The original damage ring pattern at the exit surface of fused silica induced by highly modulated nanosecond infrared laser pulses demonstrates the time dependence of damage morphology. Such a damage structure is used to study the dynamics of the plasma issued from open cracks. This pattern originates from electron avalanche in this plasma, which simultaneously leads to an ionization front displacement in air and a silica ablation process. Experiments have shown that the propagation speed of the detonation wave reaches about 20 km/s and scales as the cube root of the laser intensity, in good agreement with theoretical hydrodynamics modeling. During this presentation, we present the different phases and the associated mechanisms leading to this peculiar morphology: • During an incubation phase, a precursor defect provides energy deposit that drives the near surface material into a plasma state. • Next the silica plasma provides free electrons in the surrounding air, under laser irradiation an electron avalanche is initiated and generates a breakdown wave. • Then this breakdown wave leads to an expansion of the air plasma. This latter is able to heat strongly the silica surface as well as generate free electrons in its conduction band. Hence, the silica becomes activated along the breakdown wave. • When the silica has become absorbent, an ablation mechanism of silica occurs, simultaneously with the air plasma expansion, resulting in the formation of the ring patterns in the case of these modulated laser pulses. These mechanisms are supported by experiments realized in vacuum environment. A model describing the expansion of the heated area by thermal conduction due to plasma free electrons is then presented. Next, the paper deals with the two damage formation phases that are distinguished. The first phase corresponds to the incubation of the laser flux by a subsurface defect until the damage occurrence: an incubation fluence corresponds to this phase. The second is related to the damage expansion that only refers to the energy deposit feeding the activation mechanism up to the end of the pulse: an expansion fluence corresponds to this phase. A striking feature is that the damage diameters are proportional to the fluence of expansion at a given shot fluence. Indirectly, the fluences of incubation by the precursors are then determined.
The influence of vacuum on nanosecond laser-induced damage at the exit surface of fused silica components is investigated at 1064 nm. In the present study, as previously observed in air, ring patterns surrounding laserinduced damage sites are systematically observed on a plane surface when initiated by multiple longitudinal modes laser pulses. Compared to air, the printed pattern is clearly more concentrated. The obtained correlation between the damage morphology and the temporal structure of the pulses suggests a laser-driven ablation mechanism resulting in a thorough imprint of energy deposit. The ablation process is assumed to be subsequent to an activation of the surface by hot electrons related to the diffusive expansion of a plasma formed from silica. This interpretation is strongly reinforced with additional experiments performed on an optical grating in vacuum on which damage sites do not show any ring pattern. Qualitatively, in vacuum, the intensity-dependent ring appearance speed <i>V</i> ∝ <i>I</i><sup>1/2</sup> is shown to be different than in air where <i>V</i> ∝ <i>I</i><sup>1/3</sup> . This demonstrates that the mechanisms of formation of ring patterns are different in vacuum than in air. Moreover, the mechanism responsible of the propagation of the activation front in vacuum is shown to be outdone when experiments are performed in air.
The morphology of laser-induced damage sites at the exit surface of fused silica is tightly correlated to the mode
composition of the nanosecond laser pulses at 1064 nm. In the single longitudinal mode (SLM) configuration, a molten
and fractured central zone is surrounded by a funnel-shaped surface modification. Ring patterns surround the damage
sites when these are initiated by multiple longitudinal modes (MLM) laser pulses. In this last mode configuration, the
pulses temporal profiles as well as the damage ring patterns differ from pulse to pulse. The appearance chronology of the
rings is found to be closely related to the temporal shape of the laser pulses. This supports that the damage morphology
originates from the coupling of a laser-supported detonation wave propagating in air with an ablation mechanism in
silica. In our experiments, the propagation speed of the detonation wave reaches about 20 km/s and scales as the cube
root of the laser intensity, in good agreement with theory.
The laser induced damage densities measured on fused silica surface are found to be higher when produced with multiple longitudinal mode pulses than those produced by single longitudinal mode pulses at 1064 nm. The enhancement of the three-photon absorption due to the intensity spikes related to longitudinal mode beating might favor the damaging process at this wavelength. At 355 nm the picture is different. The absorption is supposed to be linear and an opposite behavior occurs. Evidences of a process leading to the possible annealing of a part of absorbent defects are discussed in this paper.
Photoluminescence excited by 325 nm laser light is used to investigate defect populations existing in different surface
flaws in high purity fused silica and to achieve a better understanding of laser damage mechanisms. Luminescence bands
peaking at 1.9, 2.1, 2.3, 2.7 and 3.1 eV have been detected in the spectral area ranging from 1.6 up to 3.6 eV. According
to the literature, the 2.3 eV band would be due to STE's (Self Trapped Excitons) relaxation. In order to study this
hypothesis, temperature dependent experiments have been driven in the 90 K-300 K range. For indentations as well as
laser damage, we show the evolution of luminescence spectra with temperature. Contrarily to the well known behavior
of STE's, which shows a change of several orders of magnitude for luminescence intensity, the 2.3 eV band is weakly
influenced by temperature decrease, from the ambient down to 90 K. The Gaussian decomposition of spectra allows
dividing the five luminescence bands in two categories. The first one corresponds to bands showing a significant
intensity enhancement with temperature decrease, and the second one to bands remaining insensitive to the fall in
temperature. That classification may provide helps in order to establish links between luminescence bands and defects.
This paper deals with the relation between fracture mechanics and 355 nm laser damage at the surface of fused
silica. It is organized in 3 parts. First, we discuss about the link between cracks and laser initiation of surface damage. A
1D model was proposed last year to explain how a nanometer wide, clean, uncontaminated crack could trigger a
macroscopic damage event. Here, using the model, we try to express a damage criterion able to reproduce experimental
In a second part, we consider the relationship between laser damage and mechanical damage by indents or
impacts. From Auerbach's law, it is straightforward to derive an energy density threshold for Hertzian crack initiation.
With the laser fracture interaction model, a laser fluence threshold of cone crack formation can be calculated. When cone
cracks are present, a series of shot at moderate fluence will increase their length exponentially. This is a possible
explanation for exponential damage growth at the exit surface of fused silica.
Numerous experimental and theoretical contributions in the past have stressed the detrimental effect of fractures
in the generation of surface laser damage sites in fused silica illuminated at 351 nm. However, two very important steps
lack for the moment on the way towards a scientific understanding of the role of fractures.
1. a physical model must be developed to predict damage events starting from real defect sites
2. a reproducible measurement must be obtained and compared with calculations.
Here we present the theoretical work realized to reach the first goal. Contrary to previous discussions on fractures, the
electromagnetic configuration is calculated in the case of a real material, with electronic surface states, bulk defects, and
defects dynamics. Due to electromagnetic field enhancement in the fracture, surface defects absorb a sufficient part of
laser energy, able to heat silica above the vaporization temperature. This is the initial event that triggers production of
more excited states during the pulse, and steep increase of temperature and pressure fields. Comparisons with available
experimental results are positive. Calculated fluences of damage initiation are very near those of measured events on
engineered fractures, or on real defects in polished samples.
The interaction of intense femtosecond laser pulse with model samples containing gold nanoparticales embedded in dielectrics is studied in order to understand the role played by nanodefects in optical breakdown of dielectrics. A theoretical study of the conduction electrons dynamics in the laser field predicts an efficient injection of carriers from the metallic inclusion to the conuction band of the dielectric, which leads to a strong local increase of the optical
absorption in the initially transparent matrix. This prediction is tested experimentally by using time -resolved spectral interferometry to measure excitation densities as a function of the laser intensity in silica and samples doped with gold nanoparticles, which are compared with similar measurements in pure silica.
In order to understand the role played by nanodefects in optical breakdown of dielectrics, the interaction of an intense laser field with model dielectric samples containing metallic nanoparticles is studied both theoretically and experimentally. A theoretical study of the metal conduction electrons dynamics in the laser field predicts an efficient injection of carriers from the metallic inclusion to the conduction band of the dielectric, which leads to a strong local increase of the optical absorption in the initially transparent matrix. This prediction is tested experimentally by using time-resolved spectral interferometry to measure excitation densities as a function of the laser intensity in silica samples doped with gold nanoparticles, which are compared with similar measurements in pure silica.
Laser energy deposition and redistribution in metal nanoparticles embedded in SiO<SUB>2</SUB> glass is studied by a kinetic model, which takes into account photon absorption, electron-electron and electron phonon interactions, as well as heat transfer to the glass matrix. The collision operators are usually written in an integral form. In this work, we transform those in differential operators with the use of Landau approximation. This approach allows to perform kinetic calculations beyond the nanosecond time scale. For a laser intensity relevant to high power lasers, the energy deposition on the electron population can lead to a significant Fermi smearing within very short times. An important part of the electron population is driven beyond a typical 10 eV energy, and consequently this can result in the creation of a plasma around the particle.
In this paper we describe the interaction of an intense laser beam with metallic nanoparticles embedded in glass. Energy deposition in the metal is calculated on the basis of Mie's scattering theory, using an accurate model for the dielectric function which involves interband transitions. As is shown by two-temperature modeling of the laser-heated metal, nonequilibrium thermodynamics must be used to describe such systems, even in the nanosecond laser pulse range. Taking into account the particle cooling process by heat diffusion in the glass matrix, the model provides a useful tool for the understanding of laser damage initiation by metallic nanoinclusions.