System simultaneously uses loss, gain

Light absorption or loss, usually seen as a disadvantage in optical metamaterials, actually can have useful applications.

Physicists at Friedrich Alexander University Erlangen-Nuremberg borrowed a concept from quantum field theory in designing a new metamaterial. By applying the abstract idea of “parity-time (PT) symmetry,” they altered the way light is transported and produced unusual optical behavior – invisibility in one direction, for example.

Previous experimental work in this area had been limited to small-scale systems; the work by Ulf Peschel and colleagues is the first experimental observation of light transport in a large-scale synthetic material.

“Our experimental results represent a step in the application of concepts from parity-time symmetry to a new generation of multifunctional optical devices and networks,” the team wrote in its paper, which appears in Nature (doi: 10.1038/nature11298).

Physicists have applied the abstract idea of “parity-time (PT) symmetry” to alter the way light is transported. Here, a spatial fiber coupler network equivalent to the temporal PT-symmetric lattice is realized in the Friedrich Alexander University experiment. Gain/loss regions are shown in red/blue. This is the first experimental observation of light transport in a large-scale synthetic material.

Currently, metamaterials are based mainly on the manipulation of light refraction in the subwavelength range, and optical invisibility cloaks work according to the same principle. Only recently did scientists discover that light propagation also can be influenced substantially by adjusting amplification and loss. This involves maintaining the PT so that light amplification and loss merge into each other in a space-time-reflection.

In their setup, the physicists injected a sequence of light pulses into two connected optical fiber loops designed to exhibit PT symmetry. By alternating gain and loss in the two loops, they attained the imaginary part of the refractive-index profile. The real component of the profile was introduced using phase modulators.

The approach “can be easily extended to on-chip configurations,” the team wrote, “paving the way for the realization of PT synthetic devices and effective media with new and unexpected optical properties.”

An artist’s illustration of experimental data demonstrating unidirectional invisibility: If the ray of light hits the medium from the left, the reflections at the red and blue scattering bodies made of PT-symmetrical material are even stronger than the ray of light itself. If the same ray of light hits the active elements from the right, the reflection is suppressed, and the ray can travel through the elements without any obstacles, which means that the scattering bodies are invisible from the right.

Similar concepts can be effectively used in other areas, such as plasmonics and metamaterials, where the harmonic coexistence of gain and loss is ultimately required, they added.

Experiments showed that in loop mirrors with controlled periodical amplification and loss, light travels fundamentally different from 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.

The work was done in collaboration with scientists from the university’s Institute of Optics, Information and Photonics, the Cluster of Excellence Engineering of Advanced Materials, the Erlangen Graduate School in Advanced Optical Technologies, the Max Planck Institute for the Science of Light and the University of Central Florida.