**3. Characteristics of femtosecond laser**

Before we describe the processing methods, we need to know about the characteristics of FSL. Compared with the conventional laser microfabrication techniques, which use continuous-wave lasers or long-pulse lasers, the processing of ultrafast lasers, including nano- and femtosecond lasers, has several advantages. The short laser pulse (< electron–phonon coupling time of the laser-matter interactions) makes the laser energy absorbed by electrons and rapidly transferred, which reduces the thermal effect of ultrafast laser. This characteristic of ultrafast lasers is one of the fundamental advantages, which results in higher precision that enables the

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

### **Figure 2.**

*Laser processing of a transparent material by single and multiphoton absorption and their electron excitation process.*

fabrication of fine structures. This characteristic is especially important when the peak intensity of the laser has to be controlled near the ablation threshold with a low repetition rate.

The second important advantage of ultrafast lasers is related to the non-linear electron excitation mechanism that is unique to these lasers and results in the induction of strong absorption, even in transparent materials. In the conventional linear single-photon, absorption (long-pulsed or CW lasers) requires photon energies greater than the material's band gap for excitation of an electron from the valence band to the conduction band by absorbing a single photon. Because of the surface absorption, the transparent materials cannot be internally modified. In ultrafast lasers, absorption of the extremely high density of photons (multiphoton) enables the interaction between the laser pulse and a transparent material only near the focal point (**Figure 2**). Therefore, no out-of-focus absorption occurs by focusing FSL beam on transparent bulk material. In the next subtitle, we describe the mechanism of nonlinear and multiphoton absorption processes of FSL in detail.

The combination of the two above-mentioned characteristics (heat-affected zones suppression and non-linear multiphoton absorption) shapes the third advantage of ultrafast laser processing, i.e. a resolution far beyond the diffraction limit. This factor is related to the Gaussian spatial profile of the laser beam intensity, which makes the effective absorption coefficient for *n*-photon absorption proportional to the *n*th power of the laser intensity that causes a narrower absorbed energy distribution for multiphoton absorption. The fabrication resolution can be additionally improved by adjusting the laser intensity at a threshold above which a reaction occurs on absorption.

The last important characteristic of FSL to be mentioned here is the spatially selective manner of tuning or altering the physical and chemical properties of a material. This characteristic, along with the multiphoton effect, is used in 3D femtosecond laser direct writing for the integration of multiple functions on a single substrate [6].
