Laser damage phenomena in fused silica are currently under study because of numerous related high power laser applications. Nano-sized defects are believed to be responsible for some laser damage initiation. In order to predict and to quantify this initiation process, engineered submicronic gold defects were embedded in silica. The study of these samples by localized pulsed irradiation of isolated gold particles coupled with Nomarski, atomic force and photothermal microscope observations permits us to discriminate between two distinct stages of material modification: one detectable at the surface and the second in the neighbourhood of the embedded particle. Comparison between the observations and simulations results in good agreement if we assume that inclusion melting initiates the damage.
© 2003 Optical Society of America
The understanding of laser damage phenomena in optical components remains an outstanding problem. The use of high power lasers in different applications requires improvements of optical materials. In this paper we focus our attention on silica material capable of supporting high UV fluxes from pulsed lasers. Various prior works have assumed that defects, typically a few nanometers in size, were responsible for laser damage initiation, either as surfaces defects (scratches, polishing debris) or in bulk silica (bubbles, metallic or dielectric inclusions) [1, 2, 3]. Even if we are able to determine the density of nano-defects by observing the damage probabilities they create when irradiated [4, 5, 6], the true nature of these nano-defects as well as the detailed process of laser damage initiation is still unknown.
In order to provide a firm basis for a systematic study of nanometric sized defects, the artificial insertion of defects in silica samples has been performed [7, 8]. In particular, Papernov and co-workers, using gold spheres of diameter 5 nm, have demonstrated that the silica surrounding the inclusions must also become absorbent, in order to explain the size of the craters that were observed. Subsequent work  where gold spheres of diameter 3 nm were inserted in silica, indicated that substantial amounts of gold were emitted by the sample before any appreciable surface damage was visible. We shall refer to this situation in which no visible damage has occurred, but where the inclusion has undergone considerable modification, as a “precursor”. Moreover, Papernov and co-workers have investigated the damage threshold as a function of gold particle size (2–19 nm range). They showed that even particles a few nanometers in diameter can lead to significant threshold reduction. They introduced the term «nanoscale» damage threshold as a laser fluence causing localized melting without significant vaporization . The purpose of the present article is to refine the previous study by restricting our attention to a larger single defect and performing a more detailed observation of the damage initiation process. In addition, we compare our results to the predictions of a numerical simulation which takes into account the physical processes leading to damage.
2. Experimental procedure of test and results
For this study, we again used silica samples in which gold particles were embedded at a depth of 5 µm. A relatively large particle size (600±200 nm in diameter) was chosen. Indeed, this size facilitates the observation of the damage initiation with a microscope as well as the determination of the laser damage threshold for a single particle. All the samples used were prepared by the Laboratoire d’Electronique de Technologie et d’Instrumentation (LETI) . The parameters and geometry of these samples are shown in Fig. 1, together with an atomic force microscopy (AFM) image of the dome over an inclusion. The density of these inclusions is 120±20 spheres/mm2. Two different studies have been performed using both a small (8–12 µm) and a large (0.7–1.4 mm) diameter spot size for the laser illumination. Although these yield quite similar results, we choose to present in this paper only the results obtained with the small spot size. Indeed the use of a small beam permits us to observe and to select isolated particles before the shot. Moreover, the results obtained with the smaller spot size are not subject to collective effects among inclusions.
The set up involves two different 3 ns (FWHM) pulsed YAG lasers whose wavelengths are 1064 and 355 nm, respectively . In order to perform a localized study, the lasers are focused with adapted objectives on the surface of the sample. Accurate control of the fluence is possible thanks to a pyroelectric detector associated with the spatial profile of the focused beam. An in-situ imaging system using an optical microscope permits us to follow in real time any surface modifications caused by the laser irradiation. Both a Nomarski microscope and an AFM were used to obtain images of the sample on the same particle, thanks to a repositioning system.
In Fig. 2, Nomarski images of three inclusion sites (focus on the surface and at 5 µm depth) are shown at left, and the corresponding AFM linear scans appear on the right. Figure 2(a) shows a site which was not irradiated; the undamaged inclusion is clearly visible in the 5 µm deep focus. Figure 2(b) shows an irradiated inclusion at fluence of 6 J/cm2 at 1064 nm where no surface damage has occurred, but where one sees a clear modification of the Nomarski contrast in the 5 µm deep focus. Finally, in Fig. 2(c) for higher fluence (12 J/cm2) a crater has formed, as is clearly visible in the AFM image.
If we consider that the usual strict criterion of laser damage corresponds to some (even minimal) surface modification  we can refer the situation shown on Fig. 2(b) as pre-damage. In order to determine the threshold values for pre-damage observation (labelled Tp) and surface crack apparition (labelled Ts), and because of the statistical nature of the laser damage threshold, we made a measurement on 30 selected particles for each fluence. Thanks to the small laser spot size, we can also test the silica coating resistance in a region containing no inclusions with the same procedure. The corresponding threshold value (labelled Tc) is defined as the intensity at which the first surface crack appears. The results obtained at the two wavelengths are presented in the Table 1.
We point out that an accurate determination of Tp is difficult because of the weak Nomarski contrast at this low range of fluence. We first notice a large difference between the threshold values obtained on the silica coating and on particles. This exhibits the expected role of the gold inclusions as damage initiators. However, we maintain that our most significant result is the establishment of the pre-damage state, where the inclusion has been strongly modified by the laser radiation, with no accompanying surface damage.
In addition to these microscopic observations of damage we also used a photothermal microscope to measure the local absorption change in the neighbourhood of a single particle. The microscope is based on photothermal deflection (PD) of a transmitted probe beam (He-Ne laser) which is collinear and focused through the same objective as a CW pump beam (1064 nm wavelength) . In our set-up the motor stage resolution is 0.1 µm, and the diameters of the probe and pump beams on the sample surface are about 1 µm . An example of photothermal mappings of 20 µm×20 µm shown in Fig. 3 is obtained on a single gold particle before and after two different irradiations below Ts. In this case, Phototothermal signal is given in arbitrary units because calibration is difficult. However PD signal can be interpreted in a first approximation in terms of optical absorption at 1.06µm wavelength [14,15]. Before shot we can observe the high absorption due to the metallic inclusion (one thousand more than in silica matrix). The results obtained on several 600nm-particles show that the signal is highly reproducible (95%). Furthermore we observe a decrease of the particle optical absorption after irradiation. The decrease of the optical absorption after shots is a signature of the modification of the gold particle as could happen in fusion and associated diffusion processes. Consequently, photothermal microscope clearly confirms the situation of a pre-damage observed with the Nomarski microscope.
3. Numerical simulation
In order to test our understanding of these two observed stages in the initiation process, we have compared the experimental result to numerical simulations. We use the 1-D hydrodynamic DELPOR code in spherical geometry to determine the laser absorption by a spherical nano-particle. Energy deposition is calculated by solving the Helmholtz equation thanks to the Mie theory generalized to media where the indices of refraction depend on the radius [16–18]. The code uses the multiphase Equation of State (EOS) of gold. Thermal conduction, radiative transfer and ionization by UV light emitted by the heated metallic particles are also simulated in DELPOR. In this code, mechanical stresses are not taken into account. In Fig. 4 we show the maximal gold temperature reached during the pulse (3ns) as a function of the fluence, for both wavelengths 355 nm and 1064 nm. The melting temperature and a range of temperatures at which boiling occurs (at high pressure) are indicated. One sees that the curves for the two wavelengths remain parallel until the vaporization temperature is reached. Once this temperature is reached, significant changes in transport coefficients occur, which cause a sharp increase of absorption at wavelength 1064 nm. We point out also the existence of narrow plateaux corresponding to the transition from solid nano-particle to liquid gold. The same simulation provides a prediction for the maximum transverse stress in the silica surrounding the inclusion. It shows that the melting of the inclusion and the fractures appearing in the silica matrix are correlated. If we compare the experimental results to the theoretical data, the observed pre-damage situation (Tp on the figure) is very close to the melting point for the two wavelengths. Confined gold melting could induces large stress in the fused silica. We can also notice that even for the first surface modification (Ts on the figure) the vaporization temperature is not reached. These two points constitute essential information on the initiation mechanism around an absorbing inclusion.
All the results obtained in the UV and IR show that by studying gold particles embedded in silica, it is possible to discriminate between two stages in the initiation process. The prediction from simulation is in good agreement with the experimental result, showing that the melting of the gold particles is the first step in the initiation process. To go further in the investigation, and perform a quantitative study of this observation of pre-damage, we are coupling the localized pulsed laser with a photothermal microscope. This new apparatus, which will allow in-situ correlation between gold diffusion and absorption, represents a very promising method for investigating laser damage initiation. Moreover, this study will be completed by performing detailed analyses of the surface and of the neighbourhood of the inclusion.
We gratefully acknowledge the financial support from the program Laser Megajoule at CESTA, and the LETI for the preparation of the samples. Discussions with H. Bercegol and J. Donohue are also appreciated.
References and links
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