Light Travels a One-Way Road

A new magnetic class of photonic crystals allows electromagnetic waves to flow freely in one direction only, a phenomenon that could lead to zero scattering loss in photonic devices such as optical waveguides.

Light readily bounces off obstacles in its path, and the laws of nature allow for the propagation of light in both directions. If a light beam is observed propagating in a particular direction, one can also shine a light beam to propagate in the opposite (backward) direction. Light attenuation and scattering loss are two of the problems that occur when information is transferred from one place to another via light waves sent through waveguides (tunnels or "roads" for guiding light).

In a magnetized photonic crystal (the array of the translucent rods) supporting topological states, light travels along the edge of a wall (black) around a protruding object (not shown) without any reflection or dissipation. (Images courtesy Zheng Wang, John D Joannopoulos, Marin Soljacic)

Photonic crystals are materials with periodic variations of their refractive index comparable to the wavelength of the light passing through them. Just as the electronic band structure of semiconductors means that no electrons can be found containing energies within a range known as the "bandgap," photons in photonic crystals with frequencies falling within the "photonic bandgap" are prevented from flowing inside the material.

The new materials designed by MIT research scientist Zheng Wang and his colleagues (recent MIT PhD recipient Yidong Chong, professor John Joannopoulos, and assistant professor of physics Marin Soljacic) produce the photonic equivalent of the electronic phenomenon known as the quantum Hall effect, where electrons at the edge of a two-dimensional system in a magnetic field flow unimpeded in one direction only. By designing a special kind of photonic crystal out of magneto-optical ferrite rods that allow electromagnetic waves flow unidirectionally and without scattering at its edges, the researchers essentially produced a photonic analogue of the electronic effect.

"We have now found a way to make light travel without bouncing back, by shining it through an array of small ceramic rods placed in a strong magnetic field," said Wang, a lead author of a paper on the work appearing in the Oct. 8 issue of Nature.

"Once a particular forward direction of the light travel is chosen, no backward travel is permitted," said Chong, also a lead author of the paper. Therefore, light can never bounce back or reflect. Rather, it effortlessly routes around any obstacles and defects in its path without incurring any dissipation.

"Loosely speaking, the waveguide acts as a perfect cloak of the defect or obstacle in the path of the light," said Joannopoulos. "The only difference is a phase shift of the guided light."

A variety of practical applications, such as optical isolation and optical information storage, could potentially benefit from the novel and unparalleled one-way photonic behavior observed by the MIT team. Numerous applications that require strong interactions between light and matter could also gain from such an efficiency boost.

Their work also verifies that of Haldane and Raghu of Princeton University, who theorized in Physical Review Letters last year that the edge states of photons in photonic crystals could behave like electrons do in electronic systems by allowing electromagnetic energy to flow in one direction only. It also highlights the universality of the quantum Hall effect.

In a magnetized photonic crystal supporting topological states, light travels along the edge of a wall (black) around a protruding object (not shown) without any reflection or dissipation.

"With such a low-loss waveguide, the possibility would then exist for one-hop transoceanic communication across 10,000 kilometers -- about the distance from San Francisco to Tokyo -- without the current requirement for electronic repeaters or amplifiers," wrote Eli Yablonovitch in a "News and Views" piece accompanying the research in Nature. Yablonovitch is a professor in the electrical engineering and computer sciences department at the University of California, Berkeley.

The MIT team conducted its experiments at gigahertz frequencies and said the terahertz range might be achieved using metamaterials to resonantly enhance the magnetic activity. "Further extension to the optical regime is challenging, given the losses and weak gyrotropic effects in currently known materials," the researchers wrote in their Nature paper.

The research was supported in part by the Materials Research Science and Engineering Program of the US National Science Foundation and the Institute for Soldier Nanotechnologies at the US Army Research Office.

For more information, visit: http://www.mit.edu/