This experimental set-up was used to show that it is possible to make a transistor that acts using laser beams, not electric currents (Image: Martin Pototschnig) An artist’s impression of a molecule acting as a transistor that makes it possible to use one laser beam to tune the power of another (Image: Robert Lettow)

An optical transistor that uses one laser beam to control another could form the heart of a future generation of ultrafast light-based computers, say Swiss researchers.


Conventional computers are based on transistors, which allow one electrode to control the current moving through the device and are combined to form logic gates and processors. The new component achieves the same thing, but for laser beams, not electric currents.

A green laser beam is used to control the power of an orange laser beam passing through the device.

This offers another possible route to light-based rather than electronic, computing. Such “photonic” computing is desirable because components using optical fibres carrying light could be much faster than those using wires to carry electricity.

However, previous attempts to make optical transistors for such circuits only produced very weak effects. The new device could change that.

Crystal matrix

To make their device, Vahid Sandoghdar and colleagues at the Swiss Federal Institute of Technology in Zurich, suspended tetradecane, a hydrocarbon dye, in an organic liquid. They then froze the suspension to -272 °C using liquid helium – creating a crystalline matrix in which individual dye molecules could be targeted with lasers.

When a finely tuned orange laser beam is trained on a dye molecule, it efficiently soaks up most of it up – leaving a much weaker “output” beam to continue beyond the dye.

But when the molecule is also targeted with a green laser beam, it starts to produce strong orange light of its own and so boosts the power of the orange output beam.

This effect is down to the hydrocarbon molecule absorbing the green light, only to lose the equivalent energy in the form of orange light.

“That light constructively interferes with the incoming orange beam and makes it brighter,” says Sandoghar’s colleague Jaesuk Hwang.

Chilly problem

Using the green beam to switch the orange output beam from weak to strong is analogous to the way a transistor’s control electrode switches a current between “on” and “off” voltages, and hence the 0s and 1s of digital data. And doing it with a single molecule means billions could be packed into future photonic chips.

It’s a neat trick, but the Zurich team’s work is a long way from commercially viable, says Malcolm Penn, CEO of UK electronics market researcher Future Horizons. The costs of operating at such low, cryogenic temperatures are high, he says.

But Moore’s law, which describes how the number of components on a chip roughly doubles every two years, cannot go on forever while computing is based on silicon, he acknowledges, making any novel ideas of value.

“In research labs, silicon chips with features just 15 nanometres wide are already misbehaving as quantum randomness makes devices depart from the way they should work,” he explains, “so we need new ideas like this for sure.”

Journal reference: Nature (DOI: 10.1038/nature08134)