A Zero-Threshold Polariton-Raman Laser

Raman lasers are known for their low power operation and wavelength tunability unattainable by conventional lasers. Recently ultralow-threshold Raman lasers had been realized in semiconductors with various geometries. Zero-threshold lasers were realized previously only in single atom emitter strongly coupled in a single mode cavity with spontaneous emission coupling factor of unity. However, this was not feasible in microcavities hosting isotropic Raman active materials with degenerate bare cavity modes. Here we show that a zero threshold Raman laser can be achieved when Stoke shifted polariton Raman modes are tuned within one of the anisotropic exciton-polariton bands in an optical microcavity. In addition, we demonstrate that Raman active anisotropic exciton-polaritons in a single mode microcavity offers a platform to realize a PT-symmetric non-Hermitian quantum system of two nearly orthogonal polariton modes that constitute a non-trivial band dispersion containing exceptional points. We found that the PT-symmetric phase responsible for the zero-threshold lasing can be switched to a finite threshold in PT-symmetry broken phase via cavity detuning by the variation of bath temperature, as well as by the variation of pump polarization and energy. Our discovery of zero threshold polariton Raman laser, would not only open up several new paradigms of applications in the areas of quantum optics and on-chip photonics but also offer a platform for research in quantum physics of PT-symmetric non-Hermitian system.