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Parity-time (PT) symmetry in a non-Hermitian framework can be harnessed for numerous applications in optics such as laser mode selection, non-reciprocal light propagation, polaritonic optical switches, and enhanced sensing. Here we show a zero-threshold Raman laser can be achieved in an anisotropic optical microcavity in the PT-symmetry broken phase via polarization selective optical pumping. A loss-gain mechanism between two polarized Stokes modes arises naturally via polarization dependent stimulated scattering and anisotropic Raman gain of the active layered material inside the microcavity. A microscopic theory of stimulated Raman process in anisotropic microcavity successfully explains our results at various temperatures and enabled us to predict the parameters of a proposed quantum PT-symmetric Hamiltonian. Our realization of the zero-threshold Raman laser and the proposed theory of stimulated Raman scattering in anisotropic microcavity can lead to future development of novel technologies such as efficient quantum frequency converters for applications in quantum photonics and information., Comment: Total 10 pages, 4 main figures, 11 supplementary figures

Non-Hermitian systems with parity-time (PT) symmetry have been realized using optical constructs in the classical domain, leading to a plethora of non-intuitive phenomena. However, PT-symmetry in purely quantum non-Hermitian systems like microcavity exciton-polaritons has not been realized so far. Here we show how a pair of nearly orthogonal sets of anisotropic exciton-polaritons can offer a versatile platform for realizing multiple spectral degeneracies called Exceptional Points (EPs) and propose a roadmap to achieve a PT-symmetric system. Polarization-tunable coupling strength creates one class of EPs, while Voigt EPs are observed for specific orientations where splitting of polariton modes due to birefringence is compensated by Transverse Electric (TE) -Transverse Magnetic (TM) mode splitting. Thus, paired sets of polarized anisotropic microcavity exciton-polaritons can offer a promising platform not only for fundamental research in non-Hermitian quantum physics and topological polaritons, but also, we propose that it will be critical for realizing zero threshold lasers., Comment: 18 pages, 4 figures

We show an intriguing connection between the coherence of spectral domain interference of two electromagnetic modes in Fano resonance and the resulting degree of polarization of light. A theoretical treatment is developed by combining a general electromagnetic model of partially coherent interference of a spectrally narrow and a broad continuum mode leading to Fano resonance and the cross-spectral density matrix of the interfering polarized fields of light. The model suggests a characteristic variation of the degree of polarization across the region of spectral dip and the peak of Fano resonance as an exclusive signature of the connection between the degree of polarization and the coherence of the interfering modes. The predictions of the model is experimentally verified in the partially polarized Fano resonance spectra from metal Chalcogenides systems, which emerged due to the interference of a narrow excitonic mode with the background continuum of scattered light in the reflectance spectra from the system. The demonstrated connection between polarization and coherence in the spectral domain Fano-type interference of electromagnetic modes is fundamentally important in the context of a broad variety of non-trivial wave phenomena that originate from fine interference effects, which may also have useful practical implications.

Spin-valley coupling in monolayer transition metal dichalcogenides gives rise to valley polarization and coherence effect, limited by intervalley scattering caused by exciton-phonon, exciton-impurity, and electron-hole exchange interaction (EHEI). We explore an approach to tune the EHEI by controlling excitons center of mass momentum (COM) utilizing the photon distribution of higher-order optical vortex beam. By virtue of this, we have observed excitons COM-dependent valley depolarization and decoherence which gives us the ability to measure the timescale associated with valley dynamics in the steady-state measurement. Our steady-state technique to probe the valley dynamics can open up a new paradigm to explore the physics of excitons in two-dimensional systems., Comment: 25 pages, 5 Manuscript figures, 7 Supplementary figures