PIERS ONLINE, VOL. 5, NO., 29 4 Poled Thick-film Polymer Electro-optic Modulation Using Rotational Deformation Configuration Wen-Kai Kuo and Yu-Chuan Tung Institute of Electro-Optical and Material Science, National Formosa University 64 Wenhua Rd., Huwei, Yunlin 6328, Taiwan, R.O.C. Abstract In this paper, we propose the experimental results of electro-optic polymer modulation using a rotational deformation configuration, then, the results for the conventional compressed/stretched deformation configuration is compared. The experimental results show that, the modulation index of the former is almost two times than that of the latter.. INTRODUCTION Second-order nonlinear optical (NLO) polymers are very attractive materials for light wave and integrated optics applications due to their characteristics of large nonlinearity, low-cost and easy processing [, 2]. Among these photonic applications, the most popular device is the electro-optic modulator, which is based on the linear electro-optic (EO), or Pockels, effect. This effect is classified as an NLO effect of polymer due to its 2nd order susceptibility χ (2). The susceptibility χ (2) is defined as the general relationship between the components of the induced polarization density P (at angular frequency ω) and those of the electric field(s) E (at angular frequencies ω, ω 2,...) [3]: P (ω) I = J χ () ( ω; ω ) IJ E(ω ) J + J, K χ (2) ( ω; ω, ω 2 ) IJK E(ω ) J E(ω 2 ) K +... () Here I, J, K = X, Y, Z are the Cartesian coordinates of the macroscopic frame, with linear susceptibility χ () related to optical refraction and absorption. Both the linear EO effect [χ (2) ( ω; ω, )] and the frequency doubling [χ (2) ( 2ω; ω, ω)] are the two most common effects attributed to χ (2). The above equation, describing the Pockels effect in organic material, is of an electronic nature and of microscopic origin. When this effect is explained on the molecular level, the induced polarization of the molecule is approximated as the creation of an induced dipole. For a molecule under the influence of a local electrical field(s) E, the induced polarization, or dipole moment, can be expressed as: p(ω) i = j α( ω; ω ) ij E (ω ) j + j, k β( ω; ω, ω 2 ) ijk E (ω ) j E (ω 2 ) k +... (2) Here α is the linear polarization of the material and β is the first hyper-polarizability. The indices i, j, k = x, y, z are the Cartesian coordinates in the microscope s molecular frame. Refer to χ () and χ (2) in Eq. (), α is related to the optical refraction and absorption properties of the molecule; β is the microscope equivalent of the χ (2) coefficient in a bulk material or, in other words, χ (2) is the sum of the β components of all molecules. Many hyper-polarizable molecules have a long conjugated π-electron system with an electron acceptor (A) and an electron donor (D) at opposite end. A large β component exists in this type of AπD molecule and the permanent molecule dipole moment points from A to D. In the presence of a dc poling field, the steady-state orientation distribution of the dipole is determined by the minimum Helmholtz free-energy configuration; the EO coefficient r 3 is related to the dipole orientation distribution function f(θ) [4] r 3 = Nβ 333 n 2 ε (cos 3 θ cos θ ) f(θ)dθ (3) Here β 333 is the microscopic molecular hyper-polarizability, N is the number density of the EO molecule in the polymer film, n is the refractive index, ε is the dielectric constant, and θ is the
PIERS ONLINE, VOL. 5, NO., 29 42 angle between the poling field and the dipole. The interaction energy of induced dipole moments, f(θ) is given by the following Boltzmann statistical distribution: f(θ) exp( me cos θ/kt ) (4) where m is the permanent dipole moment, E is the poling field, k is Boltzmann s constant, and T is absolute temperature. In the above equation, me cos θ is interaction energy between m and E. If the poling field is applied along the z-direction, after the poling process, in the presence of a modulation electrical field E = (E x, E y, E z ), the index ellipsoid of the poled polymer becomes [5]: ( n 2 o ) ( + r 3 E z (x 2 + y 2 ) + n 2 e + r 33 E z ) z 2 + 2r 3 E y yz + 2r 3 E x xz = (5) where n o and n e are refractive indices for ordinary and extra-ordinary rays, respectively. Under the assumption of a low poling field and rod-like molecules in thermodynamics, the other tensor element r 33 can be expressed as r 33 = 3r 3 in steady state. For most polymer EO modulation configurations, the compressed/stretched deformation (CSD) of the index ellipsoid is used to measure the EO coefficient [6], or study orientational relaxation dynamics [7]. In this paper, the rotational deformation (RD) of the index ellipsoid, which was proposed in our previous study, is applied to the EO polymer modulation [8] and because two refractive indexes, n o and n e, of the EO polymer are very close, the expected modulation index of the RD configuration has a much better performance than that of the CSD configuration. Polymer Compensator Photo-detectors HeNe Laser Polarizer ITO Glass substrate Modulation signal Analyzer Signal generator Lock-in amplifier Reference signal Oscilloscope Low noise preamplifier Figure : Schematic of the poled polymer EO modulation using rotational deformation configuration. 2. EXPERIMENT The poled polymer EO modulation using rotational deformation configuration is shown in Fig.. The top view of this test sample structure and the principal axes of the poled polymer are shown in Fig. 2. The gap between two electrodes was 2 mm. In our experiment, the sample NLO polymer is an often-used guest-host system DRl/PMMA, i.e., a mixture of azo dye 4-[ethyl(2-hydroxyethyl)amino]-6 nitroazo-benzene (DRl) and the amorphous polymer poly(methy methacrylate) (PMMA). The DR/PMMA sample (% DRl by weight) was prepared by dissolving of the mixture in chloroform. To increase the EO modulation index, a casting method for fabricating thick-film samples was developed. The casting polymer sample on an indium tin oxide (ITO) coated glass substrate was placed into a vacuum oven, set at a temperature of 65 C for 2 hours. The resulting sample, with a film thickness of approximately 7 µm, was obtained, then, sandwiched between two electrodes and contact poling was applied. Finally, we could easily peel this poled thick polymer film off the ITO substrate. Different poling temperatures (Tp) and poling voltages (Vp) were applied to these samples, then Fourier transform infrared spectroscope (FTIR) system (Model:
PIERS ONLINE, VOL. 5, NO., 29 43 Jasco FT/IR-4) was employed to evaluate their poling effects. Their FTIR measurement results are shown in Fig. 3. For the samples with higher poling voltage, their degrees of dipole orientation became higher and their absorption peaks became lower. This is because dipoles tend to rotate in the direction of the poling electric field direction (normal to the film) and the FTIR incident field (parallel to the film) is perpendicular to the rotated dipoles. For the non-heated (room temperature 25 C) samples, the poling voltage could be as high as 2 V, while for the heated sample, the highest poling voltage was approximately 8 V. The corresponding EO coefficient r 33 values of these samples can be measured by the method proposed by Shuto et al. [9] as listed in Table. The heating temperature was 2 C and poling voltage was 8 V, the sample showed the best r 33 value at approximately pm/v. Table : EO coefficient r 33 values for different poling condition samples. Poling conditions (T p/v p) r 33 (pm/v) 25 C/6 V.4 25 C/9 V 25 C/2 V 2 2 C/8 V ITO y Polymer z x Glass substrate Figure 2: Top view of the test sample structure and the principal axes of the poled polymer. 3 2.5 2.5.5 No poling Tp = 25 C,Vp = 6 V Tp = 25 C,Vp = 9 V Tp = 25 C, Vp = 2 V Tp = 2 C,Vp = 8 V 5 5 2 25 3 35 4 45 Figure 3: FTIR measurement results for different poling conditions. Following this, the polymer film was placed on a glass substrate with parallel ITO electrodes for application of the x-direction electrical field E x on the sample as shown in Fig. 2. The incident angle of the laser beam to the polymer film was approximately 3. For propagating the laser beam inside the polymer, if we only consider the component along y-direction with the electrical field E x, the index ellipse equation corresponding to this component is n 2 x 2 + n 2 z 2 + 2r 3 E x xz = (6) e
PIERS ONLINE, VOL. 5, NO., 29 44 Due to the cross-term and difference of coefficients of x 2 term and z 2 term, the principal axes are slightly rotated. This slight rotation angle θ is approximated as θ r 3E x n 2 n 2 e Since n o and n e of the poled polymer are very close, the rotation angle becomes larger. For our (7) Figure 4: Measured and simulated EO modulation signal strength versus the applied voltage. experimental set-up, the laser beam propagating the path inside the polymer has both components along y-direction and z-direction and its rotational angle can be obtained by computer numerical calculation. According to our previous study, when the laser beam in the poled polymer is p- polarized by the input polarizer, as indicated in Fig., this rotational angle can make the output light intensity between two photo-detectors to be different [8]. This is referred as the RD modulation as mentioned in the previous section. The measured n o and n e of the poled polymer sample are.592 and.598, respectively, for a wavelength of 632.8 nm of the HeNe laser. The measured and simulated EO modulation signal strength versus the applied voltage generating electrical field E x are shown in Fig. 4. To achieve a high modulation index, the optical bias point of the RD configuration required careful tuning. For comparison, the same results for the CSD configuration are shown in Fig. 4. To obtain this result, the input polarization direction was set to have equal p- and s-polarization components and the compensator was re-tuned to have the best optical bias point for the CSD configuration. Under the same conditions, it can be seen that the modulation Output signal Modulation signal Figure 5: Real-time EO modulation signal.
PIERS ONLINE, VOL. 5, NO., 29 45 index of the RD configuration is almost two times improved over that of the CSD configuration. To show the real-time EO modulation signal, a sine signal, with peak-to-peak amplitude of 2 V and a frequency of 2 KHz, was applied to the electrodes, then the output differential voltage of the photo-detectors was amplified by a low-noise preamplifier with a gain of 5. The output signal of the preamplifier on the oscilloscope had peak-to-peak amplitude of V, as shown in Fig. 5. This result shows that the new configuration can be applied to real-time non-contact EO probing technique with high sensitivity []. 3. CONCLUSION The NLO polymer modulation using rotational deformation configuration has been demonstrated successfully. Under the same conditions, this new configuration showed two-fold modulation index improvement over the conventional compressed/stretched deformation configuration. This high modulation index can be used for EO probing applications. In addition, because the modulation index of this new configuration is directly related to EO coefficient r 3 of the NLO polymer, it can be used to study those poling polymers that don t hold r 33 = 3r 3 in a steady state. Therefore, this new configuration could be a very useful tool for NLO polymer study and applications. REFERENCES. Hornak, L. A. (ed.), Polymers for Lightwave and Integrated Optics, Marcel Dekker, New York, 992. 2. Bauer, S., Poled polymer for sensor and photonic applications, J. Appl. Phys., Vo. 8, 553 5558, 996. 3. Van der Vorst, C. P. J. M. and S. J. Picken, Electric field poling of acceptor-donor molecules, J. Opt. Soc. Am. B, Vol. 7, 32 325, 99. 4. Valley, J. F., J. W. Wu, and C. L. Valencia, Heterodyne measurement of poling transient effects in electro-optic polymer thin film, App. Phys. Lett., Vol. 57, 84 86, 99. 5. Yariv, A., Quantum Electronics, 2nd ed., Wiley, New York, 987. 6. Teng, C. C. and H. T. Man, Simple reflection technique for measuring the electro-optic coefficient of poled polymer, App. Phys. Lett., Vol. 56, 734 736, 99. 7. Michelotti, F., E. Toussaere, R. Levenson, J. Liang, and J. Zyss, Study of the orientational relaxation dynamics in a nonlinear optical copolymer by mean of pole and probe technique, J. Appl. Phys., Vol. 8, 773 778, 996. 8. Kuo, W. K., et al., Two-dimensional electric-field vector measurement by a LiTaO 3 electrooptic probe tip, App. Opt., Vol. 39, 4985 4993, 2. 9. Shuto, Y. and M. Amano, Reflection measurement technique of electro-optic coefficients in lithium niobate crystals and poled polymer films, J. Appl. Phys., Vol. 77, 4632 4639, 995.. Takahashi, H., S.-I. Aoshinma, and Y. Tsuchiya, Sampling and real-time methods in electrooptic probing system, IEEE Tran. on Instrum. and Meas., Vol. 44, 965 97, 995.