Lasing at the limit

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How small, how energy-efficient can a laser be? The search for the ultimate nanolaser for the information technology of the future is engaging research groups worldwide.

As part of an international collaboration, Prof. Dr. Stephan Reitzenstein from the Institute of Solid State Physics at TU Berlin and his project partners have not only succeeded in building an extremely small and highly efficient nanolaser but have also, for the first time, conclusively demonstrated its laser properties through quantum optical measurement of the emission statistics.

"Energy efficiency is not only a concern for electric vehicle manufacturers but is also a topic in so-called 'on-chip photonics,' meaning microchips on which data transmission and processing increasingly rely on light," explains Prof. Stephan Reitzenstein. "What makes future nanolasers special is that they operate at the transition to quantum optics, in the realm of individual quanta of light, called photons." In practice, this means: It is not only particularly difficult to manufacture such nanolasers. The main challenge is also to clearly demonstrate the laser emission at all.

Laser light is generally generated in a so-called optical resonator when sufficient energy is supplied to the laser medium inside it. The problem: The supplied energy, known as pump power, must exceed a certain limit – the laser threshold – so that the laser medium emits not just light, but laser light.

"This is because initially, a large part of the supplied energy is converted into photons without coupling into the intended laser beam. In conventional semiconductor lasers, like those found in every CD or DVD player, only about one in every hundred thousand photons actually couples into the laser beam. All other photons are lost. Only when the pump strength compensates for these losses can laser light be generated," explains Reitzenstein, who likes to compare the phenomenon to a perforated bucket: "The perforated bucket symbolizes the resonator. The water hose filling the bucket is comparable to the pump source that fills the resonator with photons. The goal is to reach a certain water level in the bucket, representing the laser threshold. However, water always leaks out through many small holes in the bucket – just as photons leave the resonator repeatedly without coupling into the laser mode. Therefore, the water supply must exceed a certain limit (amount of water per time) for the water level (laser threshold) to be reached. To build an energy-efficient nanolaser with a low laser threshold, the resonator must be as small and dense as possible. In the case of an ultimate thresholdless nanolaser, it is almost possible to 'plug all the holes,' so that every introduced photon couples into the laser mode."

This has now been achieved through an extreme miniaturization of the resonator. The width of the nanolaser studied here is only about 200 nm. For comparison: the diameter of a human hair is about 60,000 nm (one nanometer [1 nm] = one millionth of a millimeter). "The highly precise structure of the resonator results in more than 7 out of 10 supplied photons (and not just the hundred-thousandth as in a normal laser) being effectively usable for laser operation," explains Stephan Reitzenstein. "We are already very close to the ultimate thresholdless laser," he adds.

For the characterization of the nanolaser, highly sensitive detectors and elaborate analysis methods were used: A quantum optical experiment was conducted to determine the photon statistics of the emitted light, which is characteristic of laser emission. Only through this complex setup was it possible for the first time to conclusively prove that the light from the nanoresonator is indeed laser light and not merely functioning as an LED.

"In particular, we demonstrate that established 'lasing criteria' for nanolasers lose their significance and that laser light can ultimately only be detected through quantum optical methods," explains Stefan T. Jagsch, who conducted the experimental work as a doctoral student under Prof. Reitzenstein's guidance.

The work was carried out within the framework of a third-party funded project supported by the DFG and the Swiss National Science Foundation (SNF), in close cooperation with leading groups in the field of semiconductor processing (Prof. Nicolas Grandjean, École Polytechnique Fédérale de Lausanne), nanolaser theory (Dr. Christopher Gies and Prof. Frank Jahnke, University of Bremen), and characterization of nitride semiconductors (Prof. Axel Hoffmann, TU Berlin). The research has been published in the current issue of the renowned open-access journal Nature Communications.