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Chirped Pulse Amplification
Published in Chunlei Guo, Subhash Chandra Singh, Handbook of Laser Technology and Applications, 2021
where n2 is the non-linear refractive index and I(z) is the on-axis intensity as a function of propagation distance, z, through the medium. In the laser community, the intensity is defined as power per unit area (in the optics field, this is referred to as irradiance). Fluence is defined as energy per unit area, so that intensity is equal to the fluence per unit time. As will be discussed in Section 22.4.1, efficient energy extraction requires that a laser amplifier be seeded at the saturation fluence or higher. Therefore to maintain efficient amplification while reducing the intensity to decrease the B-integral, the pulse duration must be increased. At the time CPA was invented, the common rule was to keep the B-value below 5 to avoid damage of the laser rods from self-focusing. Now it is more typical in CPA systems to keep the B value even lower to minimize beam and pulse distortions.
Fundamental Concepts and Quantities
Published in Shaheen A. Dewji, Nolan E. Hertel, Advanced Radiation Protection Dosimetry, 2019
The macroscopic cross section is used as an indication of the interaction probability for a particular mass of material, takes into account the shape and dimensions of the material, and has units of cm−1. Many cross sections have an energy dependence that decreases by the square root of the incident neutron energy (e.g., 1/E=1/v). The neutron mean free path, λ is the inverse of the macroscopic cross section and describes the average path length a neutron will travel in a material before undergoing a reaction. The number of total reactions in a volume can be determined by combining the probability of a reaction occurring with the neutron fluence, ϕ. The fluence is a measure of particle density and is typically given for a specific energy. For a given position, r, the neutron reaction rate, R, is given by R=∫EminEmaxϕ(E,r)Σ(E)dE
Pulsed laser deposition of dielectrics
Published in Michel Houssa, κ Gate Dielectrics, 2003
Dave H A Blank, Lianne M Doeswijk, Koray Karakaya, Gertjan Koster, Guus Rijnders
For a specific laser and target, the fluence mainly determines the target morphology evolution under pulsed laser irradiation. The target surface characteristics can influence the ablation rate, the film stoichiometry, and its surface roughness. Depending on the exact target morphology, a rough target surface can either decrease or increase the ablation rate. The former is a result of an increased surface area and, therefore, reduced fluency, while the latter is the result of better light coupling into the target. If induced target surface structures are aligned in the direction of the laser beam, the plasma is also shifted in the direction of the laser beam. Exfoliation of fragile microstructures formed at the target surface results in rough film surfaces, because the plasma carries the loose debris towards the substrate, where it condenses onto the growing thin film. For multi-element materials, it is important to realize stoichiometric ablation [46, 47]. A first indication, though not sufficient, of stoichiometric ablation is a smooth target morphology, which also offers the possibility of a longer stable use of the target. These requirements result in a critical ablation threshold, for which a desirable target morphology is obtained.
Surface Patterning of Cemented Carbides by Means of Nanosecond Laser
Published in Materials and Manufacturing Processes, 2020
Shiqi Fang, Víctor Pérez, Nuria Salán, Dirk Baehre, Luis Llanes
Laser fluence describes the pulse energy delivered per unit (or effective) area. That is, the ratio between the pulse energy and effective spot area, given by the following equation[16,17]:
Optimization of laser parameters for improved corrosion resistance of nitinol
Published in Materials and Manufacturing Processes, 2020
K. E. Ch. Vidyasagar, Abhishek Rana, Dinesh Kalyanasundaram
The main effects plot or individual processing parameter on the output responses of Lp, Pf, and IRd on Ec, Ic, and CR is shown in Fig. 3. The main effects plots are generated using Minitab® from the output responses corresponding to the input parameter. However, the main effects plot graphs the output response mean of each parameter level connected by a straight line. It is evident that all the three input parameters Lp, Pf, and IRd contribute to decreasing Ec, Ic, and CR values. Maximum Ec values were observed at higher values of Lp (between 35 and 45 W), lower values of Pf (i.e. ~50 to 100 kHz), and higher density of laser irradiation (65% to 85%). Lower Ic values were observed at higher Lp (35 to 45 W), lower Pf (~50 to 100 kHz), and higher IRd (70% to 90%). Similarly, lower values of CR were observed at higher Lp (35 to 45 W), lower Pf (~50 to 100 kHz), and higher IRd (70% to 90%). From Fig. 3(a1, b1, c1), it was observed that Lp has a linear effect on the output responses, i.e. as Lp increases, the response tends toward decreases for CR. It was also observed that for the Lp values less than 25 W, the CR is significantly high. This is attributed to the fact that the fluence (laser energy per unit area) decreases when Lp decreases, resulting in less ablation and less formation of oxides. Hence, it is prone to corrosion. Figure 3(a2, b2, c2) indicates that as Pf increases, the pulse energy decreases, and subsequently, the laser fluence decreases. Lower fluence leads to minuscule ablation, which results in the decrease of oxide formation. So Pf has an inversely proportional effect on the output responses. Figure 3 (a3, b3, c3) represents the effect of IRd on the output responses. As the density of laser irradiation increases, energy deposition per unit area increases, which is directly proportional to the oxide formation on the surface and hence preferable.
Thermo-elasto-plasto-dynamics of ultrafast optical ablation in polycrystalline metals. Part I: Theoretical formulation
Published in Journal of Thermal Stresses, 2021
Surface morphology by ultrafast laser processing, however, does not always involve material removal through phase transition or explosion. Ultrafast ablation can involve submelting, melting, and superheating in response to the employed laser fluence [9]. It was found in a study on ablating a Cu material using a subpicosecond laser (500 fs) that when the fluence F was between 0.4 J/cm2 and 5 J/cm2, a thin molten layer appeared. The mechanism involved was the melt ejection due to the recoil pressure of the plasma [1]. With the use of pump-probe technology, a similar melting process was observed in Si that was irradiated by a femtosecond laser of high laser fluence [10]. When F > 5 J/cm2, ejection of large liquid droplets was observed. A probable interpretation is that phase explosion superheats the target and causes the explosive decomposition of the material [1], [9], and [11]. Ablation at F < 0.4 J/cm2 was observed with no indication of molten layers. This necessarily excludes phase transition or boiling explosion because lattice temperature does not exceed the melting temperature. Thus, the material removal process by ultrafast laser heating of relatively low fluence cannot be explained by phase transition or explosion. Formation of optical interference patterns and disappearance of Newton rings indicate the role mechanical deformation plays in ablation as an aftermath [12]. Investigations on machining quality using femtosecond lasers also concluded the same – no molten debris was found in the ablated area morphology [13]. And the resulted material ejection was found to be mainly caused by the spallation of the fractured layers as a result of the induced stress alternating in a tensile mode [14]. Hence, when relatively low fluence (near the threshold of ablation) is employed, ablation is primarily attributed to thermally induced mechanical damage with fragmentation ejection (photomechanical spallation) being the mechanisms for material removal. Thermal–mechanical disturbance is believed to be the root cause for the incubation effect [15] and ripples [16]–[18] by a single-laser or multi-laser shots at low fluence. A comprehensive understanding for laser–material interaction, thermally induced mechanical response, and fragmentation ejection and spallation is required to describe femtosecond ablation at low intensity and the inflicted surface morphology.