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Extreme laser shedslight on nanoworld
27 July 2002
From New Scientist Print Edition.
Jeff Hecht

Enlarge Figure 1

If nanotechnology ever makes the big time, we'll need a cheap and accessible way of measuring the features of these diminutive devices. Currently, the only way to do this is with an expensive room-sized machine called a synchrotron, as this is the only source of coherent light at wavelengths short enough to measure such tiny details. But a cheap table-top alternative is on the way.

The light needed to make accurate measurements in tens of nanometres is in the extreme ultraviolet band (EUV). In a synchrotron, a particle accelerator bends the paths of high-energy electrons in such a way that they release EUV light. But it's an energy-intensive process and only a small part of the output beam is usable.

The new approach is the brainchild of Margaret Murnane and Henry Kapteyn at the University of Colorado in Boulder. Surprisingly, their starting point is at the other side of the visible spectrum to UV - in the infrared. They use an infrared laser to generate very high-powered pulses lasting only 25 femtoseconds - 25 millionths of a billionth of a second.

Researcher Randy Bartels focused these ultrashort laser pulses into the 150-micrometre-wide core of a hollow fibre filled with argon gas at low pressure. The tremendous bursts of pulsed power rip electrons from the argon atoms. These freed electrons vibrate in the intense electromagnetic field of the laser pulse and quickly collide with other argon atoms. The collisions release energy at odd harmonics of the original laser frequency, and some of these harmonics are at EUV wavelengths.

The 10-centimetre-long hollow fibre guides these shorter waves along with the remaining infrared light. A filter blocks the infrared but transmits the EUV.

The pressure of the argon gas in the tube determines which wavelengths of EUV are generated. In Science (vol 297, p 376), Bartels reports generating EUV at 40 nanometres. Although the process converts only about 0.001 per cent of the infrared laser energy into EUV, it is still far more efficient than a synchrotron.

"The directedness of the beam is much more stable and much higher quality than other EUV sources," says Bartels. "It's as good as you can get." And the whole assembly is small enough to fit on an average desk - and costs considerably less than a synchrotron.

"This makes it possible to do nanoscale imaging on a desktop," says Bartels, who recorded simple holograms using his new laser. By comparing one hologram with another, it will be possible to measure how features in nanoscale devices change as a result of heat exposure, for example. He says modified versions could generate wavelengths as short as 13 nanometres, to allow even more accurate measurements. Bartels also wants to use EUVto probe the structures of biological cells.

From issue 2353 of New Scientist magazine, 27 July 2002, page 17

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