Improvement of EUV Spectra by Optical Thickness Control

In double pulse cross-laser irradiation configuration, we measured the extreme ultraviolet (EUV) spectral purity and emission energy around 6.x nm wavelength in laser produced Gd plasmas. It was found that the spectral purity of 4.1% within a 0.6% bandwidth (Δλ = 6.74 - 6.78 nm) at 6.76 nm to the total emission between wavelengths of 5 - 9 nm was improved by producing a low-density Gd plasma target, compared to the spectral purity of 1.6% for solid-Gd target plasma. A low-density plasma must be produced before the main heating laser pulse to enhance the spectral purity. We also reported the delay time dependence of the spectral purity irradiated by the 6-ns, 1-µm main laser pulse irradiation under the cross-laser-injection scheme.