Bose Condensation of Upper-Branch Exciton-Polaritons in a Transferable Microcavity*

Exciton-polaritons as bosonic quasiparticles are ideal objects for realizing Bose-Einstein condensation in a solid environment due to the light-effective mass, which is the crucial step towards the next-generation optical device [1-2]. Simultaneously, the advent of transition-metal dichalcogenides (TMDs) monolayers has introduced a new realm of materials for polaritons [3-5]. Here, we report the first observation of upper polariton Bose-Einstein condensation in a transferrable WS₂ monolayer microcavity [6]. The evidence has been shown of a nonlinear intensity increasing, linewidth decreasing and energy blueshift in power-dependent measurement. The upper polaritons accumulate more in the ground state above the threshold. We also observe an increase in coherent time above the threshold. The dynamic rate equation shows a crucial conversion time from upper polariton to lower polariton, which should be larger than the upper polariton lifetime, leading to the possibility of condensation. Quantum Boltzmann equation simulation shows the upper polariton condensation would happen in a critical particle density and special detuning. Our work paves the way for further understanding condensation competition in open-dissipative quantum systems while linking to practical applications, shedding light on both fundamental aspects of quantum physics and practical laser technologies.