Modelling ultrafast laser ablation Rethfeld, Baerbel; Ivanov, Dmitriy S; Garcia, Martin E ...
Journal of physics. D, Applied physics,
05/2017, Letnik:
50, Številka:
19
Journal Article
Recenzirano
Odprti dostop
This review is devoted to the study of ultrafast laser ablation of solids and liquids. The ablation of condensed matter under exposure to subpicosecond laser pulses has a number of peculiar ...properties which distinguish this process from ablation induced by nanosecond and longer laser pulses. The process of ultrafast ablation includes light absorption by electrons in the skin layer, energy transfer from the skin layer to target interior by nonlinear electronic heat conduction, relaxation of the electron and ion temperatures, ultrafast melting, hydrodynamic expansion of heated matter accompanied by the formation of metastable states and subsequent formation of breaks in condensed matter. In case of ultrashort laser excitation, these processes are temporally separated and can thus be studied separately. As for energy absorption, we consider peculiarities of the case of metal irradiation in contrast to dielectrics and semiconductors. We discuss the energy dissipation processes of electronic thermal wave and lattice heating. Different types of phase transitions after ultrashort laser pulse irradiation as melting, vaporization or transitions to warm dense matter are discussed. Also nonthermal phase transitions, directly caused by the electronic excitation before considerable lattice heating, are considered. The final material removal occurs from the physical point of view as expansion of heated matter; here we discuss approaches of hydrodynamics, as well as molecular dynamic simulations directly following the atomic movements. Hybrid approaches tracing the dynamics of excited electrons, energy dissipation and structural dynamics in a combined simulation are reviewed as well.
Fully analytic expressions, for the electric and magnetic fields of an ultrashort and tightly focused laser pulse of the radially polarized category, are presented to lowest order of approximation. ...The fields are derived from scalar and vector potentials, along the lines of our earlier work for a similar pulse of the linearly polarized variety. A systematic program is also described from which the fields may be obtained to any desired accuracy, analytically or numerically.
•Analytic expressions for the fields of an ultra-short, tightly-focused, radially-polarized laser pulse are derived.•Analytic and numerical procedures are described for the derivation of the fields to any desired level of accuracy.•The fields are derived for two initial wavenumber distributions: a Gaussian and a Poissonian.•Fields to lowest-order of approximation are discussed using pulse lengths and waist radii comparable to a wavelength.
Abstract
The forthcoming generation of multi-petawatt lasers opens the way to abundant pair production by the nonlinear Breit–Wheeler process, i.e. the decay of a photon into an electron–positron ...pair inside an intense laser field. In this paper we explore the optimal conditions for Breit–Wheeler pair production in the head-on collision of a laser pulse with gamma photons. The role of the laser peak intensity versus the focal spot size and shape is examined keeping a constant laser energy to match experimental constraints. A simple model for the soft-shower case, where most pairs originate from the decay of the initial gamma photons, is derived. This approach provides us with a semi-analytical model for more complex situations involving either Gaussian or Laguerre–Gauss (LG) laser beams. We then explore the influence of the order of the LG beams on pair creation. Finally we obtain the result that, above a given threshold, a larger spot size (or a higher order in the case of LG laser beams) is more favorable than a higher peak intensity. Our results match very well with three-dimensional particle-in-cell simulations and can be used to guide upcoming experimental campaigns.
Surface structuring is a key factor for the tailoring of proper cell attachment and the improvement of the bone-implant interface anchorage. Femtosecond laser machining is especially suited to the ...structuring of implants due to the possibility of creating surfaces with a wide variety of nano- and microstructures. To achieve a desired surface topography, different laser structuring parameters can be adjusted. The scanning strategy, or rather the laser pulse overlap and scanning line overlap, affect the surface topography in an essential way, which is demonstrated in this study. Ti6Al4V samples were structured using a 300 fs laser source with a wavelength of 1030 nm. Laser pulse overlap and scanning line overlap were varied between 40% and 90% over a wide range of fluences (
from 0.49 to 12.28 J/cm²), respectively. Four different main types of surface structures were obtained depending on the applied laser parameters: femtosecond laser-induced periodic surface structures (FLIPSS), micrometric ripples (MR), micro-craters, and pillared microstructures. It could also be demonstrated that the exceedance of the strong ablation threshold of Ti6Al4V strongly depends on the scanning strategy. The formation of microstructures can be achieved at lower levels of laser pulse overlap compared to the corresponding value of scanning line overlap due to higher heat accumulation in the irradiated area during laser machining.
Abstract
High-quality protons have many potential applications in tumor therapy, proton radiography, fast ignition and nuclear physics. Over recent decades, significant progress has been made in ...generating such proton beams with lasers. However, it is still challenging to achieve a compact proton beam with an energy of hundreds of MeV under the laser intensities currently available. Here, we propose a novel scheme to produce above-100 MeV protons in nonuniform near-critical-density (NCD) plasmas driven by a Laguerre–Gaussian (LG) laser pulse. When a linearly-polarized LG laser pulse with the intensity of ∼10
20
W cm
−2
enters the NCD plasmas with a trapezoidal density profile, a donut-like double-channel structure with an electron column on the axis is produced behind the laser pulse, together with a unique, strong magnetic field. At the end of the density plateau, the magnetic field begins to expand in both longitudinal and transverse directions. The magnetic pressure force displaces the electrons with regard to the protons, resulting in a quasi-static electric field
E
x
S
. This electrostatic field can remain robust and be sustaintained for a long time since the magnetic field decreases very slowly, i.e.
E
x
S
∝
▽
B
z
2
. Thus, the protons can be accelerated by the longitudinal electrostatic field and confined by the generated transverse focusing field. Three dimensional particle-in-cell simulations confirmed that a well-defined above-100 MeV proton beam with a cut-off energy at least 3.5 times larger than that of the normal Gaussian laser could be obtained, making the proton beams a potential source for tumor therapy in the future.
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•Laser pulse modulation determines the dwell time and affects the intermetallic compounds thickness.•Laser pulse modulation influences bubble formation in the melt pool and final ...porosity.•Two intermetallic compounds are formed, TiAl and Al3Ti.•The tensile properties correlate to the change of the IMC thickness.
Laser welding of titanium and aluminum alloys usually generates pores and brittle intermetallic compounds (IMCs), which degrade the weld mechanical properties. In this study, pulse wave (PW) Nd: YAG laser was used to weld Ti-6Al-4V to aluminum alloy 7075 (AA7075) with aluminum alloy 4047 (AA4047) filler using pulse modulation to control the formation of pores and IMCs. The effect of pulse modulation on porosity levels and IMC layer thickness were analyzed. EBSD, XRD and elemental mapping identified that IMC layers of TiAl and Al3(Ti, V) were formed. A numerical model for pulse laser welding Ti/Al alloys was developed and validated with thermocouple measurements. The numerical results revealed that the predicted cooling rates above the allotropic transformation temperature (882 ℃) of Ti for different pulse modulations have significant correlation with the measured IMC layer thickness. The optimized process parameters with pulse modulation were able to control the IMC thickness to within 5 µm and attain a maximum ultimate tensile strength of 158 MPa.
Abstract
The electron acceleration in a prepared ion channel is studied theoretically by using a radially polarized (RP) cosh-Gaussian (ChG) laser pulse. The peculiar propagation properties of ChG ...laser cause it to focus early and over a shorter time than a Gaussian laser pulse, making it suitable for accelerating electrons to extremely high energies over a small duration. The electrostatic field formed by an ion channel prevents electrons from escaping the interaction zone due to their transverse oscillations, whereas the decentering parameter of the ChG laser pulse influences electron energy gain. This combined role of RP-ChG laser and the effect of an ion channel causes an enhancement in the electron energy gain significantly to the order of GeV with laser intensity (∼
10
20
W
c
m
−
2
) in an ion density channel (∼
10
22
m
−
3
) with decentered parameter (∼2.15) for an incident laser pulse at initial phase (
ψ
0
=
π
).
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