A transmission ellipsometric method has been reformed without a spatial filtering aperture to characterize electro-optic (EO) performance of EO polymers. This method affords much simpler optical setup compared to the reflection method, and lets us easily perform detailed incident angle dependence measurements using a conventional glass substrate and an un-collimated beam. It is demonstrated that the reliable characterization with this method is possible in combination with a simple data analysis. By using the recently matured deposition technique of indium zinc oxide (IZO) on soft materials, it is possible to prepare the EO polymer sandwiched between two transparent electrodes. Thus the transmission method should be re-evaluated.
© 2013 Optical Society of America
Significant attention has been paid to the development of electro-optic (EO) polymers due to their potential applications in ultra-high-speed optical waveguide modulators and switches, digital signal processing, and electric field optical sensors [1–4]. This is due to high EO coefficients of recent polymeric EO materials in addition to their inherent ultrafast electronic response . As the importance of these materials increases, a standard and reliable method to characterize the EO performance of polymers needs to be developed. Several techniques for characterizing EO performance, such as reflection ellipsometry [6–8], second harmonic generation , attenuated total reflection (ATR) [9,10], Fabry-Perot interferometry [11,12], and Mach-Zehnder interferometry [13,14] have been developed. Each has its own advantages and much effort for improvement both in the measurement system and data analysis has been paid to improve reliability as well as simplicity in characterizing the EO performance [15–18]. Among these methods, the reflection ellipsometric method by Teng and Man (Teng-Man)  and by Schildkraut  has been most commonly used due to the simplicity of its measurement procedure since one simply measures a relative phase-shift difference between the s- and p- polarized light reflected from the sample where a voltage for modulation is applied to the EO polymer sandwiched by the two electrodes [19–23]. The sample preparation with the reflection method is also relatively simpler than other methods. When performed carefully, this method provides us with a reasonable estimate of the EO performance. On the other hand, some drawbacks of this method are pointed out both from practical and intrinsic viewpoints [15,18]. A typical sample used in the Teng-Man reflection method is an EO polymer sandwiched between a metal electrode and a transparent conductive oxide (TCO) on a thick glass substrate. A linearly polarized laser beam with 45° tilted polarization impinges on the sample from the glass side, and the first reflection beam at the air-glass interface has to be spatially blocked and the second reflection beam resulting from the reflection of the first pass at the metal and subsequent transmission at glass-air interface has to be spatially filtered and then detected, and similarly the third reflection beam has to be spatially blocked. Perfect filtering by an aperture is possible but practically difficult when a conventional glass substrate is used, although the degree of the difficulty also depends on the incident angle. The use of a sufficiently thick glass substrate and/or collimated narrow beam is required to achieve perfect filtering by an aperture . Without greater care, the imperfect filtering can lead to inaccurate results. Apart from the influence on the gross multiple reflections mentioned above, the influence on the multiple reflections at the inside EO polymer is inevitable. The multiple reflection effects inside the EO polymer can affect the data analysis and results obtained. Absorption in the sample (EO polymer and TCO) can lead to erroneous results due to the phase-sensitive nature of this measurement. The incident angle dependent measurement is also practically difficult. Most of these drawbacks originate from the high reflectivity of the metal electrode and its reflection geometry.
Lundquist et al.  introduced the Teng-Man ellipsometric method in the transmission geometry. They used a special EO polymer that has an extremely high glass transition temperature Tg (~300°C), and succeeded in depositing an indium tin oxide (ITO) electrode on the robust polymer. By using the special polymer sandwiched between the two transparent ITO electrodes, they demonstrated that in transmission measurements, errors caused by multiple reflection effects are eliminated and a reliable characterization of the EO performance is possible. Whether or not an aperture was used in the transmission measurements was not described , and detailed incident angle dependent measurements were not performed, although measurements at several incident angles were performed. The transmission method has not become a popular method since Lundquist et al. reported on it. Presumably, it was technically difficult to deposit ITO on commonly used organic materials in the past. However, the deposition techniques of TCOs on organic materials have recently matured in combination with the recent advance of organic and flexible devices [25–28]. Deposition of the indium zinc oxide (IZO) among TCOs has become popular [25,27,28]. IZO can be deposited by a RF magnetron sputtering technique that has been widely used due to advantageous features such as the simple apparatus, high deposition rate, and low deposition temperature . We have actually deposited IZO on a conventional EO polymer, which is described in the experimental section. Under these circumstances, we think that the transmission method should be re-evaluated.
In this paper, we have developed the transmission ellipsometric method without an aperture. This method affords much simpler optical alignment compared to the reflection method. In addition, it enables us to perform the detailed incident angle dependence measurements for characterization of the EO performance using a conventional glass substrate and an un-collimated beam. We also present a simple data analysis that is useful for our method. Finally, it is demonstrated that a reliable characterization of the EO performance is possible with this simple and convenient method, which helps us screen new EO polymers and develop new materials.
2. Experimental procedure
Chemical structures of materials used in our EO polymer are shown in Fig. 1(a). The EO chromophore is comprised of amino-benzene with a benzyloxy group as the donor unit, thienyl-di-vinylene as the π-electron bridge, and 2-(dicyanomethylene)-3-cyano-4-methyl-5-phenyl-5-trifluoromethyl-2,5-dihydrofuran (CF3-phenyl-TCF) as the acceptor unit, which has improved nonlinear optical properties compared with the benchmark EO chromophores without the benzyloxy group [29,30]. The EO polymer used in our study is a guest/host polymer, 10 wt% EO chromophore in poly(methyl methacrylate) (PMMA). The polymer material of 1.83-μm-thick film was fabricated by spin-coating on a conventional glass substrate of 0.7 mm coated with a thin ITO of 7 nm. The thickness of the EO polymer was determined by a profilometer (Dektak 6M, ULVAC, inc.) with uncertainties of a few percent. IZOTM (Idemitsu Kosan Co., Ltd.) of 100 nm was deposited on the polymer material by RF magnetron sputtering technique at ambient temperature: thus, the IZO was successfully deposited on the conventional polymer. All materials used in our studies are amorphous. The polymer material was poled electrically with 100 V/μm at 90°C which is slightly below the glass transition temperature to generate the EO effect.
It is known that IZO shows high transmittance in visible and near infra-red regions . Figure 2 shows transmittance spectra of IZO (100 nm) deposited on a fused quartz substrate, EO polymer (1.83 µm) deposited on a fused quartz substrate and ITO (7 nm) deposited on a glass substrate, indicating the high transmittance at the optical communication wavelengths (1308 nm and 1550 nm) generally used to investigate the EO performance.
A schematic of the setup for the transmission method is also shown in Fig. 1(a). A linearly polarized beam with 45° tilted polarization impinged on the EO sample at an angle θ from the glass side that was placed on a rotation stage. The transmitted beam was detected by a photo diode detector after passing through a Babinet-Soleil Compensator (BSC) that makes an additional controllable phase retardation Ω and the analyzer with the cross-polarized configuration. The transmitted beam intensity at the incident angle of 30° was measured as a function of the phase retardation Ω that is adjusted by BSC setting, and the normal optical bias curve was obtained. The settings of BSC at half-maximum points in the optical bias curve were determined, where the curve is in its most linear region for small modulation. The transmitted intensity It and modulated intensity Im under the AC voltage of 10 V at each incident angle are recorded by using a lock-in amplifier. An aperture and optical components to make a collimated narrow beam and a spatial filter for eliminating a double-reflected beam were not used in our transmission method.
3. Results and discussion
Figure 1(b) schematically shows the optical geometry and beam propagation through the sample. The thicknesses of glass substrate, ITO, EO film, and IZO are about 0.7 mm, 7 nm, 1.83 μm, and 100 nm, respectively. The simple model ignores the properties of the TCO layers (ITO and IZO) and simplifies the multilayer structure to the air-sample-air structure. The simple model in the transmission method is equivalent to the simple model [6–8,15,19–23] in the reflection method; the air-sample-metal structure, although the metal is replaced with the air in the transmission method. Here, the simple analysis in the transmission geometry without an aperture is described. We consider the first main path and second additional path shown in Fig. 1(b), as the correction for the gross multiple reflections, which are not considered in the reflection geometry with an aperture. The incident wave is assumed to be a linearly polarized plane wave, it impinges on the sample with the incident angle θ as shown in Fig. 1(b), and the polarization direction after passing through a polarizer makes a 45° angle with respect to the incident plane. In our setup, a BSC and an analyzer are located before the detector. The output intensity It is given byEq. (1), It is approximately given as
The modulated intensity Im is obtained by differentiating Eq. (2). Im is approximately given by
By assuming r33 = 3r13 for the EO polymer, δΨsp is given by
Figures 3(a) and 3(c) show the It(π/2), and Im(π/2) that was measured from 30° to 60° by a step of 0.01°, respectively. Figures 3(b) and 3(d) show the It(π/2), and Im(π/2) enlarged between 45° and 45.5° in Figs. 3(a) and 3(c), respectively. As expected from Eqs. (3) and (6), the solid black lines in Figs. 3(a) and 3(c) as well as the closed black circles in Figs. 3(b) and 3(d) show the high frequency oscillations on the incident angle superimposed on the main term. Figures 3(e) and 3(g) show the calculated It(π/2) and Im(π/2) from 30° to 60° by using appropriate parameters. The parameters used are 1.5 for the refractive index n of the sample (EO film and all the other parts), 10 V for AC voltage, 0.7 mm for the total thickness, 1308 nm for the wavelength, and 30 pm/V for r33. The solid black lines in Figs. 3(e) and 3(g) were the calculated It(π/2), and Im(π/2) including all terms in Eqs. (3) and (6). Behaviors of the experimental It(π/2), and Im(π/2) in Figs. 3(a) and 3(c) are almost reproducible by the calculation as shown in Figs. 3(e) and 3(g). Figures 3(f) and 3(h) show the calculated It(π/2) and Im(π/2) enlarged between 45° and 45.5° in Figs. 3(e) and 3(g). The frequency of the experimental It(π/2) and Im(π/2) on the incident angle as well as the relative phase relations between the experimental It(π/2) and Im(π/2) on the incident angle in Figs. 3(b) and 3(d) are almost reproducible by the calculation as shown in Figs. 3(f) and 3(h)
As far as the amplitude of oscillation is concerned, there is a difference to some extent between experiments and calculations. In the calculation the perfect parallel of the sample and the plane wave of the incident beam are assumed, while in the actual experiments, the parallel of the sample is not perfect and the Gaussian beam as the incident beam is used. Additionally, in the experiments, the overlap between the beam on the first main path and that on the second additional path is altered with the incident angle. These factors and their incident angle dependences affect the amplitude of the oscillation produced by the gross multiple reflections. Therefore, the amplitude of the oscillation in experiments is smaller than that in calculations although the behaviors are similar. As far as the phase of oscillation is concerned, there is a slight difference between experiments and calculations. The difference mainly originates from the uncertainty of the thickness of the glass substrate. As the glass substrate is relatively thick, tiny uncertainty in the thickness leads to the difference in the phase of the oscillation between calculations and experiments. Thus, the simple analysis can nearly explain the experimental results. The solid red lines in Figs. 3(e)–3(h) represent the calculated It(π/2)|main and Im(π/2)|main of the main term in Eqs. (3) and (6). As shown in Figs. 3(f) and 3(h), the values in the small incident angle regions (solid red lines) are almost the same. Therefore, we can perform a curve fitting of the data (closed black circles) in Figs. 3(b) and 3(d) by a simple trigonometric function with a constant. The obtained constants are shown as solid red lines in Figs. 3(b) and 3(d). These constants correspond to the experimental It(π/2)|main and Im(π/2)|main in the small region. The curve fitting was also performed for all the other small regions between 30° to 60°. All resultant constants are plotted as closed red circles in Figs. 3(a) and 3(c). Thus, the experimental It(π/2)|main and Im(π/2)|main as the contribution of the main path beam can easily be extracted from the data.
It(3π/2) and the absolute value of Im(3π/2) that was measured from 30° to 60° by a step of 0.01°. Behaviors of It(3π/2) and Im(3π/2) are almost the same as those of It(π/2) and Im(π/2). The same analysis with the case at Ψsp + Ω = π/2 was performed, and the experimental It(3π/2)|main and Im(3π/2)|main were extracted as shown in Fig. 4.
Using the data of It(π/2)|main, Im(π/2)|main, It(3π/2)|main, and Im(3π/2)|main, r33 is readily given by
Figure 5 shows r33 values obtained at each incident angle region. Closed red triangles and closed red squares represent the r33 values obtained by using the data of It(π/2)|main and Im(π/2)|main and of It(3π/2)|main and Im(3π/2)|main, respectively. Closed red circles represent the r33 values obtained by using both data, which analysis is commonly used in the reflection geometry for extracting only the phase modulation term from the measurement and minimizing the contribution of the multiple reflections effect in the EO polymer. In the transmission method without an aperture, we obtained nearly the same values on r33 of triangles, squares and circles at the incident angles between 30° and 60°. Thus, we demonstrated that a reliable characterization of the EO performance is possible. The transmission geometry without an aperture also affords much simpler optical alignment and much easier angular dependence measurements compared with the reflection method. After confirming the validity and reliability of the transmission method without an aperture, we do not need to measure at the wide incident angle regions. The measurement in the very small incident angle region at only π/2 provides us with the reliable r33 value. Actually, the deviation of the r33 value obtained by It(π/2)|main and Im(π/2)|main among several-times measurements between 45° and 45.5° is within 0.8%. Therefore, it is compatible with in situ r33 measurement during the poling process. The reliability and convenience in the measurements provide us with quick screening of new EO polymers and developing of new materials. The transmission method without an aperture is also highly compatible with other kinds of in situ monitoring, such as second harmonic generation.
We developed a transmission ellipsometric method without an aperture. This method provides us with much simpler optical alignment and convenience in experiments. The precise incident angle dependence measurement was performed for characterizing the EO performance using the sample with a conventional glass substrate and un-collimated beam. A simple data analysis useful for our method was presented and it can explain the data obtained. It was demonstrated that a reliable characterization of the EO performance is possible. By using the recently matured deposition technique of IZO on conventional organic materials, it is possible to prepare the EO polymer sandwiched by two transparent electrodes regularly. Thus, the transmission geometry of ellipsometric method was reformed and re-evaluated. This method is practically significant.
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