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Order from Disorder

Using Structural Disorder to Control Light on a Nanoscale

Findings in Nature Physics paper could lead to more precise information transfer in computer chips and other applications.

A breakthrough by a team of researchers from UCL, Columbia University, UCLA and other institutions could lead to a more precise transfer of information in computer chips, as well as new types of photonic materials for light emission and lasers. The researchers including Dr Nicolae Panoiu (UCL) who performed the theoretical and computational analysis for the project, were able to control light at tiny lengths around 500 nanometers — smaller than the light’s own wavelength — by using disordered photonic crystal lattice structures to counteract light diffraction. The discovery could begin a new phase in our understanding of how light interacts with disordered media and thus lead to new technological applications.

Think of shining a flashlight against a wall. As the light moves from the flashlight and approaches the wall, it spreads out, a phenomenon called diffraction. The farther away the light source is held from the wall, the more the beam diffracts before it reaches the wall. The same phenomenon also happens on a scale so small that distances are measured in nanometers. For example, light could be used to carry information in computer chips the same way it is used in fiber-optic communications systems. But when diffraction occurs, the transfer of data is not as clean or precise as it is required to be. Technology that prevents diffraction and more precisely controls the light used to transfer data at the nanoscale could therefore lead to advances in optical communications and enable optical signal processing using devices with remarkably small footprint.

To control light on the nanoscale, the researchers used a photonic crystal superlattice, a lattice structure designed in such a way that it allows light through in a certain spectral band. Structural disorder was then gradually introduced by patterning the photonic material with thousands of nanoscale heptagonal, square and triangular holes. These holes, each smaller than the wavelength of the light traveling through the structure, serve as guideposts for a beam of light.  Scientists had understood previously that uniformly patterned holes can control the spatial diffraction to some extent. But the researchers found in the new study that, somewhat surprisingly, the structures with the most disorderly patterns were best able to trap and collimate optical beams into a narrow path, and that the structure worked over a broad part of the infrared spectrum. The effect of disorder-induced wave localisation, known as Anderson localization, was first proposed in 1958 by Nobel laureate Philip Anderson. It is the physical phenomenon that in solid state physics explains the conductance characteristics conducting materials. The new study was the first to examine transverse Anderson localization in chip-scale photonic crystal media, as well as certain similarities between electron wave dynamics in conductors and the propagation of photons in nano-patterned photonic (meta)materials. It is published in a recent paper in Nature Physics; an interesting comment on this work can be found in the same issue of the journal.

The sample fabrication was carried out at the Brookhaven National Laboratory in New York and at National Cheng Kung University in Taiwan. The research was supported primarily by a grant from the U.S. Office of Naval Research. Additional support was provided by the National Science Foundation, the Department of Energy and EPSRC.