Space–time symmetry renders optical systems invisible

FAU researchers examine the interplay between light amplification and light loss

For the first time ever, physicists from Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU) have succeeded in proving that an optical system can be ‘invisible’ from one side and act like a mirror from the other side. The study has recently been published in the renowned journal ‘Nature’.

The development of photonic materials and complex systems with new properties and great potential for application is one of the core topics of present-day optics. Currently, such metamaterials are mainly based on the manipulation of light-refraction in the sub-wavelength range—so-called ‘optical cloaks of invisibility’ work according to the same principle. Scientists have recently discovered that light propogation can also be influenced substantially by adjustingamplification and loss. This involves maintaining the parity-time symmetry (PT) so that light amplification and loss merge into each other in a space–time–reflection.

“Interestingly enough, this new concept was first developed as an alternative interpretation of quantum theory,” states Prof. Dr. Ulf Peschel of FAU’s Institute for Optics, Information and Photonics. In a joint project with colleagues from the Erlangen Max Planck Institute and from Orlando, FAU scientists have now succeeded in transferring this principle to optics and are applying the principleto light pulses in large optical networks. Experiments have shown that in loop mirrors with controlled periodical amplification and loss, light travels in a fundamentally different way than in conventional materials. The strength of optical fields can change drastically—in certain parameter ranges, the flanks of light pulses travel beyond the speed of light.

Thanks to the correlation between amplification and loss, PT-symmetrical materials can become partially invisible. “When a ray of light hits one side of the medium, it is transmitted completely without any reflection at all—just as if there was no scattering body there,” states Prof. Peschel. If the same ray of light hits the other side of the medium, there are extremely strong reflections.” The method used in the experiments can be applied directly to microstructured optical systems and thus opens up an entirely new field of applications.

The project brings together scientists from FAU’s Institute of Optics, Information and Photonics, the Cluster of Excellence ‘Engineering of Advanced Materials’ (EAM), the Erlangen Graduate School in Advanced Optical Technologies (SAOT), the Max Planck Institute for the Science of Light and the University of Central Florida, Orlando. Research results have recently been published in the acclaimed journal ‘Nature’ (DOI:10.1038/nature11298).

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