Mirror adjustment yields record short wavelengthThe Free Electron Laser Laboratory at the Electrotechnical Laboratory (ETL; Tsukuba, Japan) has broken another record--the first was in May 1998-- for the shortest-wavelength free-electron-laser (FEL) emission. A group of researchers at the laboratory used the same compact electron accelerator ring--the NIJI-IV--as in May, but this time the group has achieved emission at 212 nm (see photo on p. 58). The previous world record was set in August 1998 by researchers at Duke University (Durham, NC), who produced 217-nm light by using a large electron accelerator ring.
Free-electron lasers operate by sending a beam of electrons through a periodic magnetic field (wiggler array) and then trapping the light produced in a resonance cavity to cause lasing. The laser can be tuned continuously over a wide range of wavelengths and has the benefits of high efficiency and high output power.
The salient point of the ETL group`s new work lies in the mirror used in the resonance cavity. In the short-wavelength region, the laser gain of a FEL is small. Hence, the mirrors in the resonance cavity must be low-loss mirrors. However, the dielectric, multiple-layer mirrors commonly used in the far- ultraviolet region are made of hafnia/silica (HfO2/SiO2), which cannot be used in this experiment because the bandgap is similar to that of the 220-nm-region photons, and the energy loss is too great.
The researchers were able to break the record by changing the mirrors to alumina/silica (Al2O3/SiO2), which can also be used in the "ultraviolet vacuum" region of under 200 nm (so-called because air is highly absorbent in this region). The researchers believe that the goal of achieving sub-200-nm lasing is within striking range.
Courtesy of O plus E magazine, Tokyo