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The typical molecular ‘circuit’ consists of a molecule connecting two metallic leads. A current in this system is induced by a bias voltage that excites electrons out of the leads and onto the conducting channels of the molecule. The circuit can be turned ‘on’ or ‘off’ by inducing a change in the molecule’s chemistry.

Now, a theoretical study from Ignacio Franco and Paul Brumer at the University of Toronto in Canada and Moshe Shapiro of the Weizmann Institute in Israel suggests a different — and faster — way to drive this type of circuit with light1 . They show that the oscillating electric field from two laser beams — one twice the frequency of the other — can excite electrons within the molecule (here, trans-polyacetylene) and lead to a net current.

At first glance the idea sounds impossible, because, averaged over time, the mean electric field produced by the laser within the molecule is zero and no net current should flow. However, the number of excited (conducting) electrons depends on the amplitude of the electric field in a particular direction. In the ‘mixed’ laser configuration the theorists consider, this amplitude is higher for electrons that move toward the ‘right’ lead than the ‘left’ lead.

The direction and magnitude of the current can be tweaked just by changing the phase of one of the lasers, and the current pulses can be as fast as a few tens of femtoseconds.