Roland Piquepaille's Technology Trends
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mardi 3 août 2004

Building computer chips which use light instead of electricity will be possible in a few years, thanks to the new techniques developed by two separate research teams from the MIT and Kyoto University. Both have built photonic crystals that can be manufactured using processes suited to mass production. Technology Research News says that "the techniques could be used to make smaller, more efficient communications devices, create optical memory and quantum computing and communications devices, develop new types of lasers and biological and chemical sensors, and could ultimately lead to all-optical computer processors."

Here are some details about the MIT photonic chip.

The semiconductor industry took off with the advent of a practical and low-cost method of integrating a large number of transistors into a single chip, said Minghao Qi, a research assistant at MIT. "It is natural then to envision the possibility of integrated photonics, where information is processed fully in the optical domain [at the high] bandwidth of photons," he said.
The MIT photonic chip has seven layers that each contain two types of two-dimensional photonic crystal. One type is an arrangement of rods surrounded by air and the other type is solid material perforated with air holes. The rod slab is positioned above the hole slab in each layer, and the layers are offset to produce steps. The holes are about 500 nanometers in diameter, or about one-tenth the size of a red blood cell. The material blocks light at wavelengths of 1.3, 1.4 and 1.5 microns. Telecommunications systems use near-infrared 1.3- and 1.55-micron wavelengths.

The research work has been published by Nature. Here is a link to the abstract of the paper named "A three-dimensional optical photonic crystal with designed point defects."

Photonic crystals offer unprecedented opportunities for miniaturization and integration of optical devices. They also exhibit a variety of new physical phenomena, including suppression or enhancement of spontaneous emission, low-threshold lasing, and quantum information processing. Various techniques for the fabrication of three-dimensional (3D) photonic crystals -- such as silicon micromachining, wafer fusion bonding, holographic lithography, self-assembly, angled-etching, micromanipulation, glancing-angle deposition and auto-cloning -- have been proposed and demonstrated with different levels of success. However, a critical step towards the fabrication of functional 3D devices, that is, the incorporation of microcavities or waveguides in a controllable way, has not been achieved at optical wavelengths. Here we present the fabrication of 3D photonic crystals that are particularly suited for optical device integration using a lithographic layer-by-layer approach. Point-defect microcavities are introduced during the fabrication process and optical measurements show they have resonant signatures around telecommunications wavelengths (1.3–1.5 µm).

The Kyoto University team followed a different approach.

The Kyoto University team has advanced its existing woodpile-structured three-dimensional photonic crystal with a method to make solid areas in specific locations and have shown that the material precisely controlled light, said Susumu Noda, a professor of electronic science and engineering at Kyoto University.
The woodpile photonic crystal consists of perpendicular layers of semiconductor rods. The researchers' design calls for 200-nanometer-wide rods spaced 700 nanometers center to center. The photonic crystal controls 1.55-micron light.
The researchers also sandwiched a light source inside their photonic crystal, which is a step toward fully integrated optical devices, said Noda.

The research work has been published by Science Magazine. Here is a link to the abstract of the paper named "Control of Light Emission by 3D Photonic Crystals." (Free registration to the American Association for the Advancement of Science (AAAS) is necessary to access it.)

Three-dimensional (3D) photonic crystals containing artificial point defects have been fabricated to emit light at optical communications wavelengths. They were constructed by stacking 0.7-micrometer-period gallium arsenide striped layers, resulting in a 3D "woodpile" photonic crystal. Indium–gallium arsenide–phosphide quantum-well layers emitting at a wavelength of 1.55 micrometers were incorporated in the center of the crystal. Samples having up to nine stacked layers were constructed, and artificial point-defect cavities of different sizes were formed in the light-emitting layer. Light emission was suppressed in the photonic crystal regions, whereas cavity modes were successfully observed at the point defects and were size dependent.

Even if simple devices such as sensors based on photonic chips may appear in about three years, all-optical computer chips will not see the light before ten years.

Sources: Eric Smalley, Technology Research News, July 28/August 4, 2004; Nature 429, 538 - 542, June 3, 2004; Science, Vol. 305, Issue 5681, 227-229, July 9, 2004

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