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Diagram showing laser heating of a solid-liquid interface. IL is the laser absorption, T is the temperature distribution in both materials, and JI is the heat conduction across the interface [16]. It was shown that the laser pulse duration has an effect on both the material ablation thresholds and penetration depths. Long pulse duration or increasing laser pulse duration increases the threshold fluence and decreases the effective energy penetration depth [ 1 ]. Low-intensity long laser pulse interaction with a target material firstly heats the surface of the target due to the absorbed energy, which leads to melting and vaporisation.

It should be noted that vaporisation of the target requires much more energy than melting.

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In nanosecond laser beam interaction with material, the surface of the target material will be heated to melting point and then to vaporisation temperature. In nanosecond laser ablation regimes, there is enough time for thermal waves to propagate into the target material and to create a relatively large layer of melted material target [ 5 , 6 ]. Nanosecond laser pulses can ablate the target materials even at low laser intensities in both the vapour and liquid phases, so a recoil pressure that expels the liquid will be created due to the vaporisation process [ 6 ].

Evaporation occurrence makes challenge to precise laser processing with nanosecond laser pulses [ 18 ]. So, long laser pulse duration creates sufficient time for thermal waves to propagate within the target material, and the absorbed energy will be stored in a layer with a thickness of about L th. In this case, the target material needs much more energy to vaporise than to melt; in other words, evaporation will occur, while the energy absorbed per unit volume into the vaporised layer becomes higher than the latent heat of evaporation per unit volume, namely [ 19 ].

This figure is approximately given by the energy required to melt a surface layer of the target material of the order of L th [ 19 ]:. At low-intensity, short laser pulse interactions with a target material, due to the inverse Bremsstrahlung most of the laser energy will be absorbed by the free electrons of the target. In spite of a small energy exchange between the lattice and the electron heat conduction, the electrons are cooled [ 6 ]. In other words, when the laser pulse duration is shorter than the electron-phonon energy-transfer time, then the electron and lattice have different temperatures, meaning that they will be in a non-thermal equilibrium state.

In this case, Eq.

This method represents the lattice temperature in integral from. It can be noted from the last equation that during picosecond laser ablation, the lattice temperature remains notably lower than the electron temperature, and thus, the lattice temperature in Eq. Finally, both the lattice and electron temperature at the end of the laser pulse can be expressed by the following equation [ 5 ]:.

In this case, the electron-lattice coupling can be neglected and Eq. Thus, Eq. It has been shown that heat conduction of the target material can be neglected at the very short timescales of picosecond and femtosecond laser pulse durations; thus the target temperature at the end of the pulse within the target material can be given by [ 19 ]. The evolution of the electron temperature T e and lattice temperatures T i after the laser pulse is described by Eq. In addition, the electron temperature and lattice temperature initial conditions are given by Eq.

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Due to the energy transfer to the lattice and heat conduction of the bulk material, the electrons are rapidly cooled after the laser pulse. Since the electron cooling time is quite short, then Eq. It should be noted that the initial lattice temperature is neglected here. Fann et al. From Eq.


Laser Ablation: Effects and Applications

Then, the ablation depth per laser pulse or ablation rate L can be written as [ 5 , 21 ]:. The logarithmic dependence of the ablation depth on the laser pulse fluence is well known for the laser ablation of organic polymers and metal targets with femtosecond pulse duration [ 5 ]. It can be noted that Eqs. Therefore, the condition for strong evaporation given by 19 , the fluence threshold and the ablation depth per pulse given by 20 remain unchanged [ 5 , 18 ]. Thus, in the picosecond laser range, it is possible that logarithmic dependence of the ablation depth on the fluence exists.

Here, electron heat conduction inside the target material is neglected. In this case, laser ablation is accompanied by the electron heat conduction and production of a melted area in the target material. Even evaporation can be considered as a direct solid-vapour transition process, whereby the existence of a liquid phase in the target material reduces the precision of laser material processing.

Femtosecond laser ablation effects a direct solid-vapour transition due to the short timescales in this laser regime; as a result of this, the lattice is heated on a picosecond timescale, leading to the production of vapour and plasma phases followed by a rapid expansion in vacuum. Here, in a first approximation, 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 Chichkov et al.

A comparison of the characteristics of nano-, pico- and femtosecond lasers produced nanoparticles and materials processing have been studied [ 5 , 22 ]. Even the comparison is not a like-for-like; the experimental work should be assumed as a fare comparison between these types of commercial lasers operating at their usual operating conditions [ 22 ]. 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 ]. Laser material processing of a glass target by nanosecond laser left and femtosecond laser right ablation [4]. 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 vaporisation 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 ].

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 multi-photon ionisation process is higher at very high laser intensities. 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. For femtosecond lasers, avalanche ionisation plays an essential role in the optical breakdown. 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 ].

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 ]. 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 ]. 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. 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. Schematic diagram of ablated matter depicted in an ultra-short, that is subpicosecond, laser-material interaction.

Background gas collisional effects on expanding fs and ns laser ablation plumes - Semantic Scholar

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. In an ideal femtosecond laser ablation case, this is considerably longer than the laser pulse duration t p. 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.

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.

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 ].

Occurrence of different physical phenomena during different timescales involved in laser-material interaction. 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. Femtosecond lasers fs and nm can produce controllable size and size distribution of nanoparticles, while nanosecond laser produces relatively large, quite widely dispersed particles [ 36 ].

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.

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 ]. 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. Then, we focus on several commonly used polymer materials and compare them in detail, including the effects of polymer properties, laser parameters and feature designs.

Finally, we summarize the applications of various structures fabricated by LA in a variety of areas along with a perspective of the challenges in this research area. Overall, a thorough review of LA of several polymers is presented, which could pave the way for characterization of future novel materials. Volume 68 , Issue 8. The full text of this article hosted at iucr. If you do not receive an email within 10 minutes, your email address may not be registered, and you may need to create a new Wiley Online Library account.

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Double-pulse laser ablation of solid targets in ambient gas: Mechanisms and effects

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