are recorded as diffractive volume gratings, which enables readout of the stored data.

Encoding data consists of assembling “pages” of 1 million bits represented by 1’s and 0’s and sending them electronically to a spatial light modulator. This device is coated with pixels, each about 10 square micrometers, which can be switched rapidly to match the content of each page. When a signal beam passes through the modulator, its pixels either block or pass light, depending on whether they are set as a 1 or a 0, and the laser beam carries the message of that specific page.

Bringing competitive holographic systems to market that will exceed the traditional storage technologies will require a number of advances in recording materials. These advances include optical clarity, photosensitivity, dimensional stability, and uniform optical thickness, as well as innovations in spatial light modulators, micromirrors, and component-systems integration.

The demonstration that information can be stored on and nondestructively read from nanoclusters of only two to six silver atoms, announced earlier this year by Georgia Institute of Technology researchers, opens another potential approach to increasing data density. The Georgia Tech team exposed a thin film of the silver nanoclusters to blue light in the shape of the letter L. Two days later, they exposed the nanoclusters to green light, which caused the nanoclusters to fluoresce in the L pattern. Whether such nanoclusters can be shaped into compact arrays and handle read-write operations at the speeds of today’s computers remains a question for further study.

Electron Spin

Spintronics could lead to information storage on the same chips that process data, which would speed up computation. Data processing is based on the charge carried by electrons; data storage has relied on magnetism or optics. However, electrons also have spin, and electron spin is harnessed in magnetic storage. Spintronics seeks to manipulate electron spin in semiconductor materials for data storage and perhaps quantum computing. The key lies in devising semiconductor materials in which spin polarized electrons will function. Recent developments in spin polarizers and the synthesizing of magnetic semiconductors suggest this problem can be managed. However, making a marketable product will require ferromagnetic semiconductors that operate at room temperature—a demand not easily fulfilled.

Flat Displays and Printed Circuitry

Organic light-emitting diodes (OLEDs), a technology that offers more design flexibility and higher resolution than traditional LEDs, are now coming to



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