A team of physicists from the University of Innsbruck and Harvard University has proposed a fundamentally new way to generate laser light: a laser without mirrors. Their study shows that quantum emitters spaced at subwavelength distances can constructively synchronize their photon emission to produce a bright, very narrow-band light beam, even in the absence of any optical cavity.
In conventional lasers, mirrors are essential to bounce light back and forth, stimulating coherent emission from excited atoms or molecules, and thus light amplification. But in the new "mirrorless" concept, the atoms interact directly through their own electromagnetic dipole fields given that interatomic spacing is smaller than the emitted light's wavelength. When the system is pumped with enough energy, these interactions cause the emitters to lock together and radiate collectively-a phenomenon called superradiant emission.
The team led by Helmut Ritsch found that this collective emission generates light that is both highly directional and spectrally pure, with a single narrow spectral line, in cases where only a fraction of emitters are excited by a laser and the rest of atoms remain unpumped. Since this passive emitter fraction is not broadened by the driving laser or power broadening, it effectively acts as an optical resonator for the active emitters, in analogy with a conventional laser where the optical resonator and the gain medium are separate physical entities.
"The atoms synchronize their emission and above a certain threshold start to shine light collectively or in unison with each other," explains Anna Bychek, Postdoc from the Department of Theoretical Physics at the University of Innsbruck. "There are still many questions to be studied in future work, but it is clear that atoms build their own feedback mechanism and frequency selection via dipole-dipole interaction in free space."
Beyond its conceptual significance, this discovery points to a new class of ultra-compact light sources for nanophotonics and precision measurements. Because the emission frequency is determined primarily by the atoms themselves, such systems could provide exceptionally stable optical references for quantum sensors, clocks, or on-chip devices.
The research combines the theory of light-matter interactions with advanced numerical methods to explore how large atomic ensembles behave collectively and emit coherent radiation. The results suggest that with ongoing progress in the field, mirrorless lasing could soon move from theoretical prediction to experimental realization.
This work has recently been published in Physical Review Letters and was financially supported by the Austrian Science Fund FWF and the European Union, among others.
Publication: Nanoscale Mirrorless Superradiant Lasing. Anna Bychek, Raphael Holzinger, and Helmut Ritsch. Phys. Rev. Lett. 135, 143601 DOI: 10.1103/rbs2-2pd5
