New on DVD: Photonic Transistors

Daniel C. McCarthy

The ability of one electrical current or field to amplify another underlies the electronics industry's primary advantage over photonics: the transistor. By modulating electrical fields, transistors create the electronic pulses that serve as the ones and zeros of binary computing.

An all-optical transistor able to transfer, attenuate or amplify energy from one wavelength to another could revolutionize application markets from switching to computing and possibly reverse electronics' dominance over photonics.

Researchers at the National Institute of Advanced Industrial Science and Technology, working in collaboration with scientists at Sharp Corp. and TDK Corp., both based in Japan, demonstrated a prototype of such a transistor. The device operated through the interaction of two laser beams emitting at 405 and 635 nm and focused on a high-speed rotating disc. By controlling power at the red wavelength, the scientists amplified a modulated signal in the blue wavelength by 60 times.

To achieve this, the group recorded marks along the disc surface using technology borrowed from DVD manufacture. The marks had dimensions and periodicity below the optical diffraction limit. When rotated at high speeds, the disc strongly scattered polarized light from the lasers, causing some of the energy to run along the marks, where the trapped electrical field generated local plasmons, or groups of collectively moving electrons.
The scientists manipulated one of the beams -- usually the red -- to create a photoreaction in the disc's silver oxide layer that produced silver particles within 100 nm -- the near-field interaction range -- of the plasmon reservoirs.

These particles acted as light-scattering centers, a role analogous to an electrical transistor's gate, by coupling plasmon energy from one laser beam and transferring it to the other, thus creating an enhanced signal. The power of the control laser determined the size of the silver particles and, hence, their scattering effect. By controlling the red laser power from 1.5 to 3.5 mW, for instance, the researchers demonstrated the sixtyfold increase in the blue beam's far-field intensity.

Theoretically, that could enable photonic transistor logic systems based on amplifying one wavelength or the other, explained Junji Tominaga, who heads the institute's Laboratory of Advanced Optical Technology.

Most importantly, the device's silver oxide film eliminates the need for the prisms required for earlier demonstrations of optical transistors. Its use of silver oxide, the foundation of computer chips, indicates that the institute's device can be made smaller and designed simply enough for mass production.