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Researchers Create Tunable Photonic Bandgap Crystal

Photonics Spectra
Jan 2000
Richard Gaughan

Researchers around the world are investigating the properties of photonic crystals, the low-loss periodic dielectric lattices that influence the propagation of electromagnetic waves. Semiconductor materials with electronic bandgaps -- electron energies that are forbidden within the crystal -- were the cornerstone of the electronics revolution. Now, materials with electromagnetic bandgaps -- photon energies that cannot propagate through the crystal -- promise to lead a photonics revolution. That promise engendered enough interest to fill the November 1999 issues of both IEEE Microwave Theory Techniques and the IEEE Journal of Lightwave Technology.

A recent development comes from Sajeev John and his team at the University of Toronto: a photonic crystal with a tunable bandgap. The researchers, who reported their findings in the Aug. 2, 1999, issue of Physical Review Letters, started with an artificial opal crystal. They infiltrated silicon into the air gaps of the crystal, then etched out the opal, leaving silicon in an inverse opal crystal structure.

The inverse opal's silicon structure has periodic spherical voids. Coating the voids with nematic liquid crystal provides a tunable photonic bandgap material. Courtesy of Sajeev John.

The resultant periodic dielectric array exhibits an 8 percent bandgap at 1.5 µm; i.e., wavelengths between about 1.38 and 1.62 µm cannot propagate through the crystal. To achieve tunability, John coated the internal surfaces of the crystal with BEHA, a low-index nematic liquid crystal. The liquid crystal reduces the bandgap to 1.6 percent, but it can now be completely closed via an external magnetic field. This tunability is the key.

John said that the application of an electric field shifts the bandgap and some features of the band structure. He compares this to what happens in a semiconductor when an electric field is applied: Shifting the electronic bands controls the flow of electrical current. In the tunable photonic bandgap, light can be steered with control of the material's linear electro-optic effect.

The tunable bandgap could be used not only for simple on/off switching, but also for more localized control of the propagation. By applying electric fields to different regions within the crystal, the path of the light can be precisely controlled.

John's discovery is sparking interest in the photonic crystal community. Eli Yablonovitch commented on it in the Oct. 7 issue of Nature -- not only because of the tunability, but also because of the prospect that these materials may be easily manufactured. He said that the inverse silicon opal and the infiltration of the liquid crystal are straightforward.

According to John, only two challenges remain: precisely controlling the degree of infiltration and ensuring that the liquid crystal wets the inner surfaces uniformly. "The whole point of this project is to demonstrate that [photonic bandgap] materials could be mass-produced at negligible cost using the tricks of self-assembly," he said. If the low-cost and high-performance expectations are met, this could be another step in the photonics revolution.

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