High conductivity, mechanical flexibility and bandgaps in the visible frequency range make organic semiconductors choice materials for optoelectronic devices. But their charge carrier behaviour is surprisingly different from that of their inorganic counterparts; many observations remain unexplained. For example, when exciton polaritons in organic microcavities form a Bose–Einstein condensate — a state related to polariton lasing — a step-like blueshift occurs at the threshold. Through a series of experiments Timur Yagafarov and colleagues have now pin pointed the origin of this blueshift.
Having established a set of possible causes, the team systematically ruled out contributions from the Kerr effect and frequency pulling and ultimately identified an interplay of saturation effects and intermolecular energy migration as the origin of the blueshift. They further derived an analytical expression to evaluate the relative contributions of all the effects that play into the blueshift, which could help us understand non-linear light emission in organic cavities.