Researchers have succeeded in making electrons leave in a chosen direction using two lasers. No external electric field is needed to impose this movement. The experiment shows that a current can be controlled by light alone.
In a semiconductor, light can provide enough energy to set electrons in motion. The problem is that they usually leave in all directions. Their movements then cancel each other out, which prevents the formation of a net current.
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To avoid this cancellation, the University of Michigan team used two laser pulses of different colors. These pulses were tuned with great precision to act together on the electrons in the material.
The same electron could receive the necessary energy in two ways. It could absorb two photons of one type, or three photons of another energy. A photon is the smallest amount of energy carried by light.
These two possible paths combine according to a phenomenon called quantum interference. Their effect can add up in one direction and diminish in the opposite direction. More electrons then move to the same side, creating a directional current.
The researchers control this direction by changing the offset between the oscillations of the two lasers. This adjustment, called relative phase, makes it possible to rotate the orientation of the current. The electrons thus behave like the beam of a lighthouse that can be aimed without moving the supporting material.
The method also produces a more concentrated movement than some previous experiments. Instead of scattering widely, electrons cluster more around the chosen direction. This property could make them easier to control in future devices.
The researchers envision applications in components combining light and electronics. This control could be of interest for telecommunications, sensors, or information processing. Further tests will need to measure its consumption, stability, and compatibility with miniaturized chips.