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 how to engineer a Bose–Einstein condensation from light.

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. Polaritons are also belong to the same class of quasiparticles which forms from electronic excitations coupled to photons in a microcavity. They 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.

“Quasiparticles of light and matter may be our best hope for harnessing the strange effects of quantum condensation and superfluidity in everyday applications.”

Polariton Condensate: In free space, photons are massless. But when it is trapped in a cavity between two planar mirrors, they acquire an in-plane dispersion relation indicative of an effective mass. In a high-quality cavity, photons can reflect off the mirrors many thousands of times before escaping. If during that time the photons equilibrate to a well-defined temperature, one can consider them, for all practical purposes, to be conserved. The trickiest step in creating Bose–Einstein condensate of light is making the photons equilibrate or thermalize. One approach to do it is to make the photons interact, so they can scatter off each other. This can be done by implanting inside the optical cavity a nonlinear optical material, in which photons can interact through intensity-dependent, so-called χ(3) contributions to the index of refraction. Those contributions are ordinarily too weak to allow photons to thermalize during the cavity lifetime, but one can enhance them by tuning the photons’ energy to that of an electronic excitation in the medium.

The excitation is typically an exciton—a bound electron–hole pair—in a semiconductor quantum well, but any electronic excitation with a well-defined transition energy will do. The excitation gives rise to a resonance in the polarizability of the medium, which sharply increases the nonlinearity of the index of refraction and strengthens the photon–photon interaction.

When the coupling between a photon and an exciton becomes strong enough, one can no longer think of the two particles as separate eigenstates; instead, one must talk of a polariton—a quantum superposition of a photon and the electronic excitation. The condensation of the Polariton promises to lead to unique optical applications and new physics. Coherent polaritons in microcavities have already been used for nonlinear optical effects as optically gated amplification, optical spin Hall transport, and optical parametric generation of entangled photon pairs.