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Mini Modulator

A key device for silicon optics gets tiny

In an important step toward integrating optoelectronics into silicon chips, researchers at Cornell University have fabricated a silicon modulator – a device that converts electronic signals into optical ones – roughly 12 micrometers wide, about a thousand times smaller than previous silicon electro-optical modulators.

Why It Matters: As chip makers pack more transistors on silicon, problems such as heat generation from electrical resistance and electrical interference between closely spaced wires threaten to degrade performance. Many believe that optical connections – which transmit information in the form of light pulses instead of electric current – offer a way around these limitations. Researchers have long been striving to produce optical devices that can be easily integrated into silicon (see “Intel’s Breakthrough,” July 2005). Electro-optical modulators are vital to this plan, but current silicon versions of them are too large to fit easily onto a chip. The dramatic drop in size that Michal Lipson and her colleagues demonstrated makes a chip-based modulator seem more feasible.

Methods: To build their modulator, the Cornell researchers etched a small piece of silicon to form a 12-micrometer-diameter, 250-nanometer-tall raised ring. They positioned this ring next to a straight ridge, known as a waveguide, just 450 nanometers wide. A beam of laser light traveling down the waveguide will either pass the circular section – the “ring resonator” – without interacting with it or be diverted into it, depending on the wavelength of the light. The refractive index of the silicon and the circumference of the ring determine what wavelength of light the resonator diverts. Applying a voltage from the interior of the ring to the area just outside it creates free electrons and positively charged “holes” within the ring that change its refractive index. By using a varying voltage to either shutter light or let it pass through the waveguide, the researchers encoded information onto a laser beam at a rate of 1.5 billion bits per second.

Next Step: The researchers believe that their device will be able to modulate signals at more than five billion bits per second, once they make some refinements, such as improving the electrical contacts that supply the input signals from the rest of the circuit.

Source: Xu, Q., et al. 2005. Micrometre-scale silicon electro-optical modulator. Nature 435:325-327.

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