Low-threshold exciton-polariton condensate via fast polariton relaxation in organic microcavities*

Exciton-polaritons, in which the electronic state of an excited organic molecule and a photonic state are strongly coupled, can form a Bose-Einstein condensate (BEC) at room temperature. However, so far, the reported thresholds of organic polariton BECs under optical excitation are as high as P_th ~ 11 - 500 μJ cm^-2, which is still too high for many optoelectronic applications. Therefore, achieving a significant decrease of the BEC threshold continues to be a major challenge in the field.

One route towards lowering the condensation threshold is to increase the polariton relaxation rate (W_ep). However, the relationship of W_ep with a condensation threshold has not been fully explored. In this study, we demonstrated a room-temperature polariton condensate at a threshold pump fluence of 9.7 ± 0.1 μJ cm⁻², in a microcavity containing 4,4'-bis((E)-4-(3,6-bis(2-ethylhexyl)-(9H-carbazol-9-yl))styryl)-1,1'-biphenyl (BSBCz-EH). By using a semiclassical model to describe the polariton kinetics, we revealed that these low threshold results from the rapid relaxation rate from the dark exciton reservoir to the set of the lower polariton states forming the condensate, indicating that accelerating polariton relaxation is an important factor for realizing low-threshold polariton condensates.