Massive Photons in an Artificial Magnetic Field

An international research colla­boration from Poland, the UK and Russia has created a two-dimensional system – a thin optical cavity filled with liquid crystals – in which they trapped photons. As the pro­perties of the cavity were modified by an external voltage, the photons behaved like massive quasi­particles endowed with a spin under the influence of an arti­ficial magnetic field.

The scheme of the experiment: circular polarization of light transmitted through a cavity filled with liquid crystals depending on the direction of propagation. (Source: M. Krol, UW)

Physicists studying condensed matter have long been dealing with systems of lower dimen­sionality – two-dimensional (2D) quantum wells, one-dimen­sional (1D) quantum wires and zero-dimensional (0D) quantum dots. 2D systems have found the widest technical appli­cations – it is thanks to the reduced dimensions that efficient LEDs and laser diodes, fast transistors in inte­grated circuits, and WiFi radio amplifiers operate. Trapped electrons in two dimensions can behave com­pletely differently than free electrons. For example, in graphene, a two-dimen­sional carbon structure with honeycomb symmetry, electrons behave like photons.

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 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 two-dimen­sional graphene is such an example. These discoveries inspired 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, to study light trapped in two-dimen­sional structures – 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.

During the research the unique behavior 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. 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 spin in artificial magnetic field. Polari­zation of the electro­magnetic wave played the role of spin for light in the cavity. The behavior of light in this system is easiest to explain 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. Therefore, it was possible to build a photonic system that perfectly imitates electronic properties and leads to many sur­prising physical effects such as topo­logical states of light. The discovery of new phenomena related to the entrapment of light in optically aniso­tropic cavities may enable the implementation of new optoelectronic devices, e.g. optical neural networks and 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 can be used for quantum calcu­lations and simu­lations, solving problems that are too difficult for modern computers. The studied phenomena will open up new possi­bilities for technical solutions and further scientific disco­veries. (Source: U. Warsaw)

Reference: K. Rechcińska et al.: Engineering spin-orbit synthetic Hamiltonians in liquid-crystal optical cavities, Science 366, 727 (2019); DOI: 10.1126/science.aay4182

Link: Institute of Experimental Physics, University of Warsaw, Warsaw, Poland

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