Description

Mirrorless laser: a new light source

A team from the University of Innsbruck and Harvard University has proposed a fundamentally new way to produce laser light: a laser without mirrors. Her study shows that quantum emitters arranged at intervals below the wavelength can constructively synchronize their photon emission to produce a bright, very narrow-banded light beam, even if no optical resonator is present.

In conventional lasers, mirrors are essential to reflect light back and forth, thereby stimulating the coherent emission of excited atoms or molecules and thus the amplification of light. However, in the new “mirrorless” concept, the atoms interact directly via their own electromagnetic dipole fields, since the distance between the atoms is smaller than the wavelength of the emitted light. When the system is pumped with sufficient energy, these interactions cause the emitters to synchronize and radiate together - a phenomenon called superradiance.

The team under the leadership of Helmut Ritsch found that this collective emission produces light that is both highly directed and spectrally pure, i.e. has a single narrow spectral line, if only a part of the emitter is excited by incoherent light and the remaining atoms remain unaged. Since this passive emitter component is not increased by the external pump light or the light output, it acts effectively as an optical resonator for the active emitters, analogous to a conventional laser, in which the optical resonator and the amplification medium are separate physical units.

"The atoms synchronize their emission and begin to glow together or in harmony with each other from a certain threshold value," explains Anna Bychek, postdoctoral researcher at the Institute of Theoretical Physics at the University of Innsbruck. "There are still many questions to be examined in future work, but it is clear that atoms build their own feedback mechanism and frequency selection via dipole-dipole interactions in open space."

Beyond the conceptual significance, this discovery could enable a new class of ultra-compact light sources for nanophotonics and precision measurements. Since the emission frequency is primarily determined by the atoms themselves, such systems could provide exceptionally stable optical references for quantum sensors, quantum clocks or on-chip systems.

Work combines the theory of light-matter interactions with advanced numerical methods to study how large atomic ensembles collectively behave and emit coherent radiation. The results suggest that with ongoing progress in this field, mirrorless laser radiation could soon make the leap from theory to experimental implementation.

This paper was recently published in Physical Review Letters and financially supported by the Austrian Science Fund FWF and the European Union, among others.

The paper:
https://journals.aps.org/prl/abstract/10.1103/rbs2-2pd5

Due to strong dipole-dipole interactions between the atoms, passive emitters can significantly increase the light emission and at the same time make the emitted radiation much narrower.