Using a mirror to continually send a fraction of the emitted light back into a quantum dot microlaser, researchers were able to throw the light emission process off-kilter, prompting the microlaser to emit photons in a chaotic pulse sequence. This unique property may open doors to a new, secure form of data transmission. “This chaos is extremely interesting from a fundamental physics point of view,” said Stephan Reitzenstein, a physicist at the University of Würzburg who collaborated with Israeli colleague Ido Kanter. “It might be used in the future for optical amplifiers and switches. In addition, the negative feedback of two microlasers over a long distance is likely to provide synchronization of the chaotic pulse sequence, enabling the realization of a new, secure form of data transmission.” A quantum dot microlaser emits light that is deliberately cast back into the laser via a mirror. This disturbs the laser’s operation so much so that the emission behavior becomes chaotic. On the right, the result of a photon statistics measurement from which physicists can detect a chaotic pulse sequence, given that without chaos, no pulses would appear. (Image: Ferdinand Albert) The microlasers look like tiny towers with a diameter of less than one-tenth of a human hair, and they consist of a special sequence of extremely thin semiconductor layers. They are driven electrically using an elaborately designed ring contact. During production, quantum dots — nanostructures that emit light — are placed in the center of the microlasers. “The microlasers have been designed such that the photons emitted by the quantum dots are coupled into the lasing mode with extremely high probability and can therefore be used for very efficient laser operation,” Reitzenstein said. Because of their special design, the Würzburg microlasers can be driven with no more than a few microamperes and only around 10 quantum dots. In conventional semiconductor lasers, on the other hand, pump currents in the milliampere range as well as some 1000 to 10,000 quantum dots, are needed. “Intensive research is being conducted worldwide in order to create the ultimate micro- or nanolaser that only contains a single quantum dot as a gain medium,” said Reitzenstein. The Würzburg physicists have now come very close to this goal with their model. Such highly sophisticated quantum dot microlasers react very sensitively to fluctuations in the quantity of light particles in the laser resonator. Even the emission of a single photon can throw the laser’s operation into confusion. The scientists turned the tables and cast the emitted light back onto the microlaser with a mirror continuously, precisely and in a controlled fashion to disturb its operation. It transpired that if the lasers emit only a few tens of nanowatts of light power, the influence of the feedback cannot be measured directly. “Instead, complex photon statistics measurements are needed to prove the expected chaotic behavior,” he said. But here the physicists achieved success: They proved that disturbing the laser leads to a chaotic pulse sequence in which each light pulse contains only around 100 photons. “We are currently preparing experiments to synchronize two lasers as far as the quantum limit of just one photon moving back and forth,” Reitzenstein said. “If we succeed, we will be a step closer to a fundamental understanding of synchronization and to a secure form of data transmission.” There is also another hurdle to negotiate: At present, the microlasers work only in extremely cold conditions, at below -150 ºC. Operation at room temperature should be possible, however, if the quantum dots are optimized for this purpose. The physicists are currently pursuing this goal in a separate project. For more information, visit: www.uni-wuerzburg.de