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LASERS
Although desirable for high-resolution microscopy and holography, lasers with very short wavelengths have proven to be difficult to design. The mirrors in such lasers often aren't reflective enough, and the high energies of the radiation can damage the mirrors.
Now, a team of researchers at the National Institute of Standards & Technology in Boulder, Colo.; Sofia University in Bulgaria; and other institutions have created what they say is the first spatially coherent laserlike beam in the extreme ultraviolet (EUV) range [Science, 297, 376 (2002)]. And not only is the radiation produced in short bursts only femtoseconds long, but the unit is also only 3 feet by 12 feet, making it practical for the lab. "We created a tabletop EUV equivalent of the [visible-region] helium neon laser," NIST physicist Margaret M. Murnane says. The group produced the short-wavelength pulses using high harmonic generation (HHG). They focused a visible-wavelength femtosecond laser into a hollow fiber filled with argon gas. The laser excites the gas, which emits odd harmonics (three, five, and more times the frequency) of the exciting laser frequency, "upshifting" the light into the EUV region of the spectrum. Researchers have used HHG to create high-energy beams before, but again, have only achieved partial spatial coherence. The hollow-fiber geometry is key, Murnane says, not only to increasing the light conversion efficiency and putting the photons in phase with each other, but also to producing a straight, narrow beam. The group created holograms and diffraction patterns with the light source, which should also be useful for high-spatial-resolution nanoimaging and metrology, Murnane says. |
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