Black glasses, bulk and fibers: Obtaining information in the infrared

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Black glasses, bulk and fibers: Obtaining information in the infrared

Jacques Lucas

To cite this version:

Jacques Lucas. Black glasses, bulk and fibers: Obtaining information in the infrared. Materials Today, Elsevier, 2000, 3 (1), pp.3 - 6. �10.1016/S1369-7021(00)80002-4�. �hal-00869383�

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Black Glasses, Bulk and Fibers:

Obtaining Information in the Infrared

Jacques Lucas, Laboratoire des Verres et Ceramiques, Campus de Beaulieu, Universit de Rennes, Rennes, 35042 France

Black in the visible but highly trans- p a r e n t in the infrared, the chalcogen- based glasses have reached a maturity w h i c h makes t h e m competitive materials for several technological applications related to d e t e c t i o n in the mid IR spectral domain. As bulk IR trans- p a r e n t materials they offer an advan- tage c o m p a r e d to g e r m a n i u m crys- tals, as they are low cost materials w h i c h can be shaped into simple or sophisticated IR lenses by molding.

Many efforts have b e e n made to opti- mize the chemical c o m p o s i t i o n in o r d e r to make these glasses very resistant to moisture and o x y g e n cor- rosion or d e v i t r i f i c a t i o n . W h e n drawn i n t o o p t i c a l fibers w i t h d i f f e r e n t optical configurations, they r e p r e s e n t a n e w generation of waveguides cov- ering the 3 to 12gtm spectral d o m a i n and paving the way for the d e v e l o p - m e n t of temperature, chemical or bio- chemical sensors. Amongst these pos- sibilities, the r e m o t e i n s i t u analysis of chemical processes (such as fer- m e n t a t i o n or reactions carried out u n d e r microwave or autoclave conditions) as well i n v i v o analysis of biological tissues, are the most exciting.

The use of chalcogen glass fiber tips for s c a n n i n g near field micro-spectroscopy is also a promising field.

The glass forming ability of chalcogen-like materials has b e e n k n o w n for decades, and the p r o t o t y p e material, the arsenic trisulfide As2S 3 leads to easy glass formation. In this kind of material, built from atoms of similar electronegativity, the b o n d b e t w e e n the c h a l c o g e n S a n d the p s e u d o - chalcogen As is essentially covalent a n d is c h a r a c t e r i z e d b y a s t r o n g

overlap of the atomic orbitals, render- ing these c o m p o u n d s very stable and resistant to chemical c o r r o s i o n i n gen- eral.The field of chalcogenide glasses has b e e n recently reviewed by Elliot I and the a u t h o r of this article 2. The s e c o n d i m p o r t a n t feature c o n c e r n i n g these materials is based o n the fact that o n e lone pair of electrons o n the As atoms and two lone pairs o n the chalcogens S, Se orTe are n o t engaged in the b o n d . The e n e r g y of this non- b o n d i n g level is such that its separa- tion w i t h the u p p e r a n t i b o n d i n g level is rather weak, resulting in b a n d gap average values Eg b e i n g closed to 2eV, causing these "black" glasses to be o p a q u e in the visible. This intrinsic

p r o p e r t y is a h a n d i c a p for the trans- m i s s i o n i n the s h o r t w a v e l e n g t h r e g i o n a n d u n t i l n o w the highest t r a n s p a r e n c y o b s e r v e d in the visible c o r r e s p o n d s to s u l f u r - c o n t a i n i n g glasses that are red or yellow colored.

All the Se or Te c o n t a i n i n g glasses are black, w i t h a b a n d g a p w h i c h can b e as l o w as leV.This p o o r t r a n s m i s s i o n in the visible is n e v e r t h e l e s s c o m p e n - sated b y an IR edge w h i c h is shifted towards the long w a v e l e n g t h r e g i o n in such a way that t r a n s p a r e n c y c a n e x t e n d towards the 18 ~m r e g i o n i n glasses c o n t a i n i n g h e a v y e l e m e n t s such as Te. Consequently, these vitreous materials e x h i b i t a v e r y h i g h t r a n s p a r e n c y in the mid-IR r e g i o n in

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Polymer coatine

Transportation section cO ⁼ 450

Sensing zone

cO= 100 m

Transportation section cO = 450 btm

Figure 2. A tapered IR glass fiber used for evanescent wave spectroscopy. The diameter is reduced in changing the drawing speed or by chemical etching in order to increase the amount of the IR light propagating on the surface of the fiber. The glass used for making the fiber is a TAS glass belonging to the Te/As/Se system. In the sensing zone, the polymer coating is eliminated by a solvent.

w h i c h are located the strategic optical w i n d o w s c o r r e s p o n d i n g to the a t m o s p h e r i c t r a n s p a r e n c y ( w h i c h lie from 3 to 5 lain and 8 to 12 ~am). The s e c o n d w i n d o w is also of special interest because it c o r r e s p o n d s to the room t e m p e r a t u r e black body emission w h i c h is located at a r o u n d 10 ~tm.

The d e t e c t i o n a n d analysis of IR light has b e e n a strategic military objective for at least two decades, and m a n y night vision systems have b e e n devel- oped, b u t at a rather e x p e n s i v e cost because they include IR lenses made from germanium. A n e w generation of o p t i c a l systems w h i c h i n c l u d e m o l d e d chalcogenides glasses in their design are n o w u n d e r c o n s i d e r a t i o n for low cost thermal imaging. These could b e used in infrared cameras for civil and industrial applications (e.g.

for assisting firemen or p o l i c e m e n as well as car driving in night or foggy conditions).

The studies of chalcogenide glass compositions that have been published lead to a rather confusing situation. The objective is to select the best glass in terms of resistance to crysxallization while keeping an excellent wansmission in the 3 to 14 lure window where the oxide-treed glasses are opaque.

The challenge is to build a floppy covalent framework having the ma_Mmurn degree of fi-eedom. This would allow bending and rotation of the bonds to produce an atxuS- odic skeleton, which loses its rigidity when heated up into the liquidus region. The

restdting viscous liquid remains out of equi- libriurn when cooled down to a solid, forrrv ing a liquid of infinite viscosit3; namely a glass. The building elements used in this flamework are related to three kinds of tewa- hedral units.The X= S,Se,Te atoms with their two bonding electrons and two lone pairs, the As atom with three bonding electrons and one lone pair and the Ge atoms with its four equivalent sp3 bonds.

Consequently, glasses having different kinds of dimensionality can be pro- duced, ranging from a single dimension chain-like network to 2D or 3D network. Intermediate structural models also exist and these explain the large vitreous domain observed in systems such as Ge/As/X=S, Se.The guide-line used to shift the IR edge is to maximize the n u m b e r of heavy atoms, but the price to pay for this is weak thermomechanical properties characterized by low glass transition temperatures Tg. W h e n good mechanical characteristics are needed, tetravalent Ge is necessary to increase the dimensionality and rigidity of the glass to the detriment of a part of the IR transparency. O n the other hand the glasses exhibiting the most interesting IR edge are Ge-free and belong for example to the Te/As/Se system.

In order to follow the specifications r e q u e s t e d by the t h e r m a l imaging technology, large pieces of glass with a very h o m o g e n e o u s refractive distribu- tion have to be prepared. P i o n e e r

works in this field have b e e n initiated b y Savage 3 and have b e e n c o n d u c t e d o n an industrial level by Hilton 4. Figure 1 shows a sample of glass rod of about 12 cm diameter routinely fabricated by the French c o m p a n y Vertex 5. If cut into appropriate slices these pieces of chalcogenide glasses can be m o l d e d as aspheric and diffractive lenses u n d e r moderate temperatures and pressures.

This original t e c h n o l o g y o p e n s the way to low cost IR optical lenses for affordable IR cameras.

W h e n glass compositions are carefully optimized, the selected vitreous material offers e x c e p t i o n a l r h e o l o g i c a l properties w h i c h p e r m i t optical fiber drawing. The first step consists of the preparation of a high purity glass, pro- duced from purified starting elements, followed by a distillation of the glass liquidus. All the operations are con- d u c t e d in silica vessels and the Final rod preform is obtained after homoge- nization of the melt and cooling d o w n the silica ampoule with a rigorous temperature program. Fiber preparation from the preform in the drawing t o w e r leads to c o n s t a n t diameters w h i c h can be adjusted and modified d e p e n d i n g o n the drawing speed and the glass temperature. In m a n y applications 6 a regular c o n s t a n t diameter is sufficient, for instance in radiometric devices w h e r e the fiber will be used to catch the thermal emission of an object and carry the energy to a MCT IR detector

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F T I R

Spectrometer Detector

Detection cell with liquid to b e a n a l y z e d

Optical fiber

1

A m p l i f i e r C o m p u t e r

Figure 3. Analytical set-up for IR glass fiber evanescent spectroscopy. The organic material in contact with the surface of the tapered fiber absorbs the evanescent wave IR light and leaves its infrared fingerprints on the detector. This remote IR spectroscopy operates in the 3 to 12 iJm range, in which the IR signatures of almost all organic compounds are located.

for temperature measurements7.These same kinds of waveguides w h e n cou- pled with a CO 2 laser can be used to transfer several watts of p o w e r towards a target, such as a biological tissue w h i c h strongly absorbs the 9,3 lam radiation. That w a v e l e n g t h has b e e n selected because it corresponds to the low loss region of the fiber a r o u n d l d B / m , and b e c a u s e of its strong absorption by the tissues.

The optical configuration of the wave- guide can also be designed in a com- pletely different way b y tapering the fiber over a short distance, say a b o u t t e n centimeters. This can be achieved either by changing the drawing speed abruptly during the fibering process, or by a chemical etching process using an acidic oxidizing solution w h i c h c o n g r u e n t l y dissolves the glass. Figure 2 represents the result of this tapering o p e r a t i o n which, for example, will reduce the fiber diameter from 450 orn in the transportation section, to 100 lain in the tapered sensing zone.

I n d e e d it is well k n o w n that w h e n IR light is i n j e c t e d into such a fiber w h i c h is transparent from 3 to 12 ~tm part of the energy travels o n the surface of the guide by an e v a n e s c e n t wave or total internal reflection mech- ahism. This p h e n o m e n o n increases as the d i a m e t e r of the w a v e g u i d e decreases, as verified o n calibrated t a p e r e d fibers. Figure 3 s h o w s a remote sensing system w h i c h includes

an FTIR s p e c t r o p h o t o m e t e r c o u p l e d to an MCT detector via a tapered fiberS.When an organic material is p u t into contact w i t h the sensing zone, it will absorb the IR light propagating o n the surface of the fiber and leave its o w n IR fingerprint o n the detector.

This novel optical p r o b e called an IR fiber e v a n e s c e n t wave, IRFEW, has b e e n tested and evaluated in several analytical conditions. First of all it has b e e n verified b y using alcoholic solu- tions as a standard that the sensitivity is p r o p o r t i o n a l to the l e n g t h a n d inversely p r o p o r t i o n a l to the fiber diameter; c o n c e n t r a t i o n as low as 1%

can be easily detected. The spectral w i n d o w c o v e r e d by this s e n s o r extends from 3 to a b o u t 12/am allow- ing, for instance, the carbon-halogen vibration detection of Freon or c a r b o n tetrachloride at a r o u n d 12 pm.

Included in this spectral range are almost all the fundamental vibrations of the organic molecules and of inor- ganic materials.This favorable situation permits, for instance, i n s i t u monitor- ing of chemical reactions such as the transformation of glucose molecules into alcohol during the invaluable fer- m e n t a t i o n process leading to w i n e preparation. I n d e e d b o t h molecules have IR fingerprints w h i c h are different e n o u g h to be quantitatively detected. The same analytical methodology has b e e n applied in the milk industry to follow the fermentation of milk into

yogurt as characterized by the transformation of lactose into lactic acid.

Recently this tapered fiber p r o b e has b e e n tested in a very unusual and hos- tile situation in the form of a chemical reaction assisted by microwave irradia- tion. I m m e r s i o n of the fiber in the m i c r o w a v e o v e n has p e r m i t t e d researchers for the first time to follow

i n s i t u the evolution of the different

species, giving t h e m valuable information o n the reaction mechanism. The same kind of e x p e r i m e n t is n o w in progress u n d e r autoclave conditions.

The most interesting initiative is the aim of obtaining information o n bio- c h e m i c a l a n d biological processes, especially i n v i v o conditions, as per- mitted by the non- invasive character of this IRFEW remote spectroscopy 9.

The only difficulty lies in establishing a simple mechanical and optical contact b e t w e e n the proteins or tissues to be analyzed and the tapered fiber.

A m o n g s t the crucial targets related to m e d i c i n e is the early d e t e c t i o n of can- cer, w h i c h is k n o w n to b e associated w i t h a modification of the conforma- t i o n of c e r t a i n p r o t e i n s w h i c h change their s t r u c t u r e w h e n the m e t a b o l i s m d e r e g u l a t i o n starts. It has b e e n d e m o n s t r a t e d t h a t the d i f f e r e n c e b e t w e e n t h e h e l i x s t r u c t u r e o f h e a l t h y p r o t e i n s a n d t h e t a b u l a r s t r u c t u r e of m a l i g n a n t tissues is d e t e c t a b l e in the mid-IR by e x a m i n i n g closely the two IR a b s o r p t i o n peaks

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of the so-called amides 1 and 2 located in the 1650 c m -1 region. The same kind of observation has also b e e n noticed by biologists in ex~tmining the tissues infected b y the prions in the Bovine Spongiform E n c e p h a l o p a t h y (BSE) disease.

The sensitivity, compactness and relia- bility of such optical sensors w i l l sig- nificantly improve with the e m e r g e n c e of the n e w powerful mid-IR sources such as the Q u a n t u m Cascade Laser (QCL) w h i c h is adjustable to a specific wavelength domain. This optical system will also benefit from the rapid evolution observed in the IR detection technology w h e n bolometers (pyro- electric u n c o o l e d detector elements) arrive o n the market.

The last emerging d o m a i n in w h i c h IR fibers are also finding a small niche is due to the n e e d of very sharp fiber tips for scanning near-field IR microscopy.

It is possible to apply a chemical etching process to shape these chalcogen based glasses, and very thin and sharp fiber tips have b e e n designed and suc- cessfully tested using a free electron laser as an IR source 10.

M t h o u g h these chalcogenide glasses are n o t n e w materials in a strict sense, they appear, like m a n y exotic glasses, to be largely u n k n o w n , especially i n fields w h e r e t h e y m a y b e c o m e candidates for technological applications. In the a b u n d a n t litera- ture, only a few articles p r o m o t e the u n d e r s t a n d i n g a n d c o n t r o l of the

e n g i n e e r i n g of such materials. Almost n o t h i n g is k n o w n a b o u t p h a s e separa- t i o n problems, such as m i c r o s c o p i c b u b b l e f o r m a t i o n d u r i n g the glass processing. Optical quality require- m e n t s i m p o s e the c o n t r o l of m a n y rheological and m e c h a n i c a l parame- ters, w h i c h are solved for the m o m e n t o n l y b y e m p i r i c a l r o u t e s . F u n d a - m e n t a l w o r k c o n d u c t e d i n this author's laboratory indicate, for example, that n e w c o m p o s i t e glass-micro- crystal materials called IR vitroceram- ics will b e the n e x t g e n e r a t i o n of IR t r a n s p a r e n t materials, e i t h e r i n bulk or fibrous form. C o m p a r e d to the p u r e glassy state, for e q u i v a l e n t optical properties, the benefits in t e r m s of t h e r m o m e c h a n i c a l p r o p e r t i e s are unrivalled.

I) S.R. Elliot, 'Chalco~nide Glass~ in Gl~s and Amorphous Materials' Materi~ Science and ~chnology, 9, 374-454, 1991VCH (Weinheim) 2) J. Lucas, 'Infrared Glasses' Current Opinion in Solid State and Materials Science 4, 181-187, 1999

3) J'A" Savage"Infrared Optical Materials and their Anti'Reflecti°n C°atings' 1987 Adam Hilger (Bristol)

4) A.R Hilton and G.R. Cronin, 'Production of IR optical materials at Amorphous Materials Incorporated', SPIE proceedings 618, 184,1986 5) VERTEX, IR glass manufacturer, 24c rue des Landelles, Chantepie, 35135 France e-mail:vertex@wanadoo.fr

6) J.S. Sanghera, I.S. Aggarwall 'Development ofChalcogenide Glass Fibers at NRU J. Non-cryst. Solids 213-214, 63-67, 1997

7) E Gilbert, E Ardouin, P. Morillon, K. Lefoulgoc, X.H. Zhang, H.L. Ma, J. Lucas. 'Low temperature measurements by using IR Tex glass fibers' Proceedings SPIE, 2839, 239, 1996.

8) C. Boussard-Pledel, S. Hocde, G. Fonteneau, H . L Ma, X.H. Zhang, K. Lefoulgoc, J. Lucas 'Infrared glass fibers for evanescent wave spectroscopy', Proceedings SPIE, 3596, 91, 1999 9) N. Afanasyeva, R. Bruch, A. Katzir, 'Infrared Fiberoptic Evanescent Wave Spectroscopy: Applications in Biology and Medicine, SPIE proceedings, 3596, 152

10 D.T. Schaafsma, R. Mossadegh, J.S. Sanghera, I.D. Aggarwal, J.M. Gilligan, N.H. Tolk, M. Luce, R. Generosi, A.Perfetti, A. Cricenti, G. Margaritondo 'Single Mode chalcogenide fiber infrared SNOM probe' Ultramicroscopy 77, 77-81 ,I999.

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