Researchers claim to have fabricated the first all-optical transistor on a silicon chip. This device allows the transmission of light emitted by one laser to be governed by the intensity of another.

This novel transistor was made by researchers at EPFL in Lausanne, Switzerland, and the Max Planck Institute for Quantum Optics in Garching, Germany. According to the team, the device promises to provide another building block for constructing all-optical integrated circuits. Such circuits could dramatically improve the efficiency of telecommunication networks because they would eliminate the need to convert optical information to electrical pulses – which can be processed easily – and then back to light.

The team employed standard nanofabrication methods to make the transistor, a taper consisting of a silicon dioxide disc with a rimmed edge sitting on a silicon pillar. The ability to make devices in silicon is important because the material is widely used in the electronics industry.

To operate the device, the frequency of one laser beam (the "probe") is tuned to an optical resonance of the silicon dioxide structure. The result is that the structure behaves like an optical cavity, with the incident light bouncing endlessly around its rim. "No light is transmitted through the taper since all the light is lost in the optical mode of the cavity," explains Tobias Kippenberg from EPFL.

The beat goes on

A second "control" beam at a different frequency is then directed at the taper. Interaction between the two beams results in a beat frequency that also resonates with the disc and creates a mechanical oscillation. Interference between these three light fields results in the cancellation of the probe beam within the cavity.

"The presence of the control beam allows the probe beam to be transmitted through the taper as if it was not coupled anymore to an optical cavity. This is the optomechanically induced transparency effect," explains Kippenberg.

Cranking up the intensity of the control beam increases transmission of the light from the probe laser through the structure, but it is impossible to realize complete transmission – this would require an infinitely powerful control laser.

Kippenberg and his colleagues selected silicon dioxide for building their tapers, because this material combines very high transparency with very low optical losses.

"However, the optomechanically induced transparency effect can be realized in various optomechanical platforms that have been developed in recent years, based on many different materials such as silicon nitride and calcium fluoride."

Quantum control could be next

Kippenberg believes that the optomechanically induced transparency effect might be able to control the quantum state of the transistor. "This would be a very important step towards the realization of quantum experiments on large-scale objects and tests of decoherence on unprecedentedly large systems."

The next goal for the team is to cool the mechanical oscillator into its quantum ground state using the optomechanical interaction. Kippenberg says that this will be a first step towards the preparation and control of a mesoscopic object in various quantum states.

The team's all-optical device joins a growing band of variants on the conventional electronic transistor, including one that converts an electrical input into an electrical and a laser output. Co-inventor of this "transistor laser", Milton Feng from the University of Illinois Urbana Champaign, is not particularly impressed by the all-optical variant built by the European team: "It is science, but it will never make it in the real world of integrated circuits like the semiconductor transistor did."

The research is reported at Science DOI: 10.1126/science.1195596.