Pure-tin microdroplets irradiated with double laser pulses for efficient and minimum-mass extreme-ultraviolet light source production

Laser-driven expansion of pure-tin microdroplets was demonstrated to produce an efficient and low-debris extreme-ultraviolet (EUV) light source. The pre-expansion is indispensable for resolving the considerable mismatch between the optimal laser spot diameter (similar to 300 mu m) and the diameter (similar to 20 mu m) of microdroplets containing the minimum-mass Sn fuel for generating the required EUV radiant energy (similar to 10 mJ/pulse). Explosive expansion of microdroplets was attained by using a laser prepulse, whose intensity was at least 3x10(11) W/cm(2). The expanded microdroplet was irradiated with a CO2 laser pulse to generate EUV light. A combination of low density and long-scale length of the expanded microdroplet leads to a higher EUV energy conversion efficiency (4%) than that (2.5%) obtained from planar Sn targets irradiated by a single CO2 laser pulse. This scheme can be used to produce a practical EUV light source system.