Exciton-polariton lasers offer low-power operation, making them a promising source of coherent light for various low-energy applications. Until now, however, measurement of the spectral purity, or linewidth, of this type of laser had remained elusive. Using a commercial scanning Fabry-Pérot interferometer, researchers at FLEET, the Australian Research Council’s Center of Excellence in Future Low-Energy Electronics Technologies, investigated the energy and linewidth of exciton-polariton lasers in the single-mode regime. The researchers demonstrated that, contrary to previous assumptions, the exciton-polariton laser can maintain an ultra-narrow linewidth of 56 MHz, or 0.24 microelectronvolts (µeV) — 10x smaller than previously thought. Moreover, the measured linewidth of the laser corresponds to a coherence time of 5.7 ns. The long coherence time suggests that the macroscopic quantum state of the laser could be manipulated for quantum computing. Illustration of a trapped polariton condensate giving rise to the laser emission with an ultra-narrow spectral peak detected by a scanning Fabry-Pérot interferometer. Courtesy of Optica (2024). DOI: 10.1364/OPTICA.525961. Exciton-polariton lasers are coherent light sources generated by the decay of bosonic condensates of exciton-polaritons — hybrid particles arising from the coupling of photons and excitons. These devices achieve lasing without population inversion and at a threshold much lower than that of an equivalent conventional photon laser. Previous attempts to measure exciton-polariton laser linewidth required multiple experimental iterations and averaging over timescales that was orders of magnitude longer than the polariton lifetime and the coherence time. This affected the measurement of fluctuations in energy and linewidth. Using the Fabry-Pérot interferometer, the FLEET team directly measured the linewidth and small energy shifts of chopped, continuous-wave (CW) polariton condensates (quasi-CW pulses). The interferometer provided very high resolution, allowing the team to observe fluctuations in energy and linewidth within a single quasi-CW condensate pulse. “The interferometer enabled unprecedented levels of detail that were obscured in previous measurements using conventional techniques,” FLEET researcher Bianca Rae Fabricante said. The researchers investigated the influence of optical trapping on the linewidth of the laser. They showed that, contrary to previous studies, the excitonic reservoir responsible for creating the trap does not strongly affect the emission linewidth, provided the condensate is trapped and the pump power is well above the condensation (i.e., lasing) threshold. The excitonic reservoir was previously thought to introduce significant noise into the exciton-polariton laser, but the researchers showed that, as long as the polaritons are trapped, the effect of the reservoir remains weak. Despite its strong nonlinearity, the exciton-polariton laser has a linewidth that is comparable to that of vertical cavity surface-emitting photonic lasers (VCSELs), which are widely used in facial recognition and augmented reality. The current leading-technology, single-mode VCSELs, demonstrates a linewidth of 50 MHz. “Polariton lasers are potentially better than VCSELs for low-energy applications since they can operate at lower powers,” researcher Mateusz Król said. The laser’s long coherence time of at least 5.7 ns theoretically makes it possible to perform thousands of successive operations on the exciton-polariton laser source, to form a macroscopic quantum state of condensed exciton-polaritons for quantum information processing. “Our work not only pushes the boundaries of exciton-polariton laser technology, but also opens up new avenues for utilizing exciton-polaritons for classical and quantum computing,” researcher Eliezer Estrecho said. The exciton-polariton laser’s low-threshold operation, nonlinearity, and solid-state platform are promising for numerous low-power applications, including ultrafast optical polarization switches, modulation applications, compact sources of terahertz radiation, and logic elements. The researchers believe that exciton-polariton lasers could lead to a deeper understanding of noise spectra and spectral properties critical for various applications. “Ever since their discovery, low-threshold polariton lasers that do not require a population inversion have been waiting for practical applications,” said Elena Ostrovskaya, professor of physics at Australian National University. “Our study suggests that such applications can be broader than previously thought.” The research was published in Optica (www.doi.org/10.1364/OPTICA.525961).