**5. Conclusions**

specific area, resulting in minimal thermal damage. Both picosecond and femtosecond laser

At short and ultra-short laser pulse durations, the ablation threshold fluence of the target materials decreases and becomes very sharply defined. In addition, the excess energy in the target still can produce thermal effects on the ablation area after the pulse has ended. Further‐ more, at ultra-short laser pulse duration, the optical absorption depth reduces due to an optical breakdown caused by the ultra-short laser pulse duration; this can also cause strong absorption in otherwise transparent wide-energy band gap materials. Another advantage of picosecond and femtosecond laser ablation is that they are "separated in time from material response and ejection" [7]. In contrast, for laser production of nanoparticles in a liquid solution, the pico‐ second laser performs better than the femtosecond laser [37]. It can be seen that only the

The size and the morphology of the nanoparticles depend on the laser pulse duration, such as whether a nano-, pico- and femtosecond laser regime is being used and the nature of the target material [39]. Hamad et al. [22] showed that the picosecond laser (10 ps and 1064 nm wave‐ length) produced more spherical silver nanoparticles than those produced by both nanosecond (5 ns and 532 nm wavelength) and femtosecond (100 fs and 800 nm wavelength) lasers. Under the same conditions, smaller gold nanoparticles were produced in comparison with the silver and titania, with the latter being the largest. It was also concluded that the average size of the nanoparticles was smaller for the shorter wavelength (532 nm) nanosecond laser in relation to picosecond and femtosecond lasers, showing that the role of the laser wavelength is more significant than the laser pulse duration to control the size of the nanoparticles. It is worth mentioning that some silver nanoparticles produced by nanosecond laser have irregular (nonspherical) shapes. Dudoitis et al. [40] showed that the specific nanosecond and picosecond laser parameters can be used to produce controllable particle size distribution of nickel and

Lescoute et al. [28] observed two groups of particles of two different sizes after femtosecond laser ablation caused by the temperature achieved in the plasma plume: these could be classified into micrometer-size fragments and nanometre-size particles for temperatures lower than the critical temperature and for higher temperatures, respectively. Kabashin and Meunier [34] used femtosecond laser ablation to produce Au nanoparticles in deionised water. The researchers observed two different groups of nanoparticles: the small (about 3–10 nm) and monodispersed particles were produced due to a mechanism which, "associated with thermalfree femtosecond ablation, manifests itself at relatively low laser fluences F, 400 J/cm2

second group of particles was of a larger size and a broad size distribution, produced due to plasma-induced heating and ablation that occurs at high laser fluencies. Itina [41] showed that the ultra-short laser pulses produce small nanoparticles and a stable size distribution at optimal experimental parameters. This is because while using short and ultra-short laser pulse duration, laser radiation will not be absorbed by created plasma plume. This leads to transfer much more energy into the target material during very short time. Finally, the target ejected the cluster precursors directly to the liquid solution and the plasma plume is confined to a smaller volume. Chakravarty et al. [42] concluded that the smaller size of silver and copper

ablation can be viewed as a direct solid-vapour transition [19].

femtosecond laser avoided the laser-bubble interaction [22].

aluminium in argon gas.

31816 High Energy and Short Pulse Lasers

Pulsed laser ablation for material processing and production of nanoparticles in liquid environments have been used due to the unique properties of the laser beam in marking and machining materials. There is a lack of knowledge of the optimal laser beam parameters in this field, as different laser pulse durations respond differently to different materials. A critical comparison of different laser pulse durations in micro- and nanomaterial processing was therefore sought in order to obtain higher quality and productivity. Femtosecond and picosecond lasers were found to be superior to the nanosecond laser for precision material processing. In addition, in spite of the short and ultra-short duration of their laser pulses, these lasers were observed to be more effective for the production of nanoparticles than those with longer pulse durations. Finally, laser wavelength was found to play significant role in the production of nanoparticles. The advantages of femtosecond laser ablation for micro- and nanotechnology are due to non-thermal interaction with solid target materials.
