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Posted: April 30, 2008

NTT develops compact low-power optical memory with photonic crystal

(Nanowerk News) Nippon Telegraph and Telephone Corp (NTT) announced that it extended the memory retention time of photonic-crystal-based optical bit memory up to a maximum of 150ns.
With the adoption of InGaAsP for a material of photonic crystal, the retention time was increased to 60 times longer than the existing record of 2.5ns, which the company achieved with the use of Si material.
The maximum Q value of the latest memory is 130,000. The value indicates the degree of optical confinement. The minimum value of the bias light intensity required for the memory retention was reduced to 40µW. This is one several tenth of that achieved by the existing optical memory using semiconductor laser.
InGaAsP photonic crystal device
Structure of NTT's new InGaAsP photonic crystal device
Photonic crystal is obtained by fabricating a nanostructure with a periodicity substantially equivalent to the light wavelength in a semiconductor or other materials, and it is capable of confining light. In the latest prototype, air holes with a diameter of 200nm are arranged in the form of a triangular lattice with a periodicity of 420nm in a 200nm thick semiconductor crystal.
A linear region with no holes is formed in one of the rows so that it will serve as a waveguide. Then, a wider region is formed by displacing the positions of holes on both sides of the waveguide by several millimeters. This region will act as a resonator. The volume of the region to confine the light of resonator is only 0.1µm3.
In order to rewrite the On/Off state, the latest memory utilizes the optical bistability phenomenon induced by "optical nonlinearity," a variation in refractive index of a material caused by light.
Thus far, NTT confirmed the optical bistable behavior in a Si-based photonic crystal resonator. But it was reportedly unsuitable for use as an optical memory because it only had a retention time of 2.5ns. Compared with the Si material, the InGaAsP material is harder to process although it has a greater optical nonlinearity. Thus, it has been difficult to form a photonic crystal with a high optical confinement efficiency, NTT said.
By leveraging a high precision process technology, the company was able to develop a high performance resonator through the use of InGaAsP material.
NTT hopes to further lengthen the memory retention time by optimizing the resonator structure and the material parameters in order to apply the latest technology to optical information processing chips such as switches and memory. It is also necessary to develop technologies relating to the memory integration and the bit-string addressing, according to NTT.
The development was commissioned by Japan's National Institute of Information and Communications Technology (NICT).
NTT plans to present the latest achievement at Conference on Lasers and Electro-Optics (CLEO) 2008, which runs from May 4 to 9, 2008, in San Jose.
Source: Nikkei Electronics (Yousuke Ogasawara)
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