1. Introduction

The electro-optic (EO) polymer has considerable advantages because of its wide bandwidth and low driving voltage, which result from its low dielectric dispersion and high EO coefficient. Electro-optic polymer modulators have been demonstrated for the widest 3 dB bandwidths of up to 113 GHz [1] and the lowest half-wave voltage (V_π) values of 0.65-1.0 V with a relatively low insertion loss of 20 dB at transverse magnetic (TM) modes [2,3]. The lowest half-wave voltage of the electrode length (L_e) product for the hybrid EO polymer/sol-gel silica waveguide modulator was 1.56 V·cm (electrode length of 2.4 cm) using the cross-linkable EO polymer AJ309 with a total optical insertion loss of 18-20 dB [3], including fiber-butt coupling loss with the standard single mode fiber SMF-28. Commercialized complementary metal oxide semiconductor (CMOS)-compatible applications require not only a low V_πL_e but also a low V_π with a wider bandwidth and a moderate range of optical insertion losses. Therefore, the higher in-device EO coefficients in the polymer modulators are particularly advantageous for this application.

The dielectric dispersion (refractive index dispersion) of the EO polymer is less than one-tenth of that of semiconductors and LiNbO_3, which enabled the widest bandwidth for the optical modulator. The highest in-device EO coefficient of 142-170 pm/V at a wavelength of 1550 nm has been realized in the hybrid modulators, which was five-times higher than that in the LiNbO₃ modulators. Si modulators have been extensively studied for on-chip optical interconnections in CMOS circuits. Their performance still requires additional improvement of the half-wave voltage, bandwidth and optical insertion loss, although many reports of Si modulators have recently shown a V_πL of 2-3 V·cm with a 3 dB bandwidth of <15 GHz [4]. Because the optical propagation loss of the Si modulator is high and typically in the range of 19 dB/cm in the active region, increasing the active region to be greater than a few mm to obtain a lower V_π is difficult. Recently, a low-loss Si modulator based on a pipin diode was demonstrated with a V_πL of 3.5 V·cm (active length of 4.7 mm) and an optical insertion loss of 6 dB [5].

EO polymer modulators are better candidates for future on-chip optical interconnections and analog radio-over-fiber communications at a wavelength of 1.55 μm because of the potentially higher EO coefficient of >200 pm/V at 1550 nm, which reduces V_π to less than 0.1 V. The refractive index of EO polymers is typically between 1.6 and 1.7, which is smaller than that of Si (index of 3.5), and it was difficult to perform higher mode confinement in the EO polymer core with standard cladding materials such as sol-gel silica (index of 1.50) and other passive polymers. The high index contrast between the core and cladding, which is the higher mode confinement, results in a lower V_π because of the higher mode overlap integral (Γ) and further reduction of the electrode distance. To reduce the electrode distance, the optical confinement in the EO polymer should be increased while avoiding additional losses from the electrodes for these applications. Si slot waveguides enable higher mode confinement in relatively lower index materials, such as EO polymers when the EO polymer is filled in a high-index Si slot waveguide. EO polymers have been used with Si-based waveguides as cladding materials on Si₃N₄ [6] cores and between Si slot waveguides [7]. The Si/EO polymer slot waveguide modulator has a narrow 3 dB bandwidth of <500 MHz and a high propagation loss of 35 dB/cm [8] because a highly doped Si slot reduces performance, even though doped Si is beneficial in reducing the electrode distance (lower V_π of 0.69 V) [7, 9].

We previously demonstrated the first EO polymer/TiO₂ multilayer slot waveguide modulator for reducing V_π , improving the bandwidth and optical propagation loss [10]. The 300-nm-thick EO polymer layer was sandwiched between double high-index TiO₂ slot layers (TiO₂/EO polymer/TiO₂) on sol-gel silica cladding. The modulator produces a V_π L of 3.25 V, an in-device EO coefficient of 60 pm/V, and a propagation loss of 12-13 dB/cm at a wavelength of 1550 nm. The multilayer slot waveguide modulators consist of all-dielectric materials, which would be applied for a bandwidth of >100 GHz. Mesoporous sol-gel (MPSG) silica has been used for various applications, such as low-k materials [11], using the standard template method. To the best of our knowledge, mesoporous sol-gel silica has not been applied for low-index cladding materials in optical waveguides. The refractive index of MPSG silica produced using a template method ranges from 1.2 to 1.3. Therefore, when the materials are employed as a waveguide cladding, the high contrast between the indices of the core and the bottom cladding layers results in a higher mode confinement in the EO polymers.

Here, we report a hybrid TiO₂/EO polymer multilayer waveguide modulator on MPSG silica cladding. The low-index cladding enables higher mode confinement in the EO polymer for the EO polymer/TiO₂ multilayer waveguides and higher poling efficiency, which lowers V_{π .} The low-index cladding in the bottom cladding increased the mode confinement in the hybrid TiO₂/EO polymer multilayer waveguide modulators, which was shown from the V_π measurements and lower optical insertion loss in TiO₂/EO polymer multilayer slot waveguide modulators.

2. Design and fabrication of modulators on mesoporous sol-gel silica

The 100-nm-thick TiO₂ was placed on the 400-nm-thick EO polymer, which serves as a high-index core without a waveguiding mode in the TiO₂ layer. The devices with and without MPSG silica cladding were shown in Figs. 1(a) and 1(d), respectively. Four micrometer-thick sol-gel silica was placed on a 0.55-μm-thick MPSG silica cladding. When the thickness of the MPSG silica was increased to greater than 0.55 μm, cracking in the MPSG silica cladding frequently occurred during calcining at 400 °C. When light was coupled and guided through the core in the modulators, we observed that the 0.55-μm-thick MPSG silica cladding was too thin to prevent optical absorption from the lower electrode. Therefore, we combined the 0.55-μm-thick MPSG silica cladding with the standard sol-gel silica cladding used previously.

 

Fig. 1 Schematic cross section of the TiO₂/EO polymer multilayer waveguide modulator on low-index mesoporous sol-gel silica cladding. (a) Cross-sectional view of the modulator. (b) Cross-sectional view of the calculated waveguiding mode at the active region using the 3D FDTD method. (c) The mode shape in the vertical (Y) direction at the center of the waveguide (X = 0). Schematic cross section of the modulator without the mesoporous sol-gel silica cladding. (d) Cross-sectional view of the modulator. (e) Cross-sectional view of the calculated waveguiding mode. (f) The mode shape in the vertical (Y) direction at the center of the waveguide (X = 0). The blue dotted lines show the boundary between the sol-gel silica and the mesoporous sol-gel silica. The red dotted lines show the boundary between the TiO₂ and the CYTOP cladding, the upper green dashed line shows the boundary between the EO polymer and the TiO₂, and the lower green dashed line shows the boundary between the EO polymer and the sol-gel silica.

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The waveguiding mode was calculated using a three-dimensional finite difference time domain (3D FDTD) method. The calculated mode in the cross-section for the device with and without MPSG silica cladding is shown in Figs. 1(b), and 1(e), respectively, and the optical waveguiding power in the vertical (Y) direction at the center of the waveguide (X = 0) for the devices with and without MPSG silica cladding is shown in Figs. 1(c) and 1(f), respectively. In the calculation in Fig. 1(b), thickness of Cytop^®, TiO₂, EO polymer, sol-gel silica, and MPSG silica was 1.2, 0.1, 0.4, 1.5, and 0.55 μm, respectively. In the calculation in Fig. 1(e), thickness of sol-gel silica cladding was 6 μm. Refractive index of Cytop^®, TiO₂, EO polymer, sol-gel silica, and MPSG silica was 1.328, 2.567, 1.621, 1.487, and 1.32, respectively. The width of the core was 4 μm. In Figs. 1(c) and 1(f), the red dotted lines indicate the boundary between the TiO₂ and the Cytop^® upper cladding, and the green dashed lines indicate the boundary between the EO polymer and the TiO₂ and that between the EO polymer and the sol-gel silica. The blue dotted lines indicated the boundary between the sol-gel silica and MPSG silica claddings. The mode confinement factor (mode overlap integral Γ) was calculated to be 49% and 44% for devices with and without MPSG silica cladding, respectively. We confirmed that the mode tail in the vertical direction did not reach the lower electrode when the MPSG silica cladding was employed, as shown in Fig. 1(c).

For the dual-driven Mach-Zehnder (MZ) waveguide modulator, we fabricated separated 100-nm-thick Au/10-nm-thick Ti lower electrodes on a silica (6 μm)-on-silicon substrate, as shown in Fig. 1(a). A standard organosilicate sol-gel solution was prepared for the 1-4-μm-thick lower-cladding layers on the MPSG silica cladding and for the 4-μm-thick side-cladding. The sol-gel silica solution consists of methacryloyloxy propyltrimethoxysilane (MAPTMS) and an index modifier (zirconium(IV)-n-propoxide) with a molar ratio of 95(MAPTMS)/5 mol%. HCl (0.1 N) was used as a catalyst to hydrolyze the sol-gel silica. Irgacure 184 (Ciba) was mixed with the side-cladding solution as the photoinitiator for subsequent wet etching in isopropanol.

An MPSG silica film solution was prepared following the standard template method [11]. The surfactant was dissolved in 20 wt% ethanol, which serves as a template for the mesoporous structure when the coated sol-gel solution is calcined at 400 °C. Tetraethylorthosilicate (TEOS) was diluted with ethanol at an equal ratio and mixed with 0.05 N HCl for hydrolysis. After the solution was stirred for a few hours at 60 °C, the diluted and hydrolyzed TEOS solution was mixed with the surfactant-ethanol solution. After the TEOS solution with template was stirred overnight at room temperature, the solution was coated on the substrate as the cladding layer. The coated film was calcined at 400 °C for three hours to remove the surfactant template and to fabricate the mesoporous structure in the sol-gel cladding. After calcining the MPSG silica, the layer was confined in a box that was filled with HMDS (hexamethyldisilazane) vapor for 20–30 minutes. This process removed water from the MPSG silica and strengthened the porous structure. The refractive index of the mesoporous, 0.55-μm-thick sol-gel film was measured to be 1.21-1.26 at a wavelength of 800 nm using an ellipsometric method. A few minutes before the standard 1-4-μm-thick sol-gel silica cladding was coated on the MPSG silica, collimated ultraviolet light with an intensity of 12 mW/cm² at i-line (center wavelength of 365 nm) in a mask aligner was radiated on the MPSG silica to increase the electrical conductivity of the MPSG silica cladding. In a previous report [12], it was shown that electrical conductivity of MPSG silica film is increased with the increased amount of interior SiOH or H₂O. In a single hydrophilic MPSG silica film, Fourier transform infrared (FTIR) spectrum showed increased amount of Si-OH at a wave number of 3000 – 3750 cm⁻¹ in the UV-irradiated silica film. HDMS vapor changed amount of molecular of Si-OH to Si(CH₃)₃ in the MPSG silica film. The FTIR spectrum showed that the UV-irradiation returned the molecular of Si(CH₃)₃ to Si-OH in the film. We decided UV-irradiation time for the sol-gel silica in the modulator from these measurements. After the side-cladding layer was spin-coated, ultraviolet light was radiated through a photo mask set in a mask aligner. UV-exposed regions accelerate the hydrolysis of the silica and cross-linked silica network, and the irradiated parts became insoluble in isopropanol, which was used as the etchant during the wet etching process.

For the first trial, we fabricated EO polymer/TiO₂ multilayer slot waveguide modulators on the 1.8-μm-thick sol-gel silica/0.55-μm-thick MPSG silica cladding. However, because the poling efficiency was insufficient on the low conductivity TiO₂ on the cladding structure, we simply removed the lower TiO₂ core layer from the double TiO₂ slot waveguide (TiO₂/EO polymer/TiO₂) to optimize the poling efficiency and to obtain a lower V_π. The side cladding was wet etched to create a 4-μm-wide window for the EO polymer/TiO₂ multilayer core. After the sol-gel silica waveguide was hard calcined at 150 °C for one hour, UV light was radiated again on both cladding layers. A low-index guest-host EO polymer, SEO125 (35 wt% chromophore doped in amorphous polycarbonate, index of 1.621 at 1550 nm), was spin-coated to form a 400-nm-thick layer on the cladding and calcined overnight at 80°C in a vacuum oven. For the high-index core layer, a 100-nm-thick TiO₂ layer, which does not have a waveguiding mode in the TiO₂ core layer, was sputtered and exhibited low optical intensity in the TiO₂ region, as shown in Fig. 1(c). This waveguiding mechanism in the thin (no mode in the 100-nm-thick layer) and high-index TiO₂/low-index EO polymer waveguiding is the same as our previously demonstrated TiO₂/EO polymer/TiO₂ multilayer slot waveguide modulator that consists of an EO polymer sandwiched between double TiO₂ layers (TiO₂/EO polymer/TiO₂) on sol-gel silica cladding [10]. In addition, we fabricated the TiO₂/EO polymer waveguide structure on a standard 4-μm-thick sol-gel silica cladding without MPSG silica cladding as a benchmark for comparing V_π.

The multilayer structure of the 400-nm-thick EO polymer and the 100-nm-thick TiO₂ core was laterally confined by the side cladding to obtain higher mode confinement. The poling Au electrode was sputtered directly onto the EO polymer, and a poling voltage of 300-400 V was periodically applied to the 400-nm-thick EO polymer without the TiO₂ layer on the UV-irradiated silica claddings. When the silica claddings were UV-irradiated for 30 s, the poling current density was almost ten times greater than that without UV irradiation, as shown in Fig. 2. After poling the EO polymer and removing the poling electrode, 1.2-μm-thick Cytop^® (Asahi Glass) was coated as a buffer layer for the consecutive deposition of the Au upper electrode. The refractive indices of the sol-gel cladding, Cytop^®, and the sputtered TiO₂ at 1550 nm were 1.487, 1.328, and 2.567, respectively.

 

Fig. 2 Poling current density when a poling voltage of up to 350 V at a poling temperature of 150 °C was applied to the TiO₂/EO polymer multilayer waveguide modulators on the MPSG silica cladding. Red circles indicate the poling current density with UV-irradiated MPSG silica cladding. UV irradiation at the center wavelength of 365 nm was used for each standard sol-gel silica/MPSG silica cladding layer at an intensity of 12 mW/cm² for 30 s. The blue triangles indicate the poling current density without UV irradiation at the MPSG silica cladding.

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3. V_π measurement for the EO modulator

We measured V_π for the TiO₂/EO polymer multilayer waveguide modulator on both the 4-μm-thick sol-gel silica/550-nm-thick MPSG silica cladding and on the standard 4-μm-thick sol-gel silica cladding. The coupled light at a wavelength of 1550 nm from the single-mode and polarization-maintaining fiber directly excited the transverse magnetic (TM) mode in the modulators after the polarization direction was controlled. The output light from the modulators was collected by a microscope objective lens and focused onto a detector. A triangular-shaped voltage signal was applied between the electrodes in the modulator at a frequency of 1 kHz. Simultaneously, the optical signal from the detector was monitored on an oscilloscope along with the voltage signal. From the transfer function between the voltage signal and the optical signal, V_π was measured for both modulators at a wavelength of 1550 nm, as shown in Fig. 3. V_π was 6.0 V for the electrode length (L_e) of 5 mm (V_π L_e = 3.0 Vcm) in the dual-driven MZ modulators with MPSG silica cladding. We also measured V_π to be 10.3 V (d = 5.7 μm, L_e = 5 mm) for the modulator without MPSG cladding as the benchmark. We successfully demonstrated a lower V_π for the modulator on the MPSG silica cladding, resulting from a higher poling efficiency, compared to that (V_π = 10.3 V for 5-mm long electrode at 1550 nm) of TiO₂/EO polymer waveguide modulators on the standard sol-gel silica cladding without the MPSG silica cladding. The in-device r₃₃ for the modulator with MPSG silica cladding was the highest for the low-index guest-host EO polymer SEO125, which was 75 pm/V at 1550 nm from the calculated Γ of 49% in the modulators. This value was higher than the 60 pm/V in our previous TiO₂/EO polymer/TiO₂ multilayer slot waveguide modulator using the same EO polymer SEO125 [10]. This structure and process also exhibited a better poling efficiency for SEO125 compared to a singly poled EO film on ITO. Because the refractive index of low-index EO polymers such as SEO125 usually have lower EO coefficients (<80 pm/V), we are attempting to modify the slot waveguide structure to employ the high-index guest-host EO polymer SEO100 (refractive index = 1.705 at 1550 nm wavelength), which has an in-device r₃₃ of 160 pm/V according to our previous report [13]. We obtained the optical insertion loss of 24 dB for the 14-mm-long MZ modulator. The propagation loss was in the range of 13 dB/cm, and the coupling loss was 6 dB/facet for the butt coupling between the SM fiber and the modulators.

 

Fig. 3 Half wave voltage (V_π) measured for the hybrid TiO₂/EO polymer multilayer waveguide modulators under dual-drive operation at 1 kHz. Top: Triangular voltage waveform applied for the modulator. Bottom: Optical output waveform from the modulator. V_π = 6.0 V (d = 6.25 μm, L_e = 5 mm) at a wavelength of 1550 nm.

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4. Conclusion

We successfully obtained an in-device EO coefficient of 75 pm/V for the low-index EO polymer SEO125 and demonstrated a lower V_π of 6.0 V in a TiO₂/EO polymer multilayer waveguide modulator on low-index MPSG silica cladding. The in-device r₃₃ exhibited the highest poling efficiency of the EO polymer when the EO polymer was poled without the TiO₂ layer between the EO polymer and the UV-irradiated silica cladding layers. The modulators also consist of all-dielectric materials, which is useful in both telecommunications and as CMOS-compatible high-speed EO modulators.