Proceedings of International Forum for Green Asia 2019
5th International Exchange and Innovation Conference on Engineering & Sciences (IEICES 2019)

GA21 / P33
Thermal stability control of electro-optic (EO) polymer based norbornene for hybrid
EO modulator
Alisa Bannaron1, Andrew Mark Spring2, Shiyoshi Yokoyama2
1
IGSES, Kyushu University,
2
IMCE, Kyushu University
Email: s_yokoyama@cm.kyushu-u-ac.jp
Abstract: Electro-optic (EO) polymers have been considered as promising materials using in several EO applications
including telecommunication. This is due to the urgent demand of EO modulators providing both high EO coefficient aswell as high thermal stability. In this work, derivatives of norbornene were polymerised into EO polymer consisting ofboth EO active and thermal control units. These EO polymers based norbornene were conveniently prepared under mild
condition with less procedures that was suitable for large scale production. Moreover, they showed several physicaladvantages over traditional PMMA, such as higher thermal stability as well as well-control molecular weight. The EO
polymers were then fabricated into hybrid EO modulator with silicon and were poled at evaluated temperature. The main
characteristic of EO modulator, half-wave voltage (Vp ), will be further discussed.
Keywords: EO polymer, norbornene, norbornene-dicarboximide, optoelectronics, EO modulator.
groups which are cyclohexyl (R1) and hexyl (R2). Phenyl
vinyl thiophene or FTC chromophore (highlighted ingreen) was originally prepared by our research group,exhibiting high hyperpolarizability and thermal stability[3].

1. INTRODUCTION
Electro-optic (EO) polymers have been extensively
considered as promising materials in wide range ofoptoelectronics
applications
includingtelecommunication. This is because organic EO
polymers have shown several advantages over traditional
inorganic lithium niobate, such as adjustable EO
coefficient as well as compatibility with other opticalelements. Since then, development of EO polymers,
containing EO chromophore, has been intensively
investigated to meet Telecordia standard tests forcommercial optical materials.

ONRO

OOO

NO

R

CHCl3, RT
24 hours

NnOOO

NnON

Grubbs 3rd generation
ONOOO

Variety of poly(methyl metacrylate) (PMMA)s has been
one of the most widely used polymers using as a host in
EO polymers. However, synthesis of PMMAs has to beperformed using radical polymerisation using radicalinitiatorwhich
decomposeelectron-rich
EO
chromophore. Therefore, EO chromophore is generally
connected to PMMA backbone after polymerisation.
This two-step synthesis additionally requires dialysis asa removal of unreacted chromophore [2].

SS

CN
CN
F3C
CN

F3C
CNO
CNO
CNFig. 1. Polymerisation of norbornene derivatives viaROMP.
The evaluation of thermal stability was investigated not
only by pendant R, but also length of hydrocarbon chain
linking between norbornene-dicarboximide (NDI)
backbone and FTC chromophore. Loading density of theactive chromophore was varied from 30 wt% to 60 wt%.

Thus, in this work, we have purposed derivatives ofnorbornene as main polymer backbone due to its ease ofsynthesis, high glass transition temperature (Tg), andexcellent optical transparency which are critical needs for
EO polymer-based modulators.

2.2 Fabrication of EO polymer-based modulator
Fabrication process of EO modulator was performed onsilicon wafer with Al and Au as bottom and top
electrodes, respectively. The cladding layers to prevent
polymer breakdown under evaluated temperature is solgel. After that, the complete modulator was poled underhigh temperature close to Tg and strong electric field at400V. This was to orient the alignment of EO
chromophore to be in non-centrometric symmetry
resulting in EO effect under applied electric field. The
half-wave voltage (Vp), the important characteristic offabricated EO modulator, was then measured using input
polarised light at 1550 nm. The output light was thencollected at the other fiber and channeled to aphotodetector.

2. EXPERIMENTAL
Experimental consists of polymerisation of several
norbornene derivatives and their thermos-physicalproperties, as well as the evaluation designs for thermalstability control. In addition, the preliminary experimentof EO polymer-based modulator was also studied.
2.1 Polymerisation of norbornene derivatives andevaluation of thermal stability
Fig. 1 showed synthesis of norbornene-based EO
polymers via ring opening metathesis polymerisation(ROMP) using Grubbs 3rd generation catalyst (G3) as an
initiator under inert atmosphere. R refers to pendant- 57 -3. RESULTS ANS DISCUSSION
3.1 Thermos-physical properties of EO polymers andthermal stability control
Fig. 1 showed physical properties of series of EO
polymers. Actual loading density was ranging from 20 to
57 wt% as the amount of chromophore-NDI monomer
increased. All polydispersity (PDI) were under 2.00
indicating well-control molecular weight of polymers
which will be significant issue in device fabrication.
Nevertheless, glass transition temperatures needed to belower than decomposition temperature of FTC
chromophore (210 °C) to prevent EO breakdown ofpolymers during poling process. Thus, connecting carbon
chain connecting between NDI backbone and FTC
chromophore were extended to 4 and 6, in order toincrease mobility of polymers. However, with exact
polymerisation condition with 3a, 3e (n=4) and 3f (n=6)
did not show significant change in thermal stability.

Poling temperature was varied from 140, 150, 160 and
170°C to examine the most applicable poling efficiency.
Unfortunately, the devices cracked as poling at 160 and170 °C. This is probably due to higher poling temperaturethan that of glass transition temperature of polymer.
Poled modulator at 140 °C showed Vp at 8.80 V (Fig. 2)
which is relatively high confirming that poling efficiency
was inadequate to align chromophore molecules to be innoncentrosymmetry. However, Vp of poled device at 150°C will be measured and further to be discussed.

ElectricalOptical

Table 1. Physical properties of polymers 3a-d (R1, n=2)
and 3e-f (R1, n=4 and 6)
TgMnPolymers n%CrPDI(g/mol)(°C)
3a22215200
1.19 219
3b23321600
1.27 2433c24230200
1.44 245
3d25736800
2.00 238
3e42116100
1.28 216
3f62416600
1.26 211

Fig. 2. The measured transferred function of the EOmodulator
4. CONCLUSION
EO polymers based norbornene derivatives were
efficiently prepared via ROMP which is mild condition
for active FTC chromophore. This could be considered
as potential preparation for short period of time as wellas large volume production. Moreover, even though theevaluation of thermal control of EO polymers remains tobe studied, EO polymers based norbornene derivatives
showed advantages, such as molecular weight control
and higher thermal stability, over PMMA. Theseparameters are critical in fabrication process as they
could noticeably effect film quality of EO polymer. As
hybrid modulator made of EO polymer and silicon, thedevice was poled at evaluated temperature to examine
best poling efficiency. This is the ongoing experimentand will be discussed later.

The attempt to control thermal stability was further
optimized by replacing pendant group R1 to R2 which ishexyl chain. As shown in Table 2, polymer 4a remained
showing relatively high Tg, this is probably because ofintermolecular interaction between p-rich FTC
chromophore among each polymer chains. However, bylengthening hydrocarbon linking segment clearly showedthat Tg of polymers notably decreased.
Table 2. Physical properties of polymers 4a-c (R2)Tg
MnPolymers n%CrPDI(g/mol)(°C)
4a23817400
1.72 235
4b33222600
1.55 *
4c
43528600
3.64 154
4d53522500
1.54 157
4e62828900
2.50 *

5. REFERENCES[1] L.R. Dalton, P. A. Sullivan, D. H. Bale, Electric fieldpoled organic electro-optic materials: State of the art
and future prospects, Chem. Rev., 110 (2010) 25-55.
[2] H. Miura, Q. Feng, A. M. Spring, T. Kashino, T.
Kikuchi, M. Ozawa, H. Nawata, K. Odoi, S.
Yokoyama, High thermal stability 40 GHz electrooptic polymer modulators, Optical Express, 25 (2017)28643-28649.
[3] X. Piao, Z. Zhang, Y. Mori, M. Koishi, A. Nakaya, S.
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* DSC results did not show clear Tg due to their amorphous property.

Although the control of thermal stability of EO polymers
remains needed to be improved, EO polymers based
norbornene showed several potentials to be used in EOapplications, including ease of preparation in large
volume. Moreover, in comparison with traditional
PMMA, polymers based norbornene showed well control
molecular weight which will be advantage in fabrication
process.
3.2 Preliminary result of EO polymer-based
modulator
As a preliminary study of performance of EO polymer
based norbornene, polymer 4c having Tg 154 °C wasfabricated into modulator in the silicon hybrid waveguide.
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