A hybrid electro-optic polymer and TiO2 double-slot waveguide modulator

An electro-optic (EO) modulator using a TiO2 slot hybrid waveguide has been designed and fabricated. Optical mode calculations revealed that the mode was primarily confined within the slots when using a double-slot configuration, thus achieving a high EO activity experimentally. The TiO2 slots also acted as an important barrier to induce an enhanced DC field during the poling of the EO polymer and the driving of the EO modulator. The hybrid phase modulator exhibited a driving voltage (Vπ) of 1.6 V at 1550 nm, which can be further reduced to 0.8 V in a 1 cm-long push-pull Mach–Zehnder interferometer (MZI) structure. The modulator demonstrated a low propagation loss of 5 dB/cm and a relatively high end-fire coupling efficiency.

connections. Direct coupling from silica fiber to the SOH waveguide facet is difficult without the use of special couplers. Both inverse tapered couplers and gratings have been proposed to improve the coupling efficiency 6-12 . In order to overcome these obstacles there have been innovations targeted at waveguide design, fabrication, and materials development. Recently alternative high refractive index materials have been incorporated into photonic devices [13][14][15] . Titanium dioxide (TiO 2 ), the material of choice in this study, has been shown to demonstrate a high transparency at the telecommunications band, good nonlinear optical activity, and can be used to fabricate a nanowire waveguide [14][15][16] . Its good transparency in the amorphous form ensures a low propagation loss of ,1 dB/cm at 1550 nm. High-quality deposition of the thin film can be performed by using a conventional sputtering technique under controlled gas pressure and low temperatures 17 . Such versatility allows for the integration of photonic structures on various substrates and the blending with organic and polymer materials. The refractive index of TiO 2 is lower than that of Si, but enough higher than that of EO polymer. Such a moderate index can be applied for the fabrication of a nanowire structure. Indeed, the loss improvement is suggested by using a thin TiO 2 layer on the SOH device 18 . The electrical resistivity and dielectric constant are essential factors in the hybrid EO polymer. The electrical resistivity of TiO 2 is several orders lower than that of the EO polymer, so that there is no electric barrier during the poling process 1,17 . Furthermore, the dielectric constant of TiO 2 (.30) is higher than that of the EO polymer (,2). Such a large contrast might induce the enhanced DC field in the slot waveguide modulator.
In this work, we have fabricated a hybrid EO polymer and TiO 2 slot waveguide, and investigated its prospective properties for the modulator application. The dimensions of the TiO 2 slot were designed to enable the best optical mode confinement by using the beam propagation method (BPM) calculation. In addition, the distribution of the DC field across the slot is characterized by using the electromagnetic field calculation. Both DC field and optical mode calculations enable quantitative analysis of the overlap integral numerically. By the use of these calculations we successfully fabricated a double-slot modulator with the optimal EO properties. We measured a phase modulation with a V p of 1.6 V at 1550 nm for the 1.5 cm-long electrode, which corresponds to an in-device r 33 of 140 pm/V. The driving voltage can be converted to V p 5 1.1 V for the TE mode, and therefore reduced by half in the MZI application. The insertion loss of the fabricated double-slot modulator was 25 dB and the waveguide propagation loss was 5 dB/cm. Since the effective refractive index of the EO polymer/TiO 2 slot is ,1.69, direct coupling of the light from a silica fiber to the waveguide facet is possible without any special coupler. This is a significant practical improvement towards a direct fiber-to-device connection, while the loss can be further reduced by the attachment of taper couplers.

Results
Modulator design and simulation. Figure 1  thicknesses of the EO polymer and Au electrode are 1.0 mm and 0.6 mm, respectively. Based on this schematic the TE 0 mode distribution can be calculated using 3D BPM in Rsoft. In order to evaluate the best condition for the mode confinement, calculations were made for the slots by changing their width (d) between 100 and 500 nm. Figure 1(b) shows the mode confinement in the slots by setting d as 150 nm. In calculation, we used the measured refractive indices of 2.3 and 1.6 for TiO 2 and EO polymer at 1550 nm, respectively. The calculation indicates that the cross-section of the optical mode is 2.0 mm and 1.0 mm in horizontal and vertical directions, respectively. The simulated mode size is much larger than the cross-section of the TiO 2 strip-lines, whereas a large part of the optical mode extends into the EO polymer cladding (51%). In particular, it can be seen that an intense optical field is concentrated within the slots. The mode confinement was calculated as 31% in the slots, which exploits the increased optical interaction between the optical mode and the EO polymer. Combined with an effective refractive index of 1.69 in the EO polymer/TiO 2 slot, end-fire coupling using a lensed silica fiber is possible with a sufficient coupling efficiency.
Here, it is worth to point out the utility of this double-slot modulator over the common slot structure. For comparison, the TiO 2 single-slot is given in Fig. 1(c), and its TE 0 mode distribution is shown in Fig. 1(d). The dimension of the TiO 2 line is similar to that used in the double-slot, while d is set slightly larger for the optimization. The calculation indicates that the confinement factor of the optical mode in the slot is 25%, which is 6% smaller than that observed in the double-slot case. Calculation suggests that the double-slots offer a higher light interaction with the EO polymer than that in the single-slot structure.
The electrode geometry is another important parameter to induce the noncentrosymmetric orientation of the chromophore by applying the DC electric field, and to realize the modulator with a high indevice r 33 . The optical mode is parallel to the TE polarization, so the coplanar electrodes are placed across the slots with the certain distance. Since the electrical resistivity of the TiO 2 (10 5 Vm) is several orders lower than that of the EO polymer (10 627 Vm), a voltage applied on the electrode can almost entirely drop to the EO polymer. Therefore, the narrow-gap electrodes are beneficial to perform effective poling and to achieve a low-voltage modulator driving, however too narrow electrodes may contribute to the optical loss due to Au absorption. In order to obtain a modulator with the best performance, i.e. low V p and low loss, the optical mode should be concentrated around the core as much as possible, as the result the inter-electrode gap can be set for the minimum.
In order to clarify the double-slots geometry and its suitable interelectrode gap, the confinement factors of the optical mode are calculated for the slots with different d. The optical losses are also obtained under the same slots' condition, in which the slot-electrode distance is set as 2.0 mm. In Fig. 2, there is a clear trade-off between the confinement factor and the optical loss between d 5 100 and 500 nm. The highest confinement factor of 33% is obtained for d 5 100 nm. In this case, however, the optical loss is rather large (0.7 dB/cm). The optical loss steeply diminishes as the slot-gap is slightly increased, so that 0.25 dB/cm is obtained for d 5 150 nm, while the confinement factor remains high (31%). For d . 250 nm, the change in the optical loss reaches a plateau and finally decreases to the smallest level for d 5 500 nm, the confinement factor decreases linearly in wider slots. Therefore, we chose d 5 150 nm for the fabrication, expecting a small optical loss and a high enough confinement of the optical mode in the double-slots.
In the hybrid EO polymer and TiO 2 slot waveguide, discontinuous electric field distributions can be expected at the boundary between the TiO 2 and the EO polymer. Therefore, we clarified the DC field across the electrodes and considered the achievable r 33 as a result of the poling process. The DC field in the double-slots modulator was simulated using COMSOL 4.4, and its relative field intensity across  the slots is shown in Fig. 3(a). In the calculation, the reported dielectric constants of 3.0 and 60 are used for the EO polymer and the TiO 2 , respectively 3,19 . For comparison, the calculation for a single-slot modulator is given in Fig. 3(b). In both slot structures, the highcontrast field discontinuity can be seen at the boundary between the TiO 2 and the EO polymer. Beside the TiO 2 lines, a relatively high DC field appears in the EO polymer. In particular, the slots cause a significant increase in the DC field intensity, in which ,35% enhancement can be estimated relative to other EO polymer regions. By comparing Fig. 3 with Fig. 1(b) and (d), one can see that the DC and optical fields in the slots overlap well. Such an emphasis of the DC field is beneficial to both the poling process and the low-voltage driving of the modulator.
Modulator Test. Figure 4 (b) and (c) shows the top view and a crosssection of the TiO 2 double-slots waveguide taken by SEM. The EO polymer, structure shown in Fig. 4 (a), was spin-coated on the waveguide to form a 1.0 mm-thick film. To induce an EO effect, the EO polymer in the modulator was poled at 140uC by applying an electric field of 80 V/mm to the electrodes. The fiber-to fiber insertion loss at 1550 nm for the TE polarization was 25 dB as the sum of the propagation loss and the coupling loss. The propagation loss was measured as ,5 dB/cm by using the cut-back method. We found little change in the loss before and after poling. The propagation loss mainly originates from the EO polymer absorption (3 dB/cm) and the scattering at etched TiO 2 sidewalls which may be reduced by improving the etching process. Since the spot diameter (4 mm) from the lensed fiber is a little larger than the calculated mode size (2.0 mm 3 1.0 mm), the insertion loss can be reduced by the introduction of taper couplers.
The fabricated phase modulator was tested as an intensity modulator through a cross-polarization setup. The laser light at 1550 nm with a 145u linear polarization was coupled into the modulator through a polarization-maintaining lensed fiber. Output light from the modulator was passed a 245u polarizer, and then collected by a photo-detector. The light intensity was measured with an applied triangular voltage waveform at a frequency of 1 kHz as shown in Fig. 5(a). From a clear modulation output function of the modulator, a V p of 1.6 V was measured. Considering r 33 5 3r 13 in an EO polymer 20,21 , the true V p in the TE mode can be reduced to 1.1 V by a factor of 2/3. For the reference experiment, the TiO 2 single-slot waveguide was fabricated and the modulator was prepared after poling the EO polymer in the same manner. However, the measured V p in the TE mode was 5 V. This result shows clear evidence that the EO activity is significantly enhanced in the double-slot configuration than the single-slot. We can convert the TE mode double-slot waveguide modulator into the push-pull MZI modulator, which allows for a V p 5 0.8 V in a 1 cm-long electrode configuration. Such a predicted figure-ofmerit is at a competitive level with the state-of-the-art SOH photonic crystal slot modulators 7,11 . Here, it is worth noting that using the TiO 2 double-slot EO modulator enables the operation with a large optical bandwidth, which is generally restricted in the photonic crystal devices due to their narrow stop band property [10][11][12] .
By using the measured V p , the electrode length, and the overlap factor of ,45% between the optical field and the applied electrical field, an effective in-device r 33 was estimated to be ,140 pm/V. Obtained in-device r 33 is 55% higher than the in-film r 33 measured by using the Teng-Man reflection method 20 . This overall improvement in the in-device r 33 can be attributed to two important functions of the TiO 2 slots. The first is the DC field enhancement around the TiO 2 slots as shown in Fig. 3(a). The second is the charge injection blocking effect by the TiO 2 strip-lines, which limits the excess charge injection from the electrode into the EO polymer and reduces the current leakage 22 . The EO polymer can therefore be highly pooled to provide the greatest EO activity.
In order to evaluate the modulator response to the RF signal, a sinusoidal voltage was applied to one electrode and the other used as the ground. The output optical signal was measured by a photodetector connected to an oscilloscope. Figure 5(b) is the obtained modulator response to 36 MHz by fine tuning the device with an applied bias voltage of 0.9 V. In this lumped modulator, the electrode structure is not specially designed and fabricated for the high-RF purpose. The signal response at higher speeds became imprecise due to a large signal intensity loss. Utilizing the traveling-wave-electrodes should enable a further increase in the operating speed of up to multi-GHz 23 .
In summary, we have successfully designed and demonstrated an EO polymer cladded TiO 2 double-slot waveguide phase modulator. In the designed modulator, ,31% of the TE mode can be confined within the slots and all the poling voltage can be applied to the EO polymer. Because of the large dielectric contrast between the TiO 2 and the EO polymer, an enhanced DC field can arise in the slots. In addition, the TiO 2 can also block the excessive charge injection and reduce the current leakage. As a result, an effective poling of the EO polymer and a low V p of 1.6 V were realized. By utilizing a travelingwave-electrode in the future, the modulator can be applied in broadband modulator applications. Based on the EO polymer filled TiO 2 double-slot waveguide, other useful optical structures for EO mod-  ulators, such as ring, MZI, and photonic crystal, can also be readily realized.

Methods
The hybrid EO polymer and TiO 2 double-slot modulator was fabricated as follows. Firstly a 350 nm thick TiO 2 section was deposited onto a SiO 2 (2 mm)/Si (500 mm) substrate using RF sputtering of a TiO 2 target under controlled Ar/O 2 gas pressure. The temperature of the substrate was kept below 85uC during the deposition in order to obtain the amorphous form of the TiO 2 film. The slot configurations were fabricated using electron-beam lithography and reactive ion etching with CHF 3 gas. Subsequently, 1.5 cm-long gold electrodes were patterned by lift-off process as the designed coplanar structure. Finally, the EO polymer (structure in Fig. 4(a)) was spincoated, and then baked at 120uC for 48 hours to form a 1 mm thick film. The EO polymer was prepared according to our previous methods 24 . The polymer has a chromophore loading density of approximately 40 wt%, a glass transition temperature of 135uC, and a thermal decomposition temperature of 296uC.