Argon plasma generates coherent
DOI: 10.1063/1.2408535
Extreme UV light. In high-order harmonic generation (HHG), the electric field of a very intense laser pulse interacts nonlinearly with a gas to produce coherent emission at high harmonics of the laser frequency while ionizing the atoms. There is a cutoff energy above which HHG cannot produce emission. That cutoff has only been reached for neutral atoms, mainly because of laser defocusing effects as the gas ionizes. A group of physicists in Colorado has now overcome that limitation by putting the gas—in this case argon—into a hollow glass fiber. When an intense 1.55-eV (800-nm) laser pulse was fired into the fiber, the gas was ionized but the waveguide geometry counteracted defocusing. This method allowed the scientists to observe significant harmonic emission—up to an energy of 250 eV (5 nm)—resulting from further ionization of the argon ions. In contrast, neutral argon can generate harmonics only up to 100–150 eV. In general, HHG is useful for both ultrafast spectroscopy of materials and high-resolution imaging, including of biological and surface samples. The researchers say that coherent emission up to 1 keV will soon be seen in helium or neon ions, extending HHG to higher photon energies than previously possible. (
