In the laser treatment of a workpiece (9), e.g. for surface hardening, melting, alloying, cladding, welding or cutting, the adverse effect of reflectivity of the surface, when a pure, monochromatic laser (6) is used, is remedied by the simultaneous application of a relatively shorter wavelength beam (1). The two beams (1)(5) may be combined by a beam coupler (4) or may reach the workpiece (9) by separate optical paths (not shown). The shorter wavelength beam (1) improves the coupling efficiency of the higher- powered laser beam (5).

Laser Material Processing

We develop a theory for laser-induced periodic surface structure by associating each Fourier component of induced structure with the corresponding Fourier component of inhomogeneous energy deposition just beneath the surface. We assume that surface roughness, confined to a region of height much less than the wavelength of light, is responsible for the symmetry breaking leading to this inhomogeneous deposition; we find strong peaks in this deposition in Fourier space, which leads to predictions of induced fringe patterns with spacing and orientation dependent on the angle of incidence and polarization of the damaging beam. The nature of the generated electromagnetic field structures and their relation to the simple "surface-scattered wave" model for periodic surface damage are discussed. Our calculation, which is for arbitrary angle of incidence and polarization, applies a new approach to the electrodynamics of randomly rough surfaces, introducing a variational principle to deal with the longitudinal fields responsible for local field, or "depolarization," corrections. For a $p$-polarized damaging beam our results depend on shape and filling factors of the surface roughness, but for $s$-polarized light they are essentially independent of these generally unknown parameters; thus an unambiguous comparison of our theory with experiment is possible.

Laser-induced periodic surface structure. I. Theory

The unique characteristics of ultrafast lasers, such as picosecond and femtosecond lasers, have opened up new avenues in materials processing that employ ultrashort pulse widths and extremely high peak intensities. Thus, ultrafast lasers are currently used widely for both fundamental research and practical applications. This review describes the characteristics of ultrafast laser processing and the recent advancements and applications of both surface and volume processing. Surface processing includes micromachining, micro- and nanostructuring, and nanoablation, while volume processing includes two-photon polymerization and three-dimensional (3D) processing within transparent materials. Commercial and industrial applications of ultrafast laser processing are also introduced, and a summary of the technology with future outlooks are also given. Scientists in Asia have reviewed the role of ultrafast lasers in materials processing. Koji Sugioka from RIKEN in Japan and Ya Cheng from the Shanghai Institute of Optics and Fine Mechanics in China describe how femtosecond and picosecond lasers can be used to perform useful tasks in both surface and volume processing. Such lasers can cut, drill and ablate a variety of materials with high precision, including metals, semiconductors, ceramics and glasses. They can also polymerize organic materials that contain a suitable photosensitizer and can three-dimensionally process inside transparent materials such as glass, and are already being used to fabricate medical stents, repair photomasks, drill ink-jet nozzles and pattern solar cells. The researchers also explain the characteristics of such lasers and the interaction of ultrashort, intense pulses of light with matter.

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Ultrafast lasers—reliable tools for advanced materials processing

Amorphous metallic alloys, also called metallic glasses, are of considerable technological importance. The metastability of these systems, which gives rise to various rearrangement processes at elevated temperatures, calls for an understanding of their diffusional behavior. From the fundamental point of view, these metallic glasses are the paradigm of dense random packing. Since the recent discovery of bulk metallic glasses it has become possible to measure atomic diffusion in the supercooled liquid state and to study the dynamics of the liquid-to-glass transition in metallic systems. In the present article the authors review experimental results and computer simulations on diffusion in metallic glasses and supercooled melts. They consider in detail the experimental techniques, the temperature dependence of diffusion, effects of structural relaxation, the atom-size dependence, the pressure dependence, the isotope effect, diffusion under irradiation, and molecular-dynamics simulations. It is shown that diffusion in metallic glasses is significantly different from diffusion in crystalline metals and involves thermally activated, highly collective atomic processes. These processes appear to be closely related to low-frequency excitations. Similar thermally activated collective processes were also found to mediate diffusion in the supercooled liquid state well above the caloric glass transition temperature. This strongly supports the mode-coupling scenario of the glass transition, which predicts an arrest of liquidlike flow already at a critical temperature well above the caloric glass transition temperature.

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Diffusion in Metallic Glasses and Supercooled Melts

Periodic grating structures self-formed on a metal surface under the irradiation of a femtosecond laser pulse are characterized by grating spaces which are shorter than the laser wavelength, as well as by dependence on the laser fluence. This Brief Report presents a different interpretation of these features in terms of the process of parametric decay of laser light to surface plasma waves. Depending on the electron density, grating spaces with lengths of 680 nm to as short as 400 nm can be produced for 800 nm laser wavelength as a result of the interaction of laser pulses with laser-produced surface plasma.

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Mechanism for self-formation of periodic grating structures on a metal surface by a femtosecond laser pulse

We present a scheme for solution of the heat flow equation in one-dimension incorporating melting and vapourization produced under pulsed laser irradiation. The method can be applied to pure materials as well as multilayered structures such as deposited films. The variation of physical properties with temperature can be easily taken into account. Results of calculation are presented for aluminium and for chromium and antimony layers deposited on aluminium. As a consequence of excessive vapourization at high energy densities, the melt depth and the melt duration do not increase beyond a certain limit. The resolidification front velocity is strongly dependent on energy density and can be controlled in an experiment by a careful choice of laser parameters. Some recent experimental data on laser treated chromium films are discussed in light of our calculations.

Pulsed laser heating calculations incorporating vapourization

We report the first observation of surface ripples produced on a metal (pure and implanted A1) during pulsed laser treatment (Nd:glass laser in TEM00 mode, λ = 1.06 μm, 7 ns FWHM). Fringes with a characteristic spacing equal to the incident wavelength are readily seen near the periphery of laser spot, over a narrow region, resulting in an accurate delineation of melt threshold. Comparison with the calculated melt threshold leads to determination of the absorbed power. This method to evaluate the absorbed power can be applied to any material exhibiting surface ripples characteristic of melt threshold.

Periodic surface ripples in laser‐treated aluminum and their use to determine absorbed power

Diffusion of several impurity atoms (Cu, Al, Au, and Sb) has been studied in Zr/sub 61/Ni/sub 39/ and Fe/sub 82/B/sub 18/ amorphous alloys. A definite correlation between the diffusion coefficient (D) and the atomic size of the diffusant is seen for the metal-metal (M-M) alloy, while it is not clear for the metal-metalloid (M-Me) alloy. Based on the present data, as well as other published data in binary amorphous alloys, empirical correlations have been found between (i) the activation energy (/ital Q/) and the energy required to form a hole of the size of the diffusing atom in the host alloy, and (ii) the pre-exponential factor (/ital D//sub 0/) and /ital Q/. While the former correlation is seen only for binary M-M type of amorphous alloys, the latter correlation is more general and holds for all types of amorphous alloys. Based on the correlation between /ital D//sub 0/ and /ital Q/, it is proposed that there are two distinct mechanisms of diffusion in amorphous alloys.

Some correlations for diffusion in amorphous alloys

Laser induced surface alloy formation and diffusion of antimony in aluminium

We have investigated titanium silicide formation using high dose (∼ 2 × 10 21 ions/m 2 ) ion implantation of 30 keV, 48 Ti + ions a room temperature into two different types of Si substrates: (a) n-type 〈111〉 single crystals and (b) amorphous Si films (∼ 200 nm thick) vacuum deposited onto a thermally grown SiO 2 layer. XRD and RBS techniques were employed to characterize various silicide phases and their depth distribution in as-implanted as well as in annealed samples. We find that a mixture of TiSi, TiSi 2 and Ti 5 Si 4 silicides is formed by high dose implantation. Out of these, TiSi; was found to be the dominant phase. The composition of these silicide layers is practically uniform with depth and remains unaltered on heat treatment up to 750° C. The electrical properties of silicide layers have also been investigated using sheet resistance measurements. The resistivity of as-implanted layers is rather high ( ∼ 10 μΩ m ), but drops sharply by nearly a factor of 20 after a post-implantation anneal above 800° C. The resistivity of silicide layers thus obtained compare well with silicides prepared by other techniques.

Animesh K. Jain

Papers

Pulsed laser heating calculations incorporating vapourization

Periodic surface ripples in laser‐treated aluminum and their use to determine absorbed power

Some correlations for diffusion in amorphous alloys

Laser induced surface alloy formation and diffusion of antimony in aluminium

Formation of titanium silicides by high dose ion implantation