A new type of superconducting transistor able to be switched reversibly between on and off positions using light irradiation has been developed by a team of scientists in Japan. This switching capability, based on organic superconducting field-effect transistors (FETs), could result in a new generation of high-speed switching devices and highly sensitive optical sensors.

The researchers, led by Hiroshi Yamamoto from Japan’s Institute for Molecular Science, created the first organic superconducting FETs a couple of years ago, bringing attention to their flexibility and designability. FETs are a standard switching element that controls electrical current in electronic circuits, and are now used in many electronic devices, including smart phones and computers. Much research is being carried out into superconducting FETs as a key technology for computations using quantum states.

This study, as reported in Science [Suda et al. Science (2015) DOI: 10.1126/science.1256783], developed a novel photo-switchable transistor by replacing the gate electrode in the conventional FET with a spiropyran thin film. Spiropyran is a photo-active organic molecule that shows intra-molecular electrical polarization by ultraviolet (UV) light irradiation. On shining a pale UV light on the transistor, it demonstrated a quick decrease in electrical resistance and turned into a superconducting state after 180 seconds. However, as researcher Masayuki Suda points out, “it can be operated much faster in principle because the switching speed depends on the timescale of the photochromic reactions.”

In this process, carriers for the superconductivity can be accumulated by UV light-induced electrical polarization of the spiropyran film, while the device can be switched to the superconducting state through both gate-voltage control and light-irradiation control. This multi-mode operation indicates the high designability of organic systems.

Although superconducting transistors have been developed using electric-double layer capacitors, modulations of the carrier density have been limited to the high-temperature regime because of the freezing of ionic motion below ~200 kelvin, since the heating and cooling process is required to switch the superconductivity. For this transistor, direct in situ switching is possible even in cryogenic conditions.

The study demonstrates that a voltage source is not necessary for field-effect transistors, and that it is possible to access other energy sources such as light to operate transistors. Although still at a basic research stage, it illustrates the concept of superconductivity being switched by optical stimuli could drive innovation in the field of fast switching devices and very sensitive optical sensors.