Trapping of Polariton in Microcavities

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 made of layers of semiconducting materials. In the thin quantum-well layers, the charge carriers (electrons and holes) are confined to 2D motion. The quantum wells are then placed between two mirrors, typically made of alternating dielectric layers in a Bragg reflector structure; the arrangement makes an optical cavity that also restricts photons to 2D motion. If there were no cavity, then the absorption of a photon by a quantum well at low temperature would yield an exciton—an electron in the conduction band and a hole in the valence band, bound together by Coulomb attraction. An exciton is a composite boson, just like a Cooper pair of electrons in a superconductor, except that it is metastable, since the electron can emit a photon and fall back into the hole. An exciton is analogous to positronium, in that both are particle– antiparticle pairs with finite lifetimes. 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.

The condensation of the Polariton condensate promises to lead to unique optical applications and new physics. Coherent polaritons in microcavities have already been used for such nonlinear optical effects as optically gated amplification, optical spin Hall transport, and optical parametric generation of entangled photon pairs. Current work on electrical pumping instead of the widely used optical pumping may open a further array of potential applications, such as low- threshold coherent light generation, optical transistors (modulation of one light beam by another), and entangled photon-pair emitters. The polariton express is gathering speed.