Sébastien Maria

Tadeusz Biedroń

Rinaldo Poli

Przemyslaw Kubisa

Sebastien Maria

Przemysław Kubisa

Atom transfer radical

teaching and research institutions in France or abroad, or from public or private research centers.

Introduction

Atom Transfer Radical Polymerization (ATRP) is the most versatile method of controlled radical polymerization. [START_REF]values of the signals appearing in MALDI TOF spectrum of poly(methyl acrylate) obtained with the CH3CH(COOEt)Br -MoBr3(PMe3)3/MoBr4(PMe3)3 system. m/z obs. a m/z cald Formula b m/z obs. a m/z cald Formula b 4767 4767[END_REF][2][3][4][START_REF]Controlled/Living Radical Polymerization[END_REF] ATRP is a catalytic process based on a reversible activation-deactivation cycle, involving a low oxidation state metal complex reacting with an alkyl halide to generate radicals and a corresponding higher oxidation state metal complex, as shown schematically below: Insert Scheme 1 here -M-X + Mt-X z -M + Mt-X z+1 dormant species X = Cl, Br Mt = e.g. Cu, Ru, Fe, Mo, propagating species

Most commonly the metal complexes are copper halides but many other transition metal complexes have been successfully used including e.g. ruthenium and iron complexes.

Recently the application of molybdenum-based catalysts for ATRP of styrene has been reported. [START_REF] Stoffelbach | [END_REF]7] Not all of these complexes are well soluble in bulk monomers or in typical organic solvents. This stimulated the interest in using ionic liquids as solvents for ATRP processes.

Ionic liquids are organic salts that are liquid at ambient temperatures, the typical example being 1-butyl-3-methylimidazolium hexafluorophosphate (abbreviated as [bmim][PF6]): [START_REF] Welton | Ionic Liquids in Synthesis[END_REF][START_REF] Rogers | Ionic Liquids: Industrial Applications to Green Chemistry[END_REF][START_REF] Welton | Room-temperature ionic liquids. Solvents for Synthesis and Catalysis[END_REF] Insert Scheme 2 here is homogeneous and the residual catalyst can be easily separated from the polymer after polymerization) and may, in some instances, favorably affect the kinetics. Thus in the radical polymerization of methyl methacrylate it has been shown that the propagation rate constant is higher and the termination rate constant is lower than in bulk polymerization [11], which creates more favorable conditions for controlled polymerization. [12,13] The ATRP of acrylate esters is shown schematically below:

Insert Scheme 3 here The studies of ATRP of (meth)acrylates in ionic liquids have been so far limited to the most commonly used catalytic system, namely CuBr/CuBr2/amine ligand. In this communication we report on the attempts to perform methyl acrylate ATRP in [bmim][PF6], using molybdenum based catalysts that have been successfully used by two of the present authors for styrene ATRP in toluene solvent [7]. Preliminary results on the application of this catalytic system for acrylates ATRP have been presented at the ACS meeting. [14] The structure of the catalysts used in this work is shown in Scheme 3.

Experimental

Chemicals. Ethyl 2-bromopropionate was purchased from Aldrich and used as received.

Methyl acrylate (from Aldrich) was purified by distillation under argon. 1-n-Butyl-3methylimidazolium hexafluorophosphate was obtained according to a procedure described in the literature [15] starting from 1-methylimidazol, butyl chloride and aqueous (60%) hexafluorophosphoric acid. MoCl3(PMe3)3, MoBr3(PMe3)3, MoBr4(PMe3)3 and MoCl4(PMe3)3 were obtained as described earlier.

[16]

Polymerizations. All polymerizations were carried out under dry and oxygen-free conditions (under vacuum or under argon). The monomer and the molybdenum complex(es) were Characterization. 1 H NMR spectra were recorded on a Bruker 300MSL 300 MHz spectrometer. MALDI TOF spectra were performed on a Voyager Elite MALDI TOF spectrometer using dihydroxybenzoic acid as a matrix and nitrogen laser desorption at 337 nm. Sodium iodide was used as cationating agent. GPC measurements were performed using an LKB 2150 HPLC pump and a set of TSK-GEL G2000 HXL and G40000 HXL columns with CH2Cl2 as eluent. Double RI and MALS (multi-angle light scattering) detectors were used allowing the determination of the absolute values of the average molecular weights.

Results and Discussion

Methyl acrylate -CH3CH(COOEt)Cl -MoCl3(PMe3)3/MoCl4(PMe3)3 system

The methyl acrylate ATRP was conducted with CH3CH(COOEt)Cl as initiator and Insert Fig. 1 here

The elemental analysis results were also essentially consistent with assumption that the insoluble solid is indeed poly(methyl acrylate) (Cfound 56.00%, calcd 55.80%, Hfound 7.20%, calcd 8.13%). The polymer isolated from solution had a considerably higher molecular weight than that obtained at lower MoBr4(PMe3)3 concentration (the polymer isolated at 30% conversion had Mn = 28400 as compared to the calculated Mn = 1500). The MALDI TOF analysis of the polymer isolated from the solution showed the presence of two populations of macromolecules. In Table 1 the m/z values of the signals appearing in the spectrum are shown and the assignments are given.

Insert Table 1 here

In a control experiment, the catalyst (MoBr3(PMe3)3/MoBr4(PMe3)3) was added to methyl acrylate solution in ionic liquid in the absence of initiator. Significant quantities of insoluble polymer were formed in this system, in addition to small amounts of soluble, very high molecular weight polymer. This confirms that an unknown side-reaction occurs between MoBr4(PMe3)3 and [bmim][PF6] (no formation of insoluble polymer was observed when MoBr3(PMe3)3 was used in the absence of MoBr4(PMe3)3, but the polymerization in this system was not controlled) and the products of this reaction are able to initiate the methyl acrylate polymerization leading to the formation of branched high-molecular weight polymer in solution and eventually to cross-linked polymer. The formation of insoluble polymer on the reaction vessel walls implies that some volatile products are involved. At present we do not have enough data to speculate on the exact nature of this side-reaction.

Methyl acrylate -CH3CH(COOEt)Br -MoCl3(PMe3)3/MoCl4(PMe3)3 system It has been shown earlier by two of the present authors that MoCl3(PMe3)3 is a more active catalyst than MoBr3(PMe3)3 for the styrene ATRP in toluene (corresponding apparent polymerization rate constants at 90 0 C are equal to 1.4210 -4 min -1 and 5.4910 -5 min -1 respectively). [7] Both catalysts, however, are capable of catalyzing the styrene controlled polymerization, although the higher rate observed for MoCl3(PMe3)3 implies a higher growing radical concentration. In the case of methyl acrylate polymerization in [bmim][PF6],

MoCl3(PMe3)3 gives fast polymerization to high-molecular weight polymer even in the presence of an excess of MoCl4(PMe3)3. Apparently the deactivation reaction in this system is not effective and the polymerization proceeds as a conventional radical polymerization.

The MoBr3(PMe3)3/MoBr4(PMe3)3 system seems to be much more effective for control of the methyl acrylate polymerization in [bmim][PF6], but the side-reaction involving MoBr4(PMe3)3 leads to deviation from the controlled polymerization scheme.

Thus, in the subsequent series of experiments the mixed system was applied involving

  dissolved in[bmim][PF6] (the volume ratio of monomer to [bmim][PF6] was 2 / 1). Ethyl 2bromopropionate was then introduced to the stirred reaction under an argon atmosphere. The flask (under vacuum or under argon) was then immersed in an oil bath heated at the desired temperature. Samples of the polymerization mixture were withdrawn at different time intervals. A portion of the sample was dissolved in deuterated chloroform and the monomer conversion was determined by means of 1 H NMR spectroscopy. The remaining portion was extracted with chloroform (the chloroform extraction gave practically colorless polymer solutions) which, after the evaporation of the solvent, were used for the gel permeation chromatography (GPC) measurements.

  MoCl3(PMe3)3 / MoCl4(PMe3)3 as a catalytic system at a [monomer]0 / [initiator]0 / [Mo III ]0 / [Mo IV ]0 ratio equal to 470/11/1/1.1. At 60 0 C the polymerization proceeded fast (57% conversion after 10 min) and the polymer molecular weight (Mn ~10 5 ) was well above the calculated value. In the subsequent experiment the MoCl3(PMe3)3 concentration was reduced 5 times while the MoCl4(PMe3)3 concentration was increased (the [monomer]0 / [initiator]0 / [Mo III ]0 / [Mo IV ]0 ratio was equal to 470/11/0.2/1.8) Increasing the concentration of MoCl4(PMe3)3, in order to shift the equilibrium shown in Scheme 1 to the left-hand side and to reduce the instantaneous concentration of growing radicals, improved the outcome only slightly. The rate of polymerization indeed decreased (35% conversion after 3 h, 82% after 19 h) but the molecular weight was still considerably higher (Mn ~ 610 4 ) than calculated for a controlled polymerization. It seems therefore that under typical ATRP conditions no control can be achieved with this catalytic system in the methyl acrylate polymerization.Methyl acrylate -CH3CH(COOEt)Br -MoBr3(PMe3)3/MoBr4(PMe3)3 system Replacing chlorine by bromine both in the initiator and in the catalyst led to a significant improvement. Thus, in the polymerization conducted at a [monomer]0 / [initiator]0 / [Mo III ]0 / [Mo IV ]0 ratio equal to 500 / 10 / 1 / 1.7 at 60 0 C, almost complete conversion was reached in 24 h. The number average molecular weight was only slightly higher then the calculated value (Mn calcd = 4480, Mn (GPC) = 5730) but the dispersity was still relatively high. In order to improve the control the MoBr4(PMe3)3 concentration was increased in subsequent experiments. This, however, led to an unexpected result. The polymerization proceeded in the ionic liquid phase, as evidenced by an increasing solution viscosity, but at the same time significant quantities of a white solid formed on the ampoule walls. The recovered solid was insoluble in common organic solvents. Solid state NMR ( 13 C CP MAS) showed four signals characteristic of carbon atoms in methyl acrylate unit. In Fig.1, the13 C CP MAS spectrum of the solid product is compared with the typical spectrum of poly(methyl acrylate) (obtained by conventional ATRP in bulk) recorded in CDCl3 solution.

  bromine containing the CH3CH(COOEt)Br initiator and the chloride containing catalyst (MoCl3(PMe3)3/MoCl4(PMe3)3). The polymerization conducted at a [monomer]0 / [initiator]0 / [Mo III ]0 / [Mo IV ]0 ratio equal to 500 / 10 / 1 / 2 at 60 0 C was terminated at 80% conversion.There was a good agreement between the experimental and the theoretical molecular weights (Mn (GPC) = 3500, Mn calcd = 3640) and the dispersity was relatively narrow (Mw/Mn = 1.38). The polymer MALDI TOF analysis indicates the presence of two main populations of the macromolecules, one containing CH3CH(COOEt)-moiety as the head-group and -Br as the end-group and the other one containing the same head-group and a -Cl end-group. In addition, a third minor series is observed. Macromolecules of this series contain the same head-group and saturated -CH2-CH2(COOCH3) end-groups, formed by irreversible termination. It is interesting to note that the signals corresponding to macromolecules terminated with bromine and chlorine have similar intensities. At [CH3CH(COOEt)Br]0 / [MoCl3]0 / [MoCl4]0 ratio equal to 10 / 1 / 2 the proportion of bromine and chlorine introduced with the initiator and catalyst is 10 / 11. This observation was a little unexpected because catalytic systems containing chloride and bromide behave differently, which could indicate that the positions of activationdeactivation equilibria are different in both cases. In the system in which chloride and bromide are present in nearly equal proportion, however, the molybdenum compounds form mixed halides (MoClnBr3-n and MoClnBr4-n) and the efficiency of such mixed halides in controlling the activation-deactivation equilibria may not be related in a simple way to the efficiency of catalysts containing either only chloride or only bromine. Conclusions It has been shown earlier that the methyl acrylate ATRP can be advantageously conducted in [bmim][PF6] with CuBr/CuBr2/pentametyldiethylenetriamine (PMDETA) system. The situation is more complex with molybdenum based catalytic systems that proved to be effective in the styrene ATRP. MoCl3(PMe3)3/MoCl4(PMe3)3 is not effective in conjunction with chloride-containing initiator while for the MoBr3(PMe3)3/MoBr4(PMe3)3 catalyst in conjunction with bromine containing initiator, side-reaction (reaction of MoBr4(PMe3)3 with the cationic fragment of the ionic liquid) obscures the results. The combination of the bromine containing initiator with the chloride catalyst leads to a controlled polymerization and the polymer MALDI TOF analysis indicates that reversibly deactivated macromolecules contain both bromine and chlorine end-groups. Literature 1.Wang, J-. S.; Matyjaszewski, K. J. Am Chem Soc 1995, 117, 5614. 2.Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, 1721. 3. Patten, T. E.; Matyjaszewski, K. Acc Chem Res 1999, 12, 895. 4. "Controlled Radical Polymerization", Matyjaszewski, K., Ed.; ACS Symposium Series 685, ACS Washington DC, 1997.

  Fig.1


B.-E Benkelfat

R Ferrière

B Wacogne

P Mollier

Technological Implementation of Bragg Grating Reflectors in Ti:LiNbO 3 Waveguides by Proton Exchange

Keywords: Gratings, integrated optics, LiNbO 3, optical waveguides, tunable filters

The authors propose and demonstrate a simple and novel method for fabrication of efficient Bragg grating reflectors at telecommunication wavelengths in titanium-indiffused lithium niobate single-mode channel waveguides. This technique is based on the patterned proton-exchanged process. The validity of the method is verified by the good agreement of the spectral responses and the Bragg phase-matching condition. Reflectivity as high as 94% at 1546 nm was measured with 4-mm distributed parameter waveguides in z-cut lithium niobate.

I. INTRODUCTION

P ERIODIC and aperiodic distributed parameter waveguides have many applications in integrated optical components (IOCs) dedicated to dense wavelength-division-multiplexing (DWDM) photonic networks [START_REF] Brackett | Dense wavelength division multiplexing networks: principles and applications[END_REF]. In addition, the aperiodic structures were recognized like excellent candidates for the compensation of dispersion. Usually, fabrication methods are based on photorefractive [START_REF] Becker | Integrated optical Ti:Er:LiNbO distributed Bragg reflector laser with a fixed photorefractive grating[END_REF], [START_REF] Hukriede | Thermally fixed reflection gratings for infrared light in LiNbO :Ti:Fe channel waveguides[END_REF] and dry-etching techniques [START_REF] Söchtig | Ti:LiNbO stripe waveguide Bragg reflector gratings[END_REF]. Furthermore, UV-induced surface-relief gratings on LiNbO channel waveguides with a bandwidth of 1.8 nm at 1534.3 nm have been obtained [START_REF] Wu | UV-induced surface-relief gratings on LiNbO channel waveguides[END_REF]. More recently, a laser ablation technique has been employed for realizing gratings in LiNbO channel annealed-proton-exchanged waveguides. Using this approach, 6% reflective gratings have been fabricated [START_REF] Sidorin | Integration of Bragg gratings on LiNbO channel waveguides using laser ablation[END_REF].

In this letter, we propose for the first time, to our knowledge, a simple and reproducible method of grating fabrication in titanium-indiffused lithium niobate single-mode waveguides based on the patterned proton-exchanged technique. Proton exchange (PE) process is an attractive method for the fabrication of IOCs in lithium niobate (LiNbO ) and lithium tantalate (LiTaO ). Because of its easy implementation and large index modulation, this technique has been used to realize many useful PE-integrated optical devices such as optical frequency translator, second-harmonic generators [START_REF] Cristiani | Nonlinear characterization and modeling of periodically poled lithium niobate waveguides for 1.5 m -band cascaded wavelength conversion[END_REF], polarizers, modulators, or Fresnel lenses [START_REF] Suhara | Proton-exchanged Fresnel lenses in Ti:LiNbO waveguides[END_REF]. When compared to standard holographic and dry-etching methods, the process offers great flexibility and greatly simplifies the grating fabrication.

II. PRINCIPLE AND DISCUSSION

When Bragg gratings are used to provide feedback in titanium lithium niobate waveguides they can serve both as reflectors in Fabry-Pérot structures and as optical filters. On the other hand, the refractive index of a lithium niobate waveguide can be controlled through an applied electric voltage. The structure can hence be tuned electrically. According to the coupled mode theory, efficient coupling between the forward and backward traveling waves occurs when the phase-matching condition [START_REF] Yariv | Optical Electronics in Modern Communications[END_REF] is satisfied [START_REF] Brackett | Dense wavelength division multiplexing networks: principles and applications[END_REF] where is the effective index of the propagation mode at , is the grating period, and is the Bragg-scattering order. Wavelengths not satisfying this condition will be transmitted without attenuation. Under phase-matching conditions, the reflectivity and the bandwidth are expressed as

(2) (3)
where and are the coupling coefficient between the contrapropagating modes and grating length, respectively. The coupling coefficient is directly related to the index perturbation. Moreover, the dependence on Bragg order is represented by the multiplicative factor , where is the ratio of the tooth width to the period for a rectangular grating.

In order to obtain maximum reflectivity with a high-order Bragg grating, it will be necessary to implement a periodic dielectric waveguide with a significant index and/or length perturbation. The filter bandwidth is inversely proportional to the length of the grating provided the product . One of the ways to increase and control the coupling coefficient of the grating is to produce a large and variable index modulation. The proton exchange process is expected to produce large changes in the refractive indexes: the extraordinary index increases (

) while the ordinary index decreases by 0.04. This large and controllable change in refractive index will allow fabrication of high efficiency Bragg filters.

Results on fabrication and characterization of proton-exchanged optical waveguides in x cut, y cut, and z cut LiNbO have already been presented [START_REF] Micheli | Independent control of index and profiles in proton exchanged lithium niobate waveguides[END_REF]. PE takes place when the substrate is immersed in a proton source such as molten benzoic acid. Proton-exchanged waveguides present a step-like refractive index profile. They support only TM mode in z-cut material and TE mode in x and y cut materials. The depth of the proton-exchanged layer is determined by the immersion time and the diffusion coefficient at different temperatures. Several authors [START_REF] Dawar | Fabrication and characterization of titanium indiffused proton exchanged optical waveguides in z-cut LiNbO[END_REF] have reported the diffusion coefficients of titanium-indiffused proton-exchanged (TIPE) layers for various melt temperatures. The main drawback of the proton-exchange process is a large scattering loss as compared to the titanium diffusion process. One of the ways to reduce the undesirable effects is to anneal the samples in wet air at a temperature between 250 C and 400 C with the time ranging from minutes to hours. Such an annealing process increases the depth of the proton-exchanged layer while reducing . This results in more controllable refractive index profiles.

III. FABRICATION PROCESS

Proton-exchanged grating waveguides were implemented in two steps (Fig. 1). In the first step single-mode optical waveguides in the 1550-nm window were fabricated on z-cut (y-propagating) LiNbO substrates by standard diffusion of 80-nm-thick and 7m-wide titanium layers at 1100 C during 10 h in a wet Ar-O atmosphere. Next, a 2000-Å-thick SiO layer was deposited on the waveguide for the proton-exchange mask layer.

The SiO was chosen for its excellent proton diffusion blocking, good adhesion to the substrate, and ease of deposition and etching. Moreover, this transparent mask allows an approximate measurement of the grating characteristics before its removal. The Bragg grating pattern was directly written by UV exposure through a Cr mask in a resist layer. The pattern was transfered to the SiO layer by chemical (or reactive ion) etching. The proton exchange was performed in molten benzoic acid, and finally the SiO mask was removed by chemical etching. The waveguide was subsequently annealed in wet air. As a consequence, the electrooptic effect would be restored and propagation losses reduced. The experimental parameters of the proton exchange and annealing process are based on the preliminary investigations (e.g., refractive index profile) which have been carried out on TIPE optical waveguides in z-cut LiNbO substrates. The samples were proton-exchanged at temperature between 150 and 230 C with the time ranging from 1 to 4 h. The annealing process was performed at various temperatures (from 250 C to 400 C) with different times (from 10 mn to 3 h).

IV. EXPERIMENTAL RESULTS

To assess the spectral responses of the Bragg gratings, we used a narrow-linewidth tunable diode laser with linearly polarized output power between 1510 and 1590 nm and a power meter to measure the output power (Fig. 2). A Glan polarizer was placed in front of the grating waveguide to selectively probe TM mode. The preliminary results were obtained for Bragg gratings with periods equal to 7.9 and 8.1 m. The process was realized in benzoic acid for 2 h at 210 C, and the gratings were annealed at 400 C for 20 min. Fig. 3 shows transmission spectrums of a 2-mm-long high-order proton-exchanged Bragg grating reflectors in single-mode channel Ti:LiNbO waveguide. The spectral responses show that the resonance wavelengths are in good agreement with the Bragg phase-matching condition. At the Bragg wavelength the relative transmission drops to 25%. A 3-dB (FWHM) bandwidth of approximatively 9 nm at 1558 nm was achieved. Furthermore, the reduced transmittivity for wavelengths shorter than 1558 nm is attributed to the guided-to-substrate mode coupling. An estimate of the excess losses, due to the presence of the grating, is obtained by comparing two adjacent waveguides, one with the grating and the other without. The excess grating reflector loss is approximatively 0.4 dB. Narrower bandwidth and higher reflectivity can be obtained by optimizing the coupling coefficient and grating length.

Fig. 4 shows the measured transmission spectrum of distributed parameter Ti:LiNbO waveguide with period m which was proton-exchanged during 3 h at 230 C and annealed at 400 C for 1 h and 10 min. Reflectivity as high as 94% at 1546 nm is hence deduced showing a bandwidth of 2 nm for 4-mm grating length. The oscillations observed around the transmission dip are related to processing inhomogeneities and undesirable effects in the measuring setup.

V. CONCLUSION

We have reported the first implementation of the proton exchange technique for the fabrication of Bragg grating reflectors in Ti:LiNbO waveguides. This process offers great flexibility and simplifies the fabrication steps involved in standard holographic and dry-etching methods. The experimental results are in good agreement with the modeling of the grating for which, in particular, a reduction of its bandwidth is expected with optimized technology parameters. The annealing process reduces the refractive index modulation ( 0.12) induced by proton exchange and then allows the increase of the effective length of the distributed parameter waveguide. This results in reduction of the bandwidth.

Investigations are being carried out in order to improve the reflector uniformity and to reduce the grating period. Patterned proton-exchanged technology can also be used to realize optical waveguide reflectors and tailored optical filters with specific spectral response characteristics.

  B.-E. Benkelfat is with the Institut National des Télécommunicatons, Département Electronique et Physique, 91011 Evry Cedex, France (e-mail: badreddine.benkelfat@int-evry.fr). R. Ferrière, B. Wacogne, and P. Mollier are with the Laboratoire d'Optique P.M Duffieux, Faculté des Sciences et Techniques, Université de Franche-Comté, 25030 Besançon Cedex, France (e-mail: richard.ferriere@univ-fcomte.fr).

Fig. 1 .

 1 Fig. 1. Fabrication steps of patterned proton-exchanged Bragg grating in Ti:LiNbO waveguides.

Fig. 2 .

 2 Fig. 2. Measurement setup to characterize Bragg grating reflectors.

Fig. 3 .

 3 Fig. 3. Wavelength dependence of the measurement transmission spectrum of distributed-parameter Ti:LiNbO waveguides, reflectivity < 75%. The samples were annealed at 400 C for 20 min.

Fig. 4 .

 4 Fig. 4. Spectral response of a 4-mm-long periodic distributed-parameter Ti:LiNbO waveguide, reflectivity < 94%. The sample was proton-exchanged during 3 h at 230 C and annealed at 400 C for 1 h 10 min.

ACKNOWLEDGMENT

The authors would like to thank Prof. J. Dudley, Prof. A. Ramdane, and Dr. Q. Zou for their interest in this work.