Influence of spot size on extreme ultraviolet efficiency of laser-produced Sn plasmas

We have investigated the spot size effects on the extreme ultraviolet conversion efficiency (CE) of CO(2) laser-produced Sn plasmas. The estimated CE of the laser to 13.5 nm radiation, within a 2% bandwidth, using a 10.6 mu m CO(2) laser with various pulse widths (25-55 ns) showed a double hump structure during a target-lens scan, where the CE is nearly 25% lower at the best focal position. Density analysis of the CO(2) laser-produced plasma showed steep density gradients at the best focal position, and a reduction in CE at the best focal position, which is explained by the lack of efficient coupling between the laser and the plasma.