Development of thermoelectric devices, which can convert unused thermal energy into electric energy, is highly expected. In future, broad area sensor network will be constructed and enormous number of sensor nodes will be distributed to monitor a variety of objects. Energy harvesting will be essential technology for energy autonomous. We collaborate with University of Freiburg in Germany to fabricate highly efficient Si thermoelectric devices and monitoring systems using both nanotechnology and micromachining technology.

Artificial control of the thermal conductivity is not only the important technology for thermoelectrics but also is an interesting topic in fundamental physics. By forming thick nanowires or form nanocomposite in the structures, one can reduce the thermal conductivity due to increased surface and boundary scattering processes. These scattering processes can be understood by particle picture of a phonon. We are interested in phonon transport control using ballistic characteristic of phonons (Ray phononics). On the other hand, manipulation of phonon transport by phononics, which is based on the band engineering is possible. This technique is based on the wave picture of phonons. We investigate phonon transport in Si nanostructures, which is a key to higher conversion efficiency, with a dimension below 100-nm fabricated by top-down lithographic technology. We originally developed micro time-domain thermoreflectance system, which can measure thermal conductivity of nanostructures with very high throughput. The goal of this research topic is demonstration of highly efficient thermoelectric devices using the abovementioned approach.

Hybridized quanta called polariton will provide completely novel physics. One type of such polariton is the surface phonon-polariton (SPhP), the hybridized eigen state of a photon and phonon. The SPhP are essentially evanescent waves that propagate along the surface of polar dielectric membranes. In our study, we experimentally measured the in-plane thermal conductivity of amorphous SiN membranes at different temperatures. Our results demonstrate that the reduction of the phonon thermal conductivity of nanomaterials can be compensated by the increase of its counterpart driven by the propagation of surface phonon-polaritons and even double the thermal energy transport. Thus, this work uncovers a new channel of heat transport along polar dielectrics and lays the foundations for improving the heat dissipation in microelectronics and efficiency in silicon photonics.