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Femtosecond diffraction dynamics of laser-induced periodic surface structures on fused silica
(2013)
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 transillumination femtosecond time-resolved (0.1 ps-1 ns) pump-probe diffraction approach. This allows to reveal the generation dynamics of near-wavelength-sized LIPSS showing a transient diffraction at specific spatial frequencies even before a corresponding permanent surface relief was observed. The results confirm that the ultrafast energy deposition to the materials surface plays a key role and triggers subsequent physical mechanisms such as carrier scattering into self-trapped excitons.
The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica with multiple irradiation sequences of parallel polarized Ti:sapphire femtosecond laser pulse pairs (160 fs pulse duration, 800 nm central wavelength) was studied experimentally. For that purpose, a Michelson interferometer was used to generate near-equal-energy double-pulse sequences allowing the temporal pulse delay between the parallel-polarized individual fs-laser pulses to be varied between 0 and 40 ps with ~0.2 ps temporal resolution. The surface morphologies of the irradiated surface areas were characterized by means of scanning electron and scanning force microscopy. In the sub-ps delay range a strong decrease of the LIPSS periods and the ablation crater depths with the double-pulse delay was observed indicating the importance of the laser-induced free-electron plasma in the conduction band of the solids for the formation of LIPSS.
The formation of laser-induced periodic surface structures (LIPSS) upon irradiation of fused silica and silicon with multiple (NDPS) irradiation sequences consisting of linearly polarized femtosecond laser pulse pairs (pulse duration ~150 fs, central wavelength ~800 nm) is studied experimentally. Nearly equal-energy double-pulse sequences are generated allowing the temporal pulse delay Δt between the cross-polarized individual fs-laser pulses to be varied from -40 ps to +40 ps with a resolution of ~0.2 ps. The surface morphologies of the irradiated surface areas are characterized by means of scanning electron and scanning force microscopy. Particularly for dielectrics in the sub-ps delay range striking differences in the orientation and spatial characteristics of the LIPSS can be observed. For fused silica, a significant decrease of the LIPSS spatial periods from ~790 nm towards ~550 nm is demonstrated for delay changes of less than ~2 ps. In contrast, for silicon under similar irradiation conditions, the LIPSS periods remain constant (~760 nm) for delays up to 40 ps. The results prove the impact of laser-induced electrons in the conduction band of the solid and associated transient changes of the optical properties on fs-LIPSS formation.
The formation of laser-induced periodic surface structures (LIPSS) on titanium upon irradiation with linearly polarized femtosecond (fs) laser pulses (τ = 30 fs, λ = 790 nm) in an air environment is studied experimentally and theoretically. In the experiments, the dependence on the laser fluence and the number of laser pulses per irradiation spot has been analyzed. For a moderate number of laser pulses (N < 1000) and at fluences between ~0.09 and ~0.35 J/cm², predominantly low-spatial-frequency-LIPSS with periods between 400 nm and 800 nm are observed perpendicular to the polarization. In a narrow fluence range between 0.05 and 0.09 J/cm², high-spatial-frequency-LIPSS with sub-100-nm spatial periods (~λ/10) can be generated with an orientation parallel to the polarization (N = 50). These experimental results are complemented by calculations based on a theoretical LIPSS model and compared to the present literature.
We investigated the initial modification and ablation of crystalline silicon with single and multiple Ti:sapphire laser pulses of 5 to 400 fs duration. In accordance with earlier established models, we found the phenomena amorphization, melting, re-crystallization, nucleated vaporization, and ablation to occur with increasing laser fluence down to the shortest pulse durations. We noticed new morphological features (bubbles) as well as familiar ones (ripples, columns). A nearly constant ablation threshold fluence on the order of 0.2 J/cm2 for all pulse durations and multiple-pulse irradiation was observed. For a duration of ,100 fs, significant incubation can be observed, whereas for 5 fs pulses, the ablation threshold does not depend on the pulse number within the experimental error. For micromachining of silicon, a pulse duration of less than 500 fs is not advantageous.
The ultrafast laser ablation of silicon has been investigated experimentally and theoretically. The theoretical description is based on molecular dynamics (MD) simulations combined with a microscopic electronic model. We determine the thresholds of melting and ablation for two different pulse durations =20 and 500 fs. Experiments have been performed using 100 Ti:Sap-phire laser pulses per spot in air environment. The ablation thresholds were determined for pulses with a duration of 25 and 400 fs, respectively. Good agreement is obtained between theory and experiment.
The formation of near-wavelength laser-induced periodic surface structures (LIPSS) on silicon upon irradiation with sequences of Ti:sapphire femtosecond laser pulse pairs (pulse duration 150 fs, central wavelength 800 nm) is studied theoretically. For this purpose, the nonlinear generation of conduction band electrons in silicon and their relaxation is numerically calculated using a two-temperature model approach including intrapulse changes of optical properties, transport, diffusion and recombination effects. Following the idea that surface plasmon polaritons (SPP) can be excited when the material turns from semiconducting to metallic state, the 'SPP active area' is calculated as function of fluence and double-pulse delay up to several picoseconds and compared to the experimentally observed rippled surface areas. Evidence is presented that multi-photon absorption explains the large increase of the rippled area for temporally overlapping pulses. For longer double-pulse delays, relevant relaxation processes are identified. The results demonstrate that femtosecond LIPSS on silicon are caused by the excitation of SPP and can be controlled by temporal pulse shaping.
The formation of nearly wavelength-sized laser-induced periodic surface structures (LIPSSs) on single-crystalline silicon upon irradiation with single or multiple femtosecond-laser pulses (pulse duration τ=130 fs and 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-principles theory of Sipe and co-workers with a Drude model in order to account for transient intrapulse changes in 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 LIPSSs 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 femtosecond-laser irradiated silicon, and it is demonstrated that these LIPSSs can be formed in silicon upon irradiation by single femtosecond-laser pulses.
The formation of nearly wavelength-sized laser-induced periodic surface structures (LIPSS) on
single-crystalline silicon upon irradiation with single (N = 1) and multiple (N ≤ 1000) linearly
polarized femtosecond (fs) laser pulses (pulse duration τ = 130 fs, central wavelength λ = 800 nm)
in air is studied experimentally. Scanning electron microscopy (SEM) and optical microscopy are
used for imaging of the ablated surface morphologies, both revealing LIPSS with periodicities close
to the laser wavelength and an orientation always perpendicular to the polarization of the fs-laser
beam. It is experimentally demonstrated that these LIPSS can be formed in silicon upon irradiation
by single fs-laser pulses—a result that is additionally supported by a recent theoretical model.
Two-dimensional Fourier transforms of the SEM images allow the detailed analysis of the
distribution of the spatial frequencies of the LIPSS and indicate, at a fixed peak fluence, a
monotonous decrease in their mean spatial period between ~770 nm (N = 1) and 560 nm (N
= 1000). The characteristic decrease in the LIPSS period is caused by a feedback-mechanism acting
upon excitation of surface plasmon polaritons at the rough silicon surface which is developing under
the action of multiple pulses into a periodically corrugated surface.
A new approach is presented to quantify the so-called "heat affected zone" (HAZ) during
femtosecond laser pulse processing. Ablation of titanium nitride (TiN) thin films (~3 μm
thickness) by multiple femtosecond laser pulses (τ=130 fs, λ=800 nm) in air environment was
studied by means of two different surface analytical methods both being sensitive to chemical
alterations at the surface. Scanning Auger electron microscopy was applied for a visualization of the
spatial distribution of specific elements (Ti, O) within the laser-modified areas. The chemical state
of the irradiated surface was revealed by complementary x-ray photoelectron spectroscopy. Both
methods were used for a depth-profiling chemical analysis (tracking the elements Ti, N, O, and C)
using an Ar-ion beam for surface sputtering. In a narrow laser fluence range slightly below the
ablation threshold of TiN significant superficial oxidation can be observed leading to the formation
of substoichiometric TiO2-x. At fluences above the ablation threshold, an increased titanium
concentration is observed within the entire ablation craters. Following upon sputter removal the
elemental distribution into the depth of the nonablated material, the results allow an estimation of
the heat-affected zone for femtosecond laser ablation in air environment. According to our analyses,
the HAZ extends up to a few hundreds of nanometers into the nonablated material.