Trapped Light in Anisotropic Cavities Could Lead to Optoelectronic Advancements

A research collaboration from Poland, the United Kingdom, and Russia has created a two-dimensional system in which they trapped photons. The system, a thin optical cavity filled with liquid crystal, was modified with an external voltage that caused the photons to behave like massive quasiparticles endowed with a magnetic moment, called “spin,” under the influence of an artificial magnetic field.

The scheme of the experiment: circular polarization of light (marked in red and blue) transmitted through a cavity filled with liquid crystal depending on the direction of propagation. Courtesy of M. Krol, University of Warsaw Physics.

The researchers filled the cavity with a liquid crystal material that acted as an optical medium. Under the influence of an external voltage, molecules of this medium can rotate and change the optical path length. Because of this, it was possible to create standing waves of light in the cavity, whose energy was different when the electric field of the wave was directed across the molecules and different for polarization along their axis, a phenomenon called optical anisotropy.

During the research, conducted at the University of Warsaw, the trapped photons behaved like mass-bearing quasiparticles. Such quasiparticles have been observed before, but have historically been difficult to manipulate, as the light does not react to electric or magnetic fields. The researchers noted that in this case the optical anisotropy of the liquid crystal had changed: The trapped photons behaved like quasiparticles endowed with spin in an artificial magnetic field. Polarization of the electromagnetic wave played the role of spin for light in the cavity.

The researchers described the behavior of light in this system using the analogy of the behavior of electrons in condensed matter. The equations describing the motion of photons trapped in the cavity resemble the equations of motion of electrons with spin, which served as inspiration for the experiment. The researchers found that it was possible to build a photonic system that perfectly imitated electronic properties and led to many surprising physical effects such as topological states of light.

Electrons in a crystal interact with each other and with the crystal lattice, creating a complex system whose description is possible thanks to the introduction of the concept of so-called quasiparticles. Properties of these quasiparticles, including electric charge, magnetic moment, and mass, depend on the symmetry of the crystal and its spatial dimension. Physicists can create materials with reduced dimensions, discovering “quasi-universes” full of exotic quasiparticles. The massless electron in 2D graphene, which behaves like a massless particle such as a photon, is such an example.

The discovery of new phenomena related to the entrapment of light in optically anisotropic cavities may enable the implementation of new optoelectronic devices, for example, optical neural networks that perform neuromorphic calculations. There is particular promise to the prospect of creating a unique quantum state of matter, the Bose-Einstein condensate. Such a condensate could be used for quantum calculations and simulations, solving problems that are too difficult for modern computers.