Abstract
The semiconductor polariton laser promises a new source of coherent light, which, compared to conventional semiconductor photon lasers, has input-energy threshold orders of magnitude lower. However, intensity stability, a defining feature of a coherent state, has remained poor. Intensity noise many times the shot noise of a coherent state has persisted, attributed to multiple mechanisms that are difficult to separate in conventional polariton systems. The large intensity noise, in turn, limits the phase coherence. Thus, the capability of the polariton laser as a source of coherence light is limited. Here, we demonstrate a polariton laser with shot-noise-limited intensity stability, as expected from a fully coherent state. This stability is achieved by using an optical cavity with high mode selectivity to enforce single-mode lasing, suppress condensate depletion, and establish gain saturation. Moreover, the absence of spurious intensity fluctuations enables the measurement of a transition from exponential to Gaussian decay of the phase coherence of the polariton laser. It suggests large self-interaction energies in the polariton condensate, exceeding the laser bandwidth. Such strong interactions are unique to matter-wave lasers and important for nonlinear polariton devices. The results will guide future development of polariton lasers and nonlinear polariton devices.
- Received 25 July 2015
DOI:https://doi.org/10.1103/PhysRevX.6.011026
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Published by the American Physical Society
Physics Subject Headings (PhySH)
Focus
Polariton Laser Upgrade
Published 11 March 2016
The emission from a polariton laser shows the coherence that is common to conventional lasers, a step toward using them as high-efficiency alternative light sources.
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Popular Summary
Semiconductor polaritons are half-light, half-matter particles formed in a high-quality optical cavity. They can condense into their ground state via a thermodynamic phase transition and form a coherent matter wave, and they can produce coherent light emission known as polariton lasing. Polariton lasers do not require electronic population inversion, and they therefore hold promise as a new source of coherent light with energy thresholds orders of magnitude lower than those of traditional semiconductor lasers. However, intensity stability and phase coherence, the defining properties of laser light, have been poor in polariton systems, which will limit the utility of polariton lasers. It has remained unclear whether such properties are intrinsic to polariton lasers. Here, using an optical cavity with high-mode selectivity, we achieve a polariton laser with full intensity stability and, based on its phase coherence properties, confirm its matter-wave origin.
We create a single-mode polariton laser using a cavity that integrates a high-reflectance, polarization-selective mirror a few microns across in size. With this system, which we maintain at 10 degrees above absolute zero, we demonstrate polariton lasing with Poisson-intensity noise over a wide range of condensate occupancies, as expected of a fully coherent state. We also expand our analyses to multimode polariton lasers. At high occupancies, the strong condensate nonlinearity is manifested in the Gaussian decay of the intrinsic phase coherence, which is characteristic of matter-wave lasers.
This work provides the proof of principle of a polariton condensate as a matter-wave-based source of coherent light. We expect that our findings will guide the future development of polariton lasers and nonlinear polariton devices.