One problem hindering the use of two-dimensional (2-D) photonic crystals in integrated optical devices is the difficulty of achieving a complete bandgap. Although high-refractive-index materials such as gallium arsenide permit complete bandgaps, the ability to use lower-index materials such as glass would allow researchers to make low-loss photonic-crystal waveguides out of common optical materials.
Theory shows that the use of quasiperiodic crystals—or quasicrystals—can be a way out of this dilemma. Researchers have previously built 2-D photonic quasicrystals that operate at microwave frequencies (see Laser Focus World, Nov. 1999, p. 9), proving that the concept works, but the optical region of the spectrum has remained unexplored. A group at the University of Southampton (Southampton, England) has now demonstrated a photonic quasicrystal that produces complete bandgaps in the near-infrared, opening up applications at 1.5 µm and other telecommunications wavelengths.
Quasicrystals have aperiodic lattices with long-range order. Depending on their geometry, they can also have higher orders of symmetry than standard 2-D crystals, which peak at sixfold symmetry for a hexagonal lattice (the higher the symmetry order, the less angle-dependent—and more complete—the bandgap becomes).
The Southampton group chose a geometry with 12-fold symmetry, fabricating the quasicrystal by etching air holes through a silicon nitride slab waveguide clad with silicon dioxide (top). Although at first glance the holes appear to be randomly jumbled, they actually lie at the vertices of squares and triangles placed in a random ensemble. The resulting bandgaps are wider and flatter than those produced by ordinary photonic crystals, says Greg Parker, one of the researchers. "This could have important consequences for those trying to make tortuous waveguides or sharp waveguide bends using photonic crystals," he notes.
Illuminating the quasicrystal with a 633-nm collimated beam produces diffraction at angles that correspond to theory (bottom). The researchers use a 450-1800-nm continuum produced from ultrafast laser pulses passed through 1 mm of sapphire to measure spectral transmission of the quasicrystal. Extinction at the center of the main bandgap reaches to 10-3.
"We are currently working on using the wide bandgap to cut out competing radiative transitions in rare-earth-doped glasses," says Parker. "We will thus make a rare-earth-doped laser running the line we want using a quasicrystal structure." Parker also says that raising the symmetry order beyond 12 is another one of the group's goals. (Images originally appeared in Nature 404, 13 April 2000, p. 741. Used with permission.)
John Wallace | Senior Technical Editor (1998-2022)
John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.