Semiconductors Stack Up as Photonic Crystals

KYOTO, Japan -- The same structure that one might use to build a bonfire is also heating up photonics. By alternately stacking strips of semiconductor material, scientists at Kyoto University and the Ministry of International Trade and Industry in Tsukuba have created three-dimensional photonic crystals with a nearly perfect bandgap at telecommunications wavelengths.
In a repetitive two-step process, the researchers grew GaAs or InP on a III-V semiconductor substrate in a striped pattern, with the period, width and thickness of the pattern dictated by the bandgap wavelength. They then fused two striped wafers to the layer in a configuration rotated 90° with respect to the one below. Wet chemical etching selectively and sequentially removed one of the substrates and the etching stop layers, leaving a pile of micron-size "logs." The team shifted subsequent layers by half a period from their parallel counterparts on other layers, using a laser-diffraction alignment method to precisely control the placement of the structures.
An eight-layer crystal, described in the July 28 issue of Science, showed attenuation of >40 dB, corresponding to 99.99 percent reflection at the full bandgap of 1.3 to 1.55 µm. The researchers also demonstrated an air waveguide with a 90° bend that they constructed in a 12-layer crystal.
Team leader Susumu Noda, a professor of science and engineering at Kyoto University, said the researchers hope to produce a zero-threshold laser in an optical integrated circuit with sharp waveguides. "To do this," he said, "we have to design a single defect that is appropriate as a cavity and confirm that zero-threshold oscillation occurs -- something that's not been demonstrated yet in the real world.