Scientists control light echoes for the first time

‘Wie man in den Wald hineinruft, so schallt es heraus’ (‘shout into the forest and a similar echo will return’, or ‘What goes around, comes around’), is not only a well-known German proverb, but is also literally true. When a sound wave is reflected, the echo is heard. When exactly it comes back, however, depends on the ‘forest’ – but first and foremost on the distance between the caller and the place of reflection,” explains Professor Torsten Meier from Paderborn University. “Just imagine that you could tailor when you wanted the echo to come back to you,” continues the physicist. A team of scientists has now achieved just this. Not for acoustic signals, however, but for optical signals: The scientists succeeded in controlling photon echoes emitted by semiconductor quantum dots with sub-second precision.

Meier explains: “Optical echoes are somewhat different to conventional acoustic echoes, because they are not generated by the reflection of waves, but rather in a non-linear optical process. Two short laser pulses are sent to a sample: The first represents the signal and the second the forest. This provides for the reflection. When the lag time of these pulses is doubled, a new light pulse, the photon echo, is emitted by the system exposed to the light.” Using a further control pulse, the researchers were able to control this photon echo within the picoseconds range (i.e. 10-12 of a second), and thereby delay it to a desired point in time. Such control is particularly pertinent for nanophotonic circuits in which multiple optical systems need to be precisely synchronised with each other.

The phenomenon is set to be further developed in the future

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