Peculiarities of laser-induced material transformation inside diamond bulk

• Laser-induced modification of diamond is studied at different distances from the sample surface.
• Irradiated diamond is totally converted into nanocrystalline graphite only within thin surface layer.
• Deep-laying regions are transformed into composite material containing nanocrystalline diamond and amorphous graphite.
• Properties of microstructures produced by laser inside diamond bulk essentially differ from graphite.

Structure of a material modified by short laser pulses inside a diamond sample at different distances from the sample surface was investigated with Raman spectroscopy. It is shown that transformation of diamond into nanocrystalline graphite reported earlier takes place only within a thin surface layer. Beyond the layer, just part of sp3 sites is converted into sp2 sites at the same irradiation conditions. The residual sp3 phase forms nanometer-scale crystals, while the generated sp2 phase is rather amorphous. Suppression of the graphitization process far away from the sample surface seems to be a result of permanent compression of the modified region by the surrounding diamond matrix due to change of the material density under the phase transition. The observed total graphitization of diamond near the sample surface can be explained by efficient relaxation of the compressive stress due to mainly inelastic mechanisms.