Self-organized nanostructures (ripples) on the target surface after multi-pulse femtosecond laser ablation exhibit, obviously,
a positive multi-pulse feedback in the self-organization process. Experiments on different targets (CaF2, Si) investigate
this feedback in more detail, in particular its dynamics. The influence of pulse number and time separation between
successive pulses on both the size and the complexity of the nanostructures as well as the size of the modified surface
area is studied. In addition to a dependence on the coupled dose, confirming incubation effects previously observed
on ablation efficiency, both modified area as well as pattern feature size and complexity decrease with increasing pulse-to-
pulse delay between 1 ms and 1 s, indicating an unexpectedly long lifetime of the feedback. Further, for silicon, a persisting
modification of the crystalline structure is found well beyond the ablation spot, though no apparent change in
surface morphology can be seen. Mapping the band-to-band photoluminescence displays a spatially modulated dramatic
increase of non-radiative recombination compared to unaffected material.
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.
The impact of intense femtosecond laser pulses on dielectric targets results in a non-equilibrium state of the
surface. We consider the influence of this instability on ablation and surface relaxation phenomena. Important
consequences of the laser-material coupling and energy dissipation are addressed such as transient and permanent
modification of the surface. From experiments on ablation products kinetics, Coulomb explosion upon multiphoton
surface ionization has been established as the initial mechanism for desorption of fast positive ions from dielectric
surfaces. We refer to the role of surface defects responsible for ion yield enhancement and the nature of defects by
detecting laser induced fluorescence. Additionally, observations point to a set-in of a thermal emission process at higher
laser intensity. Investigating the dynamics of particle emission, we find ultra-short timescales for the coherence of
electronic excitation and energy relaxation via transient phases, the latter related to the coupling strength of the various solids. The surface morphology after ablation is modified, with regular nano- and micro-structures of features originated
from self-organization of surface instabilities.
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.
Thin films of high-k materials praseodymium oxide (PrxOy) and hafnium oxide (HfOx) were deposited on silicon (100) surfaces by pulsed laser deposition (PLD), using the third harmonic of a Nd:YAG laser. The two materials are compared with respect to their morphology, in dependence on the substrate temperature during deposition, and their chemical composition and crystalline structure, in particular at the interface. The films of both oxides exhibit a grainy structure when deposited at substrate temperatures below 750°C, with the grain size increasing from ≈ 40 nm at room temperature to ≈ 100 nm at 750°C. However, the PrxOy films are much more uniform than hafnia, the latter exhibiting increasingly larger holes, reaching several nm into the silicon substrate. For a substrate temperature of 900°C, the film morphology for PrxOy completely changes to much larger crystalline areas, while for HfOx the role of holes in the film becomes substantial. Also, the interface chemistry is significantly different for both materials: a silicate formation for PrxOy and a rich abundance of SiO2 and a silicide for hafnia. Finally, our PrxOy-films exert compressive stress on the substrate, for HfOx-films tensile stress is observed which, however, may also result from interfacial SiO2.
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.
Interference upon temporal overlap of two ultrashort pulse laser beams (80 - 100 fs) in a wide bandgap material (BaF2) instantaneously creates an index grating via the optical Kerr effect. This grating causes the diffraction both of the generating beams as well as of further incident beams with an overall efficiency of > 23%. Strong third harmonic generation is observed with an efficiency of up to approximately 3% due to an automatic self phase-matching process. The introduction of a third beam leads to a diffraction pattern simulating digital logic gates (e.g. decoder, demultiplexer, AND-and OR-logic), demonstrating the possibility of simple logic operations on a femtosecond time scale
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.
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.
Upon irradiation of barium fluoride (111) crystals with 100- fs pulses at 800nm, explosive emission of singly charged positive ions (Ba+, F+, and larger molecules and clusters) is observed with a kinetic energy of about 100 eV and an energy distribution corresponding to a temperature of only 1 eV, independent of the ion mass. Pump-probe experiments demonstrate that ion emission is always the consequence of preceding multiphoton surface ionization, resonantly enhanced by defect states within the band gap. Yet, the ionization process appears not to be noticeably slowed down by the resonances. Electron and light microscopic investigation of the desorption crater revealed frozen surface waves with the periodicity on the order of some microns. Superimposed, we found a ripple-like periodic fine structure, with a periodicity of 100-400 nm depending on the incident laser intensity. We suppose that this should be the consequence of self-organized relaxation of the surface, rather than the consequence of an interference effect as in the classical model for ripples formation.
We report on the realization of an all-optical full logic unit switching with a response time on the order of one femtosecond. The device is based on instantaneous transient gratings induced in a thin slide of barium fluoride by up to three femtosecond laser beams. The interference pattern from every tow of the non-collinear beams is, via optical Kerr effect, translated into a transient index grating which reacts within one optical cycle on the input since the applied light frequency is far from resonance in the material. As one consequence of the grating two strong beams of self phase matched third harmonic are generated, emerging in the middle between the two transmitted fundamentals. By introducing a third femtosecond beam we are able to emulate a demultiplexer, a decoder and simple adder. Since the output consists of the third harmonic of the input beams, the system works with uniquely high contrast.
Femtosecond second harmonic generation from the surface of as-grown 6-inch silicon wafers is used as a tool for in-situ characterization. Czochralski-grown crystal are not homogeneous over their cross section. A central zone of vacancy-rich and as outer zone of interstitial-rich crystal are separated by a ring of stacking faults. Gate oxide layers grown on such substrate show different quality in their dielectric properties, making it desirable to locate these zones before producing different devices. Exploiting the symmetry sensitivity of surface SHG, we use a particular two-pulse arrangement, similar to a conventional pump-probe setup, to obtain a non-destructive, in-situ information about the location of the three different crystal zones. Further, we demonstrate the potential of surface SHG to monitor external stress, exerted on the sample for instance by improper mounting, providing a tool for on-line optimization of process parameters. Finally, the applicability of the technique for on-line analysis and control during the growth of different types of gate dielectrics is discussed.
Femtosecond laser ablation of positive ions from transparent ionic crystals is studied by time-of-flight mass spectrometry. We find an explosive emission of positive ions. The ion yield dependence eon the laser fluence is highly nonlinear. The material is emitted in characteristic bursts, depending chaotically on the number of laser pulses hitting the sample. The mean kinetic energy of the positive ions is on the order of 100 eV while their temperature is only around 1 eV, very similar to supersonic expansion of a molecular jet. The last observation is independent of the ion species, indicating that all ions were born at the same instant and kicked out of the material simultaneously with identical kinetic energy. Negative ions, on the contrary, appear considerably later and are much slower. Al ablation is preceded by effective electron emission. We suggest that the laser generates a high-density plasma. The resulting electrons may possibly escape, due to the short pulse duration, without being disturbed by the build-up of a space charge zone. Subsequently, positive ions are expelled by Coulomb explosion of the unstable surface .Negative ions may be produced much later form the hot sample or by secondary processes.
Calibration procedures for lidar and DIAL systems are discussed. Resulting requirements for the standardization of the technique are reviewed. Possible schemes for on-line calibration, based on a standard reference cell, are presented.
During multishot ablation with 248 nm excimer laser pulses for each single laser shot the shock wave emerging from the ablated material was monitored by the acoustic mirage effect. The shockwave parameters turned out to depend sensitively on the nature of the ablated material. In particular during ablation of a polymeric film/Si02/Si multilayer system distinct changes in the deflection signal were found when the ablation was driven through the interface between layers. Inspection by optical microscopy and depth profil ing was used as cross check. 1.
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