Basics of Ultra-Short Laser-Matter Interactions

The “ultra-short pulse” definition in the frame of this book relates to the

pulses with duration shorter than major relaxation times for electron-to-

lattice energy transfer, heat conduction and hydrodynamic expansion. In

practice these are pulses with duration of 100 fs or less. In order to create

conditions for the above-defined “ultra-short pulse laser-matter interaction”,

the laser pulse should comply with several strict conditions. First, the high-

energy contrast should be achieved that means there should be no pre-pulse

or after-pulse accompanying the main pulse in time domain, which may con-

tain an amount of energy sufficient to produce the phase changes affecting

those produced by a main pulse. In practical terms, the ratio of the aver-

age intensity during the main pulse to that in the pre-pulse (contrast ratio)

should constitute ∼109. The intensity distribution over the focal spot also strongly affects the final results of a laser-mater interaction experiment as

we demonstrate in the following chapters. In this chapter, we assume that

the intensity is constant in the whole space over the focal spot and jump to

zero at the focal spot boundaries. The laser pulse in this case has the so-called

“flat-top-hat” spatial intensity distribution over the focal spot. The set of the

initial laser parameters sufficient for the description of laser-matter interac-

tion includes the laser wavelength, energy per pulse, pulse duration, and the

focal spot diameter, assuming that the focal area is a circle. Useful charac-

teristics are the average intensity during the pulse time (defined as the pulse

energy divided per pulse duration and focal area), or the so-called fluence

that is laser surface energy density (pulse energy per focal area). These laser

characteristics allow to describe many details of a single pulse laser-matter

interaction. In the multiple-pulse interaction whenmany pulses hit the same

focal spot the laser repetition rate and laser beam scanning speed over the

sample surface should be accounted for.