Florenta Costache

Ripples revisited: non-classical morphology at the bottom of femtosecond laser ablation craters in transparent dielectrics

Abstract Studying femtosecond laser ablation of wide band gap insulators, with BaF2 as a representative, we observe a complex structure of fine ripples at the bottom of the ablated crater, which are oriented perpendicular to the beam polarization. A second, wider periodic structure oriented parallel to the beam polarization appears superimposed on the first one. To check the idea of an interference pattern translating into ripples, a controlled interference was created in the target in a non-collinear two-beam experiment. However, no signature of it was observed in the ablated spot. This calls the classical interpretation for ripples formation into question. More likely, we assume that the ripples structures are due to self-organization structure formation during the relaxation of the highly non-equilibrium surface after explosive positive ion emission.

Surface patterning on insulators upon femtosecond laser ablation

Abstract The crater morphology upon femtosecond laser ablation from BaF2 and CaF2, exhibits several periodic structures of characteristics that cannot be explained by interference phenomena. More likely, the ripple pattern presents features of self-organization from a chaotic state. The ablation under the applied conditions is due to Coulomb explosion of the surface, which indicates that local thermal effects should play a minor role in the ripple formation. In this paper, we present morphological structures that point toward instabilities and self-organization.

Formation of Self-Organized Regular Nanostructures upon Femtosecond Laser Ablation

At the bottom of ablation craters produced in many materials, e.g. dielectric and silicon crystals, by the impact of femtosecond laser radiation, regular periodic structures are observed with a feature size at the order of a few 100 nanometers, much smaller than the incident wavelength. Their orientation depends strongly on the laser polarization but not on any intrinsic crystalline parameters. An increasing number of shots results in higher contrast, better developed structures, indicating a positive feedback. The region around the impact is shown, by micro Raman spectroscopy, to undergo phase transformations like under high pressure. The structure spacing appears to depend crucially on the depth of the perturbed volume, i.e. the incident (and absorbed) energy. All observations suggest that the structures form by self-organization from instabilities induced in the material by the laser input. A general picture suggests that the irradiation results in a rapid, non-equilibrium destabilization of the crystal structure, which should not be confused with melting as a classical thermodynamic process (i.e. temperatures defined as equilibrium properties). Relaxation from this instability results in the self-assembly of the observed structures. Theoretical simulations demonstrate the feasibility of this model, which also is corroborated by comparison to other unstable situations.

Femtosecond Laser Interaction with Solid Surfaces: Explosive Ablation and Self-Assembly of Ordered Nanostructures

The fundamentals of interaction between intensive laser pulses and solid surfaces are reviewed. In order to distinguish the relevant phenomena from secondary effects, e.g., laser heating of the plasma plume formed upon ablation, emphasis is placed on the action of ultrashort pulses. The present picture of energy absorption and dissipation dynamics is discussed, and transient and permanent modification of the surface, in particular its morphology, are considered.

Ultra short laser pulse ablation from sodium chloride—the role of laser induced color centers

Ablation from sodium chloride single crystals by ultra short laser pulses is investigated, recording the emitted charged particles (electrons, negative and positive ions) by time-of-flight mass spectroscopy. The influence of irradiation parameters is analyzed, such as laser intensity and the number of shots per ablation site. The ablation craters are inspected by optical microscopy. In addition to material removal from the surface, the laser irradiation induces a coloration of the crystal along the beam path. The corresponding absorption spectrum reveals the generation of F color centers and their aggregates F2, F3, F4, probably decaying after minutes to Na-colloids. Our observations agree well with our earlier results on sapphire and barium fluoride, assuming multiphoton surface ionization followed by Coulomb explosion of the surface, due to positive charging. In addition, several laser generated defect types are identified (color centers, Na colloids and mechanical stress), and their role for emission behavior is discussed.

Self-Assembled Surface Patterning and Structural Modification upon Femtosecond Laser Processing of Crystalline Silicon

Upon femtosecond laser ablation (Ti:Sapphire; 800 nm, 100 fs, under ultra-high vacuum) from crystalline silicon (001), the surface morphology and structural changes w er examined ex-situ by optical, scanning electron microscopy, and Raman spectroscopy. Af ter repetitive illumination with several thousand laser pulses at an intensity below the single hot damage threshold (10 12 W/cm), self-assembled periodic nanostructures with periods of 200 nm resp. 60 0-700 nm develop at the crater bottom. Raman spectroscopy reveals a phase transfor mation inside the crater from Si-I to the polymorphs Si-III, Si-XII, hexagonal Si-wurtzite (Si-IV) , and amorphous silicon. The ablation dynamics was monitored by time-of-flight mass spect ros opy, showing the emission of superthermal positive ions with a kinetic energy of about 7 eV. The r esults suggest that the ablation, leaves behind a severely perturbed crystal surface. The resul ting instability relaxes by a selforganization, independent of the initial, and surrounding, crystal structure.

Femtosecond laser-induced nanostructuring and phase transformation of crystalline silicon

Surface morphology and structural changes upon femtosecond laser ablation from crystalline silicon (001) were examined ex-situ by optical, scanning electron, and atomic force microscopy, as well as Raman spectroscopy. After repetitive illumination with several thousand laser pulses at an intensities below or near the single shot damage threshold (2x1012 W/cm2), self-assembled periodic nanostructures with periods of 200 nm resp. 600-700 nm develop at the crater bottom. Micro-Raman spectroscopy reveals phase transformations inside the crater from Si-I to the polymorphs Si-III, Si-XII, hexagonal Si-wurtzite (Si-IV), and amorphous silicon, pointing to substantial pressure and volume changes during the interaction. The ablation dynamics was monitored by time-of-flight mass spectroscopy, showing the emission of superthermal positive ions with a kinetic energy of several eV as well as significant contributions at lower kinetic energies. The results suggest that the ablation is associated with considerable recoil pressure and leaves behind a severely perturbed crystal surface. The resulting instability relaxes by a self-organization, independent of the initial, and surrounding, crystal structure.

Explosive ion emission from ionic crystals induced by ultrashort laser pulses: influence on surface morphology

The dynamics of femtosecond laser ablation from wide bandgap insulators (Al2O3, BaF2 and CaF2) at intensities below the single shot damage threshold (1011 - 1013 W/cm2) is characterized by efficient surface ionization, followed by the explosive emission of positive ions and small clusters, with a kinetic energy of about 100 eV (Coulomb explosion). The multiphoton coupling of the laser to the transparent material is strongly promoted by defect resonances within the bandgap, eventually generated during a considerable number of incubating pulses before a steady ablation regime is reached. At the bottom of the ablation crater, produced by an accumulation of several thousand laser pulses, periodic surface structures are developed, with a typical scaling in the nanometer range. Occasionally, these structures exhibit features like bifurcations or columns growing out of plane. The feature size and shape appears to be more sensitive to the applied laser intensity resp. irradiation dose than to wavelength or angle of incidence. The ripples cannot be explained as a result of an inhomogeneous energy input, e.g. due to interference. Instead, we suggest that the ripples are a consequence of the surface relaxation via self-organization.

Surface morphology after femtosecond laser ablation of insulators

The crater morphology in transparent insulators upon femtosecond laser ablation was investigated by ex-situ optical and electron microscopy. After multishot irradiation (several thousand shots), a superposition of up to three differently spaced ripple patterns developed at the crater bottom, the finest one running perpendicular and the next larger one parallel to the laser polarization. The ripples periods do not show any relation to the incident laser wavelength. On the contrary, they appear to be strongly influenced by the incident intensity, regardless of the wavelength. The coarsest structure exhibits features of plastic surface waves, reflected at the boundaries of the crater as well as at individual irregularities inside the crater. The finest ripples exhibit strong features of chaotic self-organization and percolation, such as bifurcations. Together with the fact, that ablation under the applied conditions is due to Coulomb explosion of the surface, our observations indicate that local thermal effects can be ruled out as the origin of the ripples formation, in contrast to the classical interference picture of ripples formation. This is further confirmed by two-pulse interference experiments.

Femtosecond laser induced surface instability resulting in self-organized nanostructures

The fundamental mechanisms and dynamics of laser ablation are reviewed, based on experiments with femtosecond laser pulses to exclude secondary effects like the interaction of the incident laser light with the ablation plume or with a target preconditioned during the initial slope of the laser pulse. It is shown that the incident energy drives the target into a state of instability, far from thermodynamic equilibrium. The subsequent ultra-rapid relaxation results in the formation of self-organized regular nanostructures in the irradiated and ablated area.