The Russian-built OK-4 free-electron laser at Duke University in Durham, N.C., emitted 5 mW of continuous-wave, 193.7-nm vacuum ultraviolet (VUV) light, the province of ArF excimer lasers. Vladimir Litvinenko, associate professor of physics at the university and associate director for light sources at its free-electron laser laboratory, said that physicists had viewed 200 nm as a technical barrier because the tunable laser's mirrors tended to degrade at short wavelengths.

The laboratory's group, which included graduate student Seong Hee Park and research scientists Igor Pinayev and Ying Wu, developed new mirrors for the optical cavity with the Lumonics Optics Group of Nepean, Ontario, Canada, and modified the OK-4's electron beam to augment its gain.

The lab will upgrade the systems in the laser, including increasing its input power by a factor of 20, and will boost its output in the VUV to 1 W. Litvinenko said the researchers' plans include the construction of a 70-ft-long OK-5, which will incorporate longer undulator magnets to further increase the gain.