February 2005

Synopsis: Nanotechnology

Continued from page 2

By Monya Baker (edit)

Trip the Light Fantastic
Teaching old optics new tricks could lead to novel sensors and interfaces

Context: Thousands of kilometers of optical fibers have been laid under roads and even oceans, where they quietly and competently transmit telephone calls, Internet downloads, and television shows in the form of light pulses. Traditionally, these glass fibers have been simple conduits; the less they interact with the rest of the world, the better they perform. Now researchers in the laboratory of Yoel Fink at MIT have turned that notion on its head, creating a fiber that detects as well as transmits light and demonstrating a much more economical way to make large-scale light sensors.

Methods and Results: In the conventional ­fiber-drawing process, a relatively thick slug, or preform, of glass is heated in a furnace and carefully stretched. To create their light-sensing fiber, Fink and Mehmet Bayindir's team used a complicated preform with three main active ingredients, including a semiconductor whose conductivity improves dramatically when it is illuminated; tin wires, which help conduct electricity; and a nanostructured mirror, which selectively confines light of a specific wavelength to the fiber's core. The original preforms were approximately 30 millimeters across, and they were drawn out to form fibers a millimeter or less in diameter.

A light shone on the resulting fiber at any point along its length can be detected as a change in electrical conductivity more than ten meters away. The researchers wove a collection of these fibers together to create a two-dimensional grid, 30 by 30 centimeters square, which tracked the position of a light beam shone onto its surface.

Why it Matters: A typical light sensor is an array of many individual detectors. As a consequence, it's limited in size: combining thousands or millions of detectors can be prohibitively expensive. By weaving together lines of fibers instead of assembling individual dots, the MIT researchers have found a way to drastically reduce the cost of spanning large areas with light detectors.
That could lead to the development of completely new kinds of optical devices and interfaces, such as a projection screen that can respond to a user's laser pointer rather than requiring a conventional computer mouse.

Source: Bayindir, M. et al. 2004. Metal-insulator-semiconductor optoelectronic fibres. Nature 431:826-829.