**3. Ultrashort laser interaction with dielectrics**

The high power density, in the order of tens of TW/cm<sup>2</sup> , achieved delivering laser pulses of few microjoules in short period of time, of few femtoseconds, focused beneath the surface in micron-sized areas, results in nonlinear interaction processes. Such high intensities may be manipulated to be employed for modifying the dielectric focal volume, inducing from weak refractive index changes to ultrahigh pressures which lead to void generation. These permanent structural modifications depend not only on the laser peak power but also on the focusing conditions, scanning speed, polarization and repetition rate.

> type of structural change depends on the laser features (irradiance, repetition rate, polarization), working parameters (scanning speed, numerical aperture) and material properties

> **Figure 1.** (Left) birefringence cartography in optical fiber Ge-doped preform core [80], and (right) nanovoids in sapphire

Ultrafast Laser Inscription of Buried Waveguides in W-TCP Bioactive Eutectic Glasses

http://dx.doi.org/10.5772/intechopen.79577

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Formation of glasses with improved properties and manufacturability plays an important role in many technologies. Various noncrystalline materials such as silica glasses, amorphous halides, semiconductors and metals have been used in areas for many engineering applications including biology, communications, electronics, and so on. Glass formation consists basically in avoiding crystallization by cooling from the molten state. The ability to glass formation depends on the melt composition that defines some parameters as glass transition and liquidus temperatures, on the cooling rate which has to be higher enough to avoid the nucleation and growth of the crystalline phases and also on the sample size. As the solidification technique used limits at some extent the cooling rate, the first question in the development of glasses is to determine the composition ranges in which glasses can be obtained under certain

It is known that for multicomponent systems, a strong tendency to glass formation exists near the eutectic points because they have the lower liquidus temperature. Glass formation and its relation to eutectic growth have been investigated for different metal and ceramic alloy systems in terms of the competition between the growth of crystalline phases and the formation of the amorphous phase. The maximum growth rate of a eutectic structure with two or more phases is lower than the maximum growth rate of a single crystalline phase. Therefore, the locations of glass formation in some oxide, halide or metal systems lie near the eutectic area compositions, even though the glass formation also depends on the cooling rate. High

(thermal conductivity, bandgap energy).

cooling conditions (quenching, directional solidification, etc.).

**4. Bioactive glasses**

[46].

The ultrashort laser-matter interaction process can be divided into three stages: generation of free carriers inside the material by non-linear processes such as multiphoton, tunnel ionization, or avalanche ionization, followed by energy relaxation processes and a subsequent modification of the material [3, 4]. The absorption process of ultrashort laser radiation for wide bandgap dielectric materials cannot be explained on the basis of linear absorption, since the photon energy of commonly used femtosecond laser pulses with wavelength between UV and NIR does not have sufficient energy to be linearly absorbed. On the contrary, multiphoton absorption can excite an electron from the valence to the conduction band as long as *mħω > E<sup>g</sup>* , where *m* is the smallest number of photons for which the overall energy surpasses the bandgap energy *Eg* . In addition, tunneling photoionization can also take place under an extremely strong laser electromagnetic field. This mechanism in dielectrics permits electron from the valence band to tunnel to the conduction band in a period of time shorter than the laser pulse. However, in most dielectric multiphoton, ionization dominates the excitation processes [78, 79]. On the other hand, laser photons can be sequentially absorbed by electrons excited in the conduction band by means of free carrier absorption. When the energy of an electron in the excited state exceeds the bandgap energy, the ionization of another electron from the valence band can take place, resulting in two excited electrons at the conduction band minimum. These electrons can be excited again by free carrier absorption processes, and more valence electrons can be produced by the same mechanism, leading to the electronic avalanche [3, 4]. The requirement for avalanche ionization is the existence of seed electrons in the conduction band, which can be provided by multiphoton or tunneling ionization or by thermally excited impurity or defect states.

Once the nonlinear photoionization and avalanche ionization create a free electron plasma, they transfer their energy to the lattice inducing three types of structural changes: a smooth refractive index modification for low pulse energies such as 100 nJ and 100 fs at 800 nm for 0.6 NA, a birefringent refractive index modification for pulse energies ranging 150–500 nJ and 100 fs at 800 nm for 0.6 NA, and microexplosions which result in void formation for pulse energies higher than 500 nJ and intensities greater than 100 TW/cm<sup>2</sup> [4, 78, 80], **Figure 1**. The

**Figure 1.** (Left) birefringence cartography in optical fiber Ge-doped preform core [80], and (right) nanovoids in sapphire [46].

type of structural change depends on the laser features (irradiance, repetition rate, polarization), working parameters (scanning speed, numerical aperture) and material properties (thermal conductivity, bandgap energy).
