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Guiding Light through Disorder in Adaptive Photonic Resonator Arrays

Final Report Summary - PHAROS (Guiding Light through Disorder in Adaptive Photonic Resonator Arrays)

The Pharos project, which is a collaboration between groups at Utrecht University in the Netherlands and Thales Research and Technology in France, has developed the new concept of adaptive photonic crystals.
Photonic crystals are nanostructures that strongly modify the propagation of light, being able to guide light in waveguides, or store it in resonators, which are important modalities e.g. for synchronization and filtering of optical signals. Smart photonic signal processing promises to enable wideband communications at a fraction of the energy cost of transmitting electronic signals. Unfortunately, the most exciting functionality in normal photonic crystals is limited by disorder, which is unavoidably introduced in the nanofabrication process.

Disorder in a photonic crystal leads to localization of light, a phenomenon that is scientifically intriguing, but in most cases technologically undesirable. It leads to the formation of localized modes which do not transport light and thereby strongly reduces the amount of usable light that makes it through the device. Such localized states were found to occur in any photonic crystal waveguide in the regime where it should be most effective at controlling the propagation.

In the new adaptive nanostructures the effects of disorder are compensated dynamically, after fabrication, by tuning the local properties of a waveguide or resonator. The photonic crystal nanostructures were designed and fabricated from GaInP, a material that allows for transparency from the infrared into the visible wavelength regime.
Using a spatial light modulator, blue-laser holograms are projected on the photonic crystal from outside the plane of propagation. The blue laser light locally heats the photonic crystal, thereby slightly changing its optical properties. The blue light intensity is adjusted by the holographic algorithm to make the light-induced changes exactly cancel the shifts due to disorder. A higher light intensity may be used to induce permanent corrections in the photonic crystal.

The control laser was initially used to detect and map the localized states, by “tickling” them at a very low power. Although this thermal probing has a limited resolution, using advanced data processing the profiles of the modes could be extracted. With the knowledge of the mode profiles, it became possible to push them towards each other using higher laser power, thereby forcing localized modes to join and become good transport channels for light, a process that can be compared to stringing pearls into a necklace.

An even more dramatic effect of disorder appears in coupled resonator waveguides, where resonances have been pre-defined in the nanostructure and light transport proceeds, in the ideal structure, by energy hopping between resonators. Disorder blocks this hopping process, thereby making the waveguide non-transmitting. An adaptively controlled laser was used to cancel the disorder and align the coupled resonators in frequency. As a result the hopping process was found to be restored, leading to higher transmission through the waveguide.

The methods developed in the course of this project are expected t give rise to advances in the fabrication of photonic crystal circuits, for example post fabrication tuning to improve the transmission and cancel the disorder, or dynamic tunable crystals to implement time-dependent functionality, as well as new concepts for power handling in active photonic circuits.