**4. Mechanisms of femtosecond processing**

Understanding the physical mechanisms and processes of FSL is important, as they differ between FSL fabrication (including phase change and material removal) and traditional manufacturing methods, which are essentially determined by laser– electron interactions. Hence, the regulation of laser-electron interactions or electrons dynamics is critical to the future development of femtosecond laser manufacturing, which poses a challenge for measuring and controlling at the electron level during fabrication processes. Hence, the development of theory and observation systems must be synchronized with the development of laser fabrication methods and applications. In ophthalmic surgery, laser energy is channeled as efficiently as possible. The flap thickness is determined by placing a sterile plastic foil between the laser and the cornea. The computer keeps all corneal cuts suctioned up in a total vacuum time of <40 seconds. Laser cuts the tissue by two mechanisms; some laser pulses vaporize small amounts of tissue by photodisruption process. Vaporized tissue causes multiple intrastromal cavitation bubbles of microplasma, composed of water and carbon dioxide. The bubbles disrupt the tissue at a larger radius than the plasma created at the laser focus, which dissociates the tissue and creates a lamellar corneal dissection plane. Other lasers create a dissection plane using the desired pattern (e.g., a raster or spiral pattern), controlled by the surgeon using laser software. The nature of the cutting processes differs based on the laser-tissue interaction parameters, which include (1) Pulse energy, (2) Pulse repetition rate, (3) Pulse duration, (4) Wavelength, (5) Focusing power, (6) Focus spot shape, and (7) Spatial pulse spacing.

The laser energy used may be a high or low pulse. Lasers with high pulse energy and low frequency (with pulse energies in J and repetition rates in kHz) were used earlier (**Figure 3**). Low repetition rate and pulse overlap reduce tissue bridging. In the high pulse energy laser group, the mechanical forces drive the cutting process by the expanding bubbles, which disrupt the tissue at a larger radius than the plasma created at the laser focus. Modern FSLs use low pulse energy by shortening the pulse duration

**Figure 3.** *High pulse low-frequency energy.*

### *Fundamentals of Femtosecond Laser and Its Application in Ophthalmology DOI: http://dx.doi.org/10.5772/intechopen.106701*

or reducing the focal spot size to reduce the side effects at a given wavelength. In the low pulse energy group, high pulse frequencies are applied (MHz range), which helps achieve a cutting speed as effective as in the high energy laser group. Tissue evaporation inside the plasma effectively separates tissue without a need for secondary mechanical tearing effects [5].

Several parameters play a role in the interaction between the laser and the material, including chemical, thermal, and mechanical effects via free-electron generation. Ablation of material is caused by thermal damage resulting from impurities and defects in the samples. FSL intensities ≥1014 W/cm2 are anticipated to have the same ablation mechanisms for metals and dielectric materials. Gamaly et al. also verified the ablation threshold value and ablation velocity formula of the metal and dielectric material. This interaction also differs between metallic and non-metallic materials, as metallic materials have a large number of free electrons while non-metallic materials do not. The absorbed FSL by free photons of metallic materials results in heated photons and their collision with other electrons that transfer energy to each other and increase the interaction with the lattice. The heated lattice results in the phase transition of the material, at picosecond (10−12 s) to tens of nanoseconds (10−8 s), resulting in micro- to millisecond-scale plasma. Conversely, in non-metallic materials, electrons are bound in the valance band. The processing of FSL in non-metallic materials includes ionization and phase change. Ionization involves photoionization (which includes tunnel ionization and multiphoton ionization) and impacts ionization mechanisms. Photoionization is the main mechanism of seed-electron generation. The super-strong electromagnetic field generated by FSL reduces the Coulomb potential barrier of valence-band electrons by the tunneling effect, which results in the transfer of electrons into the conduction band and become free electrons. Multiphoton ionization refers to the absorption of multiple photon energies by the valence-band electrons, which obtain higher levels of kinetic energy, collide with other valence electrons, and transfer into the conduction band, causing a chain reaction similar to avalanche, resulting in free electrons. In other words, avalanche ionization involves free carrier absorption followed by impact ionization. As the electron's energy exceeds the conduction band minimum (more than the band-gap energy), it can ionize another electron from the valence band, which results in two excited electrons that can be heated by the laser field through free carrier absorption. This process repeats as long as the laser field is present and intense enough, leading to an electronic avalanche.

The phase change follows the first part of ionization and electron heating. In this part, the accumulation of a large number of free electrons inside the non-metallic material and the lattice-electron interaction results in the exhibition of transient metallic characteristics. In the temporal scale of picosecond (10−12 s) to tens of nanoseconds (10−8 s) and the spatial scale of tens-of-nanometers-to-tens-of-micrometers, the phase change includes thermal phase transitions (melting and gasification; when the lattice temperature rises above the melting point of the material) and non-thermal phase transitions (Coulomb explosions and electrostatic ablation; based on plasma expansion). The pump-probe technique can be used to observe the ablation processes on this temporal and spatial scale. The thermal damage depends on the pulse width and intrinsic parameters of the material (like melting point, thermal expansion coefficient, thermal conductivity coefficient, and tensile strength). As a general rule, only the fraction of energy within a laser pulse, absorbed inside the tissue, is responsible for interactions with tissue. The emitted laser energy is redistributed from a surgical laser device at the end of the tissue dissection process (**Figure 4**), and only non-linear

### **Figure 4.**

*Redistribution of energy in a pulsed laser process for tissue dissection.*

absorbed energy, which constitutes only 10–15% of FS pulses, contributes to the tissue dissection process. Similarly, in transparent dielectric materials, the laser creates free carriers inside materials by non-linear absorption processes. For transparent materials, similar models have been described (as for non-metallic materials), including multiphoton, tunnel ionization, and avalanche.

It has to be noted that the material-laser interaction is complex, and different theoretical models have been suggested for its description, including time-dependent density function theory, molecular dynamics model, plasma model, and improved two-temperature model. But, each of these has its own limitations in special and temporal scales. For example, molecular dynamics combines the thermal and nonthermal phase transition mechanisms to explain material ablation due to lattice phase transition. However, some aspects have been described simply in this model, such as the alteration of the interaction force between atoms during the ablation process and approximate simplifications in the first-principles model, such as a time-dependent exchange-correlation, which influence the accuracy of theoretical predictions. The plasma model is used to describe photon absorption, plasma generation, and recombination of electronic systems before lattice phase transitions and offers a good explanation for the non-metallic ionization process. However, the phase transition process is not discussed in this model. Two-temperature model, described by Anisimov and colleagues for the interaction between ultrafast laser pulses and solids, is widely used for the prediction of the electron and phonon temperature distributions in laser processing. However, in this model, the electron density is set to a constant value; therefore, it is only applicable in metallic materials [3].

The models were extended by other researchers for more accurate calculations, such as the multiscale theoretical model and observation system, applicable for problems associated with each temporal scale to cover the overall scope of laser fabrication. The imaging system, such as sequentially time all-optical mapping photography, can be integrated with FSL fabrication system to realize real-time continuous observation and feedback of the fabrication process. Other theoretical models, based

### *Fundamentals of Femtosecond Laser and Its Application in Ophthalmology DOI: http://dx.doi.org/10.5772/intechopen.106701*

on simplified rate equations or a kinetic approach to Boltzmann's equations, have also been described. Models considered for the multiphoton non-linear optical process include two-photon absorption (which occurs when two photons are absorbed simultaneously by material) and two-photon polymerization (TPP; which occurs when two photons are absorbed on photosensitive material). This process changes the material and leads to polymerization by activating the photoinitiator-activated free radicals in the resist. Fabrication resolution of true 3D micro/nanostructures by TPP varies based on the laser energy, exposure duration, and concentration of the free radical [7].

Micro/nanostructures induced in transparent materials have been classified by Qiu et al. into four types based on optical coloration, refractive index modification, micro-hole creation, and micro-crack creation. FS pump-probe interferometry technique, with 100 fs temporal resolution, allows measuring the modification of refractive index induced by ultra-short intense laser pulses. When a dielectric material is subject to intense FSL, a large number of excited electrons may be generated by laser pulses, which produce intrinsic defects that make FSL an ideal tool for highprecision material processing. A complex secondary process of high-temperature and high-pressure plasma at the tightly focused laser point would induce a phase or structural modification of the material. The splitting and self-(de)focusing of FSL pulses inside dielectrics are the topics that researchers have used for the calculation of the excitation and relaxation channels of FS-induced carriers. Despite various models and explanations for laser-material interactions, the dynamics of specific phenomena are realized only partially. To quantify dynamics of laser-excited carriers, direct visualization of FSL dynamics using ultrafast imaging and spectral interferometry techniques has been designed and implemented successfully [8].
