**4. Comparison of different pulse durations for the ablation of materials and production of nanoparticles**

A comparison of the characteristics of nano-, pico- and femtosecond lasers produced nano‐ particles and materials processing have been studied [5, 22]. Even the comparison is not a likefor-like; the experimental work should be assumed as a fare comparison between these types of commercial lasers operating at their usual operating conditions [22]. Nanosecond laser ablation of materials occurs due to "melt expulsion driven by the vapour pressure and the recoil pressure of light". The melted material will solidify again because of the instability of this process, in which the fluid phase dynamics and the driving vapour conditions are very complex. As a result of this, the ablated area on the surface of the material target is not precise and uniform in comparison with that produced by a femtosecond laser (see **Figure 3**). Furthermore, nanosecond laser ablation creates a heat-affected zone (HAZ) [4]. It was shown that the HAZ of Al sample for the nanosecond laser ablation was about 40 mm wide, whereas for the femtosecond laser there was not observable because it was <2 mm wide [23].

**Figure 3.** Laser material processing of a glass target by nanosecond laser (left) and femtosecond laser (right) ablation [4].

In the case of short (picosecond laser) and ultra-short laser pulses (femtosecond lasers), their pulse duration (τ) is considerably shorter than the timescale required for energy transfer between the lattice and the free electrons of the material target. As a result of this, very high temperatures and pressures are produced at a very shallow depth in the range of microns. Conversely, irradiated material is heated rapidly by pulsed laser and directly reaches the vapour phase with high kinetic energy without passing through the melting point temperature due to the absorbed energy. In other words, the material ablation will take place by vaporisa‐ tion without producing a recast layer on the ablated area. As shown in **Figure 3**, the ablated area is very precise and smooth without forming any observable heat-affected zone (HAZ) [4]. The target materials with a high thermal conductivity are very important for the femtosecond laser ablation process because of the stable properties and chemical composition of the area ablated with femtosecond laser ablation [24].

thermal conduction into the target material can be neglected during all of the aforementioned processes. Due to the advanced properties of picosecond laser ablation, highly precise and pure laser material processing can be achieved, as has been experimentally demonstrated by

**4. Comparison of different pulse durations for the ablation of materials and**

A comparison of the characteristics of nano-, pico- and femtosecond lasers produced nano‐ particles and materials processing have been studied [5, 22]. Even the comparison is not a likefor-like; the experimental work should be assumed as a fare comparison between these types of commercial lasers operating at their usual operating conditions [22]. Nanosecond laser ablation of materials occurs due to "melt expulsion driven by the vapour pressure and the recoil pressure of light". The melted material will solidify again because of the instability of this process, in which the fluid phase dynamics and the driving vapour conditions are very complex. As a result of this, the ablated area on the surface of the material target is not precise and uniform in comparison with that produced by a femtosecond laser (see **Figure 3**). Furthermore, nanosecond laser ablation creates a heat-affected zone (HAZ) [4]. It was shown that the HAZ of Al sample for the nanosecond laser ablation was about 40 mm wide, whereas

for the femtosecond laser there was not observable because it was <2 mm wide [23].

**Figure 3.** Laser material processing of a glass target by nanosecond laser (left) and femtosecond laser (right) ablation

Chichkov et al. [5].

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[4].

**production of nanoparticles**

For ultrafast lasers, laser beam energy deposition happens on a timescale that is quite short in comparison with atomic relaxation processes. After the laser energy is absorbed by the electrons, cold ions will be produced, leading to the occurrence of thermalisation at the end of the laser pulse. In addition, the femtosecond laser intensity is quite high and is sufficient to drive highly non-linear absorption processes in the target materials which the laser wavelength cannot absorb. At these high intensities, multi-photon ionisation becomes considerably strong [4]. Because of the very high flux of the femtosecond laser photons, several photons collide and become bound electrons; this is multi-photon ionisation. When the amount of total photon energies absorbed is higher than the ionisation potential, the bound electrons will absorb several photons. As a result, the bound electrons become free from the valence band. The multiphoton ionisation process is higher at very high laser intensities. When the laser intensity (photon flux) is above 1013 W/cm2 , multi-photon ionisation becomes very strong and seed electrons are not required to initialise ionisation in high-energy band gap materials or wide band gap materials (see **Figure 4a**) [4].

**Figure 4.** Non-linear ionisation processes of multi-photon absorption (a) and avalanche ionisation (b).

If the kinetic energy of the free electron is very high, some of the energy might be transferred to a bound electron in the target material by collision, thus overcoming the ionisation potential and generating two free electrons; this process is known as (collisional) impact ionisation. Thus, more free electrons will be produced from the bound electrons after the free electrons have absorbed photons. This is called avalanche ionisation, in which free electron density exponentially increases. It is worth mentioning that avalanche ionisation is highly dependent upon free electron density. Energy loss by phonon emission and energy gain by inverse Bremsstrahlung competition indicate the efficiency of this process. It is responsible for the high-energy gap transition materials at laser intensities below 1012 W/cm2 (see **Figure 4b**) [4]. For femtosecond lasers, avalanche ionisation plays an essential role in the optical breakdown. Briefly, avalanche ionisation produces a macroscopic breakdown of the target materials and multi-photon ionisation produces seed electrons at a critical density of about 1020 cm−3 [25].

The amount of material removed during laser ablation depends on the amount of energy absorbed by the bulk material target. The dissipation of the absorbed laser energy will usually occur after the laser pulse duration. There are two major mechanisms to explain material removal by laser ablation: thermal vaporisation, where the local temperature increases to above the vaporisation point due to the electron-phonon collisions, and the occurrence of a Coulomb explosion, where the bulk materials release the excited electrons and produce a relatively strong electric field which revokes the ions inside the impact area [4, 26]. According to these mechanisms, femtosecond laser material removal can be classified in two ways: the first is "strong ablation dominated by thermal vaporisation at intensities significantly higher than the ablation threshold" and the second is "gentle ablation governed by the Coulomb explosion near the ablation threshold" [4]. As shown in **Figure 5**, the long-pulse lasers have more heat-affected zones and shock waves in comparison with the shorter picosecond and femtosecond lasers. The main differences between them are due to the mechanism or the basic principles of laser-induced target material removal processes [27].

**Figure 5.** Long-pulse and ultrafast-pulse laser interaction with target material [27].

Laser pulse duration is an important parameter in the laser ablation process. There are quite large differences between the long pulse duration (nanosecond laser) and ultra-short laser pulse duration (femtosecond laser) for the ablation of materials [26]. As shown in **Figure 6**, during nanosecond laser ablation, plasma will be produced during the laser pulse, but during femtosecond laser ablation, it will be produced after the laser pulse has ended. During nanosecond laser ablation, plasma is a part of the pulse duration, so the pulse serves to reheat the plasma. This leads to higher persistence of the plasma for nanosecond laser ablation than for femtosecond laser ablation [26].

exponentially increases. It is worth mentioning that avalanche ionisation is highly dependent upon free electron density. Energy loss by phonon emission and energy gain by inverse Bremsstrahlung competition indicate the efficiency of this process. It is responsible for the

For femtosecond lasers, avalanche ionisation plays an essential role in the optical breakdown. Briefly, avalanche ionisation produces a macroscopic breakdown of the target materials and multi-photon ionisation produces seed electrons at a critical density of about 1020 cm−3 [25].

The amount of material removed during laser ablation depends on the amount of energy absorbed by the bulk material target. The dissipation of the absorbed laser energy will usually occur after the laser pulse duration. There are two major mechanisms to explain material removal by laser ablation: thermal vaporisation, where the local temperature increases to above the vaporisation point due to the electron-phonon collisions, and the occurrence of a Coulomb explosion, where the bulk materials release the excited electrons and produce a relatively strong electric field which revokes the ions inside the impact area [4, 26]. According to these mechanisms, femtosecond laser material removal can be classified in two ways: the first is "strong ablation dominated by thermal vaporisation at intensities significantly higher than the ablation threshold" and the second is "gentle ablation governed by the Coulomb explosion near the ablation threshold" [4]. As shown in **Figure 5**, the long-pulse lasers have more heat-affected zones and shock waves in comparison with the shorter picosecond and femtosecond lasers. The main differences between them are due to the mechanism or the basic

(see **Figure 4b**) [4].

high-energy gap transition materials at laser intensities below 1012 W/cm2

31412 High Energy and Short Pulse Lasers

principles of laser-induced target material removal processes [27].

**Figure 5.** Long-pulse and ultrafast-pulse laser interaction with target material [27].

Laser pulse duration is an important parameter in the laser ablation process. There are quite large differences between the long pulse duration (nanosecond laser) and ultra-short laser pulse duration (femtosecond laser) for the ablation of materials [26]. As shown in **Figure 6**, during nanosecond laser ablation, plasma will be produced during the laser pulse, but during femtosecond laser ablation, it will be produced after the laser pulse has ended. During

**Figure 6.** Approximate timescale comparison of pulsed laser energy absorption and ablation, along with the various processes, for nanosecond laser (10 ns) and femtosecond laser (50 fs) ablation in an ambient gas.

As shown in the schematic diagram in **Figure 7**, Lescoute et al. [28] showed that during subpicosecond laser ablation three phases occur behind the shock: liquid, vapour and plasma. The shock waves produced can compress the solid target material. The plasma plume effect is more predominant in picosecond laser ablation than in femtosecond laser ablation [22]. In general, laser-plasma interaction during laser-material ablation is strongly dependent on laser wavelength and the excitation wavelength is a very important parameter in nanosecond laser ablation, as in femtosecond laser ablation. The production of nanoparticles through plasma plume condensation occurs in the microsecond-millisecond timescales.

**Figure 7.** Schematic diagram of ablated matter depicted in an ultra-short, that is subpicosecond, laser-material interac‐ tion.

Another advantage of the femtosecond laser is its short timescale which prevents the material target from being heated; this leads to a reduction in the thermal effects in comparison with the nanosecond laser ablation. The duration of the energy transfer process from electron to ion (*τei*) is approximately 1–10 s of a picosecond; this is in the same order as the heat conduction time (*τheat*). In an ideal femtosecond laser ablation case, this is considerably longer than the laser pulse duration (*tp*). (*τei* ~ *τheat* >> *tp*) [26]. This will depend on the type of material sample.

Femtosecond laser energy transfer in the material for ablation will occur through energy absorption by electrons, leading to ionisation and then redistribution of the laser energy to the lattice. Ionisation is either produced by multi-photon ionisation or collisional impact. These processes can be labelled multi-photon ionisation (MPI) and collisional impact ionisation (CII) [26]. Multi-photon ionisation (MPI) occurs when multiple photons provide sufficient energy to the electrons in the valance band; as a result, the electrons are free to reach metastable quantum states by excitation. Collisional impact ionisation (CII) will take place when bound electrons gain energy from free electrons by collision; as a result, the bound electrons will be released.

As shown in **Figure 8**, when a single high fluence fs laser pulse is incident onto a material, the timescale of the physical phenomena involved in laser-material interaction can take place as follows: after 1 fs, photo-ionisation, such as multi-photon absorption and/or tunnelling ionisation, will occur; after 50 fs, avalanche ionisation will occur; and after 100 fs, thermalisa‐ tion of the electrons of the metal target will take place. Longer laser pulse durations lead to more physical phenomena; after 1 ps, energy transformation from electron to lattice will result, then after 10 ps, some thermodynamic processes such as thermal diffusion, fusion and explosion will occur. In much longer laser pulses, after 1 ns, photochemical processes such as chemical reactions and phase transformation will take place [29].

**Figure 8.** Occurrence of different physical phenomena during different timescales involved in laser-material interac‐ tion.

Different types of nanoparticles have been produced by nanosecond [30, 31], picosecond [32, 33] and femtosecond [28, 34, 35] lasers in different liquid environments, vacuum and gas media. A physical aspect of laser ablation in liquid environments is the liquid breakdown phenomena; it has been shown that the conversion efficiency of the radiation energy to a cavitation bubble during radiation focusing of different wavelengths in water is much lower for femtosecond lasers (at 100 fs, efficiency 1–2%) than for nanosecond lasers (at 76 ns, efficiency 22–25%). Femtosecond lasers (120 fs and 800 nm) can produce controllable size and size distribution of nanoparticles, while nanosecond laser produces relatively large, quite widely dispersed particles [36].

Another advantage of the femtosecond laser is its short timescale which prevents the material target from being heated; this leads to a reduction in the thermal effects in comparison with the nanosecond laser ablation. The duration of the energy transfer process from electron to ion (*τei*) is approximately 1–10 s of a picosecond; this is in the same order as the heat conduction time (*τheat*). In an ideal femtosecond laser ablation case, this is considerably longer than the laser pulse duration (*tp*). (*τei* ~ *τheat* >> *tp*) [26]. This will depend on the type of material sample.

Femtosecond laser energy transfer in the material for ablation will occur through energy absorption by electrons, leading to ionisation and then redistribution of the laser energy to the lattice. Ionisation is either produced by multi-photon ionisation or collisional impact. These processes can be labelled multi-photon ionisation (MPI) and collisional impact ionisation (CII) [26]. Multi-photon ionisation (MPI) occurs when multiple photons provide sufficient energy to the electrons in the valance band; as a result, the electrons are free to reach metastable quantum states by excitation. Collisional impact ionisation (CII) will take place when bound electrons gain energy from free electrons by collision; as a result, the bound electrons will be

As shown in **Figure 8**, when a single high fluence fs laser pulse is incident onto a material, the timescale of the physical phenomena involved in laser-material interaction can take place as follows: after 1 fs, photo-ionisation, such as multi-photon absorption and/or tunnelling ionisation, will occur; after 50 fs, avalanche ionisation will occur; and after 100 fs, thermalisa‐ tion of the electrons of the metal target will take place. Longer laser pulse durations lead to more physical phenomena; after 1 ps, energy transformation from electron to lattice will result, then after 10 ps, some thermodynamic processes such as thermal diffusion, fusion and explosion will occur. In much longer laser pulses, after 1 ns, photochemical processes such as

**Figure 8.** Occurrence of different physical phenomena during different timescales involved in laser-material interac‐

chemical reactions and phase transformation will take place [29].

released.

31614 High Energy and Short Pulse Lasers

tion.

The generation of large nanoparticles in a liquid environment by nanosecond laser pulse durations and longer is due to the essential target material melting and laser pulse interaction with the cavitation bubbles that are then produced in the liquid environment [37]. The long pulse duration is sufficient to allow photon coupling with both the electronic and vibrational modes of the sample material. This case will be more predominant when the target material has low reflectivity at the laser wavelength, a large absorption coefficient, a low thermal diffusion coefficient and a low boiling point [38]. Long pulse duration heats the target material continually during the pulse duration. This causes the target material to start boiling and subsequently leads to evaporation, which produces a considerable melt layer. As a result, heat will be transferred to the target that prevents the production of very small structures and small nanoparticles. In nanosecond laser ablation, the laser power heats the target material to melting point and then vaporisation temperature; as such, this process can be considered an indirect solid-vapour transition but a solid-melt-vapour transition [19]. Nanosecond laser ablation ejects an ablation plume which creates a shielding on the surface of the material target, leading to a reduction in the laser power induced to the target. Chemistry material responses, cumu‐ lative changes in the target material's surface texture and morphology will occur due to surface melting and a combination of ablation and thermally activated processes [7].

However, the generation of small nanoparticles by short laser pulse duration is due to minimisation of both the thermal effects and the laser pulse interaction with the cavitation bubble [37]. Here, thermal conduction of the target material can be neglected because of the very short timescale and the heated area of the material being of the order of the laser absorp‐ tion length α−1, where α is the target material absorption coefficient [19]. At short and ultrashort laser pulse durations, because the laser interacts with materials due to very quick excitation of the electron distribution, electron-electron coupling leads to a rapid rise in the electron temperature, followed by lattice heating "at a rate dependent upon the electronphonon coupling strength, and eventual vaporisation of the transiently heated target". Electronic contributions "can manifest themselves as unexpectedly large ion yields and/or supra-thermal propagation velocities within the expanding plasma plume" [38]. At short and ultra-short pulse durations, because the energy transfer from the electrons to the lattice happens on a longer timescale than for the short or ultra-short pulses, the pulses do not heat the target material continually. This limits the heat conduction within the target material and produces a superheated layer the size of the irradiated volume. This leads to ablation in a specific area, resulting in minimal thermal damage. Both picosecond and femtosecond laser ablation can be viewed as a direct solid-vapour transition [19].

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

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 aluminium in argon gas.

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 ". The 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 nanoparticles can be produced by ultra-short laser pulse duration in comparison with the subnanosecond laser pulse duration. It was shown that broader surface plasmon resonance (SPR) and smaller reflection/transmission of silver and copper nanoparticles deposited by femtosecond laser in comparison with those deposited by subnanosecond laser indicating smaller size particle generation by ultra-short pulse durations. In contrast, Hubenthal et al. [43] concluded that the tailoring the size and shape of nanoparticles with nanosecond pulsed laser is more achieved in comparison with both continuous wave (CW) and femtosecond pulsed lasers.
