Interaction of a Laser Pulse and a Sample

 The interaction between the laser pulse and the target metal can be simplified into 3 main steps:

1.    At first, the free electrons inside the sample absorb part of laser light (the rest is reflected back to the surrounding). The light absorption penetration depth is determined by the material's optical properties and the laser wavelength.

2.   In the second step, the target material in the focal region heats up to a temperature depending on the laser light parameters (laser intensity, wavelength, pulse width, pulse peak power, pulse shape etc’). If the pulse fluence (energy per surface area) is sufficiently high, the metal can be melted or vaporized and ablation is achieved.

3.   At last, the melted liquid is expelled from the laser focal region by the recoil action and the vapor is removed directly from the focus, leaving eventually a pattern on the target.

 An illustration of the behaviors mentioned above is shown in the following cartoon...

 Irradiation of a metallic sample by an ultra-short laser pulse, namely hundreds of femtoseconds, will heat up the free electrons rapidly while leaving the lattice cold. This happens because the time of achieving thermal equilibrium between electrons and lattice phonons is in the order of several picoseconds. The fast heating of a relative small volume in the sample bulk can cause an effective vaporization of the irradiated volume and drill into the sample.