*2.1.1.2.1. Influence of wavelength and pulse duration*

PLD can be applied to vaporize and deposit thin film from any kind of material if the absorbed power density is high enough. The amount of material that is ablated during laser irradiation can be estimated from the thermal diffusion depth, <sup>=</sup> , where *D* is the heat diffusivity in the solid target and *τ* is the pulse laser duration. The thermal diffusion depth decreases with the duration of the laser pulse.

The delivered laser energy is absorbed by the target material in a layer with thickness given by the formula, <sup>=</sup> <sup>1</sup> , where *ls* is known as the optical penetration depth and α is the absorp‐ tion coefficient for the respective laser wavelength.

The energy delivered by ultra‐short laser pulses is absorbed in a thinner layer as compared to ns laser pulses, thus producing higher temperatures at surface level and faster vaporization of the target material [16].

The efficiency of laser beam absorption into the target is closely related to the wavelength that will be used. However, for numerous materials, absorption coefficient dependence on the wavelength can be more complex due to different absorption mechanisms, such as network vibration, free carrier absorption, impurities, and bandgap.

For exemplification, we present the case of Mg film deposition using laser sources with different wavelengths and pulse duration: 308 nm XeCl excimer laser (generating pulses of 30 ns) and 248 nm KrF excimer laser (with 5 ps and 500 fs). Electron microscopy analysis showed that the droplets spread and the density decreased, when using laser pulses with shorter duration [17].

The films deposited using an ns laser source had their surface covered by droplets (**Figure 6a**). These droplets were spherical and had an average diameter of 10 μm. Their presence and morphology are indicative of expulsion of molten material from the surface of the target [18]. In this case, the optical penetration depth = 2 μm, (Mg ablated at 308 nm), was less than the thermal diffusion depth, = 17 μm.

When using ps or fs pulses, the morphology of the Mg film surface changed from droplets covered to smooth surfaces, as shown in **Figure 6b** and **c**. When using ps laser pulses there were still particulates on film surface (not larger than 200 nm) but they completely disappeared when ablation was conducted with fs pulses. The smoothness of these film surfaces is a consequence of the removal of ablated material with reduced expulsion of melted particles [17].

**Figure 6.** SEM images of Mg thin films deposited by PLD in different regimes: ns (a) ps (b) and fs (c) (Reproduced with permission from Ref. [17]).

### *2.1.1.2.2. Influence of the laser fluence*

The laser pulse fluence can be defined as the optical energy that is delivered to a selected area on the target. Therefore, the fluence can be varied by changing the laser energy or the dimen‐ sion of the spot area on target.

The coupling of the laser energy to the target surface is dependent on pulse parameters (duration and energy profile), and target characteristics (surface roughness, porosity and density). The fusion and vaporization processes occur only when the laser beam intensity is higher that a *threshold value* defined as the minimal energy of the laser pulse per surface unit that generates plasma ignition.

**Figure 7.** Typical SEM micrographs of TiO2 nanoparticles deposited on carbon cloth substrate at a laser fluence of 5 (a) and 1 J/cm2 (b) respectively (Reproduced with permission from Ref. [20]).

For a fixed wavelength and a chosen material, the fluence on the target will have a major effect on the particulate size and density [1]. We present an example where in order to obtain a porous gas diffusion layer, TiO2 nanoparticles have been deposited at two different laser fluences on carbon

cloth. In the case of 5 J/cm2 laser fluence, a uniform spatial distribution of nanoparticles over the substrate surface with dimensions of tens to hundreds of nanometres (**Figure 7a** and **b**). De‐ creasing the laser fluence to 1 J/cm2 (**Figure 7b**), the number of nanoparticles was considerably reduced and film protuberances were smoother [19].
