David Snoke's PRL Article Highlighted in Physics Viewpoint
Matter-Light Condensates Reach Thermal Equilibrium
Making use of improved microcavities, hybrid condensates of matter and light can be tuned to reach a thermal equilibrium state, despite their finite lifetime.
In a laser, coherent light is created by stimulated emission of photons from an “inverted” state of matter that is significantly out of thermal equilibrium. “Inverted” means that excited states of the matter are more occupied than lower energy states, so that emission is more likely than absorption. The coherence of laser light is closely related to a quite different, and less commonly encountered, state of matter—a Bose-Einstein condensate (BEC). In the textbook description of a BEC, at low enough temperatures or high enough densities, a large number of particles occupy the same state, producing a coherent state of matter. In contrast to laser light, the textbook BEC is in thermal equilibrium. Condensates of polaritons—half-light, half-matter quasiparticles—have so far been found in conditions halfway between those of an equilibrium BEC and those of a laser. Work by David Snoke and colleagues now shows that such polariton condensates can be tuned to reach a thermal equilibrium state. With this tunability between an equilibrium and nonequilibrium state, researchers can explore how the character of phase transitions evolves between the two limits.
Snoke and colleagues created a system of polaritons in a semiconductor microcavity based on distributed Bragg reflectors as mirrors. The mirrors consist of alternating semiconducting layers and trap photons in the cavity. The polaritons are superpositions of these trapped photons and excitons that are confined in the middle semiconductor layer. A ring-shaped pump laser beam (red) creates a ring of hot polaritons (darker red), which both feeds and traps a cold polariton condensate (blue) in the middle, with increasing density (darker blue) towards the center.