# Progressive Poling of Large Area, High r\({}_{33}\) Electro-Optic Polymer SEO100c
## 4 Poling of Electro-Optic Polymers  
Poling an EO-polymer to align the chromophores, is a critical step in creating an active EO polymer. The data sheet provided by Soluxra LLC specifies the maximum alignment for SEO 100c occurs at an electric field strength of 100 V/\(\upmu\)m across the electro optic layer which would produce an \(r_{33}\) of 140 pm/V [15]. In the test device the electrodes had 3 layers  
Figure 1: Completed polymer stack for the waveguide devices.  
between them, a bottom cladding layer 4.3 \(\upmu\)m thick of NOA 73, a 2.7 \(\upmu\)m core layer of SEO 100c, and a top cladding layer 4.3 \(\upmu\)m thick of NOA 73. The resistivities at 135 \({}^{\circ}\)C of NOA 73 and SEO 100c are known to be 3.3 \(\times\) 10\({}^{10}\)\(\Omega\)cm and 3.0 \(\times\) 10\({}^{10}\)\(\Omega\)cm, respectively [6]. Equation (1) shows how the voltage across the core layer of SEO100c was calculated.  
\[V_{core}=V_{total}\frac{\rho_{core}}{\rho_{core}+2\rho_{clad}\Big{(}\frac{d_{ clad}}{d_{core}}\Big{)}} \tag{1}\]  
where \(V_{core}\) is the voltage applied to the core, \(V_{total}\) is the voltage applied to the full stack. \(\rho_{core}\) and \(\rho_{clad}\) are the resistivities of the core and cladding layers. Finally, \(d_{core}\) and \(d_{clad}\) are the thicknesses of the core and cladding layers, 2.7 \(\upmu\)m and 4.3 \(\upmu\)m, respectively. Applying the voltage divider rule, the strength of the electric field across the EO-polymer was determined to be 21% of the total electric field strength. Although this is a low value it can be increased in future devices by utilizing thinner cladding layers, or through substitution of the cladding material to a material with a lower resistivity.  
Figure 2 shows the setup used for poling. A small oven was used to heat the device. A high voltage DC power supply had the positive lead connected to top electrode via a probe and the negative lead connected to a pico-ammeter through a HV protection circuit, then to the bottom electrode of the device. The current data from the ammeter and temperature data from a k-type thermocouple were digitally logged during the poling process.  
The device was placed in the oven, elevated to allow heating from both the top and the bottom, and connected with probes to a high voltage power supply. The oven was turned on and the device began to be heated to reduce thermal shock. When the temperature reached 50 \({}^{\circ}\)C. A DC voltage was applied across the device and heating continued until the oven reached 135 \({}^{\circ}\)C. The hold temperature of 135 \({}^{\circ}\)C ensures that the glass transition temperature of 135 \({}^{\circ}\)C is not exceeded while maximizing r\({}_{33}\). At the glass transition temperature chromophores can move freely to orientate themselves in the direction of the applied electric field. Exceeding the glass transition temperature will result in dielectric breakdown, causing the device to electrically short, destroying the device. As soon as 135 \({}^{\circ}\)C was reached the heating element of the oven was switched off and the device was allowed to cool. Once the oven had cooled to 100 \({}^{\circ}\)C the oven was  
Figure 2: The poling setup, with a view of the probe configuration inside the oven.  
opened to facilitate rapid cooling. The voltage was turned off when the device was below 50 \({}^{\circ}\)C and then allowed to cool to room temperature. Leaving the voltage on while the device is cooling prevents the relaxation of chromophore alignment while the device is at an elevated temperature. Figure 3 shows the temperature and current data collected from poling with increasing poling voltage. The poling current is not correlated with the electro-optic coefficient, but it does indicate that charge moves more freely at elevated temperatures. Seeing a temperature-dependent current spike as the polymer approaches the glass transition temperature is a positive indication that the device is heating during the poling process as the resistivity of the polymer layers decrease with temperature.