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

Abstract

The presence of a pulse train structure of an oscillator-type free-electron laser (FEL) results in the immediate damage of a solid target upon focusing. We demonstrate that the laser-induced damage threshold can be significantly improved by gating the mid-infrared (MIR) FEL pulses with a plasma mirror. Although the switching pulses we employ have a nanosecond duration which does not guarantee the clean wavefront of the gated FEL pulses, the high focusablity is experimentally confirmed through the observation of spectral broadening by a factor of 2.1 when we tightly focus the gated FEL pulses onto the Ge plate.

Characterization of non-Gaussian mid-infrared free-electron laser beams by the knife-edge method

“…This suggests that the KU-FEL beam has very good focusing quality. The measured M 2 value is consistent with our recent experimental findings that we have demonstrated the high damage threshold and focusability of KU-FEL pulse gated by a plasma mirror with nanosecond switching pulses [6].…”

“…For this reason we have recently measured the micropulse duration and wavelength stability of KU-FEL at 12 µm under the presence of unknown amount of chirp by a new method, which is a variant of the fringe-resolved autocorrelation [4], and the single-shot spectra of temporally selected micropulses from KU-FEL at 11 µm using the sum-frequency mixing technique [5]. Most recently, we have demonstrated that the KU-FEL pulses gated by a plasma mirror with unusually long (nanosecond) switching pulses have the high focusability [6], which results in nonlinear spectral broadening by focusing the beam into the nonlinear target. Although the fact that we have observed the nonlinear spectral broadening clearly implies that the high intensity has been achieved upon focusing, we still do not know the spatial beam quality of the incident FEL beam.…”

Plane photoacoustic wave generation in liquid water using irradiation of terahertz pulses

“…Figure 2 B shows the single THz-micropulse picked up from the pulse train by the plasma mirror with nanosecond gating 29 , 30 . We employed a GaAs wafer irradiated by an intense femtosecond Ti:sapphire laser pulse as the nanosecond plasma mirror.…”

Time-resolved detection of structural change in polyethylene films using mid-infrared laser pulses

“…amorphous) in PE films using a mid-IR FEL developed at Kyoto University (KU-FEL). [9][10][11] To induce and detect the structural change in PE films, we employ two laser pulses: Q-switched Nd:YAG laser pulse to heat the PE film on an NaCl crystal and mid-IR FEL pulse to probe the structural change of PE. Note, however, that PE itself is practically transparent in the visible $near-IR range, and the laser-heating with the Nd:YAG laser would not be efficient.…”