Nanowire/photonic crystal structure

Researchers in the US and Korea are using active semiconductor nanowires to light up photonic crystal waveguides for the first time. According to the group, its nanowire/photonic crystal arrangement represents a significant step towards all-optical processing in nanoscale integrated photonic circuits (Nature Photonics doi:10.1038/nphoton2008.180). "In this work, we have combined active semiconductor nanowires and photonic crystal waveguides, for the first time," Hong-Gyu Park, a researcher at Korea University, South Korea, said. "The result is a hybrid  anowire/photonic-crystal structure that can offer efficient injection of light on a chip into photonic crystal waveguides."  Figure shows schematic of a CdS/CdSe core/shell nanowire facing two different straight line-defect waveguides. The left waveguide is designed to guide red light and the right waveguide is designed to guide green light (Image Credit: Hong-Gyu Park).

The efficient delivery of photons from light sources to photonic circuits is central to any fibre-optic or integrated optical system. Coupling light emitters with optical fibres or waveguides strongly affects the performance of photonic devices for optical communication and information processing. "Many solutions have been proposed to improve impedance matching in LED-to-fibre or photonic crystal cavity-to-waveguide systems," commented Park. "However, the efficient coupling of integrated light sources into nanophotonic circuits remains a challenge." According to Park, photonic crystal waveguides show great potential for optical communication. Semiconductor nanowires can act as a nanoscale light source that offers high radiance from a small emitting area and low power consumption. By combining the two technologies, the team has fabricated a hybrid structure that could simplify nanoscale integrated optical processing as well as encouraging practical applications of single-photon sources.

"We combined a bottom-up synthesized single-crystal semiconductor nanowire emitter with a top-down fabricated photonic-crystal waveguide," explained Park. "The nanowire is comparable in size to the photonic crystal, which leads to impedance matching in the two structures and thus an increase in device efficiency. Further optimization of the structure should also allow enormous reduction of the optical losses." In the setup, single-crystal semiconductor nanowires were randomly dispersed onto a silicon-nitride transparent film. Light is emitted from the semiconductor nanowire by either optical or electrical pumping and then efficiently guided by the photonic bandgap in the horizontal direction, and by total internal reflection in the vertical direction. The next step for the team is to improve device efficiency via further optimization of the structure. "A careful study of the surface roughness of the holes, which is the main reason for the waveguide loss, is necessary," concluded Park.