Youichi Sakakibara, a senior researcher of the Photonics Research Institute of the National Institute of Advanced Industrial Science and Technology (AIST) and Madoka Tokumoto, a group leader of the Nanotechnology Research Institute, AIST carried out research in collaboration with the Femtosecond Technology Research Association (FESTA), a recipient of research commission from the New Energy and Industrial Technology Organization (NEDO) under the Industrial Technology Research and Development Program of the Ministry of Economy, Trade and Industry (METI), and the Tokyo Metropolitan University (TMU) and succeeded in discovering that carbon nanotubes exhibit a saturable absorption effect in the near-infrared(IR) range around 1.55mm i.e. the wavelength region used for optical communications. This effect entails a drop in the optical absorption rate with increasing light intensity. This discovery has a promising application potential for the use of the carbon nanotube as all-optical switches capable of being actuated by light signals alone instead of the conventional opto-electronic switchs which require the conversion of the light signals to electronic signals for switching operation. The new technology thus opens up new possibilities for the realization of next-generation broadband ultra-high capacity optical communications.

At the dawn of the full-fledged broadband network age, it is of the greatest importance to develop optical switching materials that can be used for the next-generation ultra-high capacity optical communications systems. Materials exhibiting a saturable absorption effect are promising candidates for optical switch applications as they will either pass or stop (absorb) light signals according to the light intensity (on/off status) of the control signals. Under the Femtosecond Technology Research and Development System, AIST and FESTA had searched for organic materials with a saturable absorption effect in the near-IR range used for optical communication. Our assessment of the potential of the semiconducting single-wall carbon nanotube revealed that it has a strong optical absorption capacity essentially required for causing a saturable absorption effect. Based on this forsight, we carried out experiments to verify the exhibition of saturable absorption effect in collaboration with the TMU. We succeeded in observing a saturable absorption effect, confirming that when the intensity of the incident laser light exceeds a certain threshold value, the light absorption rate of the nanotube decreased with the increase in the intensity of the laser light. It was also revealed that the efficiency of the saturable absorption effect depends on the wavelength of the incident light.

The discovery of this effect is a new landmark for the carbon nanotube in terms of its potential for optical applications, considering that, so far, its various functions have been devoted mainly for mechanical and electronics applications. The successful results have major implications that may trigger a revolutionary progress toward the development of the all-optical switch for next-generation broadband ultra-high capacity optical communications.