Finite-difference Time-domain Modeling of Laser-induced Periodic Surface Structures

Laser-induced periodic surface structures (LIPSSs) consist of regular wavy surface structures with amplitudes the (sub)micrometer range and periodicities in the (sub)wavelength range. It is thought that periodically modulated absorbed laser energy is initiating the growth of LIPSSs. The “Sipe theory” (or “Efficacy factor theory”) provides an analytical model of the interaction of laser radiation with a rough surface of the material, predicting modulated absorption just below the surface of the material. To address some limitations of this model, the finite-difference time-domain (FDTD) method was employed to numerically solve the two coupled Maxwell's curl equations, for linear, isotropic, dispersive materials with no magnetic losses. It was found that the numerical model predicts the periodicity and orientation of various types of LIPSSs which might occur on the surface of the material sample. However, it should be noted that the numerical FDTD model predicts the signature or “fingerprints” of several types of LIPSSs, at different depths, based on the inhomogeneously absorbed laser energy at those depths. Whether these types of (combinations of) LIPSSs will actually form on a material will also depend on other physical phenomena, such as the excitation of the material, as well as thermal-mechanical phenomena, such as the state and transport of the materia

Microscopy study of ripples created on steel surface by use of ultra short laser pulses

This paper concentrates on ripples on the surface of steel that arise from lasermaterial interaction. In particular we have observed two different sets of ripples on steel samples that were machined by 210 fs laser pulses with 800 nm wavelength at normal incidence. Small ripples were found with spacing of about 250 nm lying longitudinal to the vector of laser beam polarization. Big ripples exhibited at a much larger distance of about 500 nm and they are perpendicular to the polarization vector. The laser treated surfaces were investigated with\ud Scanning Electron, Confocal and Atomic Force Microscopy. The laser-material interaction could be divided into three subsequent steps: absorption of laser light via electron gas excitations, transfer of heat into the lattice followed by a thermal expansion of material. From our microscopic observations it is concluded that the small ripples are formed by solidification of liquid material present as a thin layer near the interface of solid bulk material

Modification of Cu surface with picosecond laser pulses

High purity, mirror-polished polycrystalline Cu surface was treated with single picosecond laser pulses at fluence levels close to the single-pulse modification threshold. The induced surface topography and sub-surface changes were examined with scanning and transmission electron microscopy, respectively. The analysis showed an increased absorption of laser energy on the microscopic surface topography inhomogeneities, even at a fluence level below the modification threshold. Many features, like spikes, bubbles, spheres, as well as small periodic ripples at the bottom of scratches, reveal a significant influence of melting and eruptive relaxation of the absorbed laser energy on the final appearance of the surface. Further, it was found that thermal stresses result in twinning to a depth of few tens of nanometers under the surface. Voids at this depth have been observed as well. The results of the observations provide new insights into the early stages of the picosecond laser pulse modification of metals, especially metals witha weak electron-phonon coupling. (C) 2014 Elsevier B.V. All rights reserved

Modeling laser-induced periodic surface structures: Finite-difference time-domain feedback simulations

A model predicting the formation of laser-induced periodic surface structures (LIPSSs) is presented. That is, the finite-difference time domain method is used to study the interaction of electromagnetic fields with rough surfaces. In this approach, the rough surface is modified by “ablation after each laser pulse,” according to the absorbed energy profile, in order to account for inter-pulse feedback mechanisms. LIPSSs with a periodicity significantly smaller than the laser wavelength are found to “grow” either parallel or orthogonal to the laser polarization. The change in orientation and periodicity follow from the model. LIPSSs with a periodicity larger than the wavelength of the laser radiation and complex superimposed LIPSS patterns are also predicted by the mode

Modeling laser-induced periodic surface structures: Finite-difference time-domain feedback simulations

A model predicting the formation of laser-induced periodic surface structures (LIPSSs) is presented. That is, the finite-difference time domain method is used to study the interaction of electromagnetic fields with rough surfaces. In this approach, the rough surface is modified by “ablation after each laser pulse,” according to the absorbed energy profile, in order to account for inter-pulse feedback mechanisms. LIPSSs with a periodicity significantly smaller than the laser wavelength are found to “grow” either parallel or orthogonal to the laser polarization. The change in orientation and periodicity follow from the model. LIPSSs with a periodicity larger than the wavelength of the laser radiation and complex superimposed LIPSS patterns are also predicted by the mode

Laser-induced periodic surface structures: fingerprints of light localization

The finite-difference time-domain (FDTD) method is used to study the inhomogeneous absorption of linearly polarized laser radiation below a rough surface. The results are first analyzed in the frequency domain and compared to the efficacy factor theory of Sipe and coworkers. Both approaches show that the absorbed energy shows a periodic nature, not only in the direction orthogonal to the laser polarization, but also in the direction parallel to it. It is shown that the periodicity is not always close to the laser wavelength for the perpendicular direction. In the parallel direction, the periodicity is about λ/Re(ñ), with ñ being the complex refractive index of the medium. The space-domain FDTD results show a periodicity in the inhomogeneous energy absorption similar to the periodicity of the low- and high-spatial-frequency laser-induced periodic surface structures depending on the material's excitation

Modification of Cu surface with picosecond laser pulses

High purity, mirror-polished polycrystalline Cu surface was treated with single picosecond laser pulses at fluence levels close to the single-pulse modification threshold. The induced surface topography and sub-surface changes were examined with scanning and transmission electron microscopy, respectively. The analysis showed an increased absorption of laser energy on the microscopic surface topography inhomogeneities, even at a fluence level below the modification threshold. Many features, like spikes, bubbles, spheres, as well as small periodic ripples at the bottom of scratches, reveal a significant influence of melting and eruptive relaxation of the absorbed laser energy on the final appearance of the surface. Further, it was found that thermal stresses result in twinning to a depth of few tens of nanometers under the surface. Voids at this depth have been observed as well. The results of the observations provide new insights into the early stages of the picosecond laser pulse modification of metals, especially metals with a weak electron–phonon couplin

Laser-induced periodic surface structures: fingerprints of light localization

The finite-difference time-domain (FDTD) method is used to study the inhomogeneous absorption of linearly polarized laser radiation below a rough surface. The results are first analyzed in the frequency domain and compared to the efficacy factor theory of Sipe and coworkers. Both approaches show that the absorbed energy shows a periodic nature, not only in the direction orthogonal to the laser polarization, but also in the direction parallel to it. It is shown that the periodicity is not always close to the laser wavelength for the perpendicular direction. In the parallel direction, the periodicity is about λ/Re(ñ), with ñ being the complex refractive index of the medium. The space-domain FDTD results show a periodicity in the inhomogeneous energy absorption similar to the periodicity of the low- and high-spatial-frequency laser-induced periodic surface structures depending on the material's excitation