the inner cladding. The absorption of a progressively increasing amount of reflecting pumping energy leads to the amplification of the light signal confined into the core. The fact that this system supports the propagation of light at different wavelengths enables the adoption of high power diode lasers as pumping sources.
2.3 Laser processing of materials 51
Fig. 2.6 Reflectivity vs wavelength in some common metallic materials [120]
With respect to the dependence from the temperature, the absorptivity rises when the temperature grows up, which is particularly noticeable since the laser processing of the materials has the effect to increase significantly the material temperature. In fact, the primary excitation energy of the radiation absorbed within the material degrades and is converted into heat, resulting in the typical aforementioned macroscopic effects associated to the laser processing of materials, presented in Fig. 2.7.
The arising of a specific effect rather than the others is depending from the combina-tions of laser intensity and time of interaction with the material. With refer to the general phenomenon of the heating of the material, Fig. 2.8 depicts how the temperature varies in function of the time and of the depth z (the example concerns the case of copper irradiated for 1 µs with a laser power density equal to 1010 W/m2). The main trend which is worth noting by Fig. 2.8 is the one exhibited by the time at which the maximum temperature occurs in function of the depth: higher the depth, higher the range between the time corresponding to the maximum temperature and the time pulse, at which the maximum temperature on the surface (z=0) is achieved [121].
Fig. 2.7 Schematic representation of the main effects induced by the heat generation following the absorption of a laser radiation within the material [113]
Fig. 2.8 Variation of temperature in function of time and depth in the case of copper irradiated for 1 µs with a laser power density equal to 1010W/m2[121]
2.3 Laser processing of materials 53
When the laser power density oversteps a value, approximately equal to 105 W/cm2, besides the heating of the material even the melting point of the material can be reached by the temperature, which is why that intensity value is often called melting threshold. In [113]
Fig. 2.9 is offered to analyze the temporal evolution of the depth of melting during laser irradiation of a surface.
Fig. 2.9 Temporal evolution of depth of melting: (a) surface temperature vs time, (b) temper-ature vs depth below the surface during heating and cooling, (c) depth of melting vs time [113]
Keeping constant the value of time, the plot of the temperature in function of the depth below the surface during heating and cooling (Fig. 2.9, panel (b)) can provide information about the position of the solid-liquid interface: in the example, the maximum depth of it (zmax)
is reached at the time tp. Finally, plotting the identified positions of the liquid-solid interface in function of time, it is possible to evaluate the temporal range in which the absorption of a laser radiation provokes the melting of the material. In the example of Fig. 2.9, panel (c), before t2only the material without phase change is present, at t2the liquid-solid interface is on the surface, from t2to tpthe extension of the melting depth increases from the surface to zmax, from tpto t3finally the cooling takes over, bringing back the liquid-solid interface position to the surface at time t3. In Fig. 2.10 the increase of the depth of melting with the intensity and the pulse time, respectively and keeping one parameter fixed while varying the other, is shown.
Fig. 2.10 Trend of depth of melting in function of the laser intensity at constant pulse time (a) and in function of the pulse time keeping the laser intensity fixed (b) [113]
When the surface temperature overcomes the boiling point of the material, the initiation of the surface evaporation takes place. Once the depth of melting reaches zmax and the surface evaporation initiates, further increases of laser intensity or pulse time do not result in a further shift of the liquid-solid interface position but only in a removal of a certain amount of material from the surface by evaporation.
After the initiation of the surface evaporation, the subsequent interaction between evapo-rated material and the laser beam results in the important phenomenon of the ionization of
2.3 Laser processing of materials 55
vapor. The alternative mechanisms of formation of this ionized vapor (usually called plasma) are essentially the cascade ionization and the multiphoton absorption. Given the laser power density Ipat which the ionization occurs, when the laser intensity is just above Ipthe plasma forms near the surface and remained there confined, phenomenon usually referred to as plasma coupling. This one can significantly improve the absorptivity of laser radiation by the material when its reflection is high and in presence of long wavelength laser radiation. When the laser intensity becomes considerably higher than Ip, the expansion of plasma moves it away from the surface, interrupting the energy transfer and making the laser radiation being absorbed in the plasma instead in the dense material. This behavior is called plasma shielding [122].
Finally, the ablation process can be caused by a photo-termal or photo-chemical inter-action between laser radiation and material and results in the formation of visible craters and grooves over the surface. When the former mechanism takes place, the process is called thermal ablation and consists in the removal of the surface material by means of the onset of thermal stresses or vaporization induced by the increase of the temperature [122]. The photo-chemical induced ablation, referred to as photoablation, mainly involves the organic materials and arises when the energy of the photon is responsible for the breaking of the bond in the molecular chains, resulting in the molecular fragmentation of the material [113].
Often, the actual mechanism leading to the ablation of the absorbing material results from the simultaneous effect of thermal ablation and photoablation. To encourage the occurrence of this last kind of ablation, which minimizes the thermal damage, the energy of the photon should be greater than the one of the bond (even if for ultraviolet radiation with longer wavelengths this problem is surpassed by means of multiphoton mechanism) and the pulse time should be shorter than the thermal relaxation time, defined in Eq. 2.5
τ = d2
4α (2.5)
where d is the absorption depth and α is the thermal diffusivity of the material. Therefore, the most effective laser ablation processes are the ones in which the laser radiation is characterized by short wavelengths and works with micro-second pulses mode. As in the cases of melting and vaporization, even in the case of ablation the word "threshold" is employed to point out the ablation level defined as the minimum energy required for the ablation to take place: if the absorbed energy is below the ablation threshold energy only the heating effects are present, while when the threshold is overcome the ablation is promoted by the bond breaking. When this last circumstance happens, the depth of ablation has been predicted to increase with the energy of the laser beam, even if it can be affected by other phenomena like the possibility of the existence of a plasma shielding effect or the variation
of the material absorption coefficient induced by the radiation, as reported in [123]. Moving to consider which are the main parameters influencing the ablation rate, an increase of the ablation rate, beyond the ablation energy threshold, occurs in agreement with the trend of the laser fluence F, which is defined in Eq. 2.6 as the optical energy provided per unit area
F = Pm
frA0 (2.6)
where Pmis the laser nominal average power, fris the pulse repetition rate and A0is the area of a cross section of an ideal Gaussian beam. The energy provided with a single pulse is represented by the hatched area in Fig. 2.11 showing the evolution of the laser power with time during a multipulse irradiation working mode and it is provided by Eq. 2.7
Pm fr =
Z tp
0
Pdt (2.7)
Fig. 2.11 Temporal evolution of laser power during a multipulse irradiation working mode
An experimental evidence of the existence of a threshold value for the fluence under which no ablation apparently takes place has been found [124]. In this regard, as it is