chapter  5
62 Pages

Ultra-Short Laser-Matter Interaction Confined Inside a Bulk of Transparent Solid

There is a fundamental dependence of the laser-matter interaction on the

focusing conditions: either the laser beam is tightly focused inside a trans-

parent material or it is focused onto the surface. In the laser-surface interac-

tion temperature is a maximum at the outermost atomic layer at the sample-

vacuum boundary. Therefore if the absorbed energy density in the skin layer

is in excess of the ablation threshold the ablated atoms expand and leave

the laser-excited zone. In the confined interaction mode the zone contain-

ing the high energy density is enclosed inside a bulk of cold and dense solid.

For this reason the laser-affected material remains confined inside pristine

crystal. There are different ways for inducing the changes in properties of a

bulk solid by laser action. First, non-destructive and reversible phase transi-

tions (photo-refractive effect, color-centers, photodarkening in chalcogenide

glasses etc.) can be induced by lasers at the intensity below the damage

threshold. In this case an optical probe can detect the laser-affected zone

afterwards. If the structure is very small ( μm3 in volume) it can be used as amemory bit for high-density three-dimensional optical storage. The irre-

versible structural changes may be produced at high intensity well above the

optical breakdown threshold. It was demonstrated [Juodkazis et al., 2006;

Gamaly et al., 2006] that the unique conditions of extremely high pressure

and temperature with record high heating and cooling rates are created in

the energy deposition region that may result in formation of new states of

matter. Strong shock wave is generated in the interaction region propagating

into the surrounding cold material and compressing it. Shock wave propaga-

tion is accompanied by compression of the solid material at the wave front

and decompression (rarefaction wave) behind it leading to the formation of

a void inside the material. Compressed material in the form of a shell sur-

rounds the void. Careful investigation of thematerial in the compressed shell

recently revealed formation of new crystal non-existent before [Vailionis

et al., 2011].