**2. Pulsed laser ablation**

but also for precise micro- and nanomachining. Generating lasers with short and ultra-short pulse duration leads to high-precision laser processing. Lasers with pulse durations in the femtosecondtopicosecondrangedemonstrateasignificantdevelopmentinqualityfordifferent materials in comparison with nanosecond or longer laser pulses [1]. In addition, using pulsed lasers to produce nanoparticles in liquid environments is a promising alternative to chemical

In general, for laser material processing, two different laser pulse duration regimes are used: long pulse duration, such as nanosecond pulse duration, which generates quite a significant heat-affected zone in the material because "the pulse duration is longer than the thermalisation time of most metals" [3]. This type of laser is suitable for removing materials or ablation. Short pulse duration (picosecond laser) and ultra-short pulse duration (femtosecond laser) yield better results, suited to the production of high-precision micro- and nanomachining. Typical laser pulse durations for precise laser material processing are 10 ps or less. It has been shown that suitable laser pulse durations for the micromachining of copper and stainless steel are in

However, there is little evidence that researchers have approached the issue of lasers in nanotechnology in terms of precise and controllable ablation and their ability to generate nanoparticles from different materials. Consequently, the aim of this chapter is to provide an overview of how different laser pulses can be used in laser-material interaction and the

Two important parameters used to describe lasers are their pulse duration (width) and pulse repetition rate (PRR). As shown in **Figure 1a**, laser pulse duration can also be described as fullwidth at half of the maximum (FWHM) amplitude of the laser pulse. Pulse repetition rate or pulse repetition frequency refers to the number of pulses emitted per second. For 1 kHz of

Two further parameters of pulsed lasers, which are especially relevant for ultrafast lasers, are the laser's peak power (Ppeak), which is equal to the laser pulse energy divided by laser pulse

methods for the production of totally ligand-free colloidal nanoparticles [2].

the range of 10–100 ps [3].

3042 High Energy and Short Pulse Lasers

production of nanoparticles.

PRR, the time period *T* would be 0.001 s (see **Figure 1b**).

**Figure 1.** Pulse width or pulse duration (a) and pulse repetition rate (b) of a laser.

Ultra-short laser pulse duration in the range of femtosecond laser pulses and a few picoseconds can be used to produce high-quality and precise material processing. Ultra-short pulse duration can only interact with electrons but longer pulse duration interacts with lattice. It is worth mentioning that during the interaction of an ultra-short laser pulse with materials, heat conduction is limited [5]. As a result of this, the material will be ablated within a spatial or well-defined area with minimised mechanical and thermal damage of the ablated area on the target. In contrast, longer (nanosecond) pulse duration irradiation on the materials leads to continuously heating the target material. The laser pulse energy will then be spread by heat conduction to an area outside the laser spot size, causing the irradiated target to boil and evaporate. Boiling and evaporation of the target material leads to the production of an uncontrollable melt layer [6]. In the case of nanosecond laser pulse duration, this problem may be caused by imprecise machining or marking.

There is existing research on laser-material interaction [6–8]; however, these show how differently pulsed lasers can be used for material processing, especially in nanotechnology. Furthermore, there are relevant findings concerning laser ablation of nanoparticles in liquid environments [9–12], and they are somewhat showing the effects of different laser pulse durations on the ablation of nanoparticles. Even most previous research on laser-material interaction and laser-generated nanoparticles tends to highlight laser beam parameters and experimental setups to produce small and well-distributed nanoparticles and to precision material processing, there is little emphasis on the optimal laser parameters to apply in laser material processing. Moreover, little attention has been given to the conceptualisation of the structure and phase of the nanoparticles produced by laser ablation in a liquid solution. Consequently, an understanding of the optimal pulse regime for these applications is critical in order to support and enhance the performance of laser material processing in nanotechnol‐ ogy.
