  Ultrafast lasers, with pulse durations below a few picoseconds, are of
significant interest to the industry, offering a cutting-edge approach to
enhancing manufacturing processes and enabling the fabrication of intricate
components with unparalleled accuracy. When processing metals at irradiances
exceeding the evaporation threshold of about 10^10 W/cm2 these processes can
generate ultra-short, soft X-ray pulses with photon energies above 5 keV. This
has prompted extensive discussions and regulatory measures on radiation safety.
However, the impact of these ultra-short X-ray pulses on molecular pathways in
the context of living cells, has not been investigated so far. Using laser
pulses of 250 fs and 6.7 ps, along with pulse repetition rates exceeding 10
kHz, we conducted the first molecular characterization of epithelial cell
responses to ultra-short soft X-ray pulses generated during processing of steel
with ultrafast lasers. Ambient exposure of vitro human cell cultures, followed
by imaging of the DNA damage response and fitting of the data to an
experimentally calibrated model of dose rate estimation, revealed a linear
increase in the DNA damage response relative to the exposure dose. This is in
line with findings from work using continuous wave soft X-ray sources and
suggests that the ultra-short X-ray pulses do not generate additional hazard.
This research contributes valuable insights into the biological effects of
ultrafast laser processes and their potential implications for user safety.
