We report on poling of a low dielectric constant preimidized fluorinated fully aromatic guest-host polyimide nonlinear optical material including thermal stability of optical nonlinearities and waveguiding properties. We measured a second-harmonic coefficient (d33 = 4.9 +/- 0.5 pm/V at 1217 nm fundamental wavelength) which is accurately predicted by a thermodynamic model of poled polymers. The optical nonlinearity of a poled sample was thermally stable at 80-degrees-C for over 300 h. Films were observed to have negative birefringence. Optical losses for slab waveguides in lowest order TE and TM modes were greater-than-or-equal-to 7.7 dB/cm for doped waveguides at 800 nm wavelength and increased after poling

Cheng, Stephen Z. D.

Harris, F. W.

Hubbard, S. F.

Li, F.

Singer, K. D.

English

The University of Akron

The University of AkronIdeaExchange@UAkronCollege of Polymer Science and Polymer Engineering6-18-2009Nonlinear-Optical Studies of a Fluorinated PoledPolyimide Guest-Host SystemS. F. HubbardCase Western Reserve UniversityK. D. SingerCase Western Reserve UniversityF. LiStephen Z. D. ChengUniversity of Akron Main Campus, scheng@uakron.eduF. W. HarrisPlease take a moment to share how this work helps you through this survey. Your feedback will beimportant as we plan further development of our repository.Follow this and additional works at: http://ideaexchange.uakron.edu/polymer_ideasPart of the Polymer Science CommonsThis Article is brought to you for free and open access by IdeaExchange@UAkron, the institutional repository ofThe University of Akron in Akron, Ohio, USA. It has been accepted for inclusion in College of Polymer Science andPolymer Engineering by an authorized administrator of IdeaExchange@UAkron. For more information, pleasecontact mjon@uakron.edu, uapress@uakron.edu.Recommended CitationHubbard, S. F.; Singer, K. D.; Li, F.; Cheng, Stephen Z. D.; and Harris, F. W., "Nonlinear-Optical Studies of aFluorinated Poled Polyimide Guest-Host System" (2009). College of Polymer Science and Polymer Engineering. 10.http://ideaexchange.uakron.edu/polymer_ideas/10Nonlinear optical studies of a fluorinated poled polyimide guest-host system S. F. Hubbard and K. D. Singer Department of Physics, Case W-tern Reserve University Cleveland, Ohio 44106-7079 F. Li, S. 2. D. Cheng, and F. W. Harris Department of Polymer Science, The University of Akron, Akron, Ohio 443253909 (Received 24 February 1994; accepted for publication 2 May 1994) We report on poling of a low dielectric constant preimidized fluorinated fully aromatic guest-host polyimide nonlinear optical material including thermal stability of optical nonlinearities and waveguiding properties. We measured a second-harmonic coefficient (d,,=4.9t0.5 pm/V at 1217 mu fundamental wavelength) which is accurately predicted by a thermodynamic model of poled polymers. The optical nonlinearity of a poled sample was thermally stable at 80 “C for over 300 h. Films were observed to have negative birefringence. Optical losses for slab waveguides in lowest order TE and TM modes were 27.7 dB/cm for doped waveguides at 800 nm wavelength and increased after poling. The nonlinear optical properties of poled polymeric ma- terials have been researched recently.le5 The large optical nonlinearities of these materials gives them excellent poten- tial for incorporation into nonlinear optical integrated de- vices. Such devices may need to operate for long periods of time at high temperatures (~80 oC).4 Poled polymer films such as Disperse Red-l doped poly-(methylmethacrylate), will not retain their optical nonlinearities under such condi- tions due to their low glass-transition temperatures. Recently, the nonlinear optical properties of some novel high T, poled polyimides have been investigated.425 The high nonlinear op- tical susceptibilities of these materials have shown very good thermal stability when doped in a guest-host configuration. These systems, once cured and poled, have proven to be thermally stable for extended periods of time at elevated temperatures.” Different methods have been explored for the curing and poling of polyimide guest-host systems. One method used a thermal process to imidize films during poling.4*5 Room- temperature chemical imidization performed immediately af- ter poling has been seen to enhance the thermal stability of the poled electro-optic response.7 We report here on the non- linear optical properties of a poled polyimide guest-host sys- tem that is soluble even after imidization. This allows films to be easily processed and poled fully imidized. The system is fully aromatic so its glass-transition temperature is very high. This feature is useful for device applications because it lessens the effects of relaxation after poling allowing the polyimide films to retain their second-order nonlinear optical properties for extended periods of time. The polyimide we used was synthesized from 2,2’- bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 2,2’-bis(trifluoromethyl)-4,4’-diaminobiphenyl (PFMB). The glass-transition temperature of the undoped polyimide is 357 “C as determined by dynamic mechanical strain analysis. This is much higher than the glass-transition temperature of the polyetherimide Ultem, which has been reported as 210 oC.4 Very high temperatures were required to pole this polyimide so a nonlinear chromophore with very high decomposition temperature was necessary. Several mol- ecules for such applications have been studied.4T5J8 The dye we used was 2-(p-nitrophenyl)-4,5-bis(p-hydroxyphenyl) oxazole (TNON). The decomposition temperature of TNON is 305 “C! and was measured by thermal gravimetric analysis. The chemical structures of TNON and SFDALPFMB are shown in Fig. 1, and Fig. 2 shows the glass-transition tem- perature of the guest-host system plotted as a function of dye content. Considerable plasticization by the chromophore was observed. (a) OH 0’) FIG. 1. The chemical structures of (a) 6FDA.iPFMB and (b) TNON. Appl. Phys. Lett. 65 (3), 18 July 1994 0003-6951/94/65(3)/265/3/$6.00 0 1994 American Institute of Physics 265 Copyright ©2001. All Rights Reserved.360.~ I TABLE I. physical parameters of TNON and 6FDA/PFMB used in calcu- lations. E is the static dielectric constant, E, the magnitude of the poling field, n, the refractive index at the fundamental frequency, /3 the second order molecular nonlinear optical susceptibility, and /.L the dipole moment.  Both p and ,u were measured by dc-induced second harmonic generation at x=1344 urn. 340 p..... l I 320 - --*-,. 300 A.. - ‘i, 280 --%.. - ‘.._ 3 0 .-. - ., 0 260 l ol c  240 - l -.. --.... 220 - 200 - ‘..,. 180 A. - =\. 160 - ‘<-I’ ,401... f .,,, ,.L ,/,,,., I . ...,, I,1_II 0 5 10 15 20 25 30 Dye Content (%) FIG. 2. Glass-transition temperature of the TNON-6FDAE’FMB guest-host system as a function of dye content determined by DMS. The line is a least-squares fit. Films of TNON-6FDA/PFMB were spin coated onto indium-tin-oxide-coated glass and were then baked at 105 “C in a vacuum oven to remove excess solvent. After films were cast, they were corona poled in a nitrogen atmosphere with the field perpendicular to the tilm for 45 min at 300 “C. The poling field was approximated using an Isoprobe electrostatic voltmeter to measure the potential on the surface of the film with respect to ground. Second-harmonic generation was measured by placing the film on a rotation stage in a 1217 nm laser beam. This wavelength and its second harmonic are both far from reso- nance of TNON-6FDAPFMD. The absorption peak of the system is at 400 nm and was measured with a W-vis spec- trophotometer. The second harmonic produced by the film was then measured as a function of incident angle in bothp and s polarizations. In both cases the second harmonic was p polarized. All measurements were referenced to dll of quartz. The second-order nonlinear optical susceptibility of the TNON-6FDAPFMB tilms was calculated using rotational Maker fringe analysis9 Using a thermodynamic model, d,, for poled polymer films can be predicted using the molecular hyperpolarizability, ,&I, of the chromophore’ d33(-2w;qti)=Nf2wf~f~&,Z(-2~;ti,~) (1) where P= i2) where N is the number density of nonlinear optical moieties, 2o f and f o  are the local field factors at the indicated frequen- cies, E is the static dielectric constant, IZ the index of refrac- tion, and E, the poling field. The value of p  for TNON was measured by dc-induced second-harmonic generation while dissolved in 1,4-dioxane. The hyperpolarizability was calculated using an infinite dilu- tion extrapolation procedure.” Table I shows several proper- EP P P Number density E (Mv/cm) n, (=u) (em) 5.703XlO~ 2.8 2  1.57 3.6X10-” 7.7x 10-18 ties of the TNON-6FDAPFMB films used for these experi- ments. The W-vis spectrum of a TNON-6FDAiPFMB was measured for poled and unpoled films, and the height of the absorption peak at 400 nm decreased dramatically due to decomposit ion of the dye. The height of the absorption peak varies linearly with the number density of the film, and this decrease (of nearly 50%) was accounted for in the calcula- tion with the thermodynamic model. This correction leads to excellent agreement between the measured nonlinear optical susceptibility and that predicted by the model as shown in Table II. This agreement along with the measured ratio d33/d31=3.0+0.4 indicates that poling of the guest-host polyimide system leads to the maximum attainable nonlinear optical coefficient in an isotropic poling regime.’ The induced alignment in these polyimide tilms was seen to have high thermal stability. The p-p experiment was repeated several times over a period of over 300 h with a poled film stored under ambient conditions at 80 “C. The signal showed some fast decay during the first few hours, but then stabilized as shown in Fig. 3. Another sample was stored for 30 min at 200 “C after poling, and d,, decreased by a factor of approximately 4. The thermal stability of the system could be further improved by chemically attaching the chromophore to the polymer main chain rather than dop- ing in the guest-host configuration. An equilateral prism coupling technique was used to lo- cate radiation modes in TNON-6FDAPFMB tilms cast on silicon. The indicent angles at which these modes were lo- cated were used to calculate the refractive indices in both TE and TM polarizations for wavelengths ranging from 514 to 800 nm. Figure 4 shows the dispersion in the refractive index of an unpoled film and a poled film. The birefringence was observed to be negative. The magnitude of the birefringence decreased during poling because of poling-induced order perpendicular to the plane of the tilm. The birefringence of about -0.02 for the poled films makes a negligible contri- TABLE II. Comparison of the second order nonlinear optical susceptibility measured with Maker Fringes [d,, (measured) and d,, (measured)] and that predicted by the thermodynamic model based on the values of p and /.L [&a (predicted)]. Uncertainty in dsa (measured) reflects uncertainty in dst (mea- sured) which, in turn reflects uncertainty in the angles and amplitudes of the Maker Fringes. Uncertainty in d,, (predicted) is due to uncertainty in the number density. dst (measured) d,, (measured) (Pm/v) (Pm/V) dss (predicted) (Pm/v) 1.620.1 4.920.5 5.0r0.4 266  Appl. Phys. Lett., Vol. 65, No. 3, 18  July 1994  Hubbard et al. Copyright ©2001. All Rights Reserved.TABLE III. The optical loss of lowest order TE and TM modes of TNON- 6FDAiPFMB slab waveguides. _~ -..... __ ~_ 4 H 0.4 - s u 0.2 - 02 0 /-..- 1 I I I 0 50 100 150 200 250 300 350 Time (hours) FIG. 3. The decay of the optical nonlinearity as a function of time after poling with the film stored at 80 “C. bution in the calculation of the nonlinear optical susceptibility.” A right angle prism coupler and scattered light imaging were used to measure the optical loss of slab waveguides cast on silicon dioxide. The incident angle of the laser was varied until the lowest order TE or TM mode would appear as a streak in the tilm, and the image of the streak was recorded by a Cortex frame grabber and digitized for data analysis. The optical loss was calculated using the assumption that the amount of light being guided inside the film was proportional to the amount of light scattered out of the film. Measured values of the loss are shown in Table III. Microscopic pin- holes appeared on films during poling which caused large scattering losses. The optical loss was measured in undoped and doped waveguides. The undoped samples showed markedly lower loss than the doped films at 800 mn. LOSS measurements were repeated on the undoped films at 840 nm, and the loss was lower. These results indicate that the mechanism for the high loss seen in these polyimide waveguides is absorption ‘::Ar” . . ..-.- 7 ‘.Ii7 f i;; p::::::l_~ I d%2ze.m . .__ ___., 1 1.6 1 350 --I-I (,S I LPI--I 608 850 ,100 ,350 350 600 850 ,108 ,350 Wavelength (nm) Wavelength (nm) (a) (b) FIG. 4. Dispersion of the refractive index for 20% TNON-6FDA/PFMB tllms. Diamonds represent the TM indices, and circles show the TE indices-(a) is an unpoled 8lm, and (b) is a poled film. Wavelength Loss of TE mode Loss of TM mode (nm) @Ycm) (dB/cm) Undoped film 840 3.5 4.4 Undoped lilm 800 5.0 5.8 Doped unpoled film 800 8.2 7.7 Doped poled film 800 13.3 9.3 by both the chromophore and the polyimide host.‘2Y13 The loss could therefore be lowered by waveguiding at longer wavelengths which are farther from the absorption peak of the TNON-GFDA/I?FMB system. Chemical modifications to lower the optical loss in polyimide waveguides are also pos- sible and have been studied recently.r4 The thermoplastic poling of a preimidized fluorinated fully aromatic polyimide guest-host system was observed to conform with a thermodynamic model based on noninteract- ing dipoles. This demonstrates that fully imidized soluble polyimide hosts can be effectively poled using standard tech- niques and opens new processing possibilities in the con- struction of polymer electro-optic devices. The aromaticity of the system gives it a high glass-transition temperature which minimizes relaxation at temperatures well above room temperature, and the exceptionally low dielectric constant suggests that very high bandwidth electro-optic devices may be constructed. This work was supported by the National Science Foun- dation under the Science and Technology Center AICOM No. DMR89-20147. ‘K. D. Singer, J. E. Sohn, and S. J. Lalama, Appl. Phys. L&t. 49, 248 (1986). ‘D. J. Williams, Agnew. Chem. Intl. Ed. Eng. 23, 690 (1984). ‘J. Zyss, J‘. Mol. Electron. 1, 25 (1985). 4M. Stihelin, C. A. Walsh, D. M. Burland, R. D. Miller, R. J. ‘Bvieg, and W. Volksen, J. Appl. Phys. 73, 8471 (1993). ‘M. Stihelin, D. M. Burland, M. Ebert, R. D. Miller, B. A. Smith, T. J. ‘Bvieg, W. Volksen, and C. A. Walsh, Appl. Phys. Lett. 61, 1626 (1992). ‘J. W. Wu, J. F. Valley, S. Ermer, E. S. Binkley, J. T. Kenney, G. F. Lipscomb, and R. Lytel, Appl. Phys. Lett. 58, 225 (1991). 7J. W. Wu, J. F. Valley, S. Ermer, E. S. Binkley, J. T. Kenney, and R. Lytel, Appl. Phys. L&t. 59, 2213 (1991). ‘R. F. Shi, M. H. Wu, S. Yamada, Y. M. Cai, and A. F. Garito, Appl. Phys. Lett. 63, 1173 (1993). ‘M. G. Kuzyk, K. D. Singer, H. E. Zahn, and L. A. King, J. Opt. Sot. Am. B 6, 742 (1989). IoK. D. Singer and A. F. Gartio, J. Chem. Phys. 75, 3572 (1981). ‘IN. Okamoto, Y. Hirano, and 0. Sugihara, J. Opt. Sot. Am. B 9, 2083 (1992). r*P. A. Cahill, C. H. Seager, M. B. Meinhardt, A. J. Beuhler, D. A. War- gowski, K. D. Singer, T. C. Kowalczyk, and T. Z. Kosc, Proc. SPIE 2025, 5s (1993). 13A. Skumanich, M. Jurich, and J. D. Swalen, Appl. Phys. Lett. 62, 446 (1993). “T. C. Kowalczyk, T. Z. Kosc, K. D. Singer, P. A. Cahill, C. 0. Seager, M. B. Meinhardt, A. J. Beuhler, and D. A. Wargowski, J. Appl. Phys. (to be published) . Appl. Phys. Leil., Vol. 65, No. 3, 18 July 1994 Hubbard et al. 267 Copyright ©2001. All Rights Reserved.

Nonlinear-Optical Studies of a Fluorinated Poled Polyimide Guest-Host System

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Nonlinear-Optical Studies of a Fluorinated Poled Polyimide Guest-Host System

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