[acs symposium series] polymers for second-order nonlinear optics volume 601 || advances in organic...

Chapter 32

Advances in Organic Polymer-Based Optoelectronics

R. Levenson, J. Liang, C. Rossier, R. Hierle, E. Toussaere, N. Bouadma, and J. Zyss

France Telecom, Centre National d'Etudes des Télécommunications, Paris B, BP 107, 196 Avenue Henri Ravera, 92225 Bagneux, France

Design and fabrication of low loss polymer waveguides with an attenuation of the order of 1 to 2 dB/cm are discussed. Passive functions such as optical combiners or splitters have been achieved with a polystyrene core waveguide. Electrooptic devices such as phase modulators or Mach Zehnder interferometers have been fabricated and demonstrated respectively with MAGLY, a new variety of crosslinked polymers and the classical methyl methacrylate-Disperse Red One (DR2-MMA) side chain polymer. The linear and nonlinear properties of MAGLY are characterized in comparison with DR1-MMA. Polymer films are shown to sustain, after combined curing and poling processes a high electrooptic coefficient of 12 pm/V at 1.32μm during several weeks at 85°C. The fabrication and characterization of Buried Ridge Structure (BRS) lasers monolithically integrated with a butt coupled polymer based buried strip waveguide are presented. The device exhibits a total waveguide insertion loss of less than 5dB.

The limitations of current semiconductor or lithium niobate based technologies in terms of efficiency, integrability, and cost can be surpassed by calling on the remarkable properties of fimctionalized polymers.^H5! The major asset of this new family of materials is the unlimited flexibility of potentially available structures resulting from a predictive molecular engineering approach. Furthermore, adequately defined poling and processing technologies are shown to be compatible with hybrid polymer/semiconductor integration. In that perspective, the fabrication technology of unimodal low loss waveguides will be described as well as resulting passive and electrooptic applications. We will review the linear

0097-6156/95/0601-0436$12.00/0 © 1995 American Chemical Society

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In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

32. LEVENSON ET AL. Advances in Organic Polymer-Based Optoelectronics 437

and nonlinear properties of side-chain and crosslinked poled polymeric thin films. The wavelength dispersion of refractive index and absorption are obtained by spectroscopic ellipsometry. The second order susceptibility tensor %(2)(-co3; cOj, cOj) is jointly estimated by transverse second harmonic generation (co1=co2=co, a)3=2a>) and modulated reflection measurements (co co, co^Q, co3=co+Q where Q corresponds to a low frequency voltage). Comparison between the two approaches is in-keeping with a quantum two-level model of the molecular quadratic nonlinearities. Dynamical orientation and relaxation behaviour is inferred from second harmonic generation combined with in-situ corona poling. A waveguided phase modulator with 2-D confinement based on a crosslinked polymer strip waveguide over a doped silicon substrate has been demonstrated with a half wavelength voltage of 30V at 1.06|um for an electrode length of 1.2cm, corresponding to an r 3 3 coefficient of 4pm/V. In the rapidly developing field of photonic integrated circuits (PICs), monolithic integration of active and passive optoelectronic components is becoming increasingly important as a tool to produce low cost and high functionality optical modules with application in a wide range of systems. One of the key elements in such PICs is the connection of a laser diode to an external waveguide in a monolithical configuration. t 6 H 10] We report here the monolithic integration of a laser diode with a polymeric based waveguide as a first step towards the development of monolithically integrated photonic devices.

I. STRUCTURE AND FABRICATION OF THE WAVE GUIDES:

The basic architecture of the waveguides comprises a strip of a high index polymer eventually endowed of nonlinear properties, depending on the applications in which it will be involved, buried between passive buffer polymers of lower refractive index and deposited on a semiconductor substrate (see Figure 1) [HI. The strip is designed by classical photolithography an dry etching techniques. The fabrication process is summarized in Figure 2. t 1 2 l . When the guiding core is made of a thermally stable electrooptic polymer, the electrical poling process occurs at step 2 under the optimal conditions that will be detailed in section III. In step 3, a classical photolithography process, currently used for microelectronics, is applied over the silicon nitride (Si3N4) layer. The photosensitive resin is spin-coated and insulated through a mask with UV light. The insulated resin is then dissolved with a developer leading to the structure shown in Step 3 of Figure 2 is then obtained. To achieve the configuration in step 4, the selective reactive ion etching technique is used : Oxygen and tetrafluorocarbon plasmas were chosen to etch respectively the organic layers (the photosensitive resin and the electrooptic polymer) and the Si 3 N 4 layer. The sample is plasma etched three times: first based on the structure of step 3, a CF 4 plasma is used to etch the exposed Si 3 N 4 section with no photosensitive resin on top. The 0 2 plasma is then used to etch the organic layers through the Si 3 N 4 mask. CF 4 is then used again to etch the Si 3 N 4 on top of the electrooptic guide core.

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438 POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

Figure 1: Transverse cross section of the waveguide.

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LEVENSON ET AL. Advances in Organic Polymer-Based Optoelectronics 439

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A top buffer layer is spin-coated and cured at a temperature close to 100°C. In step 4, a passive polymer waveguide is obtained. For the electrooptic devices, a lOOOA-thick gold electrode is deposited by evaporation, with a final photolithography step whereby a thicker photosensitive resin is exposed through the electrode pattern mask. After photolithography, a potassium iodide solution removes the gold layer that is not covered with resin. A final UV insulation and application of a developer cleans the residual resin over the gold electrode. An electrooptic polymer modulator is finally obtained at step 6. A propagation loss of 2 dB/cm or less depending on the guiding material, has been measured by the cutback method for such electrooptic waveguides.

These steps applied at 100°C for about 3 hours should not destroy the electrooptic properties of the film. This method is only valid for electrooptic polymers thermally stable at 100°C. For an electrooptic polymer unstable at 100°C, like MMA-DR1, electrical poling can only be applied at the end of the fabrication process, namely after Step 6.

ILPASSIVE FUNCTIONS

Light propagation in the polymer waveguides is simulated by BPM-CNET (ALCOR) [13M15] , software developed by CNET FRANCE TELECOM following the Beam Propagation Method (BPM). The propagation losses in a S-bend waveguide is shown in Figure 3. The propagation losses of 9 S-bend structures have been measured . Photolithography masks which include a series of S-bends with different angles (from 1° to 26°) and a series of Y junctions have been first fabricated. The loss is inferred from comparison between the output signal of a S-bend wave guide and a straight-line waveguide, the ratio of the two outgoing signals evidencing the losses due to the curvature radius of the waveguide with results presented in Figure 4.

Experimental results are in good agreement with the simulation . The curvature radius of S-bends is a function of the angle given by R=e/(20)2 where e is the distance between input and outputs of the S-bend waveguide.

Figure 4 shows that the propagation losses are negligible when the angle is less than 10°, corresponding to a curvature radius of lOOum. This value is one order of magnitude smaller than for LiNb03 which limits typical S-bend angles to values of the order of 1°. A polymer based integrated device may thus be down scaled to a much smaller size. Firstly, the waveguide width is about 2um instead of 10pm in LiNb03 (5) and the gap between parallel waveguides is of the order of 10-20um instead of 100-200um. Secondly, the transition length of a low loss S-bend is shorter, 100-200um instead of l-2mm. The combination of these two advantages opens interesting perspectives for polymers in optronic devices. It permits the design of new integrated optics devices achievable with polymers but out of reach for LiNb03, the latter technology being limited by the bulk crystal configuration and the small refractive index step as from titane in-diffusion.

Based on this technology, 1 to 4 junctions have been achieved with asymetric or symmetric outputs (see respectively Figures 5a and 5b).

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Figure 4: Losses in S-Bends of different angles.

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HI. ELECTROOPTIC APPLICATIONS

ffl.l MATERIAL:

III.l.a)Structure of the cross-linkable polymer

Nonlinear side-chain polymers have been widely studied over recent years towards applications in electrooptic modulation in integrated optics format. The most current side-chain polymer is DR1-MMA (see Figure 6a) 1161. The molar concentration of chromophores in the polymer matrix is given by x. This polymer presents a high quadratic susceptibility (d33=56pm/V at 1.32pm), but suffers from a poor thermal stability leading to a strong decrease of the susceptibility at 70°C . This temperature is too low for telecommunication applications which require stability up to 85°C at least. In order to improve the thermal stability of DR1-MMA, a cross-linkable polymer known as "Red-acid Magly" has been synthesised and studied [CNET patent n° 9310572]. Pyroelectric relaxation and Thermally Stimulated Depolarisation experiments have permitted to evidence, within the framework of the Kohlrausch-Williams-Watt (KWW) model, the higher activation energy E A of the Magly polymer. A pEAvalue, where P is the stretching coefficient and EAthe activation energy for the relaxation lifetime, of 3.2 ± 0.6 eV has been measured by thermally stimulated depolarisation and is to be compared to 1.7± 0.06 eV for a DRl-styrene copolymer and 1.1 ± 0.4 eV for a DR1-PMMA guest host systenJ17]. Its structure is shown in Figure 6b.

Thermal crosslinking is achieved between a carboxylic acid function (COOH) located on the nonlinear chromophore and an epoxy side group. There can be no disruptive chemical reaction between chromophores.

III. Lb) Electrical poling of polymer films:

The polymer powder is dissolved in trichloro 1,1,2 ethane at 10% concentration in weight. One micron thick films are prepared by spin-coating (1000 to 2000 rpm). At this stage, the orientation distribution of the nonlinear polarizable units inside the polymer film is centrosymmetric. In order for these 151ms to display electrooptic properties, centrosymmetry must be broken by electric poling. In the case of side-chain polymers, the electric poling procedure is achieved in two steps.

Firstly, the side-chain polymer film is heated near its glass transition temperature Tg, thereby enhancing the orientational mobility of the chromophores which are oriented through electrostatic interaction of their dipole moment with an electric field induced either by corona discharge or by planar electrodes. In the second step, the samples are cooled down to room temperature thus freezing the molecular orientation before the electric field is finally tumed-off.

In the case of crosslinked polymers, the first step is modified since the polymer must simultaneously undergo a poling and a crosslinking process. Crosslinking will considerably improve the film stability due to the anchoring of

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chromophores to the polymer matrix. However this procedure also competes with the poling process since it will partly disorient the molecular dipoles as a result of chemical attachment, thus decreasing the nonlinearity of the film.

Since this competition depends on the temperature, we have further decomposed the poling and the crosslinking procedures into two steps. The first step takes place at low temperature, it favours the poling of the chromophores over the thermal crosslinking procedure which remains very slow. The second step takes place at high temperature, thus enabling complete crosslinking of the polymer while maintaining the same poling field. The optimal poling conditions are as follows:

-first the polymer is heated at 70°C during 1 hour while the corona voltage is maintained at 5kV. This step corresponds essentially to a poling procedure.

-secondly the temperature is increased to 130°C during 3 hours under the same 5kV corona discharge. The polymer is thus crosslinked with the molecular dipoles oriented perpendicular to the substrate.

After these two steps, the films are cooled down to room temperature in presence of the electric field.

After crosslinking, the polymer does not dissolve in any solvent. The transparency of the film is increased, and the colour turns from purple to red.

A decrease in the absorption peak and a blue shift of from 520nm to 488nm are observed in accordance with the observed increased transparency and colour change. The blue shift of Amax observed on the absorption spectrum shown in Figure 7 is due to the crosslinking process which modifies the electronic configuration of the chromophore. A possible mechanism may involve the influence of the acid group: before cross linking it acts as an electron acceptor acid group which adds its effect to that of the nitro group. Subsequent cross linking neutralizes or at least reduces its influence, hence the blue shift.

The efficiency of the thermal crosslinking procedure is a function of temperature. The higher the temperature, the faster the crosslinking proceeds. Table 1 shows the duration needed to reach complete crosslinking at different temperatures for Magly with a 30% chromophore molar concentration as observed by Differential Scanning Calorimetry.

Table 1: Cross-link duration for Magly 30% at different temperatures.

Temperature Cross-link duration

130°C 170mn 180°C 30mn 230°C 2mn

After cross linking, the glass transition temperature of the polymer reaches 160°C. Its decomposition temperature is 250°C.

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H 5 C 2 - N

Figure 6a Figure 6b

Figure 6: Structure of the NLO chromophores.

A Absorption

400 500 600 Wavelength [nm]

Figure 7: Absorption spectrum of Red Acid Magly before and after cross linking

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III.l.c) Ellipsometric measurements:

Spectroscopic ellipsometry is an efficient method to determine refractive index, absorption, and thickness of thin birefringent polymer filmsJ18H19] in Table 2 we have listed the refractive indices at the four wavelengths used in our experiments:

Table 2: Refractive index at different wavelengths for Magly at 8%, 17% and 30% molar concentrations.

670 nm 860 nm 1.06 urn 1.32um Magly 8% (COPOl) 1.59 1.55 1.545 1.54

Magly 17% (COP08) 1.66 1.60 1.58 1.57

Magly 30% COP09b 1.7 1.62 1.62 1.62

We observe an increase in the refractive index with the concentration. The spectroscopic dispersion curves of the refractive index n and the absorption coefficient k of Magly 30% are shown in Figure 8. Real and imaginary parts of the refractive index follow the Kramers-Kronig relation. The ellipsometric absorption curve is in good agreement with the direct absorption measurement shown in Fig.3.

IILl.d): Second Harmonic Generation measurements and thermal stability evaluation:

The stability of the polymer films was investigated by measuring the dependence of the quadratic susceptibility involved in the second harmonic generation of a 1.32um YAG laser.

Second-harmonic generation is a reliable method to characterize the nonlinear properties of polymer films.[2°H21] Th e signal detection is much more sensitive and is unambiguously of nonlinear origin as opposed to the modulated reflection method described in the next paragraph, which may contain other contributions such as of electrostrictive origin.

An important short term decay of the quadratic susceptibility d 3 3 of a DR1-MMA film (29% molar concentration) was evidenced by the SHG method at 1.32pm .When comparing the d 3 3 values just after poling and 24 hours after poling at room temperature, the chromophore relaxation brings the nonlinear efficiency from 56pm/V down to a stable value of 29pm/V. On the contrary, no significant decay is observed for Red-acid Magly in the same conditions, showing that short term relaxation in these crosslinked polymers is much weaker than in analogue side-chain polymers.

In order to assess the thermal stability of polymer films, the SHG response from a polymer film is measured following a constant in-situ heating rate. The

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0.0 I * 1 *•—< »—-i -J *—4 I I 1 - 4 — 4 ^ ! i I ! : ; In ' I j ' I

0.3 0.4 0.5 0.6 0.7 0.8 0.9

WaveL. (vim)

Figure 8: Ellipsometric measurements :wavelength dispersion of the real (n) and imaginary (k) parts of the refractive index.

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temperature dependance of the SHG signal I 2 t 0 for DR1-MMA 29% and Magly 30% are shown in Figure 9.

Thermal stability is increased in the case of Magly 30%: for the same amplitude of I 2 f 0 , a 30°C improvement can be noted.

Polar relaxation of the different polymers was also investigated at different temperatures. Figure 10 shows the decay of d33( at 1.32pm ) as a function of time at 55°C for DR1-MMA 29% and at 85°C for Magly 30%. Magly 30% displays a good stability at 85°C whereas DR1-MMA 29% exhibits comparable stability only up to 55°C. A 30°C improvement in thermal stability is observed here again in isothermal conditions when comparing the stability of the side-chain polymer to that of the crosslinked polymer.

We also measured the susceptibilities of different copolymers of the Red-acid Magly family prepared with different chromophore concentrations and compared them to DR1-MMA 29%.

Table 3: d33 and d13 values ofDRl-MMA 29% and Red-acid Magly of different molar concentrations at a fundamental wavelength of 1.32pm.

polymer d„ d n

DR1-MMA 29%(relaxed)

29pm/V -

Magly 8% 10.2pm/V -Magly 17% 24pm/V -Magly 25% 24pm/V -Magly 30% 29pm/V 9.5pm/V

This table shows that d 3 3 increases with the chromophore concentration, within the range of the studied concentrations.

Comparing d 1 3 and d 3 3 of Magly 30% shows that the d33/di3 ratio coming close to 3 agrees well with the results of a one-dimensional molecular model^ for this crosslinked polymer following the process described above.

Ill.l.e) Electrooptic measurements:

For the determination of the electrooptic r 3 3 coefficient, we have used a modulated reflection method t 2 2 l , whereby the electrooptic coefficients difference r3 3-r1 3 is deduced from the modulation of the intensity of low power cw laser beams reflected by the samples under test. The r 3 3 coefficient can be inferred assuming a r 3 3/r 3 1 ratio of 3. This method remains a fast assessment technique which provides the electrooptic coefficient of polymer films to be compared to the susceptibilities obtained from SHG measurements.

The measurements, carried out at two wavelengths after eventual short term relaxation of the molecular orientation, for both DR1-MMA 29% and Magly 30%, yield similar results for both types of polymers : (Table 4):

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r33~13pm/V

T(°C) (dT/dt = 5°C/mn)

Figure 9: In situ SHG measurement :Thermal stability of the orientation as measured by second harmonic generation for Magly 30% and DR1-MMA.

d33(pm/V) 35 i

2 0

1 5

1 O

5

O ' 1

O 2 4 e 8 1 0 1 2 1 4 1 6 Time (days)

Figure 10: Stability of the %(2) tensor at 85°C for Magly, and at 55°C for MMA-DR1.

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Table 5: Electrooptic coefficients r33 of DR1-MMA 29% and Magly 30%.

r„ at 1064nm r^at 1320nm DR1-MMA 29% 13.5 pm/V 12.4 pm/V

Magly 30% 14.8 pm/V 12.6 pm/V

The quantum two-level model of the molecular quadratic nonlinearities [23]-[25] leads to the following relation between the second harmonic coefficient %(2)

zzz(-2a); GO,co) and the electrooptic coefficient r33(-co; GO, 0).

r33(-Q);co,0) = — T

= 2 f° (3cpg-a)2)(a)g-4a)2)^(2)

nif2° 3Q) 2 K-co 2 ) Z™(-2(D;CDf(D) Eq.l

where GO is the fundamental laser frequency, co0 is the absorption peak frequency of the polymer, n^ the refractive index at frequency GO and f2® (resp.f0) are Lorenz-Lorentz (resp. Onsager) local field factors respectively given by:

r = nl+2 f 0 = e(0)(fi!+2) 3 2e(0) + w2

with e(0) ~ 4.5 and %V>m(-2to\ GO, ©) = 2d3 3

Experimental d 3 3 and r 3 3 values respectively equal to 29pm/V and 12.6pm/V are in good agreement with this model for the crosslinked polymer.

III.2: ELECTROOPTIC DEVICES

III.2.a) MMA-DR1 integrated Mach-Zehnder modulator

The nonlinearity of the side chain MMA-DR1 has been demonstrated in an amplitude modulator with integrated Mach-Zehnder geometry. For that application, the bottom buffer layer is SOG a commercial Allied Signal planarization resin (n~1.44) and the top buffer layer is pure PMMA (n~1.48). As MMA-DR1 exhibits poor stability above 60°C, the poling process is performed after the fabrication of the modulator.

The best figures obtained are V7t=15V at 1.06pm with a modulation rate of 60% (the structure is not unimodal at that wavelength) and V7t=18V with a modulation rate of 80% at 1.32pm for the Mach-Zehnder modulator operating in TM polarisation (figure 11).

These values should be decreased by an optimization of the poling conditions of the multilayer as it corresponds to a r 3 3 coefficient of about 6.5 pm/V whereas the best 1^3 measured for the same polymer in thin film geometry is 13pm/V.

IIL2.b) MAGLY phase modulator

Here the bottom buffer layer is unchanged and the top buffer layer is a fluorinated PMMA. (n=1.43)

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The best figures obtained so far at 1.06um are Vn = 30V for the phase modulator inserted between crossed polarizers. Here V^ represents the voltage necessary to induce by electrooptic effect a phase difference of n between the TE and TM modes. Electrooptic modulation was also observed at 1.32pm. The experimental set-up and the electrooptic modulation signal are presented in Figure 12. The modulation rates are the same as for the Mach-Zehnder configuration.

Here again, the r 3 3 value in the phase modulator, as inferred from the relation r 3 3 =3n7d(2Vn L n3>- is of the order of 4pm/V. This value is weaker than in Table 4 for two reasons: thermal disorientation during the waveguide fabrication process (3 hours, at 100°C) and weaker poling efficiency for multilayers (Step 2 in Fig.2) than for single layers in thin film measurements.

VI . INTEGRATION OF A LASER DIODE WITH A POLYMER BASED WAVEGUIDE:

The monohthically integrated laser/waveguide device shown schematically on Figure 13 was prepared via two distinct processing steps : first the laser wafer with Buried Ridge Stripe structure (BRS) was fabricated using MOVPE and reactive ion beam etching (RIBE) ^technique. After the p and n contact metallisation, the Fabry-Perrot laser mirrors have been made by CH4/H2/Ar based RIBE.t28! Vertical and extremely smooth facets have been achieved. Single mode polymer waveguides were then fabricated using the process previously described. A bottom cladding layer of PMMA (n=1.48, 1.5um thick and cured at 170°C) and a core layer of polystyrene (n=1.6, lum thick cured at 200°C) were spin coated onto the substrate. In order to improve the coupling efficiency, care has been taken to butt-joint couple the laser to the waveguide active layer, by photolithographically etching the bottom of the cladding layer to clear the laser facet. t 2 9 l Finally after the etching of the core ridge, a Teflon AF upper cladding layer was deposited. The light output power was then measured at the cleaved laser facet and compared to that emitted at the end of the waveguide. The laser cavity and the waveguide lengths were respectively 250um and 600um. The laser threshold current is 15mA and the optical power output from the laser and the waveguide facets were respectively 11 mW and 5mW at 100mA. Significant improvements may be expected by developing polymeric materials with low loss figures and by optimizing the processing steps. The integrated device shows high thermal stability against temperature after heating at 250°C for lh, with no significant decrease in the waveguide power output.

V CONCLUSION

In conclusion, we have evidenced the improved stability of a thermally stable crosslinked nonlinear polymer and demonstrated a waveguide phase modulator based on this polymer. This polymer maintains its electrooptic coefficients over at least several weeks at 85°C. This efficiency-stability trade-off is believed to correspond to the current optimum for this class of materials and devices. A decrease of V^ by a factor of 3 to 4 can be expected from further

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Figure 11: DR1-MMA based integrated Mach-Zehnder intensity modulator.

Figure 12: Magly based integrated phase modulator.

Figure 13: Monolithic integration of a laser diode with a polymer passive waveguide.

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optimization such as a higher poling efficiency and better thermal stability of the crosslinked polymer. A waveguide fabrication process has been set-up and shown to lead to several passive or active applications. Furthermore, a 1.3um BRS laser and polymer waveguide have been monolithically integrated using a high performance potentially low cost technology. The device exhibits a low threshold current and a total insertion loss smaller than 5dB. These results open the way to a variety of integrated III-V active functions together with passive or electrooptic polymer based functions such as optical combiners for W D M devices, splitters, switches and various other PICs.

V I A C K N O W L E D G E M E N T S

The authors gratefully acknowledge P.Boulet, M.Carre, J.Charil, S.Grosmaire and F.Huet for technical assistance in the fabrication of the demonstrators and A.Rousseau and F.Foll from the Ecole de Chimie de Montpellier for supplying the electrooptic polymers.

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