Optica Applicata, Vol. XII, No. 3 -4 , 1982
Changes of refractive index in glass induced by UV-irradiation
and a new possibility of their determination
Stanisław Gębala, Ireneusz Wilk
In s titu te o f P h ysics, T echn ical U niversity o f 'Wrocław, Wybrzeże Wyspiańskiego 27, 50-570 Wrocław, Poland.
1. Introduction
In the course o f e x p lo ita tio n the g la s s elements are usually su b ject t o ir r a d ia tio n by o f d iffe r e n t wavelengths· Most frequ ently the ir r a d ia tin g lig h t spectrum ranges from v i s i b l e - to UV-ragion but a ls o the ir ra d ia tio n by high quantum energy ra d ia tion lik e X-rays and gamma-rays may happen. This ir r a d ia tio n may r e s u lt in some changes in o p t ic a l p r op erties o f g la s s which may worsen the qu a lity o f the o p t ic a l elements and systems (made o f i t ) during th e ir e x p lo ita tio n *
On the other hand, i t becomes more and more c le a r that some kinds o f ir ra d ia tio n may crea te new p o s s i b i l i t i e s o f production o f ranted e f f e c t s in g la ss in a c o n tro lle d way* Here, the photochrome g la sses may be mentioned a s an obvious example [1 , 2 ] · These g la s ses seem to be in growing demand.
O bviously, i f the wanted changes in glass* are to be co n tro lle d the problem o f s u it able measurement method becomes o f considerable importance* While the changes in ab sorp tion are r e la t iv e ly easy to measure, the measurement o f changes in r e fr a c tiv e index d is tr ib u tio n as a fu n ction o f the wavelength may be connected with seriou s d i f f i c u l t i e s · The purpose o f th is paper i s to in d ica te a p o s s ib ilit y o f measuring the very small changes in r e fr a c tiv e index provided that they are o f p e r io d ic nature·
2. Experimental method
The sta rtin g p oin t o f the method suggested i s the ir r a d ia tio n o f the examined g lass p la te s v ia a p e r io d ic mask o f s p a tia l frequency, say 5 p a ir lineB/mm. This irra d ia tio n induces a complex in tern a l stru ctu re determined, among oth ers, by changes in r e fra ctiv e index and absorption w hile both these changes depend lo c a lly upon the rad ia tion dosis used to ir r a d ia tio n . In order to v isu a liz e these changes as w ell as to examine them
464 S . GgBALA, I . WIIX
q u a n tita tiv e ly the fa c t may be employed that the lig h t beam propagating in a medium tends to c o l l e c t the inform ation about the macrostructure o f the l a t t e r . In our case the p la te mas illum inated by a monochromatic collim ated l i g h t beam in the setup shown in
F ig . 1. O p tical setup to ir r a d ia tio n o f the g la s s p la te s : 1 - HBO-200 lamp, 2 - mask (muster g r a tin g ), 3 - g la ss p la te to be irra d ia ted
F ig . 1 which i s here reproduced from paper [ 1 ] . As a r e s u lt a d if f r a c t io n pattern appear^ a t a given distance from the pla te examined. This pattern contains informs» tio n about the structure o f the irra d ia ted g la ss p la t e , in p a rticu la r about the ampli tude o f the r e fr a c tiv e index d is tr ib u tio n induced by ir r a d ia tio n .
3. Experimental results
Soma examples o f d if f r a c t io n patterns generated by the g la ss p la te s form erly irra d ia ted by UV lig h t beams v ia a muster gra tin g were given in [ 3 ] . The inform ation presented in Fig·. 2 are o f the same character as those reported in [3 ] and are given here f o r the sake o f in trod u ction . T hey.include: the in te n s ity d is tr ib u tio n in d if f r a c t io n pattern gener ated by a muster g ra tin g ( mask) recorded on a photographic p la te ( curve 1 ) , ( th is mask was 'used to modify su itab ly the UV lig h t beam used f o r inducing ir r a d ia tio n o f the g la ss ‘ p la t e s ) , the in te n sity d is tr ib u tio n in the d if f r a c t io n pattern produced by BK101 g la ss
p la te form erly UV-irradiated v ia the m uster-grating (curve 2 ) , the same f o r HK107 g la ss pla te (curve 3 ).
In Figures 3 and 4 the dependences o f the am plitude-to-background r a t i o in the d i f fr a c tio n patterns generated by the g la ss p la tes with induced g ra tin g stru ctu res upon the used wavelength X are shown f o r BK101 and BK107 g la s s samples, r e s p e c tiv e ly (curves 1 ). For the salQa o f comparison the increment n ( \ ) has been ca lcu la ted on the base o f Kra
mer s-K ronig fw ptulae, the absorption changes w ithin the 200-400 nm in te r v a l being given [4J and the r e s u lts are presented in the form o f the curves 2 . A d d ition a lly , the de pendence o f the p o s itiv e increments o f the absorption c o e f fi c i e n t A K(cm~1) upon the wavelength X i s shown (curves 3)*
. In Figure 5 tlie amplitude-to-background r a tio i s shown fo r the case o f a g la ss pla te irra d ia te d f i r s t by*gamma-radiation (curve 1 ), and next exposed to v is ib le ir ra d ia tio n v ia mask using the XB0-100 Xenon lamp with a 400 nm edge f i l t e r to cut out the u ltra v io le t . Th.a ir r a d ia tio n o f g la ss by gamma-radiation re su lts in soma changes in o p t ic a l density D, the dependence o f which on the wavelength i s illu s tr a t e d with the help o f the curve 2 . The curve 3 g iv e s , in turn, the r e s u ltin g change in absorption c o e f f i c i e n t .
Letters to the Editor 485 F ig . 2 . D if fr a c t io n p a tt e r n s -g e n e r a te d r e s p e c ti v e ly b y * th e m a sk (c u r v e 1 )» th e B K 10 1 g la s s p la te (c u r v e 2 ), th e B K 1 0 7 g la s s p la te (c u r v e 3 ). I n te n s it y g iv e n in co n v e n ti o n a l u n it s
s.
GgBAU, I. aniK486
f i g . Ad Dependences o f the am- plitude-to-background r a t io a /b ( c u r v e .l) » An (curve 2 ) , and AK(cm" 1)(cu rv e ‘3.) upon the wave length X , a l l f o r BK107 p la tes
F ig· 5· Dependences o f the am plitude-to-bac kgroun£ r a t io (cu rts 1) , o p t ic a l density D (curve 2 ) , and ^ ( c n r 1) (curve 3 ) , a l l
f o r BK g la s s p la te s ex posed formerly to gaama- ir r a d ia tic p
The change i s negative and corresponds to a decrease o f the g la ss c o lo r a tio n · Since the maxim al o f the absorption band introduced by the gamma irradiation is located in the near UV range, i t may be b e liev ed that' A n ( \ ) in the v is ib le range in o f p o s itiv e sign and o f normal run· The ir ra d ia tio n o f the g la ss pla te by v isib le light beam resu lted in dim inishing o f the absorption and A n (\ ) both «induced by former irrqdiaftion by gamma ra y s.
A r e la t io n · sim ila r to that presented i n Figure 5 f o r BaK4 gla ss, shown in Figure 6 f o r 2X1 g la ss p la tes of. thickness d * 3 am· These p la tes were also formerly irra d ia ted by gamma ra d ia tion (F ig · 6 b ). In the pla te ( t ) the g ra tin g was induced by i t s ir ra d ia tio n with a HBO-200 lamp in the way analogous to that applied to 3K1C1 a id BK107 g la s
-letters to the Editor
4e rr
F ig · 6. Dependences o f the am plitude- to-background rat i o ( a / b ) , D and AK(c■ * ') upon* X.' in two 2X1 g la ss
p la te s exposed form erly to gamma i r ra d ia tion , and next irra d ia te d by v is ib le + UV lig h t beam ( f i r s t sample - curves 1 ) . and the v is ib le (o n ly ) lig h t bean (secon d sample -O u r v e e .2 )
se s . As a r e s u lt O f t h it ir r a d ia tio n a p o s itiv e increment o f absorption was obtained which may be observed im *wrve 1 in F ig . 6 c. For th is sample ( 1 ) the dependences on
the wavelength o f the fo llo w in g qu an tities are given i the amplitude-to-background r a t io (cu rve 1 ) in F ig . 6a , o p t ic a l den sity D (curve 1 in F ig . 6 b ), and absorption in cre ment A K (curve 1 in F ig · 6 e )« For the pla te 2 the same HB0-200 lamp was used as a lightj source but the u ltr a v io le t region was cu t out by a su ita b le edge f i l t e r . T h is resu lted in negative increment o f ib a e rp tio n . For th is sample the three analogous dependences are represented by the curves 2 i d the same fig u r e s .
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0.1
760
F ig . 7 . Dependence o f the ampli tude-to-background r a t i o upon the wavelength X 1 f o r HX101 glass p la tes su b ject to annealings 1 - i n i t i a l measurement, 2 - measure ment a ftflr the f i r s t 0 .5 h period o f annealing at the 473 X tea p er- a tu re, 3 - measurement a f t e r the second 0 .5 h period o f annealing at the 573 K temperature, 4 - measurement a ft e r the th ird 0 .5 h period o f annealing at the temper ature 673 X, 5 - measurement a f te r the fou rth 0 .5 h period o f an nealing a t the temperature 773 X
488 S . GgBAU, I . VIIX
In Figure 7 the amplitude-to-baekground r a t io a /b v s · the wavelength \ i s shown f o r the BK101 p la te · The ourve 1 r e fe r s to the p la te w ith induced g ratin g stru ctu re im m ediately a ft e r the inducing exposure· Next, the p la te was su bject to annealing in a
furnace f o r fou r subsequent halve-an-hour p eriods o f t in e , while the annealing tempera ture was changed stepw isely from 475 K to 773 K, each step being equal to 100 deg· The b ra cin g -a ction o f the elevated annealing temperature with resp ect t o the induced gratin g Structure in the p la tes nay be w ell observed.
4. Conclusions
The s i n i la r it y o f the curves a /b ( X) and A n ( \ ) in d ica tes th e ir strong c o r r e la t io n · The s a a ll values o f A n (\ )w s r e d i f f i c u l t to measure by using tr a d itio n a l methods· T h erefore, the computational methods were usually used in stead· By measuring the am plitude-to-back- ground r a tio and taking advantage o f the c o r r e la tio n in d ica ted above the wanted fu nction A n ( X) may be determined experim entally. Even the very small changes o f r e fr a c tiv e index An(\)ma> be measured in th is way provided that they are p e r io d ic . Above we have given
only an idea· The tech n ica l aspects o f the methods may be e a s ily improved·
Reference·
[ 1 ] *0X0 J · , ANCKLL C .A ., Glass Structure by Spectroscopy, Ed. It· Dekker In c . New York 1976·
[ 2 ] Glass It In te ra ctio n w ith Electromagnetic R adiation, Ed· M· Tomosawa, R.H. Doremua, Academic P ress, New York 1977·
[ 3 ] GfBALA S . , WIIX I , O ptica A pplicata H ( l 9 8 l ) , 321-326. [4 ] KUCHARSKI K ·, Acta Pbys. Polon. (19 79 ). 891.
