Research

By using semiconductor quantum dots, it is possible to realize a new photoelectric information conversion system that mutually converts the spin information of electrons and the spin information of light, i.e., circular polarization properties. Currently, spintronics (electronics technology using spin states) using metallic ferromagnetic materials such as iron (Fe), which can generate electron spin information at room temperature, is being actively researched. Spin is a property of a magnet, so to speak, that has information on the polarization of its direction (called spin polarization) and is kept stable in ferromagnetic materials. This means that energy-saving electronic circuits, such as memory, can be realized that do not require power consumption.

On the other hand, since optical devices cannot be fabricated from metallic ferromagnetic materials, we are developing spin-polarized light-emitting diodes (spin LEDs) that can directly convert electron spin information into circularly polarized light for optical transmission.

We have recently realized, in collaboration with overseas groups, new quantum dot materials that can efficiently amplify and restore electron spin information (spin polarization) in semiconductors, which is easily lost at room temperature, over a temperature range from room temperature to even 110 degrees Celsius. We have also proposed a spin-LED that utilizes such spin-amplified quantum dots.

In conventional nonmagnetic semiconductors, controlling electron spin requires the application of a strong external magnetic field of several tesla (T). We are now working on ultrafast control of electron spin polarization by electric field manipulation, a fundamental technology in electronics, without the need for such a magnetic field. This research will lead to the development of integrated polarization modulators that can operate in the gigahertz (GHz) frequency range.

The quantum mechanical coupling of nanostructures with different dimensionality of quantum confinement for electrons through tunneling effect leads to innovative functions that can never be achieved by these nanostructures on their own. We have proposed a unique hybrid nanostructure consisting of a zero-dimensional quantum dot and a two-dimensional electron system, a quantum well, to study novel electron spin properties and optical functionality.

Semiconductor quantum dots have a strong confinement effect on electrons, so their optical performance, including laser oscillation characteristics, is maintained even at room temperature and high temperatures, making them a promising material for future optical devices. They are also beginning to be put into practical use. However, since the size of quantum dots is very small, a high-density dot array structure is essential to achieve high practical performance. In collaboration with other universities, we are applying biotechnological techniques to design the size and shape of quantum dots in a top-down approach and fabricate quantum dot optically active layers with extreme high density.

In collaboration with the Graduate School of Science, Hokkaido University, we are conducting time-resolved photoluminescence measurements of newly synthesized complex compounds to elucidate the electronic states induced by photoexcitation. Through such research, we will develop new photo-functional molecular materials that exhibit high-brightness and luminescence properties that vary depending on the chemical environment and thermal history.