Ultrashort Laser Pulse Phenomena, 2nd Edition

566 Generation of Extreme Wavelengths

There are substantial differences between the plasma generation with pulses on

the order of ps or longer and fs pulses. The physical difference is related to the

time scale of the plasma expansion for a distance equal to the absorption length,

into the surrounding vacuum. Expansion velocities are comparable with the sound

velocity, typically about 0.1 nm/fs, while the absorption length of metals is of

the order of 10 nm. The energy will thus be deposited in a time shorter than

the expansion time with a 100-fs pulse. Consequently a fs pulse interacts with

a solid or near-solid density plasma. With ps and longer pulses, the leading part

generates an expanding plasma that creates a density gradient above the target ...