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OPTICAL BISTABILITY IN MOLECULAR BEAM DEPOSITED ZnSe/BaF2 MULTILAYER
INTERFERENCE FILTERS
A. Miller, I. Muirhead, K. Lewis, J. Staromlynska, K. Welford
To cite this version:
A. Miller, I. Muirhead, K. Lewis, J. Staromlynska, K. Welford. OPTICAL BISTABILITY IN MOLEC-
ULAR BEAM DEPOSITED ZnSe/BaF2 MULTILAYER INTERFERENCE FILTERS. Journal de
Physique Colloques, 1988, 49 (C2), pp.C2-105-C2-108. �10.1051/jphyscol:1988223�. �jpa-00227640�
JOURNAL DE PHYSIQUE
C o l l o q u e C2, S u p p l e m e n t
au
n 0 6 , Tome 49, j u i n 1988OPTICAL BISTABILITY IN MOLECULAR BEAM DEPOSITED ZnSe/BaF, MULTILAYER INTERFERENCE FILTERS
A. MILLER, I. T. M U I R H E A D ( ~ )
,
K. L. LEWIS, J. STAROMLYNSKA and K.
R.
WELFORDRoyal Signals and Radar Establishment, St Andrews Road, GB-Great Malvern W R 1 4 3PS, Worcestershire, Great-Britain
A b s t r a c t
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Complete i n t e r f e r e n c e f i l t e r s h a v e been f a b r i c a t e d u s i n g u l t r a h i g h vacuum m o l e c u l a r beam t e c h n i q u e s and o p t i c a l l y a s s e s s e d . The w a v e l e n g t h dependence of o p t i c a l b i s t a b i l i t y i s compared w i t h t h e o r y between 676 and 521nm f o r a 6 u m t h i c k f i l t e r w i t h Znse/BaF2 m u l t i l a y e r r e f l e c t o r s . Both d i s p e r s i v e b i s t a b i l i t y and b i s t a b i l i t y due t o i n c r e a s i n g a b s o r p t i o n were o b s e r v e d a t 514nm.1
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INTRODUCTIONThermo-optic i n t e r f e r e n c e f i l t e r s based on I I - V I s e m i c o n d u c t o r s a r e a t t r a c t i v e a s t e s t - b e d s f o r a l l - o p t i c a l computing schemes. They can be u n i f o r m l y f a b r i c a t e d i n l a r g e a r e a s , and t h e r e f o r e o f f e r t h e p o t e n t i a l f o r l a r g e a r r a y s of o p t i c a l l y b i s t a b l e d e v i c e s w i t h t h e c o n v e n i e n c e of room t e m p e r a t u r e , v i s i b l e l i g h t o p e r a t i o n / I / . The n o n l i n e a r mechanism h a s now been w e l l c h a r a c t e r i z e d as a t h e r m a l l y i n d u c e d , d i s p e r s i v e o p t i c a l n o n l i n e a r i t y /2,3/. M o s t e f f o r t has i n v o l v e d
e v a p o r a t e d ZnSe s i n c e t h i s is a w e l l e s t a b l i s h e d t h i n f i l m c o a t i n g m a t e r i a l and p r o v i d e s a band gap e n e r g y which is r e s o n a n t w i t h t h e 514nm o u t p u t o f t h e a r g o n l a s e r . However, s t a n d a r d t h e r m a l e v a p o r a t i o n t e c h n i q u e s a r e l i m i t e d i n what c a n be a c h i e v e d i n t e r m s of m a t e r i a l q u a l i t y and c o m p l e x i t y of s t r u c t u r e . T h i s p a p e r r e p o r t s on t h e a p p l i c a t i o n of a new o p t i c a l c o a t i n g
t e c h n o l o g y , m o l e c u l a r beam d e p o s i t i o n (MBD), f o r t h e f a b r i c a t i o n of c o m p l e t e Fabry-Perot e t a l o n s . The MBD t e c h n i q u e i s c a p a b l e of producing d e n s e f i l m s , f r e e of p o r o s i t y and w i t h low c o n t a m i n a t i o n
l e v e l s which s h o u l d improve t h e s t a b i l i t y of thermo-optic d e v i c e s . An immediate a d v a n t a g e of t h e MBD t e c h n i q u e f o r o p t i c a l b i s t a b i l i t y i s t h e a b i l i t y i t p r o v i d e s t o produce much t h i c k e r ,
m e c h a n i c a l l y s t a b l e l a y e r s . T h i s i s s i g n i f i c a n t i n t e r m s o f o p t i m i s i n g t h e s t r u c t u r e f o r l o w e r s w i t c h i n g powers, r e d u c i n g t h e r e q u i r e d t e m p e r a t u r e r i s e , c o n t r o l l i n g h e a t f l o w and t h e a b i l i t y t o a c h i e v e o p t i c a l b i s t a b i l i t y due t o i n c r e a s i n g a b s o r p t i o n .
I n t e r f e r e n c e f i l t e r e t a l o n s h a v e t h e g e n e r a l s t r u c t u r e , HLHL...(mHH),.LHLH where H i n d i c a t e s a q u a r t e r w a v e l e n g t h o p t i c a l t h i c k n e s s (h/4) l a y e r of h i g h i n d e x m a t e r i a l (ZnSe, n-2.7 a t 514nm) where h i s t h e vacuum w a v e l e n g t h , and L i s a h / 4 l a y e r of l o w i n d e x (e.g ThF4, n = 1.55). The i n t e g e r , m, ( t y p i c a l l y 8 f o r t h e r m a l l y e v a p o r a t e d l a y e r s ) g i v e s t h e e t a l o n s p a c e r t h i c k n e s s . These e t a l o n s c a n be s e l f - t u n e d w i t h a l a s e r because of band edge a b s o r p t i o n i n t h e ZnSe c a u s i n g a r i s e i n l a t t i c e t e m p e r a t u r e ; t h e a b s o r p t i o n e d g e s h i f t t o l o n g e r w a v e l e n g t h s a t h i g h e r
t e m p e r a t u r e s r e s u l t s i n a n i n c r e a s e i n t h e r e f r a c t i v e i n d e x and t h e r e f o r e a l s o i n t h e round t r i p e t a l o n phase change. The MBD f i l t e r s f a b r i c a t e d f o r t h e s e s t u d i e s h a v e 6um t h i c k ZnSe s p a c e r s (m 120). E a r l i e r work on u l t r a h i g h vacuum (UHV) f i l m f a b r i c a t i o n h a s produced ZnSe f i l m s w i t h h i g h l a s e r damage t h r e s h o l d s 141. The low i n d e x m a t e r i a l chosen was BaF2 (n
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1.45). Recent work on f l u o r i d e d e p o s i t i o n h a s h i g h l i g h t e d t h e p o t e n t i a l of barium and l e a d f l u o r i d e s a s l o w i n d e x m a t e r i a l s f o r u s e o v e r a wide r a n g e of w a v e l e n g t h s 151. Techniques h a v e been d e v e l o p e d f o r t h e c o n t r o l o f t h e m i c r o s t r u c t u r e of t h e f i l m s o f f e r i n g t h e p o t e n t i a l f o r c o n t r o l l i n g t h e p r o p e r t i e s of d i f f e r e n t m a t e r i a l s w i t h i n n o v e l s t r u c t u r e s . A p r e v i o u s p a p e r h a s r e p o r t e d r e s u l t s on o p t i c a l b i s t a b i l i t y i n f i l t e r s c o n s t r u c t e d w i t h t h i c k MBD ZnSe s p a c e r s w i t h c o n v e n t i o n a l , t h e r m a l l y e v a p o r a t e d Z n s e / T h ~ q m i r r o r s / 6 / .("permanent address : E L I Optical Coetinge L t d . Dunfernline. GB-Fife KPll 5FR. Great-Britain
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988223
C2-106 JOURNAL DE PHYSIQUE
( a ) ( b )
Fig. 1
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Scanning e l e c t r o n micrographs o f c l e a v e d c r o s s - s e c t i o n s o f MBD f i l t e r s t r u c t u r e , sample AP245. The o u t e r r e f l e c t o r l a y e r s are just d i s c e r n a b l e i n ( a ) .2
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ETALON FABRICATION AND ASSESSMENTThe MBD l a y e r s were deposited i n a Vacuum Generators load-locked UHV system f i t t e d w i t h 3 Knudsen sources and i n - s i t u s u r f a c e d i a g n o s t i c s . The source m a t e r i a l s were contained w i t h i n h i g h p u r i t y g r a p h i t e c r u c i b l e s wh.lch were c a r e f u l l y outgassed f o l l o w i n g baking o f t h e e n t i r e d e p o s i t i o n chamber a t 180'. The ZnSe source was u l t r a - h i g h p u r i t y p o l y c r y s t a l l i n e ZnSe which had p r e v i o u s l y been prepared by chemical vapour d e p o s i t i o n from a m i x t u r e o f z i n c vapour and hydrogen s e l e n i d e . The barium f l u o r i d e source was high p u r i t y c r y s t a l l i n e o p t i c a l grade m a t e r i a l . Source
temperatures were adjusted t o g i v e d e p o s i t i o n r a t e s o f O.bm/h f o r ZnSe (896°C) and 0.7pm/h f o r BaF2 (1185'C) a t pressures o f 10-8mbar. The g l a s s s u b s t r a t e temperature during growth f o r t h e e n t i r e MBD deposited l a y e r was 300°C. F i l t e r s were b u i l t up by f i r s t d e p o s i t i n g a two-period s t a c k comprising h / 4 t h i c k l a y e r s o f ZnSe and BaF2 centred a t 633nm, f o l l o w e d by t h e ZnSe spacer and f i n a l l y a matching two period mirror t o complete t h e e t a l o n . The m u l t i l a y e r s were
e x c e p t i o n a l l y smooth w i t h no e v i d e n c e o f s u r f a c e t e x t u r e v i s i b l e by Nomarski i n t e r f e r e n c e microscopy. A s m a l l b u i l t - i n wedge allowed t u n i n g o f t h e e t a l o n .
Transmission s p e c t r a were recorded a t a number o f sample temperatures i n one sample, i n order t o determine t h e d i s p e r s i o n i n both t h e r e f r a c t i v e index and t h e i n d e x change w i t h temperature 6 n 1 6 ~ . The r e s u l t s are shown i n f i g u r e 2. Transmission measurements were a l s o c a r r i e d o u t a t f i v e
wavelengths using low power, focused krypton l a s e r r a d i a t i o n i n order t o determine precise maximum and minimum v a l u e s by f u l l y r e s o l v i n g t h e f r i n g e s 171, t a b l e 1. From t h e s e r e s u l t s peak mirror r e f l e c t i v i t i e s o f about 70% were deduced assuming equal v a l u e s o f r e f l e c t i v i t y f o r f r o n t ( R E ) and back ( R B ) mirrors. These r e f l e c t i v i t i e s may be compared w i t h t h e t h e o r e t i c a l l y predicted v a l u e s o f RF = 81% and RB = 74% assuming no a b s o r p t i o n i n t h e stacks. V a l u e s of ad were a l s o estimated where a i s t h e a b s o r p t i o n c o e f f i c i e n t and d i s t h e e t a l o n spacer l a y e r t h i c k n e s s . The ZnSe absorption c o e f f i c i e n t v a r i e s from about 100 t o 1000 cm-l between 676 and 521 nm. The a b s o r p t i o n edge i s l e s s abrupt than f o r b u l k p o l y c r y s t a l l i n e ZnSe, as found f o r c o n v e n t i o n a 1 , ' t h e r m a l l y evaporated t h i n f i l m s 121.
Table 1: Parameters f o r sample AP245
3
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OPTICAL BISTABILITYSpacer l a y e r s 6 pm t h i c k produce e t a l o n f r i n g e s w i t h spacing a t l e s s than lOnm i n t h e v i s i b l e . T h i s allowed o p t i c a l b i s t a b i l i t y t o be observed a t f i v e krypton l a s e r wavelengths between 676 and
V ~ 0 l u r g t h tna) Y.volurgth 0
Fig.2
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Measured dispersion in optical thickness, nd and its temperature dependence, d(nd)/d~(sample ~ ~ 2 4 4 )
521nm implying a spot size, wo = 12pm (l/e2 intensity radius). Input-output characteristics measured at 531nm are shown in figure 3a, with critical switching recorded at 1lmW. Measurements of critical switching power at the five wavelengths are plotted in figure 3b; the minimum critical power observed was 9.5mW at 56811- The optimum wavelength is a trade-off between the larger nonlinearity at shorter wavelengths due to higher absorption plus slightly larger dn/dT, against the higher absorption causing a reduction in the finesse of the etalon.
An expression giving the wavelength dependence of the critical switching irradiance, Ic, for optothermal devices has been derived by Wherrett et a1
181.
Two regimes may be considered. For large spot sizes, such that longitudinal heat dissipation into the substrate dominates cooling,This is the product of material parameters, (Ks is the substrate thermal conductivity) and an expression comprising a complicated function of etalon parameters (reflectivities, absorption coefficient and thickness). In the limit of small spot sizes and thick films, cooling is expected to be transverse with heat loss along the layers, in which case, 181,
A K
Ic = f f(%?
%,
ad) (2)(an/a~)w~ 2
where ~f is the film conductivity. In either case, for a given laser spot size, the wavelength
Input fnY) Uovelength (nm)
Fig.3
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(a) Input-output characteristics for AP245 at 531nm. (b) Measured critical switching powers for optical bistability compared to theory (line).JOURNAL DE PHYSIQUE
Input (mu)
Fig. 4
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D i s p e r s i v e and a b s o r p t i v e b i s t a b i l i t y i n sample AP245 a t 514nm.dependence o f t h e c r i t i c a l s w i t c h i n g power i s g i v e n by, PC- f
/
( 6 n / 6 ~ ) . T h i s f u n c t i o n i s p l o t t e d i n f i g u r e 3(b) f o r t h e e t a l o n p a r a m e t e r s e x t r a c t e d from t h e t r a n s m i s s i o n measurements.The l i n e shown h a s been s c a l e d v e r t i c a l l y t o o v e r l a p t h e 568nm r e s u l t . A l t h o u g h t h e d i s p e r s i o n i n 6n/6T i s small ( a b o u t a f a c t o r of two between 5 2 1 and 676nm from f i g u r e 2) i n c l u s i o n r e s u l t s i n a n e x c e l l e n t f i t t o t h e d a t a .
The s i z e o f t h e i l l u m i n a t e d a r e a and how i t i s h e a t s u n k w i l l e f f e c t b o t h t h e s w i t c h i n g power and t h e s w i t c h i n g s p e e d /9/. A l t h o u g h t h e s p o t d i a m e t e r i s f o u r t i m e s t h e f i l m t h i c k n e s s i n t h e s e measurements, t h e r a t i o of the t h e r m a l c o n d u c t i v i t y of ZnSe t o t h a t of g l a s s i s twenty. For t h e measurements a t 5681x11, e q u a t i o n 1 p r e d i c t s a c r i t i c a l s w i t c h i n g power o f 1.5mW, w h i l e e q u a t i o n 2 g i v e s l l O m W f o r ZnSe on g l a s s . Thus, s i g n i f i c a n t t r a n s v e r s e c o o l i n g i s i m p l i e d by t h e o r y under t h e c o n d i t i o n s used h e r e , and t h e h i g h e r v a l u e o f c r i t i c a l s w i t c h i n g power s h o u l d a p p l y . The measured v a l u e of 9.5mW would i n d i c a t e however t h a t c o o l i n g i s s i g n i f i c a n t b o t h a l o n g t h e l a y e r s and i n t o t h e s u b s t r a t e . A l t h o u g h some d r i f t of t h e b i s t a b i l i t y w i t h t i m e was s t i l l a p p a r e n t i n t h e s e i n i t i a l MBD s t r u c t u r e s , e s s e n t i a l l y s t a b l e o p e r a t i o n was m a i n t a i n e d f o r s e v e r a l h o u r s a t 568nm /7/. F u r t h e r s t u d i e s on t h e s t a b i l i t y of t h e s e s t r u c t u r e s are i n progress.
O p t i c a l b i s t a b i l i t y was o b s e r v e d i n t h e same f i l t e r w i t h an a r g o n l a s e r o p e r a t i n g a t 514nm. f i g u r e 4. D i s p e r s i v e b i s t a b i l i t y i s shown h e r e a t much h i g h e r powers, p a r t l y b e c a u s e t h e f i l t e r d e s i g n i s f a r from o p t i m i s e d a t 514nm, b u t a l s o b e c a u s e a much l a r g e r s p o t s i z e (approx. 100pm) was employed i n t h i s case. At h i g h e r powers a c l o c k w i s e l o o p o c c u r s due t o a b s o r p t i v e b i s t a b i l i t y . T h i s e f f e c t , due t o t h e i n h e r e n t f e e d b a c k between t h e a b s o r p t i o n edge s h i f t and t h e i n c i d e n t i n t e n s i t y , h a s been r e p o r t e d p r e v i o u s l y i n t h i c k p o l y c r y s t a l l i n e ZnSe s a m p l e s a t 474nm / 1 0 , 1 1 / , b u t n o t i n e v a p o r a t e d l a y e r s . The p r e s e n t s a m p l e s s a t i s f y t h e r e q u i r e m e n t , a d
>
0.18 a t 514nm /11/ f o r t h i s form o f b i s t a b i l i t y t o occur.REFERENCES
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