ABSTRACT
When laser light of visible, near-IR or UV spectral range hits condensed matter, it interacts with the valance and/or conduction electrons of the system under action. Depending on laser intensity and irradiation geometry, the interaction can have different far-reaching consequences such as melting, ablation, changing of optical properties, mechanical and chemical transformations. Among existing laser systems, ultrafast lasers have become an extraordinary tool for processing of any kind of materials. With proper choosing the irradiation conditions, laser action allows either inducing highly localized gentle modifications or obtaining strongly damaged material sites with desired or deleterious structures such as voids, periodic nanocracks, periodic surface structures, and craters of various shapes and dimensions. This chapter presents a review on tremendous efforts of researchers in order to achieve clearer insights
into laser-matter interactions in ultrashort irradiation regimes. The review does not pretend to completeness and aims to outline main ideas, achievements, and most intriguing findings still waiting for explanations and theoretical treatments. 3.1 IntroductionUltrafast lasers have become an extraordinary tool for processing of any kind of materials [1-4]. By proper choosing the irradiation conditions, one can induce highly localized gentle modifications or obtain strongly damaged material sites of desired structures such as voids, periodic nanocracks, periodic surface structures, and deep craters. Because of minimized heat diffusion effects, subwavelengthdiameter craters can be produced in metal films [5]. Tight focusing of infrared or visible laser pulses of sub-ps duration inside widebandgap dielectrics allows modification within the focal volume [6-11]. However, within the same material family, considerably different material modifications are achieved when applying similar laser pulses. Deep understanding of the processes triggered in laser-irradiated materials by femtosecond (fs) laser pulses is of vital need for further advance of laser material processing techniques. Over the past three decades, considerable progress has been achieved in establishing the main mechanisms of ultrafast laser interaction with metals, semiconductors, and inorganic dielectrics, which include radiation absorption, leading to creation of highly non-equilibrium thermodynamic states, material heating through equilibration of electronic and ionic/atomic subsystems, and recombination of laser-excited charge carriers (in the case of bandgap materials) with generation of various defect states. Several mechanisms of fs-laser ablation have been ascertained, such as phase explosion, mechanical spallation, and coulomb explosion (mainly for dielectric materials). A lesser understanding has been achieved in respect of thermoelastoplastic processes and plastic deformations resulted from creation of high temperature (and pressure) gradients inside matter as well as of effects of heat, stress, and defect accumulation in multi-pulse irradiation regimes. Below a brief description and a short historical overview of the main approaches developed to date are presented.
