Applied Physics A manuscript No

Applied Physics A, 2013

The formation of laser-induced periodic surface structures (LIPSS, ripples) upon irradiation of silicon with multiple irradiation sequences consisting of femtosecond laser pulse pairs (pulse duration 150 fs, central wavelength 800 nm) is studied numerically using a rate equation system along with a two-temperature model accounting for one-and twophoton absorption and subsequent carrier diffusion and Auger recombination processes. The temporal delay between the individual equal-energy fs-laser pulses was varied between 0 and ∼ 4 ps for quantification of the transient carrier densities in the conduction band of the laser-excited silicon. The results of the numerical analysis reveal the importance of carrier generation and relaxation processes in fs-LIPSS formation on silicon and quantitatively explain the two time constants of the delay dependent decrease of the Low-Spatial-Frequency LIPSS (LSFL) area observed experimentally. The role of carrier generation, diffusion and recombination are quantified individually.

2011

We consider the case of surface irradiation by a small number of femtosecond laser shots leading to the formation of surface ripples. To explain this effect, we propose a numerical model that accounts for the following processes: (i) interference of the laser irradiation with an electromagnetic surface wave propagating on a silicon sample; (ii) free carrier formation and laser energy absorption; (iii) energy relaxation and electron-phonon coupling. We perform numerical calculations taking into account the interference of a surface wave with laser; and present the obtained simulation results in order to explain formation mechanisms of the experimentally observed patterns.

Analytical and Bioanalytical Chemistry, 2010

We report on the structural investigation of selforganized periodic microstructures (ripples) generated in Si (100) targets after multishot irradiation by approximately 100-fs to 800-nm laser pulses at intensities near the single shot ablation threshold. Inspection by surface sensitive microscopy, e.g., atomic force microscopy (AFM) or scanning electron microscopy (SEM), and conventional and high-resolution transmission electron microscopy reveal complex structural modifications upon interaction with the laser: even well outside the ablated area, the target surface exhibits fine ripple-like undulations, consisting of alternating crystalline and amorphous silicon. Inside the heavily modified area, amorphous silicon is found only in the valleys but not on the crests which, instead, consist of highly distorted crystalline phases, rich in defects.

Applied Surface Science, 2012

Laser induced periodic surface structures (LIPSS) are formed by multiple irradiation of femtosecond laser on a silicon target. In this paper, we focus and discuss the surface plasmon polariton mechanism by an analysis of transient phase-matching conditions in Si on the basis of a single pulse experiment and numerical simulations. Two regimes of ripple formation mechanisms at low number of shots are identified and detailed. Correlation of numerical and experimental results is good.

Applied Surface Science

The multipulse interaction of ultraviolet femtosecond laser pulses with silicon and generation of surface structures in a large area spot (≳1 mm2) has been studied. The evolution of multiscale structures at the constant fluence strongly depends on the number of pulses, N. For N < 200, the “carpet-like” pattern of nano-, and micro-spikes is generated by the bubble explosion in a thin surface foam layer. The accumulation of bubbles and their explosion due to repetition of laser pulses cause damped membrane-like oscillations of the silicon surface. For 200 ≤ N, bifurcation of surface morphology takes place: (i) the surface tension waves of the wavelength ∼200 μm appear in the peripheral region of the spot. Generated by the surface thermal gradient in the liquid foam layer, they spread from the hot centerline towards the periphery of the spot. The change of their wavelength with propagation distance indicates onset of the Eckhaus instability caused by the phase modulation in multipul...

Journal of Applied Physics, 2009

The formation of nearly wavelength-sized laser-induced periodic surface structures (LIPSS) on single-crystalline silicon upon irradiation with single or multiple femtosecond (fs) laser pulses (pulse duration τ = 130 fs, central wavelength λ = 800 nm) in air is studied experimentally and theoretically. In our theoretical approach, we model the LIPSS formation by combining the generally accepted first-principle theory of Sipe and co-workers with a Drude model in order to account for transient intra-pulse changes of the optical properties of the material due to the excitation of a dense electron-hole plasma. Our results are capable to explain quantitatively the spatial periods of the LIPSS being somewhat smaller than the laser wavelength, their orientation perpendicular to the laser beam polarization and their characteristic fluence dependence. Moreover, evidence is presented that surface plasmon polaritons play a dominant role during the initial stage of near-wavelength sized periodic surface structures in fs laser irradiated silicon and it is demonstrated that these LIPSS structures can be formed in silicon upon irradiation by single fs laser pulses.

MRS Bulletin, 2006

In this article, we present summaries of the evolution of surface morphology resulting from the irradiation of single-crystal silicon with femtosecond laser pulses. In the first section, we discuss the development of micrometer-sized cones on a silicon surface irradiated with hundreds of femtosecond laser pulses in the presence of sulfur hexafluoride and other gases. We propose a general formation mechanism for the surface spikes. In the second section, we discuss the formation of blisters or bubbles at the interface between a thermal silicon oxide and a silicon surface after irradiation with one or more femtosecond laser pulses. We discuss the physical mechanism for blister formation and its potential use as channels in microfluidic devices.

2017

Laser-induced periodic surface structures are created on Si (100) and Si (111) wafers by 500 fs laser pulses at 248 nm. The periodic structure is concentric and highly regular. The spatial period is consistently varying between 1.1 μm and 3.3 μm in the radial direction. It is shown that the fluence of the irradiation at the same pulse number determines the size of the area where the periodic structure is created and for the same fluence the pulse number determines the regularity of the created grooves by melting processes. The origin of this structure is identified as the inhomogeneity of the laser beam profile caused by Fresnel diffraction close to the focal plane. Further improvement of the formation of periodic structure with femtosecond laser pulses is suggested.

Optics Express, 2013

Applied Physics A, 2006

Three-dimensional (3D) and two-dimensional (2D) periodic silicon nanostructures formed by polarized focused Nd:YAG laser irradiation (532 nm) with spot size less than 3 µm on Si covered by SiO 2 are presented in this paper. We observed that at a low laser intensity I range, from I = 0.9 to 1.08 W, 2D periodic coexisting of liquid and solid exists, while for 1.08 < I < 1.44 W, 3D periodic ripples were formed. However, when the light intensity is out of those ranges, either no melting was created (I < 0.9 W) or the periodicity was destroyed (I > 1.44 W). The periodicity of these periodic structures is 359 nm related to the wavelength of frequency doubled Nd:YAG laser and the index of refraction of SiO 2. We propose a model based on the fact that as the oxygen is diffusing locally from SiO 2 into the melted Si, thus forming SiO β with a lower melting point, successive pulses melt preferentially these regions giving rise to a positive feedback. This dynamic nanoscale modeling, based on variations of melting points of Si and dielectric and reflection coefficient, confirms the experimental results.