Laser wavelength and spot diameter dependence of extreme ultraviolet conversion efficiency in ω, 2ω, and 3ω Nd:YAG laser-produced plasmas -: art. no. 261502

Conversion efficiency and spectra of extreme ultraviolet radiations from a cryogenic planar solid xenon target were investigated as a function of laser wavelength (omega, 2 omega, and 3 omega Nd:YAG) and the laser focus spot size (50-700 mu m) at the intensity 10(10)-5x10(12) W/cm(2). The conversion efficiency increased with laser intensity and reached the maximum value at about 10(11) W/cm(2) for all colors. It was found that an edge effect appears more strongly at the omega-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 omega, 2 omega, and 3 omega, a spectral hump appeared in the extreme ultraviolet band around 13.5 nm region, while the spectral intensity at 10.8 nm drastically decreased. High-energy photon generation in the tail of 10.8 nm peak was found to be strongly suppressed at shorter-wavelength laser (3 omega), while the conversion efficiency at 13.5 nm was as large as that at omega. This indicates that a Xe[XI] ion-rich plasma have been efficiently produced in the ablation plasma by using 3 omega laser without overheating the underdense plasma responsible for extreme ultraviolet emission.