A semiconductor film just one atom thick is like a communal apartment: it houses negatively charged electrons and positively charged 'holes.' When light hits such a film, an electron and a hole can pair up into an exciton — a bound state of an electron and a hole, akin to a hydrogen atom. If a free electron happens to be nearby, it can join the pair, forming a trion — a trio of two electrons and one hole. A trion is heavier and less mobile than a lone electron.
Scientists considered a scenario where the film already contains free charge carriers, and light continuously generates new excitons. They found that if the system is heated with low-frequency radiation, under certain conditions two stable equilibrium states emerge. In the first, cold state, almost all free electrons are bound into trions. In the second, hot state, most trions have dissociated, leaving many free electrons and holes. The transition between these states is like a light switch: the system abruptly changes its temperature, electric current, and emission brightness.
The reason lies in the different conductivities of the two states. Free electrons and holes absorb low-frequency radiation much more efficiently than bound trions. Thus, the hot state, with its abundance of free carriers, heats up more and sustains itself. The cold state, where carriers are tied up in trions, absorbs weakly and stays cold. This creates a positive feedback loop that gives rise to bistability.
This light-driven phase transition resembles the freezing of water: at the same temperature, the system can exist in two distinct states — liquid or solid. Here, however, the control parameter is not temperature but the intensity of the incident light.
Trions were first observed in 1993 in semiconductor quantum wells, and their study in two-dimensional materials gained momentum after the isolation of graphene.
In Greg Egan's novel 'Permutation City,' materials that change their properties under light are described, vaguely reminiscent of the effect discussed here.