Polaritons

The new era of Polariton condensates

  • By Leena Aggarwal
  • 17 October 2017

In a recently published paper in Physics Today, David Snoke, a professor of physics and astronomy at the University of Pittsburgh and Jonathan Keeling who is a reader in theoretical condensed-matter physics at the University of St. Andrews in Scotland, have shown the superfluidity of light where photon treats as a gas of interacting bosonic atoms. They have demonstrated that how to engineer a Bose–Einstein condensation from light.

 

Trapping of Polariton in Microcavities

  • By Leena Aggarwal
  • 9 October 2017

Bose-Einstein Condensation (BEC) is a process which occurs at low temperature when an ensemble of bosons cools down and enters a single quantum state. In most of the experiments, this condensation process includes the atomic gases. But in solid-state systems, which has a long history of generating new types of particles and quasiparticles, BEC can occur in quasiparticles e.g. fermion-like excitations of Bose- condensed Cooper pairs in a superconductor.

One class of such quasiparticles is polaritons, which form from electronic excitations coupled to photons in a microcavity. Polaritons are not fundamentally different in character from elementary particles; they are just highly renormalized to have different properties. In particular, polaritons can be viewed as photons having an effective mass and much stronger interactions than photons in a vacuum.

“A carefully engineered coupling between light and matter could pave the way to a room-temperature Bose–Einstein condensate.”

In this work supported by the NSF and the Engineering and Physical Sciences Research Council, David Snoke and Peter Littlewood have shown the trapping and coupling of polariton with photon in microcavities. Authors have shown a typical structure for polariton experiments where polariton effect will occur with just one quantum well. If a quantum well is placed at an antinode of the confined photon mode in an optical cavity, and if the energy of the exciton is close to that of a photon in the cavity, then the exciton and photon states couple to each other. The standard effect of quantum mechanical mixing leads to two new eigenstates, each of which is a linear combination of the photon and exciton states. Those new eigenstates are the polaritons.

David Snoke's PRL Article Highlighted in Physics Viewpoint

  • By Aude Marjolin
  • 9 January 2017

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.

Breakthrough in Particle Control Creates Special Half-Vortex Rotation

  • By Aude Marjolin
  • 3 March 2015

A breakthrough in the control of a type of particle known as the polariton has created a highly specialized form of rotation. 

PQI faculty Andrew Daley and David Snoke and their colleages at Princeton University conducted a test in which they were able to arrange the particles into a 'ring geometry' form in a solid-state environment. The result was a half-vortex in a 'quantized rotation' form.