Damage threshold and focusability of mid-infrared free-electron laser pulses gated by a plasma mirror with nanosecond switching pulses

摘要

The presence of a pulse train structure of an oscillator-type free-electronlaser (FEL) results in the immediate damage of a solid target upon focusing. Wedemonstrate that the laser-induced damage threshold can be significantlyimproved by gating the mid-infrared (MIR) FEL pulses with a plasma mirror.Although the switching pulses we employ have a nanosecond duration which doesnot guarantee the clean wavefront of the gated FEL pulses, the high focusablityis experimentally confirmed through the observation of spectral broadening by afactor of 2.1 when we tightly focus the gated FEL pulses onto the Ge plate.