Modal dispersion phase-matched second harmonic generation is demonstrated in new poled polymer waveguide geometries with a nonlinear optical core consisting of two side-chain polymers with different glass transition temperatures. After poling above and between the respective glass transitions, the sign of the nonlinear optical coefficient is reversed in the two polymers, thereby improving the overlap integral. Conversion efficiencies up to 7%/W cm(2) were achieved in the first experiments

Ahlheim, M.

Bauer, S.

Bauer-Gogonea, S.

Brinker, W.

Diemeer, M.

Flipse, M. C.

Jäjer, M.

Lehr, F.

Stegeman, G. I.

Stähelin, M.

Wirges, W.

Yilmaz, S.

Zysset, B.

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University of Central Florida STARS Faculty Bibliography 1990s Faculty Bibliography 1-1-1997 Polymer waveguides with optimized overlap integral for modal dispersion phase-matching W. Wirges S. Yilmaz W. Brinker S. Bauer-Gogonea S. Bauer See next page for additional authors Find similar works at: https://stars.library.ucf.edu/facultybib1990 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 1990s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Wirges, W.; Yilmaz, S.; Brinker, W.; Bauer-Gogonea, S.; Bauer, S.; Jäjer, M.; Stegeman, G. I.; Ahlheim, M.; Stähelin, M.; Zysset, B.; Lehr, F.; Diemeer, M.; and Flipse, M. C., "Polymer waveguides with optimized overlap integral for modal dispersion phase-matching" (1997). Faculty Bibliography 1990s. 2134. https://stars.library.ucf.edu/facultybib1990/2134 Authors W. Wirges, S. Yilmaz, W. Brinker, S. Bauer-Gogonea, S. Bauer, M. Jäjer, G. I. Stegeman, M. Ahlheim, M. Stähelin, B. Zysset, F. Lehr, M. Diemeer, and M. C. Flipse This article is available at STARS: https://stars.library.ucf.edu/facultybib1990/2134 Appl. Phys. Lett. 70, 3347 (1997); https://doi.org/10.1063/1.119166 70, 3347© 1997 American Institute of Physics.Polymer waveguides with optimizedoverlap integral for modal dispersion phase-matchingCite as: Appl. Phys. Lett. 70, 3347 (1997); https://doi.org/10.1063/1.119166Submitted: 17 February 1997 . Accepted: 23 April 1997 . Published Online: 04 June 1998W. Wirges, S. Yilmaz, W. Brinker, S. Bauer-Gogonea, S. Bauer, M. Jäger, G. I. Stegeman, M. Ahlheim, M.Stähelin, B. Zysset, F. Lehr, M. Diemeer, and M. C. FlipseARTICLES YOU MAY BE INTERESTED INFabrication and structural properties of AlN submicron periodic lateral polar structures andwaveguides for UV-C applicationsApplied Physics Letters 108, 261106 (2016); https://doi.org/10.1063/1.4955033Second harmonic generation in phase matched aluminum nitride waveguides and micro-ringresonatorsApplied Physics Letters 100, 223501 (2012); https://doi.org/10.1063/1.4722941Second-harmonic generation through optimized modal phase matching in semiconductorwaveguidesApplied Physics Letters 83, 620 (2003); https://doi.org/10.1063/1.1596726Polymer waveguides with optimized overlap integral for modal dispersionphase-matchingW. Wirges, S. Yilmaz, W. Brinker, S. Bauer-Gogonea, and S. Bauera)Heinrich Hertz Institut fu¨r Nachrichtentechnik, D-10587 Berlin, GermanyM. Jäger and G. I. StegemanCenter for Research and Education in Optics and Lasers, University of Central Florida,Orlando, Florida 32816-2700M. Ahlheim, M. Stähelin, B. Zysset, and F. LehrSANDOZ Optoelectronics, F-68330 Huningue, FranceM. Diemeer and M. C. FlipseAKZO Nobel Electronic Products, Arnhem, The Netherlands~Received 17 February 1997; accepted for publication 23 April 1997!Modal dispersion phase-matched second harmonic generation is demonstrated in new poledpolymer waveguide geometries with a nonlinear optical core consisting of two side-chain polymerswith different glass transition temperatures. After poling above and between the respective glasstransitions, the sign of the nonlinear optical coefficient is reversed in the two polymers, therebyimproving the overlap integral. Conversion efficiencies up to7%/W cm2 were achieved in the firstexperiments. ©1997 American Institute of Physics.@S0003-6951~97!04925-5#Second order nonlinear optical processes in the telecom-munication windows at 1.3 and 1.55mm have regained at-tention recently for parametric amplification, wavelengthconversion, and for cascading for all-optical switching, spa-tial solitons, etc.1 Cascading is possible via second-harmonicgeneration ~SHG! and difference frequency generation~DFG! or by optical rectification~OR! and the electro-optic~EO! effect.1 Nonlinear optical~NLO! polymers seem to bevery interesting materials for waveguide SHG due to theirlarge nonresonant second order nonlinearities after poling.Efficient SHG requires phase matching~PM! so that the har-monic fields generated in different parts along the waveguideinterfere constructively at the output. There are differentways to achieve PM, such as anomalous dispersion PM,2quasi-phase-matching~QPM!,3 and modal dispersion phase-matching~MDPM!.4 Anomalous dispersion PM suffers fromlimited transparency, while periodical poling for QPM leadsto significant surface deformations and small values for theperiodically modulated nonlinearity.5 Therefore MDPMseems to be an interesting alternative for efficient SHG.Efficient modal dispersion phase-matching requires so-phisticated multilayer fabrication techniques. Because of theusual waveguide and material dispersion with wavelength,the SH appears as a higher order guided mode.6 B cause theSH field is rarely the final output for processes such as cas-cading, the resulting complicated field structure is not a limi-tation. However, it can lead to low conversion efficiencies(}uSu2) due to interference effects across the waveguide di-mensions inherent in the overlap integralS where:S5AtE0tx33~2!~z!@Ezn,v~z!#2Ezm,2v~z!dz.Here t is the core thickness,x33(2)(z) is the relevant spatiallyvarying second order nonlinear optical susceptibility tensorelement~for TM modes in our case!, Ezn,v(z) andEzm,2v(z)denote thez component of the electric field of the fundamen-tal and the second harmonic mode, respectively, andm andn~typically n50) are the respective mode numbers. BecauseEm,2v(z) changes signm times across the waveguide, theintegral is reduced unless~a! the sign of the nonlinearity isalso reversedm times or~b! the nonlinearity is zero when-ever the field is negative~or positive!. Figure 1 illustratesconcept~a! by showing the calculated electric field distribu-tion of the TM12v and TM22v modes~top left and right! andthe required nonlinearity profile across the waveguide corefor an optimized overlap integralS for TM0v→TM12v ~bottomleft! and TM0v→TM22v ~bottom right! mode conversion. Thecalculations are based on the refractive indices of the poly-mers described below. Both approaches~a! and ~b! havebeen used for slab waveguides made from Langmuir–Blodgett and polymer films.4,7–10 MDPM witha!Present address: Angewandte Festko¨rperphysik, Universita¨t Potsdam, AmNeuen Palais 10, D-14469 Potsdam, Germany.Electronic mail: sbauer@hhi.deFIG. 1. Electric field distribution of the TM12v mode ~top left! and theTM22v mode ~top right!. In order to obtain a large overlap integral, thenonlinearity must change sign once across the core thickness forTM0v→TM12v mode conversion~bottom left! and twice for TM0v→TM22v~bottom right!.3347Appl. Phys. Lett. 70 (25), 23 June 1997 0003-6951/97/70(25)/3347/3/$10.00 © 1997 American Institute of PhysicsTM0v→TM12v was demonstrated in a slab and a channelmultilayer waveguide with a guiding layer consisting of apassive and a nonlinear optically active polymer film4,7 andwith TM0v→TM22v in a channel waveguide with a guidinglayer consisting of a single nonlinear optically active layer.8Channel waveguides are required for practical applicationsand a high conversion efficiency h5P2v /Pv2L251%/W cm2 ~with P2v the power of the second harmoniclight, Pv the power of the fundamental light, andL the de-vice length! was achieved even in this latter nonoptimizedcase.8 For TM0v→TM12v mode conversion up to four timesthe efficiency may be expected by replacing the linear inac-tive region by a region of opposite nonlinearity (x33(2) coeffi-cient!. Such a steplike nonlinearity profile has been achievedin slab waveguides, for example, by using the Langmuir–Blodgett technique.9,10 In this letter we demonstrate a tech-nique for implementing sequentially inverted poled polymerlayers in channel waveguides and preliminary measurementsof SHG in them.Our approach uses the waveguides schematically shownin Fig. 2. The core of the waveguide consists of two nonlin-ear optical side-chain polymers with different glass transitiontemperaturesTg1 andTg2 with Tg1.Tg2 . In order to obtainthe desired change of sign of the nonlinear optical coeffi-cient, steplike dipole orientation profiles, as indicated in Fig.2, are required. This is achieved by a two step poling tech-nique, reported earlier in more detail.11 First the full structureis poled near theTg1 so that all of the polymer layers arepoled in the same direction. The temperature is then reducedto nearTg2 and the reversed poling field is again applied tothe full structure. Although the orientation of the~low! Tg2material is reversed, the temperature is too low to repole the~high! Tg1 material leading to the desired periodically re-versed sequence of layers. Here we report on these new poly-mer based waveguide structures with optimized overlap in-tegral for efficient TM0v→TM12v and TM0v→TM22v modeconversion.For the experiments, waveguide structures were pre-pared by multilayer spin-coating of appropriate polymer so-lutions onto silicon or ITO-coated glass substrates. In orderto separate the guided mode fields from the absorbing elec-trodes, the guiding layers were sandwiched between twobuffer layers~PC polymer from AKZO!. For the guidinglayers, two different poly~styrene-maleic anhydride! copoly-mers with chemically attached Disperse Red 1 side groups~products 9511 and 9512 from SANDOZ12! and glass transi-tions Tg5137 and 164 °C were used. In order to preventmixing of the side-chain polymers, the samples were bakedfor several hours at elevated temperatures after every spin-coating step. In addition, the viscosity of the polymer solu-tions and the rotation speed during spin-coating were opti-mized.Figure 3 shows that the PM condition can be fulfilled inthis structure only for different fundamental and harmonicmode numbers. For TM0v→TM12v mode conversion, theguiding layer thickness~below 1 mm! is very near to thecut-off thickness. Better waveguiding is obtained forTM0v→TM22v mode conversion. The effective index of thetransverse magnetic~TM! waveguide modes was calculatedas a function of the total waveguide thicknesst for a wave-lengthl51.55mm and with the refractive indices~as deter-mined previously by prism coupling measurements!n1v51.664,n12v51.725 ~core!, n2v51.544,andn22v51.553~cladding! of the polymers described below. Poling was per-formed with typical poling fields of 50 V/mm at 165 and140 °C, respectively. No attempt was made in this initialstudy to optimize the poling conditions. Finally, channelwaveguides with widths varying from 1 to 5mm were de-fined by photobleaching.The optical nonlinearity achieved during the two-steppoling process was monitoredin situ by ellipsometrical EOmeasurements.13 The recording of the EO response, the SHGresponse with a fundamental at 1.064mm, and the pyroelec-tric response upon linear heating of the polymer film verifiedthe successful preparation of steplike dipole orientationprofiles.13For the SHG experiments, 1-mm-long samples werediced for end-fire coupling. For such short waveguides, themeasured waveguide losses of 4 dB/cm @ 1.55mm wereacceptable at the fundamental, but were high at the secondharmonic wavelength~estimated to be larger than 50 dB/cm@ 0.78mm! due to the large residual absorption of the Dis-perse Red 1 chromophores. This limits the useful devicelength for SHG to onlyLmax5 ln@a(2v)/2a(v)#/@a(2v)22a(v)#51.8 mm with an assumed absorption of 50dB/cm at 0.78mm.14 The first SHG experiments exhibitedFIG. 2. Cross-section of polymer channel waveguides with optimized over-lap integral for TM0v→TM12v and TM0v→TM22v PM SHG. The steplike di-pole orientation profile indicated in the figure is obtained by consecutivethermally assisted poling above and between the respective glass transitiontemperatures. Channels of varying widths from 1 to 5mm were defined byphotobleaching.FIG. 3. Effective indexNeff vs core thicknesst of a symmetrical slab wave-guide for the lowest order transverse magnetic fundamental mode TM0v andfor the first three modes at the second harmonic frequency. The phase-matching conditions are fulfilled for TM0v→TM12v at a core thickness of 0.7mm and for TM0v→TM22v at a core thickness of 1.7mm.3348 Appl. Phys. Lett., Vol. 70, No. 25, 23 June 1997 Wirges et al.PM-SHG for both waveguide structures. Table I comparesthe SHG figures of merit and phase matching lengths formode-matched conversion processes in polymer channelwaveguides. We have found good agreement between theexperimentally measured PM wavelength and the PM wave-length calculated from the thickness of the waveguide core~10% deviation!. This is rather surprising because the smalldifferences in the refractive indices of the two nonlinear op-tical polymers as well as the poling-induced birefringencehave been neglected in the calculation. It is noteworthy thatthe maximum conversion efficiencyh achieved~which isamong the highest reported for poled polymers so far! isnearly two orders of magnitude larger than the QPM basedTM0v→TM02v mode conversion SHG. Furthermore, our re-sults compare very favorably with the published value forLiNbO3 of h527%/W/cm2.15 However, assuming that thevalues for the nonlinearity in optimized single layerwaveguides can be achieved, the measured figures of meritare more than one order of magnitude smaller than predictedtheoretically, leaving a great deal of scope for improvement.We suspect that the problem lies in nonoptimized claddinglayers which do not allow a large voltage drop to occuracross the nonlinear polymer stack. Furthermore, in order toincrease the conversion efficiency and~most important! theuseful device length for SHG, new nonlinear optical poly-mers with a larger nonresonant second order susceptibilityand a lower absorption around 800 nm~compared to DANSand DR1! are required.14To summarize, we have demonstrated modal dispersionPM-SHG by mode conversion in new polymer waveguidestructures. A significant improvement in conversion effi-ciency has been achieved compared to QPM-SHG or modaldispersion PM with nonoptimized overlap integral. Stillhigher conversion efficiencies are possible with optimizedwaveguides under optimized poling conditions.The research in the US was supported by the Air ForceOffice of Scientific Research and the National Science Foun-dation and in Germany by the Federal Minister of Researchand Technology.1G. I. Stegeman, D. J. Hagan, and L. Torner, J. Opt. Quantum Electron.28,1691 ~1996!.2T. C. Kowalczyk, K. D. Singer, and P. A. Cahill, Opt. Lett.20, 2273~1995!.3G. Khanarian, R. A. Norwood, D. Haas, B. Feuer, and D. Karim, Appl.Phys. Lett.57, 977 ~1990!.4K. Clays, J. S. Schildkraut, and D. J. Williams, J. Opt. Soc. Am. B11, 655~1994!.5M. Jäger, G. I. Stegeman, W. Brinker, S. Yilmaz, S. Bauer, W. H. G.Horsthuis, and G. R. Mo¨hlmann, Appl. Phys. Lett.68, 1183~1996!.6H. Ito and H. Inaba, Opt. Lett.2, 139 ~1978!.7M. Flörsheimer, M. Ku¨pfer, Ch. Bosshard, H. Looser, and P. Gu¨nter, Adv.Mater. Commun.4, 795 ~1992!.8M. Jäger, G. I. Stegeman, G. R. Mo¨hlmann, M. C. Flipse, and B. J.Diemeer, Electron. Lett.32, 2009~1996!.9M. Küpfer, M. Flörsheimer, Ch. Bosshard, and P. Gu¨nter, Electron. Lett.29, 2033~1993!.10T. L. Penner, H. R. Motschmann, N. J. Armstrong, M. C. Ezenyilimba,and D. J. Williams, Nature~London! 367, 49 ~1994!.11S. Bauer-Gogonea, S. Bauer, W. Wirges, and R. Gerhard-Multhaupt, Ann.Phys.~Leipzig! 4, 355 ~1995!.12M. Ahlheim and F. Lehr, Macromol. Chem. Phys.195, 361 ~1994!.13S. Yilmaz, W. Wirges, W. Brinker, S. Bauer-Gogonea, S. Bauer, M. Ja¨ger,G. I. Stegeman, M. Ahlheim, M. Sta¨helin, B. Zysset, F. Lehr, M. Diemeer,and M. C. Flipse, Proc. SPIE3006, 382 ~1997!.14A. Otomo, M. Ja¨ger, G. I. Stegeman, M. C. Flipse, and M. Diemeer, Appl.Phys. Lett.69, 1991~1996!.15M. L. Bortz, M. A. Arbore, and M. Fejer, Opt. Lett.20, 49 ~1995!.TABLE I. Comparison of SHG figures of merith and phase matchinglengthsLPM for different mode conversion processes.ModeconversionNLOchromophoreNumberofpolymerlayersPMwavelengthlPM ~nm!PMdistanceLPM ~mm!Figureof meritTM0v–TM02v~QPM!aDANS 3 1615 2 0.05%/W cm2TM0v–TM22v~PM!bDANS 3 1535 2 1%/W cm2TM0v–TM12v~PM!DR1 4 1610 2 3–4%/W cm2TM0v–TM22v~PM!DR1 5 1540 2 7%/W cm2aData from Ref. 5.bData from Ref. 8.3349Appl. Phys. Lett., Vol. 70, No. 25, 23 June 1997 Wirges et al.

Polymer waveguides with optimized overlap integral for modal dispersion phase-matching

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Polymer waveguides with optimized overlap integral for modal dispersion phase-matching

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