e n g t h t r a n s i t i o n s because of intense e x c i t e d - s t a t e absorption. Therefore m o s t p r a c t i c a l a c t i n i d e l a s e r a c t i o n w i l l be l i m i t e d t o t h e infrared spectral region. The s h i e l d i n g o f t h e 5 f e l e c t r o n s o f t h e a c t i n i d e s i s l e s s and t h e r e f o r e t h e y a r e more s e n s i t i v e t o t h e i r e n v i r o n m e n t t h a n a r e t h e l a n t h a n i d e s . T h i s has s e v e r a l c o n s e q u e n c e s : ( 1 ) t h e e l e c t r o s t a t i c ( R a c a h ) p a r a m e t e r s a r e s m a l l e r and t h e s p i n o r b i t p a r a m e t e r C 5 f * % 4 f (97_), h e n c e t h e e n e r g y g a p s b e t w e e n J states are reduced. ( 2 ) The c r y s t a l - f i e l d p a r a m e t e r s a r e a p p r o x i m a t e l y t w i c e as l a r g e a s f o r t h e l a n t h a n i d e s ( 9 7 ) , t h e r e f o r e the S t a r k s p l i t t i n g i s l a r g e r and t h e a d m i x i n g o f J s t a t e s i s g r e a t e r (thereby r e d u c i n g the e f f e c t s o f s e l e c t i o n r u l e s f o r t r a n s i t i o n s ) . ( 3 ) The a d m i x i n g o f o p p o s i t e - p a r i t y s t a t e s i n t o t h e 5 f c o n f i g u r a t i o n s i s l a r g e r and e l e c t r i c d i p o l e t r a n s i t i o n s a r e more p r o b a b l e . This i s reflected in the a c t i n i d e Judd-Ofelt i n t e n s i t y parameters which are l a r g e r b e c a u s e o f t h i s e f f e c t a n d t h e l o w e r e n e r g i e s o f t h e 6d b a n d s . The p r o b a b i l i t y o f i o n - i o n e n e r g y t r a n s f e r by e l e c t r i c d i p o l e d i p o l e i n t e r a c t i o n s ( 1 0 0 ) w i l l a l s o be g r e a t e r . F o r s e l f quenching processes t h i s i s detrimental; f o r fluorescence s e n s i t i z a t i o n i t may be b e n e f i c i a l . T h e d y n a m i c c r y s t a l - f i e l d i n t e r a c t i o n s and i o n - p h o n o n c o u p l i n g a r e a l s o e x p e c t e d t o be s t r o n g e r f o r t h e a c t i n i d e s . The n a t u r a l l i n e w i d t h s o f o p t i c a l t r a n s i t i o n s a r e g o v e r n e d by one-and two-phonon t r a n s i t i o n s between S t a r k l e v e l s . Broader l i n e s i n c r y s t a l s r e d u c e t h e p e a k c r o s s s e c t i o n s . In g l a s s e s , w h e r e i n h o m o g e n e o u s b r o a d e n i n g p r e d o m i n a t e s , an i n c r e a s e d n a t u r a l l i n e w i d t h c o n t r i b u t e s t o a more s p e c t r a l l y homogeneous t r a n s i t i o n . Only r e c e n t l y have s t u d i e s o f the homogeneous l i n e w i d t h s o f a c t i n i d e s b e e n made u s i n g f l u o r e s c e n c e l i n e narrowing techniques (101). Larger ion-phonon i n t e r a c t i o n s t r e n g t h s were observed. Increased ion-phonon coupling contributes, in addition, to increased p r o b a b i l i t y for vibronic transitions. Multiphonon processes which involve the ion-phonon c o u p l i n g t o h i g h e r o r d e r s h o u l d a l s o be m o r e p r o b a b l e f o r a c t i n i d e than f o r l a n t h a n i d e i o n s . S y s t e m a t i c s t u d i e s and q u a n t i t a t i v e d a t a on t h e r a t e s o f t h e s e p r o c e s s e s , s u c h a s e x i s t s f o r l a n t h a n i d e s (29), are s t i l l l a c k i n g f o r the a c t i n i d e s . Because of the l a r g e r p r o b a b i l i t y f o r nonradiative decay, e f f i c i e n t l a s e r action i s f u r t h e r l i m i t e d to t r a n s i t i o n s between the l o w e r - l y i n g J s t a t e s which have l a r g e energy separations. These t r a n s i t i o n s are g e n e r a l l y in the i n f r a r e d . n



300

LANTHANIDE

AND

ACTINIDE

CHEMISTRY

AND

SPECTROSCOPY

T h e s e l e c t i o n o f h o s t m e d i a h a v i n g Tow v i b r a t i o n a l f r e q u e n c i e s i s a l s o more i m p o r t a n t f o r a c t i n i d e f l u o r e s c e n c e . A l l o f the a c t i n i d e s through Cf have i s o t o p e s w i t h h a l f l i f e s o f hundreds t o t h o u s a n d s o f y e a r s . The l o n g e s t h a l f l i f e s o f Es a n d Fm a r e m e a s u r e d i n t e n s o f d a y s ; Md a n d h i g h e r a t o m i c number e l e m e n t s e x i s t f o r o n l y h o u r s o r l e s s ( 1 0 2 ) . Because o f the r a d i o a c t i v e decay, these l a t t e r elements are not c o n s i d e r e d f o r normal l a s e r a p p l i c a t i o n s . O t h e r c o n c e r n s f o r p r a c t i c a l l a s e r s a r e (1) t h e p r e s e n c e o f r a d i o a c t i v e i s o t o p e s w h i c h may c a u s e r a d i a t i o n damage a n d u n d e s i r a b l e a b s o r p t i o n bands i n t h e h o s t and (2) t h e c o s t and a v a i l a b i l i t y of adequate q u a n t i t i e s of the r e q u i r e d i s o t o p e . An e x a m i n a t i o n o f t h e t r i v a l e n t a c t i n i d e e n e r g y l e v e l schemes r e v e a l s s e v e r a l p o s s i b i l i t i e s f o r l a s e r a c t i o n . These are d i s c u s s e d i n l i g h t o f the general p r o p e r t i e s c i t e d above. O n l y c o n v e n t i o n a l b r o a d b a n d o p t i c a l pump s o u r c e s a r e c o n s i d e r e d . O b v i o u s l y w i t h s e l e c t i v e l a s e r e x c i t a t i o n and c a s c a d e l a s i n g s c h e m e s , s t i m u l a t e d e m i s s i o n f r o m many m o r e s t a t e s s h o u l d b e p o s s i b l e , but t h e s e s p e c i a l s i t u a t i o n s a r e too numerous t o be considered i n d e t a i l here. A c t i n i d e i o n s can be i r r a d i a t e d t o a c h i e v e o t h e r v a l e n c e s t a t e s . In C a F 2 i t was f o u n d t h e t r i v a l e n t Am and E s c o u l d b e r e d u c e d t o t h e d i v a l e n t s t a t e b y gamma-ray i r r a d i a t i o n ; t r i v a l e n t U , Np, P u , a n d Cm, o n t h e o t h e r h a n d , w e r e c o n v e r t e d t o t h e t e t r a v a l e n t s t a t e ( 1 0 3 ) . In t h e s u r v e y b e l o w , i o n s i s o e l e c t r o n i c w i t h the t r i v a l e n t ion under c o n s i d e r a t i o n are i n c l u d e d i n p a r e n t h e s e s ; n o t e , however, t h a t depending upon t h e e l e c t r o s t a t i c and s p i n - o r b i t p a r a m e t e r s , t h e o r d e r i n g o f t h e J s t a t e s a n d p o s s i b l e l a s i n g t r a n s i t i o n s may b e d i f f e r e n t . Usumium.

4 +

( N p ) . Uranium i s t h e o n l y a c t i n i d e which has l a s e d . T h e t r a n s i t i o n was ^ I - j i / 2 " 9 / 2 o s c i l l a t i o n occurs a t 2 . 4 - 2 . 6 UTI. H o s t s i n c l u d e d C a F 2 , 5rr~z a t t e m p e r a t u r e s r a n g i n g f r o m 4 - 3 0 0 K. B o t h p u l s e d a n d cw o s c i l l a t i o n w e r e d e m o n s t r a t e d (1_, 1 0 4 - 1 0 9 ) . T r i v a l e n t u r a n i u m h a s many a b s o r p t i o n b a n d s i n t h e v i s i b l e and n e a r - i n f r a r e d s u i t a b l e f o r x e n o n f l a s h l a m p p u m p i n g ( s e e F i g . 7 ) . T h e s e l e c t i o n o f h o s t w i l l g o v e r n how many e x c i t e d s t a t e s f l u o r e s c e w i t h h i g h quantum e f f i c i e n c y . For example, f l u o r e s c e n c e i s o b s e r v e d f r o m s e v e r a l e x c i t e d s t a t e s o f 1)3+ i n L a C l 3 ( 9 7 ) . T h e r e f o r e F 3 / ^ l 9 / 2 o s c i l l a t i o n may a l s o b e p o s s i b l e . Cascade F 3 / 2 ^ I i y 2 ^ l 9 / 2 l a s i n g i s another p o s s i b i l i t y , a l t h o u g h f - f e x c i t e d - s t a t e a b s o r p t i o n may c o m p e t e i n t h e f i r s t s t e p . I n some h o s t s t h e 6 d b a n d s may o c c u r a t e n e r g i e s a s low as 15,000 cm" , t h u s p o s s i b l e f * d e x c i t e d - s t a t e a b s o r p t i o n should l i m i t o s c i l l a t i o n to i n f r a r e d wavelengths. > 4 l

4

a n d

4

2

4

4

4

1

Neptunium. E m i s s i o n f r o m ^ 5 a n d $1$ t o I s h o u l d o c c u r w i t h high e f f i c i e n c y i n most c r y s t a l l i n e h o s t s . The l 6 - > I t r a n s i t i o n i s p a r t i c u l a r l y p r o m i s i n g f o r l a s i n g a t 5

4

5

5

4


14.

WEBER

Lanthanide

and Actinide

301

Lasers

low t e m p e r a t u r e s s u f f i c i e n t t o r e d u c e t h e t h e r m a l p o p u l a t i o n i n t h e t e r m i n a l S t a r k l e v e l s o f ^14. T h e r e a r e n u m e r o u s a b s o r p t i o n bands throughout t h e v i s i b l e a n d n e a r - i n f r a r e d f o r o p t i c a l pumping, b u t no bands f o r competing e x c i t e d s t a t e a b s o r p t i o n f r o m S j g . ^15->^14 l a s e r a c t i o n i s a l s o p o s s i b l e p r o v i d i n g e x c i t a t i o n i n t o h i g h e r - l y i n g pump b a n d s c a n r a p i d l y b r i d g e t h e15-^^15 e n e r g y g a p . C a s c a d e ^15-^^15^14 lasing i sanother p o s s i b i l i t y . Fluorescence i sobserved f o r several h i g h e r - l y i n g excited states o f Np i n L a B r 3 ( 1 1 0 ) . I n t e n s e e m i s s i o n a t * 5 0 5 nm a n d 6 2 4 nm o r i g i n a t e s f r o m t h e s t a t e a t ^ 1 9 , 8 0 0 cm-1 t o ^ 4 a n d l 5 s t a t e s , r e s p e c t i v e l y . I n t e n s e f l u o r e s c e n c e was a l s o o b s e r v e d from s t a t e s a t -17,300 cm ! a n d 14,700 c n H t o t h e g r o u n d s t a t e . I n a l l t h e s e c a s e s , o s c i l l a t i o n may b e p r e vented by f * d a b s o r p t i o n because t h e 6d bands begin a t ^25,000 c n H . A more p r o m i s i n g p o s s i b i l i t y i s t h e 4 - l e v e l l a s i n g scheme from the J=5 (^5) s t a t e a t 15,000 c n H t o ^ i ^ . 5

3 +


5

-

Concentration quenching o f the ^ 5 f l u o r e s c e n c e i s probably large. When i r r a d i a t e d w i t h u l t r a v i o l e t l i g h t , a n a r r o w e m i s s i o n p e a k a t 1.95 ym w a s o b s e r v e d f r o m a r a d i a t i o n - d a m a g e d c r y s t a l of P u - d o p e d CaFp. T h i s f e a t u r e was a t t r i b u t e d t o P u ^ (HI)2 3 8

+

+

Plutonium. ( A m ^ ) . T h e e n e r g y l e v e l s c h e m e a n d p o s s i b l e lasing transitions for Pu are very s i m i l a r t o those o f Np . Prospective t r a n s i t i o n s include Hg/2^ H5/2 Hg/^^/g* and 7 / 2 - * H C / . F o r e f f i c i e n t f l u o r e s c e n c e a n d l a s e r a c t i o n from e i t h e r ? n e H g / 2 o r ^7/2 s t a t e s , h o s t s s h o u l d have l o w phonon f r e q u e n c i e s t o r e d u c e n o n r a d i a t i v e decay by m u l t i p h o n o n p r o c e s s e s . Depending upon t h e h o s t a n d t h e e x a c t p o s i t i o n s o f h i g h e r - l y i n g s t a t e s , e x c i t e d - s t a t e a b s o r p t i o n may r e d u c e o r prevent n e t gain. E m i s s i o n was o b s e r v e d f r o m P u i n C a F 2 a t 1 . 7 8 ym c o r r e s p o n d i n g t o t h e ^9/2+^5/2 t r a n s i t i o n ( 1 1 1 ) . While this t r a n s i t i o n s a t i s f i e d theAJ = 2 r u l e f o r hypersensitive t r a n s i t i o n s , t h e U(2) m a t r i x element i s n o t l a r g e (112) a n d t h e r e f o r e s h o u l d be l e s s h o s t dependent. 3 +

3 +

6

6

5

6h

6

2

6

3 +

Kmznlcslum. ( C m ^ ) . A t t e m p t s h a v e b e e n made t o l a s e t h e 0 . 6 9 5 u n L 6 + F 2 t r a n s i t i o n o f Am + i n a P O C I 3 l i q u i d ( 1 1 3 ) a n d i n a C a W 0 c r y s t a l ( 1 1 4 ) , b o t h a t 3 0 0 K. No o s c i l l a t i o n was o b s e r v e d . T h e $1.5 f l u o r e s c e n c e l i f e t i m e s i n CaW04 v a r i e d f r o m 50 t o 8 0 y s ; i n t h e P O C I 3 l i q u i d t h e f l u o r e s c e n c e w a s weak a n d t h e d e c a y t i m e w a s v e r y s h o r t w h i c h s u g g e s t s t h e presence o f quenching. T h e 6d bands a r e l o c a t e d a t e n e r g i e s * 4 0 , 0 0 0 c n f l a n d t h e r e f o r e f-*d e x c i t e d - s t a t e a b s o r p t i o n s h o u l d not prevent o s c i l l a t i o n , although f - f a b s o r p t i o n could. A l t h o u g h t h e 0.695 ym e m i s s i o n w a s a t t r i b u t e d t o t h e \ § s t a t e a n d h a s been s t u d i e d i n L a C l 3 ( 1 1 5 ) , n o n r a d i a t i v e d e c a y t o t h e J = 1 s t a t e a t ^ 1 7 , 0 0 0 cm~l s h o u l d o c c u r i n +

5

7

3

4


302

LANTHANIDE

AND

ACTINIDE

CHEMISTRY

AND

SPECTROSCOPY

hosts with high frequency v i b r a t i o n s . T r a n s i t i o n s from t h i s l e v e l t o l e v e l s o f 'F] and F 2 a r e c a n d i a t e s f o r l a s e r action. T r a n s i t i o n s between t h e lower 7p s t a t e s a r e p o s s i b l e infrared l a s e r candidates i n hosts with low v i b r a t i o n a l f r e q u e n c i e s . T h e ^F-J-^FQ t r a n s i t i o n i s o n l y m a g n e t i c - d i p o l e a l l o w e d a n d 7F2+'FQ s h o u l d b e a h y p e r s e n s i t i v e t r a n s i t i o n , however s e l e c t i o n r u l e s a r e r e l a x e d b y J - s t a t e mixing. 7

2 +

4 +

CuAuim. (Am , B k ) . Lasing o f t h e 6(p,D) /Z* $J/Z t r a n s i t i o n should be p o s s i b l e i n a host where n o n r a d i a t i v e c a s c a d e t o ^Py/z i s e f f i c i e n t o r , a l t e r n a t i v e l y , f r o m t h e n e x t h i g h e r e x c i t e d s t a t e , J = 5/2. Pumping w o u l d b e v i a s e v e r a l v i s i b l e a n d n e a r - u l t r a v i o l e t b a n d s a t ^ 1 7 , 0 0 0 cm"'. The bands a r o u n d 25,000-27,000 c n r l a r e p a r t i c u l a r l y s t r o n g because o f t h e l a r g e matrix elements. In comparison t o t h e l a n t h a n i d e a n a l o g G d ^ , t h e o s c i l l a t o r s t r e n g t h s o f Cm3+ t r a n s i t i o n s a r e 10-100 t i m e s g r e a t e r (116). T h e ground s t a t e o f Cm3 i s an S s t a t e ; t h e s p l i t t i n g , w h i l e l a r g e r than f o r G d , i s s t i l l o n l y a f e w t e n s o f cm~l ( 1 1 7 ) a n d l o w t e m p e r a t u r e s would be r e q u i r e d f o r q u a s i - f o u r - l e v e l l a s i n g . Radiation-reduced Am i n CaF2 c r y s t a l s shows a b s o r p t i o n b a n d s b e g i n n i n g a t

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