**2. Laser interactions with biomaterials**

The interaction of light with matter can occur through several different nonlinear processes, such as two-photon or multi-photon absorption, second harmonic generation, or Raman scattering. It is useful in deep-tissue two-photon imaging. Moreover, if irradiance is high enough, any material can be machined to very high precision by using ultrashort laser pulses. This interaction is independent of linear absorption properties of the materials and which would otherwise be transparent to the laser wavelength. Ablations to micron scale precision with minimal collateral damage to the rest of the material are achieved by faster removal of material than the rate of heat conduction to the bulk.

In present clinical practices, pulses longer than a few tens of picoseconds have been utilized. It can cause damage to the dielectrics involving heating of seed electrons and transfer of this energy to the lattice. This damage occurs via conventional thermal deposition resulting in the melting and boiling of dielectric material (**Figure 1(a)** and **(b)**). Because the energy is transferred through thermal conduction, this model predicts a square root (τ1/2) dependence of the threshold fluence (Fth; energy/area) upon pulse duration (τ) (**Figure 2**) [13].

With these conventional lasers, the material is removed by thermal ablation, wherein the material is locally heated to near the boiling point. Since the boiling point of dielectrics is very high typically 1000°C, this ablation is coupled with a strong

*Ablation of Materials Using Femtosecond Lasers and Electron Beams DOI: http://dx.doi.org/10.5772/intechopen.106198*

**Figure 1.**

*(a) Ablations of retinal-tissue with 2.94 μm millisecond pulses (2 J/cm<sup>2</sup> , 400 ms), (b) retinal ablations with 1.064 μm nanosecond pulses (0.8 J/cm<sup>2</sup> , 5–7 ns), and (c) retinal blood vessel ablation with 0.8 μm femtosecond pulses (150 fs) [11, 12].*

thermal shock transferred to the adjacent material. This thermal shock often results in cracking of the adjacent material in an uncontrolled manner. These effects can be observed in **Figure 1(a)** and **(b)**, where the ablations of porcine retina using a conventional Er: YAG (λ = 2.94 μm) and ND: YAG laser (λ = 1.064 μm) are shown [11, 12]. Meanwhile, fs-pulses (800 nm) having sufficient intensity for multi-photon ionization throughout its beam waist ablate the retinal vessel walls in an extremely controlled manner (**Figure 1(c)**). The heat transfer into the surrounding material was minimal and also no thermal shock-induced cracking was observed. It could be inferred that the laser damage threshold for dielectrics with lasers having pulse durations less than a few picoseconds to femtosecond does not follow the proportionality rule of square dependence [14–16]. In the next section, various ionization mechanisms operating in biological materials are briefly discussed.
