Laser wavelength and spot diameter dependence of extreme ultraviolet conversion efficiency in ω, 2ω, and 3ω Nd:YAG laser-produced plasmas

Abstract

Conversion efficiency and spectra of extreme ultraviolet radiations from a cryogenic planar solid xenon target were investigated as a function of laser wavelength (ω, 2ω, and 3ω Nd:YAG) and the laser focus spot size (50–700μm) at the intensity 1010–5×1012W∕cm2. The conversion efficiency increased with laser intensity and reached the maximum value at about 1011W∕cm2 for all colors. It was found that an edge effect appears more strongly at the ω-laser case, indicating more lateral energy loss, while it appears only weakly for higher harmonics. Shorter-wavelength lasers generated significant conversion efficiencies even at lower laser energies; that is, with smaller laser spots. As the wavelength decreased from ω, 2ω, and 3ω, a spectral hump appeared in the extreme ultraviolet band around 13.5nm region, while the spectral intensity at 10.8nm drastically decreased. High-energy photon generation in the tail of 10.8nm peak was found to be strongly suppressed at shorter-wavelength laser (3ω), while the conversion efficiency at 13.5nm was as large as that at ω. This indicates that a Xe[XI] ion-rich plasma have been efficiently produced in the ablation plasma by using 3ω laser without overheating the underdense plasma responsible for extreme ultraviolet emission.