Advances in Organic Polymer-Based Optoelectronics - American

Chapter 32

Advances in Organic Polymer-Based Optoelectronics

Downloaded by UNIV OF ROCHESTER on April 22, 2013 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch032

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 offimctionalizedpolymers.^H ! 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 5

0097-6156/95/0601-0436$12.00/0 © 1995 American Chemical Society In Polymers for Second-Order Nonlinear Optics; Lindsay, G., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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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 % (-co ; cOj, cOj) is jointly estimated by transverse second harmonic generation (co=co=co, a)=2a>) and modulated reflection measurements (co^co, co^Q, co=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 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 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. (2)

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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 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 (Si N ) layer. The photosensitive resin is spincoated 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 N layer. The sample is plasma etched three times:firstbased on the structure of step 3, a CF plasma is used to etch the exposed Si N section with no photosensitive resin on top. The 0 plasma is then used to etch the organic layers through the Si N mask. CF is then used again to etch the Si N on top of the electrooptic guide core. 12

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

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POLYMERS FOR SECOND-ORDER NONLINEAR OPTICS

Figure 1: Transverse cross section of the waveguide.


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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 cut back 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 Sbend 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) 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 Sbend 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). 2




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


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5 Downloaded by UNIV OF ROCHESTER on April 22, 2013 | http://pubs.acs.org Publication Date: August 11, 1995 | doi: 10.1021/bk-1995-0601.ch032

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(-2to\ GO, ©) = 2d Experimental d and r values respectively equal to 29pm/V and 12.6pm/V are in good agreement with this model for the crosslinked polymer. 33

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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 coefficient of about 6.5 pm/V whereas the best 1^3 measured for the same polymer in thin film geometry is 13pm/V. 33

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 V = 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 T M 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 value in the phase modulator, as inferred from the relation r =3n7d(2V L n>- 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. n

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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 :firstthe 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/H /Ar based RIBE.t ! 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 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. 2

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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.


VI ACKNOWLEDGEMENTS 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.

Bibliography [1] R.Lytel, G.F.Liscomb, J.L Thackara . Proc SPIE 824, 152(1987). [2] Donald R. Ulrich .Mol.Cryst. Liq. Cryst. 160,1(1988). [3] J.R. Hill, P.Pantelis and G.J Davies. Inst. Phys. Conf. Ser. No 103: Section 2.5 .Conf. Materials for Nonlinear and Electrooptics. Cambridge, 1989. [4] Emanuel Van Tomme , Peter p. Van Daele, Roel G. Baets, Paul P. Lagasse. IEE Journal of Quantum Electronics 27, 3(1991). [5] Emanuel Van Tomme , Peter p. Van Daele, Roel G. Baets, G.R Möhlmann , M.B Diemmer. J. Appl. Phys. 69 (9), (1991). [6] P.J. Williams, P.M. Charles, I.Griffith, L.Considine, A.C. Carter. Electron.Lett. 26, 142(1990). [7] T.L.Koch, U.Koren. J.Lightwave Technol. 8, 274(1990). [8] T.Sasaki, I.Mito. Digest of OFC/IOOC'93, Comm. T h K l , 210(1993). [9] A.Neyer, T.Knoche, L.Muller, Electron. Lett. 29, 399(1993). [10] C.Rompf, B.Hilmer, W.Kowalsky. Tech.Dig.ECOC'93 , Comm. WeP7.5 (1993). [11] R. Pinsard-Levenson, J. Liang, E. Toussaere, N. Bouadma, A.Carenco, J.Zyss, G.Froyer, M.Guilbert, Y.Pellous and D.Bosc, Nonlinear Optics 4, 233 (1993). [12] CNET Patent n° 9114662 [13] M.Filoche,PhD Thesis, 6 March 1991, Universitéde Paris Sud. [14] G.Hervé-Gruyer, Electron. Lett. 26(17),1338(1992). [15] G.Hervé-Gruyer, M.Filoche, F.Ghirardi. ECIO 93. Neuchâtel,(1993). [16] K.D.Singer, J.E.Sohn, and S.J.Lalama, Appl.Phys.Lett. 49 ,5 (1986). [17] S.Bauer et al., Proc. 8th International Symposium on the Electrets, Paris (1994).



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[18] E.Toussaere, J.Zyss. Thin Solid Films, 234, 454(1993). [19] E.Toussaere, J.Zyss. Thin Solid Films, 234,432(1993). [20] J.Jerphagnon, S.K. Kurtz, J. Appl. Phys. 41(4),(1970). [21] D.Chemla, P.Kupecek. Revue de physique appliquée 6, 31(1971). [22] C.C. Teng, H.T. Man, App. Phys. Lett. 56, 18 (1990). [23] M.Sigelle and R.Hierle, J. Appl. Phys. 52, 6 (1981). [24] J.L.Oudar, D.S.Chemla. J. Chem. Phys., 66 (6) , 2264(1977). [25] M.Sigelle, R.Hierle. J. Appl. Phys. 52 ,6(1981). [26] K.D. Singer, M.G. Kuzyk, J.E. Sohn. J. Opt. Soc. Am. B.4 (6), (1987). [27] N.Bouadma, C.Kazmierski, J.Semo, Appl.Phys.Lett. 59,1 (1991). [28] N.Bouadma, J.Semo, J.Lightwave Technol. 12(4) ,(1994). [29] N.Bouadma, J.Liang, R.Levenson, S. Grosmaire, P.Boulet, S.Sainson. IEE Photonics Tech. Letters 6,(1994). RECEIVED March 27, 1995


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Advances in Organic Polymer-Based Optoelectronics - American

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