“…A second mechanism, active for high intensity irradiation and giving a much higher etching selectivity, is the formation of self-ordered nanocracks, perpendicular to the laser polarisation direction [37]. The physical processes underlying the formation of nanocracks have been studied in detail in [69,70] and involve the following transient nanoplasmonic model: i) in the focal volume, hot spots for multiphoton ionization occur due to the presence of defects or color centers; ii) such hot spots evolve into spherically shaped nanoplasmas over successive laser pulses due to a memory effect [71], equivalent to a reduction of the effective bandgap in the previously ionized region; iii) field enhancement at the boundaries of the nanoplasma droplets results in asymmetric growth of the initially spherical droplets, in a direction perpendicular to the laser polarization, leading to the formation of nanoellipsoids, which eventually grow into nanoplanes; iv) the nanoplanes are initially randomly spaced; when the electron plasma density inside them exceeds the critical density, they become metallic and start influencing light propagation in such a way that they assemble in parallel nanoplanes spaced by λ =n, where λ is the wavelength of the femtosecond writing laser and n the refractive index of the medium. One particular feature of this process, which is very important for applications, is that nanoplanes formed in adjacent focal volumes displaying a lateral offset tend to be coherently linked, resulting in a self-alignment of the planes (that become aligned nanocracks if a suitable fluence is used in the irradiation process).…”