Spontaneous Formations of Dynamical Steady States in Polariton Condensates

We present a numerical analysis on dynamical steady states of polariton Bose–Einstein condensates (BECs) in an incoherent exciton reservoir driven by a ring-shaped optical pump. The balance between the loss and gain of polariton BEC induces a variety of steady states with different configurations, including approximate Gaussian distribution and topological defects, such as vortex–antivortex pairs, vortices with a winding number, and solitons. Besides, the system becomes unstable under fast decay rates and small pumping ring, where BECs can no longer exist in the long-time limit. We also confirm the soliton is dynamically stable in this system, with a steady polariton current induced by the repulsive polariton–polariton and polariton–exciton interactions.

Valley depolarization in downconversion and upconversion emission of monolayer WS2 at room temperature

Benefiting from strong photon–exciton and phonon–exciton interactions in atomic thickness, transition metal dichalcogenides (TMDCs) are viewed as one promising platform for exploring elementary excitonic photoluminescence (PL) and intrinsic spin–valley properties at the monolayer limit. Despite well-studied Stokes downconversion (DC) PL, the anti-Stokes upconversion (UC) PL has been recently reported in TMDC monolayers, which mainly focus on UC mechanisms while detailed valley-related dynamical processes are unwittingly less concerned. Here, we carry out an in-depth investigation on both DC and UC emission features of monolayer WS2 at room temperature, where UC PL persists with energy gain up to 190 meV. The PL excitation and power-dependent experiments clearly distinguish the origins of DC PL and UC PL, which refer to saturated absorption and phonon-assisted transition from charged trions to neutral A-excitons. And contrast valley properties are observed in DC and UC scenarios with polarization-resolved PL and pump–probe measurements. According to the experimental facts, phenomenological dynamical DC and UC scenarios are modeled with intervalley depolarization taken into consideration, in which intermediates from spontaneous intervalley depolarization account for the observed emission and valley properties. This work can help understand the light–matter interactions and valley properties in monolayer TMDCs.