planar optical waveguides on soda-lime glass substrates...
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F. Rey-García et al, Planar optical waveguides via Sol-Gel 18/10/1224/10/10 10:0013:36 1
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Planar optical waveguides on soda-lime glass substrates obtained via 1
Sol-Gel 2
F. REY-GARCÍA(1)
, C. GÓMEZ-REINO(1)
, M.T. FLORES-ARIAS(1)
, G. F. DE LA 3
FUENTE(2)
, A. DURÁN(3)
, Y. CASTRO(3)
4
1. Unidad Asociada de Óptica & Microóptica GRIN (CSIC-ICMA), 5
Departamento de Física Aplicada, Escola Universitaria de Óptica e 6
Optometría, Universidade de Santiago de Compostela, Campus Sur 7
s/n, E-15782 Santiago de Compostela, e-mail: [email protected] 8
2. Instituto de Ciencia de Materiales de Aragón (CSIC-Universidad de 9
Zaragoza), María de Luna 3, E-50018 Zaragoza, e-mail: 10
3. Instituto de Cerámica y Vidrio (CSIC), Kelsen 5, E-28049, Madrid, 12
e-mail: [email protected] 13
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15
16
17
18
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Corresponding author: A. Durán ([email protected]) 21
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F. Rey-García et al, Planar optical waveguides via Sol-Gel 18/10/1224/10/10 10:0013:36 2
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Abstract 23
Inorganic and hybrid planar waveguides with different compositions (silica-titania, 24
methacrylate -silica-cerium oxide, zirconia-cerium oxide and silica-zirconia) have been 25
obtained by sol-gel synthesis followed by dip-coating. Soda-lime glass slides and 26
conventional commercial window glass were used as substrates. The thickness and 27
refractive index of the coatings were determined by profilometry and Spectroscopic 28
Ellipsometry. Waveguide efficiency was measured at ca. 70% with a He-Ne laser beam, 29
coupled with an optical microscope objective into and out of the waveguiding layer via 30
a double prism configuration. Thicknesses between 150 and 2000 nm, along with 31
refractive index values ranging between 1.45 and 1.99 ( = 633 nm) were obtained 32
depending on the sol composition and the dip-coating conditions. This wide range of 33
values allows designing multilayered guides that can be used in a variety of 34
applications. 35
Keywords: planar waveguides, inorganic coatings, hybrid coatings, sol-gel 36
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Introduction 37
Planar waveguides contain a guiding layer with a higher refractive index than the 38
substrate or the adjacent layers which confine it physically. Planar optical waveguides 39
have been prepared by ion-exchange, chemical or physical vapour deposition methods 40
[1-3]. Laser induced deposition and direct writing techniques have also been employed 41
to obtain planar waveguides on glass substrates of different compositions [4-11]. The 42
controlled local modification of the refractive index of commercial glass substrates is 43
the main target of these processing methods, which enable the preparation of coatings of 44
different thickness and composition. 45
The preparation of planar waveguides has attracted considerable attention because of 46
their potential applications in photonic devices, as well as in the field of 47
communications and data transmission. The generation of higher index paths in 48
commercial glass substrates should allow achieving effective waveguiding within any 49
established geometrical trace in an economically attractive fashion. 50
An alternative, convenient method to obtain planar waveguides is based on sol-gel 51
technology. It permits to obtain coatings with controlled thickness and refractive index. 52
A considerable number of papers dealing with planar waveguides obtained by the Sol-53
Gel method have been reported in the literature [12-22]. For example, Gonçalves et al 54
have reported on the sol-gel preparation and properties of multilayer, Er3+
doped SiO2-55
ZrO2, active planar waveguides [19] on Si (100) substrates. Maia et al have most 56
recently described the preparation of highly planar multilayer Er:YAl3(BO3)4 glass thin 57
films by the polymeric precursor and Sol-Gel methods, where the film thickness ranged 58
between 630 and 780 nm [20]. Moreover, there were previous reports on dip-coated, 59
thin lead titanate sol-gel waveguide crystalline films obtained on borosilicate glass [21], 60
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as well as on dip-coated, 9-micron thick sol-gel laponite (synthetic clay) waveguide 61
films obtained on both, borosilicate glass slides and Si wafers [22]. 62
From the previous literature it may be inferred that the sol-gel method has been 63
demonstrated convenient to control chemical composition, thus the refractive index, and 64
film thickness by single and multideposition techniques. Reduced costs and large area 65
coatings are additional advantages of sol-gel processes, as compared with other 66
techniques developed for the preparation of planar waveguides; in particular, those 67
requiring the use of vacuum chambers. Deposition techniques such as dipping, spraying 68
and spinning allow coating a wide variety of substrates and complex geometries via sol-69
gel. In addition, sol-gel techniques allow to incorporate organic molecules for obtaining 70
hybrid coatings, which have attracted considerable interest for the fabrication of sensor 71
waveguides as reviewed in [23]. 72
The microstructure of sol-gel derived coatings, thus their optical properties, is affected 73
by different parameters involved in their synthesis. These include starting materials, 74
solvent, and thermal processing conditions, among others [24-25]. The optimisation and 75
control of all these parameters should allow the production of efficient sol-gel derived 76
planar waveguides. 77
The aim of this work was to obtain waveguides on commercial glass substrates using a 78
sol-gel method and inorganic and hybrid (organic-inorganic) sols previously described 79
in the literature [26-29]. The optical properties of the films as a function of the organic 80
component, thickness, refractive index and multi-deposition processes have been 81
studied. In addition, the efficiency of the waveguides has been measured using a He-Ne 82
laser and evaluating the energy input and output with a double prism configuration 83
similar to that described in [30-31]. 84
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Experimental 85
Silica-titania (SiTi sol), silica-zirconia (SiZcol, SiZrZcol sols), methacrylate-silica-86
cerium oxide (TM-Ce sol) and zirconia-cerium oxide (ZrCe sol) inorganic and hybrid 87
sols were prepared by acid catalysis following different routes. Table I summarises the 88
molar ratios, composition and final concentration of the sols. 89
The inorganic ZrCe sol was prepared by mixing tetrapropoxyzirconate (TPZ) with 90
glacial acetic acid (AcH) as complexing agent to control the hydrolysis and 91
condensation reactions; cerium nitrate (Ce(NO3)36H2O was used as cerium precursor. 92
Absolute ethanol and distilled water were added drop wise to this solution to start the 93
hydrolysis and condensation reactions. Two different compositions were prepared: 94
90ZrO2:10CeO2 and 70ZrO2:30CeO2, both with final concentrations of 100 g/L. 95
On the one hand, two types of hybrid inorganic-organic sols were synthesised. Low-96
hybrid sols (SiTi, SiZcol, SiZrZcol sols) incorporate methyl groups as the organic part 97
responsible for increasing the critical thickness of the coatings. On the other hand, 98
highly organic hybrid sols (TM-Ce sol) are obtained from a hybrid alkoxide, 3-99
methacryloxypropyl trimethoxysilane (MPS), where methacrylate groups can be 100
partially polymerised in the liquid in order to develop two interpenetrating and bonded 101
organic and inorganic networks. 102
SiZcol and SiZrZcol sols were prepared using a zirconia colloidal suspension (Nyacol 103
suspension, 20% zirconia in water), methyl-triethoxysilane (MTES), tetraethyl 104
orthosilicate (TEOS) and TPZ (only in the case of SiZrZcol sol). The synthesis of SiZcol 105
sol was reported previously [26]. In the case of SiZrZcol sol, a new process was 106
followed: TPZ was first mixed with ethanol and acetic acid under stirring for 1 hour to 107
produce a chelate complex. In a subsequent step, MTES, TEOS and zirconia colloidal 108
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suspension were incorporated under vigorous stirring; concentrated HNO3 (0.6 vol %) 109
was then added to attain a final pH between 2 and 3. 110
SiTi sols were prepared by complexing tetraisopropoxytitanate (TISP) with acetic acid 111
and adding MTES in different molar ratios (Table 1) and acidified water in a dropwise 112
manner, to complete the synthesis reactions. 113
Finally, highly organic hybrid sol (TM-Ce sol) was prepared following the process 114
described in [27]. The methacrylate groups were partially polymerised in liquid using 115
2,2´-azobis(isobutyro)nitrile (AIBN) as initiator. This synthesis process may yield 116
thicker ( 5 µm) and more flexible coatings. 117
The stability of the sols was followed by measuring the evolution of viscosity with time 118
using the Ostwald method [28-26]. 119
Coatings were deposited by dip-coating varying the withdrawal rate between 10 and 50 120
cm/min on soda-lime glass slides and on 5 mm thick commercial window glass. The 121
substrates were cleaned in an ultrasonic bath with absolute ethanol for 15 min. The 122
coatings were heat-treated at 450ºC for 30 min in air, except those obtained from TM-123
Ce sol, which were cured at 150ºC during 1 h. 124
Multilayer coatings were prepared for the compositions specified in Table II. Substrates 125
were dip-coated with five-layers of the SiTi sol containing 70:30 and 50:50 molar 126
ratios, and with four-layers of the ZrCe sol (70:30 molar ratio) at a constant withdrawal 127
rate of 25 cm/min. Intermediate heat treatments at 450ºC during 15 min were applied 128
between coatings, followed by a final treatment at 450ºC during 30 min. 129
On the other hand, two sandwich type multilayer coatings were prepared by deposition 130
of alternate sols, with the objective of obtaining an intermediate layer with a lowhigher 131
refractive index, convenient for optical wave-guiding. This was achieved by depositing 132
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alternatively a coating of 133 nm from the ZrCe (70:30) sol, followed by a second layer 133
of 350 nm from the SiTi (70:30) sol, and finishing with a third layer of 133 or 138 nm 134
from the ZrCe or SiTi (70:30) sol, respectively. The latter procedure was similar to the 135
one described in the previous paragraph. 136
The coatings were characterised by optical microscopy (Zeiss, HP1, Germany) to 137
evaluate the homogeneity and detect possible defects and/or cracks. Thickness (e) and 138
refractive index (n) were respectively measured by profilometry (Talystep, Taylor-139
Hobson, UK) using a variable angle spectroscopic ellipsometer (Woollam M2000U). 140
The refractive index and coating thickness of coatings obtained from the different sols 141
are presented in Table II. 142
Microstructural studies were performed with a LEO435-VP optical microscope and a 143
Hitachi S4700 Field Emission Scanning Electron microscope. Semi-quantitative 144
elemental analysis was carried out with an EDX unit coupled to the FE-SEM. 145
146
Waveguide efficiency measurements 147
148
Two M multilayer SiTi (70-30) systemand a monolayer TM-Ce (95SiO2:5CeO2)s 149
systems with 1.2 and 4.15 μm in thickness and refractive index values of 1.59 and 1.48, 150
respectively, determined by variable angle ellipsometry at λ = 633 nm, were selected 151
to study the optical waveguide efficiency., SiTi (70-30) and TM-Ce (95SiO2:5CeO2) 152
with 1.2 and 4.15 μm in thickness and refractive index values of 1.59 and 1.48 153
respectively, determined by variable angle ellipsometry at λ = 633 nm. These 154
measurements were performed using the double prism method reported in [32-36], 155
illustrated in Figure 1 and described as follows: 156
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The beam of a He-Ne laser emitting at 633 nm with a nominal power of 35 mW was 157
introduced into an entry prism by focusing it through a microscope objective (20x, NA 158
0.4). The entrance angle into the prism (n = 1.779, NSFL-11, Edmund Optics) was 159
defined mathematically using Snell’s law, resulting in a value of 1º 42’ with respect to 160
the normal to the prism hypotenuse. A second exit prism was placed 2 cm away. 161
Coupling between the prisms and the waveguide was achieved with an index adjustment 162
liquid (n = 1.56). 163
A laser power meter (Beam Profiler, Thorlabs GmbH) was used to measure the entrance 164
(24 mW) and exit (17 mW) laser beam intensity, yielding a guiding efficiency value of 165
70.8%. This value is approximate, since coupling losses at the entry prism were not 166
evaluated, and it was assumed that 100% of the laser power was introduced into the 167
prism. 168
169
Results and discussion 170
Characterisation of the sols 171
Hydrolysis and condensation reactions of the sols were controlled in order to obtain 172
transparent and homogeneous inorganic and hybrid sols. While SiTi, SiZrcol and TM-Ce 173
sols were colourless, ZrCe and SiZrZcol sols appeared slightly yellow. 174
Stability studies show that the viscosity of SiTi sols increases slightly from 1.2 to 3.0 175
mPa.s when the amount of TiO2 changes from 10 to 50 molar %, these values are 176
maintained for more than one month stored at 4ºC indicating the high stability of the 177
sols. On the other hand, viscosity of SiZcol and SiZrZcol sols increases rapidly with time, 178
and precipitation was observed, indicating the de-stabilisation of the sols with the aging 179
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time. On the other hand, ZrCe and TM-Ce sols are very sensible to visible light and it is 180
necessary to keep them in amber bottles to maintain their stability, although the ZrCe 181
sol has a maximum stability for one week at 4 ºC. 182
Deposition and characterisation of coatings 183
SiO2-TiO2 coatings were deposited by dip-coating onto soda-lime slide-glass substrates, 184
using SiTi sols with different molar ratios and varying the withdrawal rate. After 185
sintering at 450ºC for 30 min, homogeneous and transparent coatings were obtained. 186
Figure 2 shows the critical thickness and refractive index as a function of SiO2 in terms 187
of mol %. These values are summarised in Table II, along with the corresponding sol-188
gel systems and molar compositions. A simultaneous decrease of the index of refraction 189
and increase of thickness is observed when SiO2 content is increased. This is related to 190
the presence of methyl groups coming from MTES and linked to the inorganic network. 191
On the other hand, the refractive index increases for increasing TiO2 contents, because 192
of its higher index (n= 2.1) with respect to that of SiO2 (n= 1.45). This binary system 193
allows the selection of suitable compositions to obtain rather high thickness and specific 194
refractive indexes. 195
Another method to attain coatings with high refractive index involves the addition of 196
zirconia in the sol precursor compositions, both as colloidal dense particles and/or 197
alkoxides. SiZcol, SiZrZcol and ZrCe sols were dip coated at different withdrawal rates. 198
Refractive indexes around 1.5 and 1.6 were obtained for SiZcol and SiZrZcol coatings, 199
respectively. On the other hand, the incorporation of colloidal zirconia does not increase 200
the refractive index as expected [37], although it has been recently reported that thermal 201
processing conditions play an important role in tailoring the index of sol-gel derived 202
films [38]. The highest refractive index, n 2.0, was found for ZrCe coatings with very 203
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low thickness. The thickness slightly increased when cerium content increased from 10 204
to 30 % molar, as observed in Table II. 205
Finally, highly organic hybrid TM-Ce sol was used to raise the coating thickness for 206
achieving better laser coupling. A lower sintering temperature was necessary to avoid 207
the degradation of organic groups. The presence of methacrylate organic chains that can 208
be polymerised in the liquid allows relaxing stresses during sintering, thus enabling an 209
increase in the final film thickness. Critical thickness was not achieved and a maximum 210
thickness of 2.3 µm at 60 cm/min was obtained for a single dip-coated layer. However, 211
an important limitation is observed in the refractive index value, which is lower than 212
that of the substrate (n = 1.47). This could inhibit light guiding in the waveguide. 213
In order to approach the fabrication of useful waveguides, coatings with high thickness 214
and high refractive index values have been sought. These have been obtained in two 215
ways: either by preparing multi-layer coatings using the SiTi sols, and/or by utilising a 216
coating architecture that combined layers with different compositions and refractive 217
index values. 218
To this respect, multilayer coatings with 5 layers were prepared using SiTi sols with 219
70:30 and 50:50 molar ratios, and 4 layers for the composition ZrCe (70:30). The final 220
multilayer coatings exhibit maximum thickness values of 1230 nm 613 nm and 489 nm 221
and refractive index values of 1.59, 1.75 and 2.04 foir SiTi 70:30, SiTi 50:50 and ZrCe 222
sols, respectively (Table II). 223
An example of the second route is the combination of SiTi (70:30 and 50:50) and ZrCe 224
(70:30) sols as in a “sandwich” configuration with intermediate films of lower higher 225
(???) refractive index with respect to adjacent layers. This structure combines layers 226
with different thickness (133-350 nm) and refractive indexxes (1.62 and 2.04) values, 227
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respectively, as gathered from the data given in Table II. These architectures were 228
explored in order to test the possibility of obtaining complex optical devices based on 229
the use of where different wavelength lasers. may be employed to induce beam 230
combination phenomena 231
Scanning Electron Microscopy and EDX analysis were used to characterise a typical 232
coating obtained from the SiTi (70:30) sol. Figure 3 (a) shows a transversal section of 233
the coating, imbedded in an epoxy resin. The multilayer coating has a thickness of ca. 234
1230 nm, and it appears to be well adhered to the substrate, as judged from the fact that 235
it has undergone a certain level of mechanical stress during sample preparation and 236
neither cracks, nor layer separations, have been observed within the sample. Figures 3 237
(b) and (c) show EDX spectra performed at points 1 and 2, as indicated in Figure 3(a). 238
These are consistent with the expected composition both, at the sol-gel layer, and at the 239
substrate. Some Ti was detected at point 1, nevertheless, close to the substrate/coating 240
interface. This is expected, since the volume analysed is of the same order in size than 241
the distance between points 1 and 2. 242
Light guiding was studied in TMCe and SiTi 70:30 different kinds of coatings (TMCe 243
and SiTi) using the output beam of a 35 mW He-Ne laser and the apparatus described in 244
ref. [10]. Initial results show that, apparently, no gradual change in index is found 245
across the coating-substrate interface; an expected result, taking into account the 246
presence of two materials with different refractive index and the absence of diffusion. 247
(hemos quitado todo lo de la difusión del Ti que no tiene sentido plantear) Esto 248
It is well established, however, that a minimum, wavelength-dependent thickness value 249
is required to guide visible or NIR light through a coating which acts as a planar 250
waveguide [37]. Multiple sol-gel dip-coatings from 70SiO2-30TiO2 and TMCe sols 251
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were thus prepared in order to increase the coating thickness to a minimum which 252
allows enables performing useful laser waveguiding measurements. 253
In Figure 4, light from a He-Ne laser output at = 633 nm is guided by a 1230 nm thick 254
planar sol-gel multilayer obtained in this work, derived from the SiO2:TiO2 (70:30) sol. 255
The image in the Figure has been captured at the exit end of the guide with a CCD 256
camera. The intense horizontal line thus corresponds to the cross-section of the Sol-Gel 257
multilayer planar waveguide at its exit end. Light from the He-Ne laser is concentrated 258
within the film thickness, although scattering is observed coming from within the glass 259
substrate and in the volume of air near it. 260
Figure 5 shows how light from a He-Ne laser (633 nm, 1 mW), coupled with a 261
microscope objective (40x, NA 0.65), is guided through a TMCe sol derived multilayer 262
of 4,15 µm. An intense region of light is clearly observed at the exit surface waveguide 263
layer, while the rest of the glass substrate is does not appear illuminated. Light is, as 264
expected, confined to the sol-gel derived, 4.15 µm multilayer waveguide which has a 265
higher refractive index than its adjacent glass substrate and air layers. 266
Both Figures, 4 and 5, demonstrate qualitatively the feasibility of the sol-gel method 267
and the combination of layers with different refractive index to attain planar waveguides 268
on commercial glass substrates. 269
Conclusions 270
Planar single and multilayer waveguides have been obtained by the sol-gel method, 271
employing the dip-coating technique. Sols of different compositions (SiO2-TiO2, SiO2-272
ZrO2, and SiO2-CeO3) were synthesised to obtain coatings with different refractive 273
index and thickness, in order to produce planar waveguides with different 274
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characteristics. Scanning electron microscopy observations allowed identifying 275
adequate coupling between the deposited layers and their corresponding soda-lime 276
commercial glass substrates. A wide range of values for the index of refraction and for 277
layer thickness have been demonstrated feasible, depending on the sol composition and 278
experimental conditions applied along the dip-coating process. This wide range of 279
refractive indexes and thickness values should allow to conveniently design and prepare 280
adequate multilayer waveguides. 281
282
Acknowledgements. 283
The authors acknowledge funding from MICINN (TEC2006-10469, CEN 2007-2014 284
and SURFALUX SOL-00030930), from DGA (Group of Excellence in Laser Material 285
Processing and Characterisation), and XUNTA DE GALICIA 286
(WCITE08PXIB206013PR). 287
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[34] Strasser, T.A. & Gupta, M.C., Applied Optics 31 (1992) 2041. 342
[35] Ab-Rahman, M.S. & Zaman, M.H.M, Australian Journal of Basics and Applied 343
Sciences 3 (2009) 2901. 344
[36] Okamoto, K., “Fundamentals of Optical Waveguides” Academic Press, 2000. 345
[37] F. J. Ferrer, F. Frutos, J. García-López, A. R. González-Elipe, F. Yubero, Thin 346
Solid Films 516 (2007) 481-485. 347
[38] S. Jana, P. K. Biswas, Materials Chemistry and Physics, 117 (2009) 511-516. 348
F. Rey-García et al, Planar optical waveguides via Sol-Gel 18/10/1224/10/10 10:0013:36 17
17
Captions: 349
Figure 1. Schematic drawing of the experimental double-prism set-up used to determine 350
the optical efficiency of the sol-gel waveguides obtained in this work. Elements are 351
identified in the drawing. 352
Figure 2. Evolution of SiO2-TiO2 sol-gel coating thickness and its index of refraction as 353
a function of SiO2 content. The coatings were obtained on borosilicate slide-glass 354
substrates. The lines are placed as eye-guides. 355
Figure 3. FE-SEM image corresponding to a 70SiO2-30TiO2 multilayer coating 356
obtained by sol-gel: a) transversal cross-section of the coating; EDX analysis at point 1 357
(b) and at point 2 (c) indicated in the micrograph. 358
Figure 4. CCD image from 70 SiO2- 30 TiO2 multilayer coating. Light is concentred at 359
the sol-gel derived layer and appears as a bright, horizontal line. The rest of the images 360
observed are consequences of various optical phenomena deriving from the incidence of 361
the laser beam on part of the glass substrate. 362
Figure 5. Light guiding of a He-Ne laser beam, coupled via a microscope objective, 363
through a 70SiO2-30TiO2 multilayer coating deposited onto a commercial, 5 mm-thick 364
soda-lime glass substrate. The exit guided beam is observed on the photograph as a 365
bright spot opposite the microscope objective. 366
367
368
F. Rey-García et al, Planar optical waveguides via Sol-Gel 18/10/1224/10/10 10:0013:36 18
18
Table I. Molar ratios, compositions and concentrations used for the preparation of the sols employed in this study and described in the text.
Sol Abbreviation Molar composition H2O/alkoxides Alkoxide/AcH
Concentration
(goxides /L)
Inorganic ZrCe* (90-70)ZrO2-(10-30) CeO2 1 1 - 2 100
Low-hybrid
SiTi* (90-80) SiO2- (10-20) TiO2 2 1 80
SiTi* (70-50) SiO2- (30-50) TiO2 1.5 1 50
SiZcol 75 SiO2-25 ZrO2 colloidal 7.3 100
SiZrZcol
65 SiO2- 25ZrO2 – 10
ZrO2colloidal 7.3 ---- 100
High hybrid TM-Ce 95 SiO2-5 CeO2 3.3 1 267
F. Rey-García et al, Planar optical waveguides via Sol-Gel 18/10/1224/10/10 10:0013:36 19
19
Table II. Refractive index (λ = 633 nm) and critical thickness values for the different
coatings obtained using the sols described in Table I. The critical thickness refers to the
maximum thickness attainable by dip-coating without film breakdown. (a) internal
coating adjacent to substrate (b) intermediate coating (c) external coating.
System Molar composition Refractive index Thickness (nm)
Single layers
SiTi sols
90:10 1.45 1100
80:20 1.51 401
70:30 1.63 358
50:50 1.73 138
ZrCe sols 90:10 1.98 147
70:30 1.99 203
SiZcol sol 75:25 1.53 213
SiZrZcol sol 65:25:10 1.59 215
TM-Ce sol 95:5 1.47 2330
Multilayers
SiTi sol (5) 70:30 1.59 1230
SiTi sol (5) 50:50 1.75 613
ZrCe sol (4) 70:30 2.04 489
TMCe sol (¿???) 95:5 ---¿?? 4500
Sandwiches
#1
ZrCe (a) sol 70:30 2.04 133
SiTi (b) sol 70:30 1.62 350
ZrCe (c) sol 70:30 2.04 133
#2
ZrCe (a) sol 70:30 2.04 133
SiTi (b) sol 70:30 1.62 350
SiTi (c) sol 50:50 1.73 138
F. Rey-García et al, Planar optical waveguides via Sol-Gel 18/10/1224/10/10 10:0013:36 20
20
F. Rey-García et al, Planar optical waveguides via Sol-Gel 18/10/1224/10/10 10:0013:36 21
21
Figure 1. Rey-García et al: Schematic drawing of the experimental double-prism set-up
used to determine the optical efficiency of the Sol-Gel waveguides obtained in this
work. Elements are identified in the drawing.
F. Rey-García et al, Planar optical waveguides via Sol-Gel 18/10/1224/10/10 10:0013:36 22
22
Figure 2. Rey-García et al: Evolution of SiO2-TiO2 Sol-Gel coating thickness and its
index of refraction as a function of SiO2 content. The coatings were obtained on
borosilicate slide-glass substrates. The lines are placed as eye-guides.
1.40
1.45
1.50
1.55
1.60
1.65
1.70
1.75
0
200
400
600
800
1000
1200
30 50 70 90 110
Re
fra
ctive
inde
x (
= 6
33 n
m)
Th
ickn
ess (
nm
)
mol % SiO2
Thickness (nm)
Refractive index
F. Rey-García et al, Planar optical waveguides via Sol-Gel 18/10/1224/10/10 10:0013:36 23
23
Figure 3. Rey-García et al: FE-SEM image corresponding to a 70SiO2-30TiO2 multilayer
coating obtained by Sol-Gel: a) transversal cross-section of the coating; EDX analysis at
point 1 (b) and at point 2 (c) indicated in the micrograph.
1
2
Coating
Resin
b)
c)
a)
Glass-slide
F. Rey-García et al, Planar optical waveguides via Sol-Gel 18/10/1224/10/10 10:0013:36 24
24
Figure 4. Rey-García et al: CCD image from 70 SiO2- 30 TiO2 multilayer coating. Light
is concentred at the sol-gel derived layer and appears as a bright, horizontal line. The
rest of the images observed are a consequence of light scattering phenomena deriving
from the incidence of the laser beam on part of the glass substrate.
F. Rey-García et al, Planar optical waveguides via Sol-Gel 18/10/1224/10/10 10:0013:36 25
25
Figure 5. Rey-García et al: Light guiding of a He-Ne laser beam, coupled via a
microscope objective, through a 70SiO2-30TiO2 multilayer coating deposited onto a
commercial, 5 mm-thick soda-lime glass substrate. The exit guided beam is observed on
the photograph as a bright spot opposite the microscope objective.