Up to now, nobody has succeeded in making optical transistors that can do all and can also be made out of silicon. However, Leo Varghese at Purdue University in Indiana and a few pals say they've built a device that take a significant step in this direction. Their optical transistor consists of a microring resonator next to an optical line. In ordinary circumstances the light supply enters the optical line, passes along it and then outputs. But at a specific resonant frequency, the light interacts with the microring resonator, vastly reducing the output. In this state, the output is essentially off even though the supply is on.