Scientists have devised a way to reveal the superpowers of a crystal, with a mere flash of laser light.

The superpowers in question, known as topological phases of light, include the ability to guide light along winding paths perfectly with no loss, to transmit it through a barrier or to change its colour, and to perform these processes efficiently even if the crystal contains flaws.

“Because these states are protected against losses they could be used to develop superior digital devices and energy-efficient communications technology such as integrated optical circuits and nanolasers,” said researcher Dr Daria Smirnova, from the Nonlinear Physics Centre at the Research School of Physics, who is a co-author on the Nature Physics paper reporting the finding.

Topological properties emerge in certain crystalline or judiciously designed materials, and occur in both the electronic and photonic behaviour of materials. However photonic properties are much harder to measure because photons do not interact strongly, in the way that electrons do.

This meant that complete characterisation of a material’s optical superpowers required a laborious process to map its structure from every angle using tomography, said lead author Dr Daniel Leykam from the Institute for Basic Science in Korea.

“Light in the middle of a crystal typically undergoes complicated wave interference which is highly sensitive to defects and obscures the topological properties,” he said.

The pair theorised that a simpler method was to illuminate the material with a laser pulse. Some energy would then be trapped and form a topological phase, while the rest would leak out. 

“This method is similar to distillation – modes belonging to different energy bands can be reliably separated, by exploiting their different lifetimes. The topological properties of the material are then directly imprinted on the remaining trapped light,” said Dr Smirnova.

“Our technique is superior to other methods because, by using our recipe, the topological invariants can still be correctly retrieved even for small and disordered lattices.”

To validate their theory, Dr Smirnova and Dr Leykam simulated two representative lattice models, finding perfect agreement. A collaborator is now working on experimental tests.

Dr Leykam began working on the theory in 2018 but did not immediately realise how it could be used to simply unveil material properties, which seemed thoroughly shrouded.

“The mechanism for their remarkable robustness against defects and disorder is rooted in topological properties of the medium in momentum space, that is, the existence of abstract holes in the modes of the system,” he said. 

For electrons, these abstract properties show up in measurable quantities, such as conductance at low temperature and in high magnetic fields, conditions which do not affect photons and therefore make optical properties much harder to characterise. 

However, the new technique enables easier access to the optical properties – Leykam and Smirnova propose that waveguide arrays inscribed in glass or patterned onto silicon chips will do the job.

“The technique provides a simple way to make light mimic electrons in topological condensed matter systems,” said Dr Leykam.

“It spontaneously creates topologically-ordered states, without requiring cryogenic temperatures or strong interactions, which are notoriously hard to implement in photonics.”

Probing bulk topological invariants using leaky photonic lattices was published in Nature Physics in February 2021.