The Future with Nano ICT—It's right there

Research and Development of Organic Electro-optic Polymers
Toward Practical Applications

electro-optic (EO) effect is a pheT henomenon by which the refractive
index of a material changes when an electrical field is applied to it. The EO effect isused in devices such as optical modulatorsand optical switches, which convert electrical signals to optical signals and are key
devices for optical communications. NICT isconducting research to improve the characteristics of polymers exhibiting the EO
effect (EO polymers) and their applications,developing various types of optical modulators and switches as well as terahertz generation and detection. Here, we introduce
development of EO polymer materials, research to improve their thermal stability, on
device applications, and the excellent characteristics of EO polymers.

Toshiki YAMADA

Senior Researcher
Nano ICT Laboratory,
Advanced ICT Research Institute■Introduction

After completing his doctoral course, hebecame JST-CREST researcher in 1996. He
joined Communications Research Laboratory (currently NICT) in 1999. Engaged
in research on new thin film fabricationtechniques, single molecule spectroscopy, organic electro-optic polymers. Ph. D.
(Engineering).

Recently, with further increases in the transmission capacity of communications networks,
increasing speed of data processing, and diversification of applications, both increasing thebandwidth and decreasing power consumptionof optical modulators and switching devices,which are important technical elements of op-Intramolecularhydrogen bonds

Acceptor unit
Donor unit

Figure 1 An example of high-performance organic electrooptic (EO) chromophore utilizing intramolecularhydrogen bonds

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tical communications, have become importantissues. EO polymers have a low dielectric constant in the microwave frequency region andthe difference between the refractive index for
optical waves used in optical communications
and the effective refractive index for microwave modulation can be made small, so they
are promising for ultra-high-speed operation at
100 GHz or more. Recently, the characteristics
of EO chromophores have improved dramatically, as shown below. The EO coefficient,r33, of EO polymers has exceeded 100 pm/V,
and they exceed lithium niobate (LiNbO3) interms of the figure of merit (FOM = n3r33). Assuch, EO polymer devices are attracting attention, having both ultra-high-speed response
and low power consumption. Ultra-high-speed
communication over 100 Gbit/s is also becoming needed for medium and short-distance data
communications, in addition to long distancecommunication networks, and the transition
from electrical wiring to optical "wiring" (optical interconnections) has begun. For suchapplications, reduced size and low cost areimportant, in addition to high speed and low
power consumption. For this reason, hybrid
devices combining silicon waveguides with
EO polymers are attracting attention.

■Design and synthesis of new EO
chromophores and a techniqueto evaluate EO coefficients for EO
polymers
EO chromophores are composed of an electron donor unit, an electron acceptor unit, and aπ-conjugated unit, and thus EO chromophores
have an asymmetric structure. When atoms
within the molecule create a double bond, theelectrons forming the first bond (σ electrons)bond tightly, while the electrons forming thesecond bond (π electrons) are more mobile. A
π-conjugated system, which is a collection of πelectrons, plays an important role in determining the optical and electrical characteristics
of a molecule, and it is important to consider
the length and orientation of π conjugated systems when designing EO chromophores. We
have found that EO performance can be improved by introducing an alkyloxy group at aparticular position in the electron donor unit(meta-position of aminobenzene), and using
intramolecular hydrogen bonds to stabilize thestructure of the π-conjugated system. This has
provided new molecular design guidelines forfurther increasing performance of EO chromophores (Figure 1), and we have successfully
developed one of the best EO chromophores inthis field. In the EO polymers we developed,
this type of EO chromophore is introduced asa side chain on the polymer main-chain. The
EO effect is generated in the EO polymer by
a process called polling. We also developed anew, highly-reliable method for evaluating theEO coefficient, r33, called the transmission ellipsometric method without an aperture, which
we are using for feedback into EO polymer material design (Figure 2).

■Development of new EO polymers
with better thermal stability
EO polymers are applicable in ultra-highspeed modulation at 100 GHz or more, and canbe operated with lower power consumption

Figure 3 Comparison of performance of silicon-waveguide/EO-polymer hybrid
modulator and silicon-only modulator

than existing materials, but the thermal stability has been an issue preventing practical application so far, due mostly to orientation relaxation of EO chromophores. At NICT, we havedeveloped EO polymers with a glass transition
temperature above 160℃, and found that theyshow good thermal stability in an environmental-stress test maintaining a temperature of85℃, as required for optical communications
devices.

■Development of silicon-waveguide/
EO-polymer hybrid devices

Figure 2 Measurement system for the electro-optic
constant, r33 (transmission ellipsometricmethod without an aperture)

As shown in Figure 3, silicon-waveguide/
EO-polymer hybrid modulators have promising characteristics including small size
(approx. 1 mm), ultra-high-speed (100 GHz
or greater), and low power consumption(0.1 mW/GHz or less). This is achieved by
combining a silicon waveguide, which has excellent light confinement effect due to the highrefractive index of silicon, with the strong EOeffect and ultra-high-speed operation of theEO polymer. In contrast, the operating principle of silicon modulators is the free-carrier
plasma effect (change in free-carrier density)rather than the pure EO effect, and there arelimits to reduction of power consumption andincrease of operation speed because of the current flow. Since they are not using the pure EO
effect for optical modulation, there is a correlation between phase and amplitude when
performing optical modulation, and this may
cause difficulty in practical applications.
To apply optical communications for me-dium and short distance data communications(optical interconnections), NICT is developing silicon waveguide/EO polymer hybrid
devices using organic EO materials with highperformance and thermal stability, yielding
ultra-high speed, low power consumption andsize reduction.

■Future prospects
We are developing EO polymer opticalmodulators and switches and silicon waveguide/EO polymer hybrid devices based on
our technologies for developing EO pigments
and EO polymer materials, and we are deploying them in society in collaboration with industry. Beyond applications in long, medium,and short-range communications as describedabove, EO polymers in combination with lightsources can be applied to generate ultra-highspeed pulses with low power consumption for
sensor applications. The EO polymers we havedeveloped are also very promising for terahertz generation and detection, and we expect
they will be developed in a variety of applications.

NICT NEWS JAN 20167