Question

Difference between femtosecond laser and nanosecond laser

Answer 1

Theoretical model of laser ablation (at least one of them) is called two temperature model. And it speaks separately about temperature (kinetic energy) of electrons and of nucleis. Basically it says the following: First electrons absorb the optical energy, and either in a ladder like manner (for longer pulses) or by direct multi-photon process (for shorter ones) are getting out of bound states to continuum.  Now two things can happen to the electron:

1. It will collide with a nuclei ant transfer its energy to phonons (thermalization)

After many collisions, nuclei will gain enough energy to escape the bulk

2. Electrons are gaining lots and lots of energy and leave the material (kind of multi-photon photoelectric effect). Nuclei stay cool. But after enough electrons have left, the nuclei remain unbound, and "fly away" due to room temperature energy.

The second mechanism, obviously affects much less area. In fact, being naturally multi-photon, the ablated area is proportional to intensity in the optical spot size in high power, effectively making the "hole" smaller. 

This second type of ablation is claimed to be used in medical devices for ophthalmology, where the size of the perforation is most critical. 

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