- 3-D Crystals Produced by Microassembly
Daniel S. Burgess
Employing techniques that were developed for manufacturing integrated circuits, researchers in Japan have constructed three-dimensional photonic crystals from layers of patterned semiconductor plates. The method points to the development of more efficient emitters and detectors, and may lead to nanophotonic analogues of electronic integrated circuits.
Scientists have suggested, and demonstrated, several techniques for the production of 3-D photonic bandgap structures, including the stereoscopic dry etching of III-V semiconductors, colloidal precipitation and holography, but each method has encountered problems in its flexibility of design or materials that threaten its application, said Kanna Aoki, currently of the University of Tokyo. Layer-by-layer deposition techniques have displayed good results, but they are complicated and expensive and feature etching, polishing and heating processes that can damage the constituent materials.
Aoki and her colleagues at Riken, at the National Institute for Materials Science and at Yokohama National University instead fabricate the layers of the photonic crystal separately and stack them with a micromanipulation system. In a recent demonstration of the technique, they lithographically patterned a series of 2-D photonic plates in an indium-phosphide wafer, separated the plates from the substrate with mechanical pressure and used approximately 1-µm-diameter polystyrene beads seated in fiducial holes to guide the alignment of the layers.
The assembly process, which they conducted in the chamber of a scanning electron microscope, took approximately one hour per layer. The application of mechanical pressure to a stack of plates for 30 minutes initiated self-bonding. In the demonstration, they produced stacks of up to 20 layers.
To further demonstrate the flexibility of the technique, the researchers also produced an eight-layer InP photonic crystal that incorporated a controlled defect structure. The stack, which sandwiched two plates of 0.47-µm-diameter rods between plates of 0.37-µm rods, displayed a transmission peak at the edge of the photonic bandgap that did not appear in the spectra of a photonic crystal without the defect layers.
Aoki said that the approach brings a new flexibility to the production of photonic crystals. It also promises to enable the integration of optical and electronic devices, but a more precise assembly process will be required before the technique is suitable for use with the shorter telecommunications wavelengths, which she estimated should be possible within a few years.
The mass production of 3-D photonic crystals would be more complex, and Aoki acknowledged the difficulties of developing image-recognition systems and robotic manipulators to work with micro- and nanoscale parts. She noted that approximately three decades passed between the invention of the transistor and the mass production of DRAM. "We consider that the mass production of photonic crystal devices would require shorter periods than that, but it would take more than 10 years to bring our technology to the industrial level."
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