Recent advances in chalcogenide glasses
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Journal of Non-Crystalline Solids 345&346 (2004) 276–283
www.elsevier.com/locate/jnoncrysol
Section 5. Optical properties
Recent advances in chalcogenide glasses
Bruno Bureau a,*, Xiang Hua Zhang a, Frederic Smektala a, Jean-Luc Adam a,
Johann Troles a, Hong-li Ma a, Catherine Boussard-Plèdel a, Jacques Lucas a,
Pierre Lucas b, David Le Coq b, Mark R. Riley b, Joseph H. Simmons b
a
Laboratoire des Verres et Céramiques, UMR-CNRS 6512, Campus de Beaulieu, Université de Rennes 1, Rennes cedex 35042, France
b
Arizona Materials Laboratory, 4715 E. Fort Lowell, Tucson, AZ 85712, USA
Abstract
Compared to oxide-based glasses, vitreous materials involving chalcogens form a rather new family of glasses which have
received attention, mainly because of their transmission in the mid-infrared. Indeed as low phonon compounds, these heavy-anion
glasses allow the fabrication of molded optics for infrared cameras as well as infrared fibers operating in a large spectral range. These
waveguides, when correctly tapered, allows the development of a new generation of sensitive evanescent-wave optical sensors which
have been used for biomedical applications. Here we will focus on the spectral analysis of biomolecules present in human lung cells
by measuring their infrared signatures. Because they contain heavy polarizable anions as well as lone-pair electrons, these glasses
exhibit very large non-linear properties compared to silica and are candidates for fast optical switching and signal regeneration
in telecom. Due to the technological interest in chalcogenide glasses, more information is needed on their structural organization
and 77Se NMR spectroscopy appears to be a useful tool for checking the local environment of the Se atoms.
2004 Elsevier B.V. All rights reserved.
PACS: 61.43.Fs; 42.65; 78.20. e; 76.60. k; 42.81. i; 42.81.Pa
1. Introduction modes shifted far in the IR, and rending these glasses
interesting for the fabrication of thermal-imaging sys-
The glass-forming ability of chalcogens or pseudo- tems [1]. This exceptional transparency, associated with
chalcogens combinations has been known for several a suitable viscosity/temperature dependence, creates a
decades but compared to oxide glasses, especially sili- good opportunity for the development of molded optics
cates, this class of vitreous materials is just emerging for low-cost infrared cameras. It must be noticed that
from their infancy. The main attention paid to these these chalcogenide glasses also exhibit unique properties,
materials relies on their large optical window extending such as photo-darkening [2], giant photo-expansion as
in the mid-infrared and covering usually the two atmos- well as photo-fluidity [3] when irradiated by appropriate
pheric windows lying from 3 to 5 and 8 to 12 lm. These light. These photo-induced phenomena will not be dis-
low-phonon materials have to be considered as heavy- cussed here. Therefore, these glasses, which contain large
anion glasses since sulfur, S, selenium, Se and even polarizable atoms associated with external lone-pair elec-
tellurium, Te, are the main constituents of their compo- trons, are prone to exceptional non-linear optical proper-
sitions. This situation leads to fundamental vibrational ties when irradiated by an electromagnetic field.
With a non-linear refractive index, n2, more than hun-
*
Corresponding author. Tel.: +33 2 2323 6573/5619; fax: +33 2
dred times higher than silica, they are serious candidates
2323 5611. for fast switching and signal regeneration devices for tel-
E-mail address: bruno.bureau@univ-rennes1.fr (B. Bureau). ecommunication. Some selected compositions of these
0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jnoncrysol.2004.08.096
B. Bureau et al. / Journal of Non-Crystalline Solids 345&346 (2004) 276–283 277
low-phonon glasses are very resistant to devitrification by reference to their structural organization which also
and can be drawn into optical fibers which offer excep- reflects the thermodynamical and mechanical properties
tional spectral windows, typically lying from 3 to 12 lm. of the materials. Glasses such as vitreous selenium Se or
A controlled tapering technique is essential for the devel- the TeX glasses (X = Cl, Br, I), such as Te3I2, are chain-
opment of evanescent-wave optical sensors which can be like materials with a one-dimensional (1D) framework,
applied to investigate, at a molecular level, several prob- which leads to a poor network rigidity and consequently
lems encountered in microbiology [4]. For instance, to a low glass-transition temperature Tg, usually below
recording the infrared signatures of the bio-molecules 100 C; for instance Tg for Se is 40 C. The second family
present in human living lung cells is of major importance represented by As2S3 and As2Se3 originates from the
for the early diagnostics of tumor. Depending on the met- two-dimensional (2D) connection of AsSe3 pyramids
abolic conditions and using a simple contact between the and due to the better degree of reticulation they exhibit
fiber and the cells, precious information can be obtained. more rigidity and consequently higher Tg values; for
A good knowledge of the glass structure is essential example Tg = 185 C for As2Se3. The third group repre-
to understand some mechanical, thermodynamical and sented by GeS2 or GeSe2 are 3D glasses which result
chemical properties of such materials and the difficulties from the connection of tetrahedra sharing corners and
associated with the disordered nature of the glassy state which exhibit the highest Tg.
is a real challenge. As for as Se-containing glasses are Glasses belonging to the Se/As2Se3 system, situated
concerned, 77Se NMR spectroscopy appears to be a just in between a 1D and 2D network have been investi-
powerful method to obtain indications on the local Se gated by Se NMR spectroscopy and compared to the
environment which contributes to reinforce the con- parent crystalline materials c-Se and c-As2Se3 [5]. This
struction of structural models. work was aimed at evidencing Se atoms having a coor-
dination different from those observed in the two limit-
ing cases; Se–Se–Se in the pure Se where Se atoms are
2. 77Se NMR spectroscopy, a precious tool for local surrounded by two Se and As–Se–As in As2Se3 where
structure probing Se is bonded to two As atoms. Fig. 1 represents the
77
Se NMR of the five materials: pure Se in the vitreous
A convenient way to classify the numerous glass com- and crystalline forms, the intermediate glass AsSe4.5 and
positions which are based on chalcogen combinations is the 2D As2Se3 in the crystalline and glassy variety. It is
a c
≈ 850 ppm ≈ 380 ppm
b
≈ 550 ppm
v-Se
c-Se
v-AsSe4.5
v-As2Se3
c-As2Se3
1200 800 400 0 - 400
Chemical shift (ppm)
77
Fig. 1. Se solid state NMR spectra of pure Se in vitreous and crystalline forms, the glass AsSe4.5 and As2Se3 in the crystalline and glassy variety.
278 B. Bureau et al. / Journal of Non-Crystalline Solids 345&346 (2004) 276–283
clear that the chemical shift at 380 ppm is attributed to a
Se atom coordinated by two others Se(a), Se–Se–Se, that
the shift at 850 ppm is due to a Se atom connected to two
As atoms, As–Se–As, as described in the orpiment crys-
talline structure of arsenic selenide.
The glasses intermediate between Se (1D) and As2Se3
(2D), and represented on the figure only by one repre-
senting AsSe4.5, have to be considered as chain ramified
glasses due to the trivalency of As atoms which allows
chains cross-linking. This situation leads to a new
type of coordination for the Se atoms, Se–Se–As, where
the Se are coordinated by one Se and one As: the
corresponding chemical shift Se (b) is located at
550 ppm.
By measuring the intensity of this 550 ppm peak and
comparing with the two other peaks at 380 and 850 ppm,
it is possible to estimate the degree of reticulation when
the Se glass is enriched with As or at the opposite the Fig. 2. Photograph of a chalcogenide glass rod.
loss in network dimensionality when Se is added to
As2Se3 glass.
(d) The final product must satisfy certain mechanical
and thermal specifications, which can be monitored
3. Chalcogenide glass molding for infrared cameras optics in selecting a combination of chemical elements
which allows a control of the glass network rigidity
The only materials used up to now for the design of as discussed above.
lenses for infrared thermal imaging systems is germa-
nium, Ge, an expensive material, which needs to be A good compromise between all these parameters is
grown into large single crystals. This material satisfies found in using a glass composition containing tetrava-
the main technological requirement, which has a good lent Ge, trivalent As or Sb, and divalent Se or Te
transparency in the two spectral windows where the atoms. Technical compositions have been developed
atmosphere is transparent enough to transmit the energy by one of us (X.H.Z) in our research laboratory as well
emitted by a thermal object and which are located in the as in the company Vertex, which has become Umicore
regions 3–5 lm and 8–12 lm. However germanium is an IR Glass and which offers two products: GASIR 1
expensive element and, to be shaped into complicated (standard grade) and GASIR 2 (As-free), as well as
optics such as aspheric and diffractive lenses, costly dia- molded optics [6]. A molding technique has been devel-
mond turning operations are necessary. This observa- oped at the university and Vertex under French Def-
tion explains why the price of infrared cameras is still ense contracts. Fig. 3 represents the simple molding
high and it is clear that the evolution of night-vision procedure in which a slice of glass is heated under mod-
technology towards mass production is depending on erated pressure until it reaches the appropriate viscos-
the fabrication of low-cost optics, obtained by molding ity and then plasticity in order to duplicate perfectly
IR-transmitting glasses. The selection of the suitable the surface of the mould. As shown in Fig. 4, lenses
glass composition need to obey at least four with a complicated design, such as aspheric and also
requirements: diffractive, have been obtained in single molding opera-
tions after selecting the appropriate molding material.
(a) An excellent transparency from 1 to 14 lm, which is A surface profile investigation, using a Thalysurf
rather easy to obtain, for instance in selecting mechanical profilometer has been conducted in order
heavy-anion Se-rich compositions. to verify the quality of the duplication and the rough-
(b) An exceptional resistance to devitrification, espe- ness of the molded surface. The deviation from ideal
cially when large samples of glasses are concerned, duplication is less than half a micron which is more
as represented in Fig. 2. In such samples the pres- than enough for optics operating in the 10 lm region.
ence of small crystallites in the heart of the rod, Fig. 5 represents a night vision picture obtained with
which is a slow-cooling region, would be detrimen- a camera equipped with chalcogenide-glass lenses. It
tal for the optical properties of the lenses. must be noticed that, in using asphero-diffractive lenses,
(c) The viscosity/temperature dependence needs to be the design of the optical system is simplified which
as flat as possible in order to offer an easy control consequently reduces the cost and the size of the
of the molding process. cameras.
B. Bureau et al. / Journal of Non-Crystalline Solids 345&346 (2004) 276–283 279
Fig. 3. Sketch of the molding procedure to make chalcogenide glass lenses.
Fig. 4. Molded lenses with diffractive (a) or spheric (b) surfaces.
spatial arrangement demonstrates that the lone pairs
are very active, occupying a significant volume in the
network and contributing significantly to the
polarizability.
The second consequence of these lone-pair electron
densities is to produce non-bonding levels in the energy
diagram, located between the bonding and anti-bonding
levels and giving rise to a significant lowering of the
optical band gap Eg. This observation explains why
chalcogenide glasses have a very poor transmission in
the visible, most of them being black. This remark is
of importance when measuring the non-linear refractive
index n2 which is not a constant when measured near a
resonance. The general tendency is that Eg which meas-
Fig. 5. Photograph taken with a night vision camera equipped with ures the separation between the non-bonding and anti-
chalcogenide glass lens.
bonding levels, decreases regularly from the sulfides to
the selenides and then the tellurides.
This phenomenon introduces a limitation in the uti-
4. A glass family with high non-linear optical properties lization of these glasses in optical telecommunication
which operates at 1.55 lm, not too far from the
As already mentioned, chalcogenide-glasses are band-gap edge. The tolerances in term of optical loss
made from heavy elements such as S, As, Se, Te, hav- are severe and restrictive and chalcogenide devices, be-
ing electron shells which are easily polarizable under an cause of a possible residual absorbance in the telecom
electro-magnetic field excitation as illustrated in Fig. 6. window, would have to be used only for short
Examination of the structure of the parent crystalline distances.
materials indicate clearly that the coordination around Fig. 6 portrays the general evolution of the non-linear
the chalcogen atoms is always pseudo-tetrahedral made refractive index n2 for several glass families ranging from
from two bonding electron pairs and two non-bonding the fluorides which have the lowest values, with n2 close
pairs which indicate a strong stereochemical effect due to 0.75 · 10 20 m2/W to values around 1.3 · 10 17 m2/W
to electron-pairs repulsion. The resulting tetrahedral for Se-containing glasses, which means that three orders
280 B. Bureau et al. / Journal of Non-Crystalline Solids 345&346 (2004) 276–283
2 4
4s ,4 p
n 2 (m2/W)
Low E High E
- 17
10 Selenium based Se
- 18 glasses
10 Sulfur based glasses
- 20
10 Oxide glasses
- 21 S
P = χ E+χ E +χ E
(1) (2) 2 (3) 3
10 Fluoride glasses
... 2 4
PL N
PL 3s , 3p
Fig. 6. Evolution of the non-linear refractive index n2 for several glass families.
of magnitude separate these families. As shown in Fig. restriction that the glass must have an acceptable optical
6, the highest values have been measured on Se-contain- loss at the telecommunications wavelengths in the silica
ing glasses belonging to the Ge–As–Se system; the value low loss region around 1.55 lm.
obtained at 1.55 lm [7] is very similar to our value [8]
measured at 1.06 lm, which indicates that the position
of the band gap is far enough from the measuring wave- 5. Optical fiber engineering and fiber evanescent-wave
lengths to produce erroneous values. spectroscopy
The clear message is that Eg and n2 are evolving in
two opposite ways and that the selection of a suitable The interest of infrared chalcogenide glass fibers re-
technical glass will be a compromise between these two lies on three kind of applications:
parameters.
One potential application, associated with the possi- (a) They can be used to carry the infrared signal emit-
bility of increasing the refractive index of a glass wave- ted by a thermal object towards a detector to meas-
guide during the short time of a laser pulse irradiation, ure the local temperature in remote, hazardous
is illustrated in Fig. 7. The scheme represents a Mach– places.
Zehnder interferometer, made from a non-linear chalco- (b) They can be used, as flexible waveguides, to carry
genide glass where an input signal have been split into towards a target, the energy of a laser source such
two branches. Assuming that one of the branches is illu- as a CO2 laser, operating around 10 lm.
minated by a command laser pulse, it will result during (c) They are ideal candidates for the development of
the pulse in an increase of the refractive index of the optical sensors, based on IR fiber evanescent-wave
branch waveguide which will delay the optical signal absorption, operating in situ, especially in medical
in the branch, and will permit the generation of destruc- and biological conditions. After several successful
tive or constructive interferences. Chalcogenide glasses, bio-sensing experiments on liver metabolism, tumor
with high n2 need only moderate laser power to change detection, serum analysis [9,10], we focus here on
their refractive index, and appear to be serious candi- the infrared fingerprints of human lung cells in dif-
dates for these fast optical switching devices with the ferent metabolic conditions.
Fig. 7. Sketch of a fast optical switching device. In a Mach–Zehnder interferometer made from a non-linear chalcogenide glass, a short laser pulse
modifies the refractive index and delays the optical signal in one branch.B. Bureau et al. / Journal of Non-Crystalline Solids 345&346 (2004) 276–283 281
In each of these applications the key point is the and leads to a shiny, high optical quality surface, which
selection of a glass which can be shaped into a few meter of course increases the efficiency of the sensor.
long fiber and has the broadest optical window in the
IR. It should reach, if possible, the 12 lm IR edge region
in order to carry the CO2 laser beam, to transport the 6. Infrared fiber evanescent-wave spectroscopy of
maximum of the radiation emitted by a thermal object biomolecules in human lung cells
and finally to fit with the infrared signatures of the mol-
ecules encountered in chemistry or biology and which Fig. 9 represents the analytical set-up used for the
have fundamental vibrations modes in the range from experiments, with an IR tapered glass fiber coupled to
3 to 12 lm. the input of a FTIR spectrophotometer and the output
The selected glass must also satisfy other criteria, of an MCT (Mercury–Cadmium–Telluride) infrared
such as a strong resistance to crystallization and water cooled detector. The infrared spectra are recorded after
corrosion. Our selected glass belongs to the ternary sys- establishing a contact between the tapered sensing zone
tem Te/As/Se with the composition Te2As3Se5. This and a suspension of a human lung cell line (A549) in a
TAS glass, as discussed before [11], can be drawn into 0.9% NaCl solution. These cells are typical immortalized
single-index optical fibers from a high-purity perform, mammalian lung cell line that form strong attachments
synthesized by purifying the starting elements and the to surfaces. The contact between cells and fiber is made,
glass itself by distillation. either by dipping the fiber into the cell suspension or
One recent progress we have accomplished in the field depositing one drop of the liquid on the fiber.
was motivated by the need to control the fiber diameter The lung cells are a few microns thick and about ten
in a short portion of the fiber used as the sensing zone microns long and are a complex biological system in
[12]. Indeed, tapering the fiber results in a strong in- which the cellular membrane forms a lipid-rich envelope
crease of the evanescent-wave intensity which propa- with some interspersed proteins, which together main-
gates along the fiber surface. This increases the tain an intact and viable cell.
absorption sensitivity when the fiber is put into contact The evanescent wave propagates at the surface of the
with molecules or biomolecules. Two methods have fiber at a maximum distance of a few microns. Hence it
been developed to control the diameter of the fiber along is mainly collecting the infrared signatures of the mem-
the zone which will be used for sensing, as indicated in brane. It is necessary to ensure an immersion time of
the Fig. 8. A fast increase of the drawing speed permits about two hours to permit the deposition and attach-
the reduction of the diameter, for instance from 400 to ment of the maximum number of cells on the glass sur-
150 lm on a length of about 10 cm. This operation is face. In these conditions, the intensities of the IR
possible because of the suitable rheological properties absorption band remains constant with time, as shown
of the glass which exhibits a rather flat temperature/vis- in the typical IR fingerprint spectra of a live cell repre-
cosity dependence. A second process, based on chemical sented in Fig. 10. The two main bands at 2853 and
etching, has been used to congruently dissolve the glass 2926 cm 1 are attributed to CH2 asymmetric and sym-
and control precisely the diameter of the tapered zone metric vibrations respectively, and the two lower inten-
up to 100 lm diameter, for instance. The etching process sity bands at 2871 and 2960 cm 1 correspond to the
is based on an oxidation and acidic attack of the glass symmetric and asymmetric CH3 vibrations, respectively
which dissolves the three elements, Se, Te, As at the [13].
same rate. Two type of solutions are used which contain One of the goals of this work was to collect the IR
concentrated sulfuric acid added with either H2O2 or signatures of healthy lung cells and to follow the change
potassium bichromate as oxidizing agents. This method in their optical signatures when a toxic agent was intro-
offers a second advantage because it induces a chemical duced in the solution. The surfactant Triton was used as
polishing of the glass surface by eroding the asperities a model compound as this disrupts cell membranes and
MCT
FTIR
FTIR Detector
Cell with sample
Spectrometer TAS glass to be analysed
Spectrometer tapered optical
fiber
Amplifier
Computer
Fig. 8. (a) Photograph of a fiber tapered by both mechanical handling
and chemical etching. (b) Sketch evidencing the increase of the Fig. 9. Experimental set-up used for the fiber evanescent-wave
reflection number and so the sensitivity in the sensing zone. spectroscopy experiments.282 B. Bureau et al. / Journal of Non-Crystalline Solids 345&346 (2004) 276–283
5.0
4.5 BEFORE TRITON
Asymmetric Symmetric TRITON ADDITION
4.0 CH 2 bands CH 3 bands
5 min
10 min
Absorbance (u.a.)
3.5 15 min
20 min
3.0 Symmetric 60 min
Asymmetric CH 2 bands
2.5 CH 3 bands
2.0
1.5
1.0
0.5
0.0
3025 3000 2975 2950 2925 2900 2875 2850 2825 2800 2775
Wavenumber (cm-1)
Fig. 10. Human lung cell infrared spectra recorded with the TAS glass fiber. The asymmetric and symmetric CH2 bands decrease in intensity
regularly whereas the methyl CH3 group around 2870 cm 1 increases and slightly change in position.
causes rapid cell death when present at sufficiently high
concentrations.
Fig. 10 portrays the evolution of the IR spectra of the
cells from the time of initial introduction of Triton to a
period ranging from minutes to a few hours.
It is observed that the asymmetric and symmetric
CH2 vibrational bands decrease regularly in intensities
while the methyl CH3 group around 2870 cm 1 becomes
more important and changes in position.
The biological effect of a surfactant on mammalian
cells is to solubilize portions of the cell membrane, thus
leading to leakage of intracellular contents. The environ-
ment of the phospholipids in the membrane changes
quite rapidly. Fig. 11 displays a schematic of a potential
scenario explaining the change in intensities attributed Fig. 12. Photograph of the cells deposited around the fiber.
to a round-up of the cells exposed to Triton. As por-
trayed in Fig. 12, cells have initially a large contact area 7. Conclusion
with the glass when spread over the fiber surface, result-
ing in strong absorption bands. When exposed to Tri- In spite of having thermal and mechanical properties
ton, the cells lose membrane integrity, round up, and notably inferior to those of oxide glasses, chalcogenide-
lose contact with the fiber, due probably to a strong based glasses offer potential which has just started to be
modification in the structure of the membrane lipids. explored. The fabrication of low-cost, high-quality,
This step is a prelude to the loss of cell membrane integ- molded infrared optics is still now at an early stage of
rity indicative of cell death. growth but should reach its mature level with the devel-
Despite the complexity of biological systems, infrared opment of thermal imaging systems for car driving
remote spectroscopy using fibers appears to be a power- assistance. Anti-reflective coatings have been developed
ful tool to discriminate between different metabolic situ- in order to decrease reflection losses associated with the
ations. In following the change of some important high refractive index, and hard coatings are used to pro-
functional groups, it has been demonstrated that the duce optics resistant to abrasion. The exceptional non-
evolution from healthy live cells to dead cells can be linear properties of chalcogenide glasses have just been
monitored. identified and promise a bright future. However, many
Healthy cells Unhealthy cells Dead cells
TAS
Fiber
Cells attach to fiber and
Cells “round up” Cell membranes are not intact
spread over surface
Fig. 11. Schematic representation of the cell behavior on the fiber exposed to Triton.B. Bureau et al. / Journal of Non-Crystalline Solids 345&346 (2004) 276–283 283
other phenomena need to be elucidated, among them the [2] C.R. Schardt, J.H. Simmons, P. Lucas, L. Le Neindre, J. Lucas,
strong variation of the refractive index n under a laser J. Non-Cryst. Solids 274 (2000) 23.
[3] K. Tanaka, C.R. Chimie 5 (2002) 805.
pulse either in the reversible regime or when a local irre- [4] D. Naumann, in: Infrared Spectroscopy in Microbiology, Ency-
versible modification of n is produced, making possible clopedia of Analytical Chemistry, John Wiley & sons, 2000, p.
micro-lenses or Bragg gratings fabrication. It is clear 102.
that an understanding of the interactions between pho- [5] B. Bureau, J. Troles, M. LeFloch, F. Smektala, G. Silly, J. Lucas,
tons of appropriate energy and the chalcogenide glass Solid State Sci. 5 (2003) 219.
[6] Umicore IR Glass, Z.A. du Boulais 35690 Acigné, France.
network producing exciting phenomena such as photo- Available from: .
refractivity, photo-expansion and photo-fluidity is of [7] J.H. Harbolt, F.O. Ilday, F.W. Wise, B.G. Aitken, IEEE Photon.
major importance. Technol. Lett. 14 (6) (2002) 822.
Even if the limit in infrared transmission is probably [8] C. Quémard, F. Smektala, V. Couderc, A. Barthélémy, J. Lucas,
reached with the actual glass compositions, progresses J. Phys. Chem. Solids 62 (2001) 1435.
[9] J. Keirsse, C. Boussard-Plédel, O. Loréal, O. Sire, B. Bureau
can be made in fiber and confined planar waveguide de- B. Turlin, P. Leroyer, J. Lucas, J. Non-Cryst. Solids 327 (2003)
sign, opening the way to more compact, more sensitive 430.
infrared optical sensors for medical, biological and [10] J. Keirsse, C. Boussard-Plédel, O. Loréal, O. Sire, B. Bureau
industrial applications. P. Leroyer, B. Turlin, J. Lucas, Vib. Spectrosc. 32 (1) (2003)
23.
[11] S. Hocdé, C. Boussard-Plédel, G. Fonteneau, J. Lucas, Solid
State Sci. 3 (2001) 279.
References [12] D. Le Coq, K. Michel, G. Fonteneau, S. Hocdé, C. Boussard-
Plédel, J. Lucas, Int. J. Inorg. Mater. 3 (2001) 233.
[1] V.F. Kokorina, Glasses for infrared optics, in: M.J. Weber (Ed.), [13] B. Rigas, S. Morgello, I.S. Goldman, P.T.T. Wong, Proc. Nat.
Laser and Optical Science and Technology Series, CRC Press, 1996. Acad. Sci. USA 87 (1990) 8140.You can also read
