Comparison of quasiphasematching geometries for secondharmonic generation inpoled polymer channel waveguides at 1.5 μmM. Jäger, G. I. Stegeman, W. Brinker, S. Yilmaz, S. Bauer, W. H. G. Horsthuis, and G. R. Möhlmann Citation: Applied Physics Letters 68, 1183 (1996); doi: 10.1063/1.115962 View online: http://dx.doi.org/10.1063/1.115962 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/68/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Influence of the photodepoling parameters on quasiphase-matched second-harmonic generation and opticalparametric fluorescence in polymer channel waveguides J. Appl. Phys. 96, 7112 (2004); 10.1063/1.1809770 Quasiphase matched second-harmonic generation from periodic optical randomization of poled polymer channelwaveguides Appl. Phys. Lett. 83, 1086 (2003); 10.1063/1.1597748 Quasiphasematched second harmonic generation in a polymer waveguide with a periodic poled structure Appl. Phys. Lett. 68, 1760 (1996); 10.1063/1.116658 Quasiphasematched secondharmonic generation in AlGaAs waveguides with periodic domain inversionachieved by waferbonding Appl. Phys. Lett. 66, 3410 (1995); 10.1063/1.113370 Quasiphasematched secondharmonic generation of blue light in periodically poled LiNbO3 Appl. Phys. Lett. 56, 108 (1990); 10.1063/1.103276

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Comparison of quasi-phase-matching geometries for second-harmonicgeneration in poled polymer channel waveguides at 1.5 mm

M. Jager and G. I. StegemanCREOL, University of Central Florida, 4000 Central Florida Blvd., Orlando, Florida 32826

W. Brinker, S. Yilmaz, and S. Bauera)Heinrich-Hertz-Institut fu¨r Nachrichtentechnik Berlin, Einsteinufer 37, D-10587 Berlin, Germany

W. H. G. Horsthuis and G. R. MohlmannAKZO Nobel Electronic Products, Arnhem, The Netherlands

~Received 2 October 1995; accepted for publication 20 December 1995!

We have investigated three different quasi-phase-matching approaches to second-harmonicgeneration~SHG! in DANS ~4-dimethylamino-48-nitrostilbene! poled polymer channel waveguidesat 1.5mm. Periodic photobleaching and periodically poled electrodes deposited directly on the filmproduced unacceptably high propagation losses. However, periodic electrodes on the substrate gavelow losses and useful SHG. ©1996 American Institute of Physics.@S0003-6951~96!04009-8#

The field of second-order nonlinearities in waveguideshas found a significant application in cw second-harmonicgeneration~SHG!, which enabled the fabrication of compact,blue light sources. More recently, it has gained new attentiondue to the possibility of using cascadedx (2) effects for all-optical switching, spatial solitons, etc.1 Because cascadingapplications usually require pulsed sources, frequently atcommunications wavelengths, it is desirable to both work atspecific wavelengths and minimize the refractive index dis-persion with wavelength. Of the different phase-matchingtechniques demonstrated to date, one of the most successfulhas been quasi-phase-matching~QPM! essentially because itallows any frequency to be doubled.2 It involves a periodicmodulation of the refractive index or the nonlinear coeffi-cientd(2) ~2v; v, v! such that the harmonic fields generatedin different parts along the waveguide interfere construc-tively at the output. QPM also allows the phase matching of~a! the diagonald(2) tensor elements which are typically thelargest and~b! the lowest order mode which typically leadsto the highest waveguide overlap integrals~and therefore ef-ficiency!, by choosing the appropriate periodicity for thenonlinear grating. These conditions have been demonstratedprimarily in QPM inorganic crystal waveguides to date.3

Another material class which can exhibit very largesecond-order nonlinearities is poled polymers. They couldultimately have certain advantages over ferroelectric materi-als due to ease of processing, low costs, etc. Periodicpoling,4–6 photobleaching,7 and laser ablation8 have alreadybeen used to create a periodic modulation of thed(2) coeffi-cient in polymers. These studies were all performed on dif-ferent materials and with different processing conditions. Asa result, to date no direct comparison between any of theseapproaches has been carried out. In this letter, we reportQPM-SHG implemented by periodic photobleaching and byperiodic poling with different electrode locations and com-pare their relative problems and merits in the side-chainDANS ~4-dimethylamino-48-nitrostilbene! polymer.

The samples were fabricated by multilayer spin coating

onto silicon or fused silica substrates. In order to spatiallyseparate the guided mode fields from the absorbing elec-trodes, the guiding DANS layer was sandwiched betweentwo buffer layers~PC polymer, provided by AKZO!. Allthree polymeric layers were 2.1mm thick and the aluminumelectrodes used for poling had a thickness of about 0.05mm.For the photobleaching study, a planar electrode was firstdeposited on the glass substrate, the multilayers were thenspun on and a second planar electrode was deposited ontothe buffer film surface. After poling, the top electrode wasremoved by chemical etching. A 14mm period grating wasphotobleached into the slab waveguide by illumination fromabove with an argon ion laser through a mask. The gratingperiodicity was determined fromL52p/uDbu, Db52b1

2b2, whereb1 andb2 are the fundamental and harmonicpropagation wave vectors of the waveguide. To create digitalelectrodes with this periodicity, one of the aluminum layerswas photolithographically patterned. Two sets of periodicallypoled samples were fabricated, the first on a silicon substrate~which acted as the bottom electrode! with the patternedelectrode on top of the multilayer stack, and the second withthe patterned electrode beneath the polymer stack~directlyon the fused silica substrate, see Fig. 1!. All of the sampleswere poled with an electric field of 100 V/mm for 30 min at137 °C~near the glass transition temperature of 142 °C! andsubsequently cooled down to room temperature. Ad33 of 25pm/V has been measured at a fundamental wavelength of 1.5

a!Current address: Institut fu¨r Festkorperphysik, Universita¨t Potsdam, AmNeuen Palais 10, D-14469 Potsdam, Germany.

FIG. 1. Sample geometry with periodic electrode~L514 mm! on the sub-strate.

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mm using the Maker fringe method for a poling field of 90V/mm across planar~no periodicity! electrodes. The mea-surement was performed versus the reference of quartz~0.4pm/V!. Channel waveguides with widths from 1 to 5mmwere then photobleached by exposure to blue/UV lightthrough a mask for 2 h. Finally, the samples were diced intopieces of several lengths for endfire coupling.

The waveguides were first characterized by evaluatingtheir throughputs and deducing from such measurements thelinear losses at the fundamental wavelength. Very largelosses were measured for the photobleached grating sample.At 1.5 mm, the measured loss was 25 dB/cm, more than anorder of magnitude larger than the absorptive loss in DANS.The origin was quickly identified as coherent scattering ofthe fundamental into the substrate via the resulting strongindex modulation induced by the photobleaching. That is, theregions of different refractive index act like a phased arrayantenna in converting the guided wave into radiation fields.This was confirmed by beam propagation analysis through aperiodic index structure with an index change of 0.04 be-tween alternate waveguide regions. In addition, the pho-tobleaching process changes the thickness of the polymerfilm. In order to measure these thickness changes, thesamples were studied with a phase-shift interferometer, asreported in detail elsewhere.9 The surface~not shown! exhib-ited a periodic deformation of about 80 nm or 1.3% of thetotal stack thickness.

The propagation losses for the two periodically poledgeometries were radically different. For periodic electrodeson the substrate, waveguide losses were estimated to be amaximum of 5 dB/cm. However, a surprisingly large 40dB/cm was measured for waveguides in which the periodicelectrodes were deposited onto the multilayer stack.

In order to determine the origin of this excessive loss,the samples were also studied using phase shift interferom-etry. It was found that the polymer under the electrodes wassqueezed when a strong field was applied, leading to a peri-odic thickness perturbation which scattered the light bothcoherently and incoherently out of the waveguide. Figure 2shows a three-dimensional map of the thickness variations,

where the imbedded regions correspond to the digital elec-trode grating. The maximum deformation amplitude wasfound to be more than 300 nm~or 5% of the polymer stack!in the poled waveguide device.

Systematic investigations with different grating periodsexhibited an increase in the deformation amplitude for finerelectrode gratings. For example, the deformation for a grat-ing period of 18mm was about 10%–20% smaller than thatof a 14mm grating. These results can be explained by theelectrostatic forces during the poling and the resulting vis-cous flow of the polymer in the rubbery state. For large grat-ing periods, a similar flow of the polymer is, as observed,only possible near the edges of the electrodes. It is theseperiodic deformations which led to the large propagationlosses.

For samples with a plain electrode on top, the deforma-tion of the polymer waveguide is, therefore, minimized. Thesurface~not shown! exhibits only a slight periodic deforma-tion of about 20 nm or 0.3% of the stack thickness. Further-more, it was observed that the regions that had been exposedto the UV lamp of the mask aligner during the photobleach-ing were reduced in thickness, causing the channels to standout like small ridges. The effect is still small~about 0.5% ofthe stack thickness!, and does not seem to influence the de-vice performance in our case.

The SGH measurements were performed with a synchro-nously pumped NaCl:OH2 color center laser, which pro-duces 6–9 ps pulses with tunability from 1.50 to 1.65mm.By tuning the wavelength, the second-harmonic power wasinvestigated versus the detuningDbLSAMPLE. For the peri-odically photobleached sample, the full width of 7 nm indi-cated an effective phase-matching length ofLPM50.6 mm,LSAMPLE52 mm, in good agreement with the measuredloss coefficient. Even for such small values ofLPM,P(2v)/P2(v)5531024%/W was measured. For the peri-odically poled sample with electrodes on the upper surface,the measured conversion efficiency was even less because ofthe larger propagation losses.

The best results were obtained with periodic electrodesdeposited on the substrate surface. As shown in Fig. 3, theexpected sinc2(DbLPM! dependency is observed. From the

FIG. 2. Three-dimensional surface plot of the periodically poled samplewith digital electrode on the multilayer stack surface. Maximum deforma-tion amplitude is larger than 300 nm.

FIG. 3. Normalized second-harmonic output vs detuning. The acceptancebandwidth corresponds to a phase-matching length of 0.2 cm.

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phase-matching curve and the waveguide parameters, thelength LPM over which phase matching is maintained wasdetermined to be 0.2 cm, which is very close to the samplelength ofL50.25 cm. The figure of merit for this sample is

h5P2v

Pv2L2

50.05%/W-cm2.

Although this efficiency, to the best of our knowledge, is thehighest reported for a polymeric channel waveguide, it is stillvery modest compared to that expected from the high off-resonantd33 coefficient of 25 pm/V. As discussed previouslyby Khanarian and co-workers, the spatial modulation ofd33, i.e.,Dd33(z) is small with respect to its average value inthis poling geometry.10 In order to utilize the high nonlinear-ity of polymers, a better geometry, or poling procedureand/or material has to be found that allows complete modu-lation of the nonlinearity.

In conclusion, we have investigated three QPM-SHG ge-ometries for fundamental beams at 1.5mm. It was found thatQPM under certain conditions led to large scattering losses.For example, the large radiative losses found for the indexmodulation which accompanies photobleaching made thisapproach unattractive. Periodic film deformation via polingelectrodes on the free film surface also led to large radiativelosses and proved unsuitable. These deleterious effects wereminimized by changing the waveguide design, i.e., using pe-riodic poling electrodes on the substrate surface, leading to a

figure of merit h50.05%/W-cm2 for SHG with a phase-matching length of 0.2 cm. A large improvement of the SHGconversion efficiency is still to be expected for an improvedpoling geometry that allows a better modulation depth of thed33 coefficient.

The research at CREOL was supported by AFOSR andNSF.

1Reviewed in the following: G. I. Stegeman, R. Schiek, G. Krijnen, W.Torruellas, M. Sundheimer, E. VanStryland, C. Menyuk, L. Torner, and G.Assanto,Guided-Wave Optoelectronics: Device Characterization, Analy-sis, and Design,Proceedings of the 4th WRI International Conference onGuided Wave Optoelectronics, edited by T. Tamir, H. Bertoni, and G.Griffel ~Plenum, New York, 1995!, pp. 371–379.

2M. M. Fejer, G. A. Magel, D. H. Jundt, and R. L. Byer, IEEE J. QuantumElectron.28, 2631~1992!.

3For example, D. Eger, M. Oron, M. Katz, and A. Zussman, Appl. Phys.Lett. 64, 3208~1994!.

4R. A. Norwood and G. Khanarian, Electron. Lett.26, 2105~1990!.5G. Khanarian and R. A. Norwood, inProceedings of the 5th Toyota Con-ference on Nonlinear Optical Materials, edited by S. Miyata~North Hol-land, Amsterdam, 1992!, p. 461.

6Y. Azumai, M. Kishimoto, I. Seo, and H. Sato, IEEE J. Quantum Electron.30, 1924~1994!.

7G. L. J. A. Rikken, C. J. E. Seppen, S. Nijhuis, and E. Meijer, Appl. Phys.Lett. 58, 435 ~1991!.

8G. Marowsky, E. J. Canto-Said, S. Lehmann, F. Sieverdes, and A. Bratz,Phys. Rev.48, 18114~1993!.

9W. Brinker, S. Yilmaz, W. Wirges, S. Bauer, and R. Gerhard-Multhaupt,Opt. Lett.20, 816 ~1995!.

10G. Khanarian, R. Norwood, and P. Landi, Proc. SPIE1147, 129 ~1989!.

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