The Nature of Trions and Trion-Polaritons in 2D Materials: Theory and Experiments

The traditional picture of trions as bound states of two electrons and hole has been called into question by several recent works ^[1,2]. The traditional model of trions is also inconsistent with coherent trion-polaritons ^[3]. In this work we present a many body description of trions and trion-polaritons that is valid over a wide range of electron densities. In our work, a negatively charged trion, for example, appears as a bound state of a valence band hole, two conduction band electrons, and a conduction band hole. At small electron densities, this 4-body bound state description of trions is found to be approximately correct. The binding of the conduction band hole allows coherent trion-polaritons. At much larger electron densities, bound trion states don’t exist and a many-body screened-exciton description of trions is found to be more accurate. We present experimental results for the evolution of the exciton and trion oscillator strengths and binding energies with the electron density, and also for the trion-polariton dispersion relations, in 2D-MoSe₂, and show very good agreements with our theoretical model. [1] Phys. Rev. B, 95, 035417 (2017), [2] Nat. Phys., 13, 255 (2016). [3] Nat. Phys., 14, 130 (2018)