Light trap turns photons into massive quasiparticles

November 20, 2019 //By Julien Happich
Light trap
Researchers from the University of Warsaw have created a light trap consisting of a thin optical cavity filled with liquid crystal, whose properties they could tune by applying an external voltage. The tunable light trap imparted novel properties to photons, turning them into massive quasiparticles with a spin.

While electrons trapped in two dimensions (like in graphene) can behave completely differently than free electrons, the researchers wanted to impose similar dimension constraints on photons to unveil unique or new exotic behaviours. A team of researchers from the University of Warsaw, the Polish Military University of Technology, the Institute of Physics of the Polish Academy of Sciences, the University of Southampton and the Skolkovo Institute near Moscow have set themselves the goal to study light trapped in two-dimensional structures.

Reporting their research in a Science paper titled “Engineering spin-orbit synthetic Hamiltonians in liquid-crystal optical cavities”, the scientists created an optical cavity in which they trapped photons between two mirrors. The original idea was to fill the cavity with a liquid crystal material that acts 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 (frequency of vibrations) was different when the electric field of the wave (polarization) was directed across the molecules and different for polarization along their axis (this phenomenon is called optical anisotropy). During the research, conducted at the University of Warsaw, the unique behaviour of photons trapped in the cavity was found as they behaved like mass-bearing quasiparticles. Such quasiparticles have been observed before, but they were difficult to manipulate because the light does not react to electric or magnetic fields.

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. Credit: M. Krol, UW Physics

This time, it was noted that as the optical anisotropy of the liquid crystal material in the cavity was changed, the trapped photons behaved like quasiparticles endowed with a magnetic moment, or a "spin" in an "artificial magnetic field." Polarization of the electromagnetic wave played the role of "spin" for light in the cavity. The behaviour of light in this system is easiest to explain using the analogy of the behaviour of electrons in condensed matter, the author noted, resorting to the equations of motion of electrons with spin to describe the motion of photons trapped in the cavity.

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