Preparation and properties of infrared-to-visible conversion phosphors
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R e c e n t l y phosphor m a t e r i a l s which e f f i c i e n t l y c o n v e r t i n f r a r e d (9300A) r a d i a t i o n f r o m Si : GaAs d i o d e s to r e d , g r e e n , o r blue light have been d e v e l o p e d . The light s o u r c e s which r e s u l t f r o m such d i o d e - p h o s p h o r c o m b i n a t i o n s a r e s o m e w h a t l e s s e f f i c i e n t f o r r e d and g r e e n and a r e m o r e e f f i c i e n t f o r blue than the b e s t r e p o r t e d v a l u e s f o r o t h e r solid s t a t e light s o u r c e s . The b a s i c m e c h a n i s m of the c o n v e r s i o n p r o c e s s including the i m p o r t a n t f a c t o r s c o n t r o l l i n g the e f f i c i e n c y is d i s c u s s e d followed by the d e t a i l s of the e f f i c i e n t p h o s p h o r s y s t e m s . The p r e p a r a t i o n of m a t e r i a l s and it s influence on e f f i c i e n c y i s a l s o c o v e r e d .
T H E R E a r e a n u m b e r of m e t h o d s f o r c o n v e r t i n g i n f r a r e d r a d i a t i o n to v i s i b l e light. F o r e x a m p l e , v i s i b l e c a t h o d o l u m i n e s c e n c e of p h o t o e l e c t r o n s f r o m an i n f r a re d s e n s i t i v e photocathode is used in i m a g e c o n v e r t e r s . Second h a r m o n i c g e n e r a t i o n by m e a n s of the n o n l i n e a r s u s c e p t i b i l i t y of c e r t a i n c r y s t a l s is used to p r od u ce c o h e r e n t v i s i b l e light f r o m i n f r a r e d l a s e r s . The p a r t i c u l a r m e t h o d that w i l l be d i s c u s s e d h e r e inv o l v e s the s t e p w i s e e x c i t a t i o n of an ion to a v i s i b l e f l u o r e s c i n g l e v e l . This m e t h o d h a s its r o o t s in the inf r a r e d quantum c o u n t e r p r o p o s e d by B l o e m b e r g e n l in which the c o n s e c u t i v e a b s o r p t i o n of an i n f r a r e d photon and a pump photon of a d i f f e r e n t w a v e l e n g t h e x c i t e s an i m p u r i t y ion to a v i s i b l e f l u o r e s c i n g l e v e l . T h i s w a s o r i g i n a l l y c o n c e i v e d as a m e t h o d f o r d e t e c t i n g i n f r a r e d r a d i a t i o n by c o n v e r t i n g it to the v i s i b l e . S u b s e q u e n t l y it was found that in s o m e r a r e e a r t h ions c o n v e r s i o n of i n f r a r e d r a d i a t i o n of a single w a v e l e n g t h to v i s i b l e e m i s s i o n took place. E n e r g y t r a n s f e r was found to be m o r e e f f i c i e n t in p r o d u c i n g the s e c o n d s t e p of e x c i t a t i o n than d i r e c t a b s o r p t i o n of r a d i a t i o n . 2'3 A u z e l , 4'5 and Ovsyankin and F e o f i l o v 6 then i n d e p e n d e n t l y d i s c o v e r e d that the c o n v e r s i o n p r o c e s s in c e r t a i n r a r e e a r t h ions could be s e n s i t i z e d by Yb 3+ with g r e a t l y e n h a n c e d e f f i c i e n c i e s . H o w e v e r , due to the n o n l i n e a r n a t u r e of the p r o c e s s , the c o n v e r s i o n e f f i c i e n c
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