The University of Southampton

Polaritonics research develops new artificial lattice platform

Published: 29 September 2020
3D illustration of a 1D polariton crystal optically imprinted via a spatially sculpted laser

An international research collaboration led by physicists from the University of Southampton has demonstrated a non-invasive optical method to access quantum physics in artificial lattices.

The new study builds on increasing interest over the past decade in optically responsive quasiparticles known as polaritons that originate from the strong-coupling regime between light and matter.

The results could inspire the creation of field-programmable polariton circuitry and new strategies for the robust confinement of coherent light sources, such as lasers.

Researchers from Southampton's Hybrid Photonics group, Lancaster University and the Skolkovo Institute of Science and Technology (Skoltech) in Russia have published their findings in the journal Nature Communications.

Creating artificial lattices for quantum particles enables scientists to explore physics in an environment which might not be conventionally found in nature. Artificial lattices are especially appealing since their symmetries can lead to exactly solvable models and transparent understanding of their properties.

Designing them is, however, a challenging task with limited flexibility as most methods require materials to be irreversibly engineered to get the job done.

The new research, led Southampton’s Dr Lucy Pickup, Dr Helgi Sigurdsson and Professor Pavlos Lagoudakis, and Lancaster's Professor Janne Ruostekoski, overcame this challenge by using structured laser light to synthesise arbitrarily shaped and reprogrammable artificial lattices. These cavity-polariton systems could be changed from one lattice to another without the costly need to engineer a new system from scratch.

Dr Pickup, article co-author, says: "The results open a path to study dissipative many-body quantum physics in a lattice environment with properties that cannot be reproduced in normal Hermitian quantum systems."

When laser light hits a semiconductor quantum well it excites a lot of electrons, holes and bound states of the two, known as excitons. When the quantum well is put between two mirrors, forming a trap (or a cavity) for the photons, some of the exciton particles start becoming 'dressed' in photons, forming new part-light part-matter quasiparticles known as exciton-polaritons or cavity-polaritons.

Exciton-polaritons are very interactive and bounce frequently off one another. However, they also bounce off normal electrons, holes, and excitons in the background around them.

By applying laser light in a geometrically structured fashion the exciton-polaritons start bouncing off the excited electrons, holes and excitons following the shape of the laser. In other words, the exciton-polaritons experience a synthetic potential landscape imprinted by the laser.

The laser generated potential landscapes are only felt by the exciton-polaritons and not the photons inside the cavity, making the system uniquely different from photonic crystals. By creating a laser pattern with translational symmetry the researchers produced the fundamental signature of solid state systems, the formation of crystal energy bands for exciton-polaritons, just like one would observe for electrons in solid state materials.

The produced bands can be reconfigured by simply adjusting the laser pattern, making the method applicable to a variety of potential applications from optical-based communications and information processing, to high sensitivity detectors in biomedicine and topologically protected lasers. The results also open a path to study fundamental many-body lattice physics in an open quantum environment.

Dr Sigurdsson, article co-author, adds: "This is an exciting development for the relatively new field of non-Hermitian topological physics and has only just begun."

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