The Future with Nano ICT—It's right there

Development of Organic Terahertz Devices
Towards high-performance and compact terahertz sources

waves hold promise inT erahertz
applications such as non-contact,
non-invasive sensing and ultra-high-speed
wireless communications. To promote theiruse in society, it will be necessary to reduce
the size and increase the performance ofterahertz equipment. We are developing
organic polymers with excellent properties,
as well as devices that use them, in the hopeof realizing terahertz sources and detectorsthat are more compact and have higher performance than ever before.

■Background

Takahiro KAJI

Senior Researcher
Nano ICT Laboratory,
Advanced ICT Research Institute
After completing graduate school, he was a
Specially Appointed Assistant Professor of
the Global COE Program before joining NICTin 2009, and he has held his current position
since May, 2015. He is engaged in researchon fabrication of organic photonic devices.
Ph. D. (Engineering).

Terahertz waves are electromagnetic waves
with frequencies between those of radio waves
and light (0.1 to 10 THz). They are higher infrequency than radio waves, so they are promising for applications in ultra-high-speed wireless communications. They are also highly
penetrating of objects, and materials produce
characteristic absorption patterns called fingerprint spectra, so they are also promising foruse in sensing. When using terahertz waves forsensing, the internal structure of an object canbe imaged in a non-contact and non-invasive
way, information can be obtained about things(materials, biomaterials, etc.) in the immediatesurroundings, and materials composing the ob-ject can be identified. Such information aboutobjects is difficult to obtain using conventional sensors, and using it in society will enable
increases in productivity and efficiency in all
kinds of scenarios related to security, health,medicine, the environment, science, industry
and agriculture. We will also be able to detect
dangers in society quickly.
To accelerate utilization of terahertz waves
in society for sensing and wireless communications, it will be important to reduce thesize and increase the performance of terahertz
equipment (terahertz sources and detectors)(see Figure 1). A number of current terahertz
sources generate terahertz waves by changing
the wavelength of laser light using non-linear
optical materials. However, such wavelength
conversions are extremely inefficient, so theyrequire large laser light sources with high output, and the equipment is large. Most current
terahertz wave equipment for emission anddetection is also limited to lower frequencies(approx. 0.1 to 4 THz), so for fingerprint spectra, it has been difficult to obtain many of theabsorption peaks in the higher-frequencies,
which are useful for identifying materials. To
advance the use of terahertz waves in sensing,
the bandwidth of terahertz wave emitters anddetectors needs to be increased.

爆発物 テラヘルツ波

混入異物

有害物質テラヘルツ波

テラヘルツ波テラヘルツ波Figure 1 E xpansion of the use in society by reducing size and increasing performance of terahertz equipment

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NICT NEWS JAN 2016■Terahertz technology using
non-linear optical polymers
We have developed non-linear optical polymers and devices that use them in order togreatly increase the efficiency and bandwidthof terahertz wave emitters and detectors.
Compared to inorganic and organicnon-linear optical crystal materials that havebeen used for emission and detection of terahertz waves, such as lithium niobate (LiNbO3),
zinc telluride (ZnTe) and DAST, non-linear optical polymers have a large electro-optical coefficient (> 100 pm/V) and their figure of merit(FOM) values for terahertz wave emission,considering the effects of refractive index of
the materials, exceed the values for the materials above (see Table).
For inorganic crystal materials such as
LiNbO3 and ZnTe, the bandwidth of emitted
terahertz waves is narrow because high-frequencies are absorbed due to crystal lattice
vibration. For non-linear optical polymers,
the absorption coefficient is small over a wide
range of the terahertz domain, so emission
and detection of terahertz waves over an ultra-broad band (0.1 to 20 THz) is possible.
Non-linear optical polymers can also be
used with micro-processing to fabricate waveguide devices, so we can expect to realize
devices that yield large increases in terahertz
wave emission efficiency through use of optical confinement effects.

Figure 2 Apparatus for evaluating a non-linear optical polymer terahertz wave
emitting device■Terahertz wave emitting devices
using non-linear optical polymers
To realize a compact terahertz source, weare developing a terahertz emitter device using
1.5 μm band laser light that can lase in small
semiconductor lasers and fiber lasers and canbe used in various optical communication
technologies.
Earlier, we successfully generated terahertz waves by taking laser light from a femtosecond titanium-sapphire laser, convertingit to laser light of wavelength 1.5 μm using
an optical parametric amplifier, and then exposing a non-linear optical polymer to thislight. We also built a system to evaluate a terahertz emitter device using an even smaller,
low-power 1.5 μm-band erbium-doped fiber
laser (Figure 2), and succeeded for the firsttime in observing terahertz wave emission
from a non-linear optical polymer device under weak pump light intensity conditions. To
further increase the terahertz wave emission
efficiency, we are continuing to develop tera-hertz wave emitter devices using micro-waveguide structures and conducting experiments
using a 1.5 μm-band ultra-short pulse fiber
laser able to emit terahertz waves over a very
wide bandwidth.

■Future prospects
To further improve the performance of terahertz devices, we are developing materials
that greatly reduce terahertz wave absorption
losses, and by applying the organic-silicon
hybrid optical modulator technology that ourgroup is developing, we will develop devices
with dramatically increased terahertz wave
emission efficiencies. Use of non-linear opticalpolymers will also make more-sensitive terahertz wave detectors possible. Improved performance of both emission and detection will
make it possible to implement compact terahertz equipment not seen before, and this willcontribute to a safer, more secure ICT society
using terahertz waves.

Table Comparison of non-linear optical materials used for terahertz wave emitters and detectors

NICT NEWS JAN 20169