Ultrahigh electro-optic coefficient of 170pm/V and low Vπ π of
1V at 1.55μ μm in hybrid polymer/sol-gel waveguide
Y. Enami1, C.T. DeRose1, D. Mathine1, C. Loychik1, C. Greenlee1, R. A. Norwood1, R. Stegeman1, T. D. Kim2
J. Luo2, Y. Tian2, A. K-Y. Jen2, and N. Peyghambarian1
1. College of Optical Sciences, University of Arizona, Tucson, Arizona 85721
2. Department of Material Science and Engineering, University of Washington, Seattle, Washington 98195-2120
Abstract: We demonstrated the highest electro-optic (EO) coefficient with the highest poling
efficiency (~100%) in actual modulator devices. This breakthrough was accomplished with
contact poling of a crosslinkable EO polymer with an electrically conductive sol-gel cladding.
©2007 Optical Society of America
OCIS codes: (250.2080) Electro-optic polymer, (250.7360) Waveguide modulator
Polymeric EO modulators have demonstrated high speed modulation up to 110GHz  and low voltage
operation for high speed fiber communication and RF link systems. Recently studied EO polymer films have shown
EO coefficients >100pm/V at 1.31μm. However, the highest reported EO coefficient in all polymeric waveguide
modulators was less than 36pm/V at 1550nm because of low contact poling efficiency  of ~30% for the EO
polymer core layer with a relatively high conductivity passive polymer cladding layer. In our previous work, we
proposed and demonstrated high poling efficiency and higher EO coefficient [3-7] in actual modulator devices with
help of the high conductivity (at the poling temperature) of specially formulated sol-gel waveguide and cladding
layers. Here, we discuss ultra-high poling efficiency, which leads to the highest in-device EO polymer r33
coefficient of 170pm/V at 1.55μm using hybrid EO polymer/sol-gel waveguide modulators. Efficiently contact
poled cross-linkable EO polymer devices showed a low half wave voltage (Vπ ) of 1.0V for a Mach-Zehnder
intensity modulator and 2.5V for a phase modulator at 1.55μm. The in-device EO coefficients of the EO polymers
in the hybrid modulators exceeded that of LiNbO3 by a factor of 5-6. These results will enable a new generation of
modulators for ultra low voltage digital switching, reconfigurable optical interconnection and RF photonics.
EO Polymer Core
Fig. 1. Schematic of hybrid crosslinkable EO polymer/sol-gel modulator.
2. Hybrid crosslinkable EO polymer/sol-gel waveguide modulator
Hybrid EO polymer/sol-gel waveguide modulators demonstrate high poling efficiency and low coupling loss
(1dB/end face)  due to excellent mode field matching between standard single mode fiber and the sol-gel
waveguide in passive region. The sol-gel waveguide consists of a sol-gel undercladding, core, side cladding, and
overcladding as shown in Fig. 1. Each sol-gel layer was fabricated by spin-coating and wet-etching of
methacryloyloxy propyltrimethoxysilane (MAPTMS) doped with an index modifier (zirconium(IV)-n-propoxide).
The mole ratio of index modifier in MAPTMS was controlled to obtain a mode field similar to that of single mode Download full-text
fiber . Our employed EO polymer is a thermally stable crosslinkable guest-host polymer doped with a nonlinear
optical chromophore as shown in Fig. 1. The EO polymer combined the use of both crosslinking and guest host
systems for thermal stability and high chromophore doping, which has been difficult in previous guest-host systems.
The EO polymer was spin-coated on a selectively buried sol-gel waveguide in the active region and cured at 50°C
overnight. EO polymer coated on the sol-gel waveguides was contact-poled between the Au over electrode and Ti
under electrode. The poling temperature was increased from 50°C to 135°C with applied voltage to initiate
crosslinking during the poling process. The volume conductivity of our sol-gel layer was 1.0S/m at the poling
temperature of 135°C , which is higher than that of the EO polymer by nine orders of magnitude when a voltage
of 400V is applied. When a poling voltage of 200-400V was applied between over and under electrodes; we
achieved a poling efficiency of 100%. After contact-poling, the over Au electrode was removed by wet-etching, and
an Au electrode for EO modulation was sputtered on the coated buffer layer (CYTOP Asahi Glass).
Sol Gel Cladding
3. EO coefficient in hybrid modulator
The low frequency transfer function at 1kHz was monitored to measure Vπ of the phase modulator and the
Mach-Zehnder modulator. A Vπ of 2.5V was measured in the phase modulator, which corresponded to an r33 of
170pm/V at 1550nm, using an overlap integral of 80%, inter-electrode distance of 15μm and electrode length of
24mm. This result was 5 times larger than that of previously reported all-polymeric modulators at 1550nm. The EO
coefficient in the devices was consistent with values obtained from poled single EO polymer films in Teng-Man
ellipsometric measurements. The Mach-Zehnder modulator with dual modulation showed a Vπ of 1.0V at 1550nm
as shown in Fig. 3, which corresponds to an r33 of 138pm/V. We estimated an r33 of 228pm/V at 1310nm from Vπ at
1550nm using the two level model, which predicts a Vπ of ~0.6V at 1310nm. The inter-electrode distance would be
reduced to 11μm when a tapered core  is employed for this crosslinkable EO polymer, which will lead to Vπ of
0.4V at 1310nm and 0.6V at 1550nm for the Mach-Zehnder modulator. The Vπ value of these devices did not
change when they were kept in air at room temperature for more than 20 days. Thermal and optical stability in
devices is currently under examination with initial results indicating high thermal stability.
 D. Chen et al., “Demonstration of 110GHz electro-optic polymer modulation”, Appl. Phys. Lett. 70, 3335-3337 (1997).
 Y-H Kuo et al., “Enhanced thermal stability of electrooptic polymer modulators using the Diels-Alder crosslinkable polymer”, IEEE Photon.
Technol. Lett. 18, 175-177 (2006).
 Y. Enami et al., “Polarization-insensitive transition between sol-gel waveguide and electrooptic polymer and intensity modulation for all-
optical networks”, J. Lightwave Technol., 21, 2053-2060 (2003).
 Y. Enami et al., “Hybrid electro-optic polymer and selectively buried sol-gel waveguide”, Appl. Phys. Lett., 82, 490-492 (2003).
 Y. Enami et al., “Hybrid electro-optic polymer/sol-gel waveguide modulator fabricated by all-etching process”, Appl. Phys. Lett. 83, 4692-
 C. DeRose et al., “Pockels coefficient enhancement of poled electro-optic polymers with hybrid organic-inorganic sol-gel cladding layer”,
Appl. Phys. Lett., 89, 131102 (2006).
 Y. Enami et al., “Low half-wave voltage and high electro-optic polymers with hybrid polymer/sol-gel waveguide modulators”, Appl. Phys.
Lett. 89, 143506 (2006).
Fig. 2. Cross sectional view of contact
poling and EO modulation
Fig. 3. One example of low frequency transfer
function of hybrid modulator at 1550nm.