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The dynamics of the formation of laser-induced periodic surface structures (LIPSS) on fused silica upon irradiation with linearly polarized fs-laser pulses (50 fs pulse duration, 800 nm center wavelength) is studied experimentally using a double pulse experiment with cross polarized pulse sequences and a trans illumination femtosecond time-resolved (0.1 ps – 1 ns) pump-probe diffraction approach. The results in both experiments confirm the importance of the ultrafast energy deposition and the laser-induced free-electron plasma in the conduction band of the solids for the formation of LIPSS.

Irradiation with a single nanosecond laser pulse in the melting regime can result in a characteristic change in the surface morphology of crystalline silicon. This has been verified experimentally in a variety of situations, where dimple-shaped surface topographies are produced. In this work, the dimple height, depth, and width are modeled following and extending in a more rigorous manner the approach of Wood and Giles [Phys. Rev. B 23, 2923–2942 (1981)] and that of Schwarz-Selinger and coworkers [Phys. Rev. B 64, 155323 (2001)], upon varying the laser irradiation parameters such as peak energy density, pulse duration, and wavelength. This is achieved with numerical simulations of one-dimensional heat flow as input to the analytical fluid-flow equations. [ABSTRACT FROM AUTHOR]

Cycling between the crystalline and amorphous phases of 25-nm-thick GeSb films induced by single laser pulses of duration of 100 fs or 20 ps is investigated in the 400-800 nm wavelength range. The time evolution of the phase transformations are studied with picosecond resolution real-time reflectivity measurements at a probe wavelength of 514.5 nm and also with femtosecond and picosecond pump-probe measurements.

A study has observed the laser-induced periodic surface structures (LIPSS; ripples) with different spatial characteristics after irradiation of single-crystalline indium phosphide (c-InP) with multiple linearly polarized femtosecond pulses (130 fs, 800 nm) in air. Details of two different types of ripples that are observed with an increasing number of pulses per spot, N, up to 100 are discussed.

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of silicon by multiple (N=100) linearly polarized Ti:sapphire femtosecond laser pulses (duration τ=30 fs, center wavelength λ0~790 nm) is studied experimentally in air and water environment. The LIPSS surface morphologies are characterized by scanning electron microscopy and their spatial periods are quantified by two-dimensional Fourier analyses. It is demonstrated that the irradiation environment significantly influences the periodicity of the LIPSS. In air, so-called low-spatial frequency LIPSS (LSFL) were found with periods somewhat smaller than the laser wavelength (⋀LSFL~0.7×k0) and an orientation perpendicular to the laser polarization. In contrast, for laser processing in water a reduced ablation threshold and LIPSS with approximately five times smaller periods ⋀LIPSS~0.15×λ0 were observed in the same direction as in air. The results are discussed within the frame of recent LIPSS theories and complemented by a thin film based surface plasmon polariton model, which successfully describes the tremendously reduced LIPSS periods in water. [ABSTRACT FROM AUTHOR]

Single-pulse (532nm, 8 ns) micropatterning of silicon with nanometric surface modulation is demonstrated by irradiating through a diffracting pinhole. The irradiation results obtained at fluences above the melting threshold are characterized by scanning electron and scanning force microscopy and reveal a good agreement with Fresnel diffraction theory. The physical mechanism is identified and discussed on basis of both thermocapillary and chemicapillary induced material transport during the molten state of the surface. [ABSTRACT FROM AUTHOR]

Irradiation with a single spatially Gaussian-shaped nanosecond laser pulse in the melting regime can result in a characteristic annular change in the surface morphology of crystalline silicon. This has been verified experimentally in a variety of situations, where dimple-shaped surface topographies are produced. In a recent work we have investigated the induced changes in the surface topography upon exposure to wavelengths in the visible and near infrared spectral region. Irradiation in the UV requires a more detailed analysis due to the enhanced absorption of the material. In the present analysis, we determine under which conditions our previous model can be used and the corresponding results are presented. [ABSTRACT FROM AUTHOR]

The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica with multiple irradiation sequences consisting of five Ti:sapphire femtosecond (fs) laser pulse pairs (150 fs, 800 nm) is studied experimentally. A Michelson interferometer is used to generate near-equal-energy double-pulse sequences with a temporal pulse delay from -20 to +20 ps between the cross-polarized individual fs-laser pulses (∼0.2 ps resolution). The results of multiple double-pulse irradiation sequences are characterized by means of Scanning Electron and Scanning Force Microscopy. Specifically in the sub-ps delay domain striking differences in the surface morphologies can be observed, indicating the importance of the laser-induced free-electron plasma in the conduction band of the solids for the formation of LIPSS. [ABSTRACT FROM AUTHOR]

Ablation of single-crystalline (100) indium phosphide wafer surfaces with single 10 fs Ti:sapphire laser pulses in air has been studied by means of complementary cross-sectional transmission electron microscopy, scanning force microscopy, and optical microscopy. A local protrusion of ∼70 nm height is generated within the ablation craters for fluences exceeding 0.78 J/cm2. This morphological feature has been studied in detail, revealing the material structure of the laser-affected zone and its spatial extent. The resolidified layer (60–200 nm thick) consists of polycrystalline grains (5–15 nm diameter) and is covered by an ∼10 nm thick amorphous top layer. Interestingly, the sharp boundary of the solidified layer to the unaffected crystal underneath exhibits a Gaussian-like shape and does not follow the shape of the surface topography. Evidence is presented that the central crater protrusion is formed by near-surface optical breakdown, and that the absorption in the material transiently changes during the femtosecond-laser pulse. [ABSTRACT FROM AUTHOR]

Laser-induced periodic surface structures (LIPSS) (ripples) with different spatial characteristics have been observed after irradiation of single-crystalline zinc oxide surfaces with multiple linearly polarized femtosecond pulses (150–200 fs, 800 nm) in air. For normal incident laser radiation, low spatial frequency LIPSS (LSFL) with a period (630–730 nm) close to the wavelength and an orientation perpendicular to the laser polarization have been found in the fluence range between ∼0.7 and ∼0.8 J/cm2 and predominantly for pulse numbers up to N=100. For lower fluences (0.5–0.7 J/cm2), a sharp transition from the LSFL features toward the formation of high spatial frequency LIPSS (HSFL) appears at any given pulse number below N=100. The HSFL are always parallel to the LSFL, exhibit spatial periods between 200 and 280 nm, and completely substitute the LSFL for pulse numbers N>100. Additionally, the influence of the angle of incidence has been studied experimentally for both LIPSS types revealing a different behavior. Experimental evidence for surface scattered second harmonic generation is presented in the regime of HSFL formation. Moreover, we will show that the HSFL structures on ZnO surfaces can be fully explained by an extension of the existing LIPSS theories if the photoexcitation of the dielectric material (affecting its transient optical properties) is considered in the frame of a simple Drude model along with the second harmonic generation at the irradiated surface. Based on our analysis, the current models of femtosecond laser-induced LIPSS are revisited and an explanation is proposed why HSFL are observed predominantly in the subpicosecond range for below band-gap excitation of dielectrics and semiconductors. [ABSTRACT FROM AUTHOR]

This work reports on the optical response and morphological changes of gold nanoparticles (NPs) induced by laser irradiation with single nanosecond laser pulses of different fluences and wavelengths. The as-grown specimens consist of irregularly shaped NPs (10–13 nm in average dimensions) produced by pulsed laser deposition on a substrate. They exhibit a broad optical absorption band related to the surface plasmon resonance (SPR) peaking in the range of 634–685 nm. After irradiation with fluences above a threshold value, a blueshift, a decrease of the overall absorption, and a narrowing of the SPR band are observed. The SPR peak wavelength and amplitude after irradiation at increasing fluences reach an approximately constant value, irrespective of the irradiation wavelength. This value is consistent with a homogeneous distribution of spherical gold NPs with a mean diameter close to the average initial dimensions as determined by transmission electron microscopy. The results demonstrate that the conversion of irregularly shaped NPs into spherical NPs with reduced dimension dispersion is a thermally driven process and occurs within a fluence interval defined by the melting threshold of all NPs and the ablation threshold. Whereas the useful fluence interval is controlled by the effective fluence absorbed, that is dependent on the laser wavelength and thermal properties of the substrate, the final shape and dimensions of the NPs are independent of the irradiation wavelength provided the fluence is within this useful interval. [ABSTRACT FROM AUTHOR]

The structural transformation dynamics of single-crystalline indium phosphide irradiated with 150 fs laser pulses at 800 nm has been investigated by means of time-resolved reflectivity measurements covering a time window from 150 fs up to 500 ns. The results obtained show that for fluences above the threshold of 0.16 J/cm2 thermal melting of the material occurs on the time scale of 1–2 ps, while ablation at higher fluences (>0.23 J/cm2) is precedeed by an ultrafast phase transition (“nonthermal melting”) occurring within 400 fs after the arrival of the pulse to the surface. The evolution of the reflectivity on a longer time scale shows a resolidification time typically around 25 ns after which an amorphous layer several tens of nanometer thick is formed on the surface. [ABSTRACT FROM AUTHOR]