Effect of laser irradiation on silica substrate contaminated by aluminum particles

Effect of laser irradiation on silica substratecontaminated by aluminum particles

Stéphanie Palmier,1,2,* Jean Luc Rullier,1 Jérémie Capoulade,2 and Jean Yves Natoli2

1Commissariat à l’Énergie Atomique, Centre d’études scientifiques et techniques d’Aquitaine, BP 2, 33114 Le Barp, France2Institut Fresnel, Unite Mixté de Recherche 6133, Domaine Universitaire de St Jérôme, 13397 Marseille, France

*Corresponding author: [email protected]

Received 10 October 2007; revised 24 January 2008; accepted 25 January 2008;posted 25 January 2008 (Doc. ID 88307); published 10 March 2008

A major issue in the use of high-power lasers, such as the Laser Megajoule (LMJ), is laser-induceddamage of optical components. One potential damage initiator is particulate contamination, but its effectis hard to distinguish from that of other damage precursors. To do so, we introduced artificial contami-nants typical of metallic pollution likely to be present on the optical components of the LMJ chains. Moreprecisely, aluminum particles of two different sizes were placed on a silica sample. These dots were char-acterized by optical microscopy and profilometry. Then they were exposed to a laser beam with a pulselength of 6:5ns at 1064nm and fluences in the range from 1 to 40J=cm2. Each dot was characterizedagain with the same techniques and also by photothermal microscopy. To complete the experimentalresults, we performed numerical simulations with a one-dimensional Lagrangian hydrodynamics code.We show that the particle removal by laser irradiation produces a modification of the silica surface thatdoes not evolve into catastrophic damage under subsequent irradiation. However, the effect does dependon the size of the dots. We demonstrate that a procedure exists that removes the dot and leaves the sitecapable of resisting high fluence. © 2008 Optical Society of America

OCIS codes: 140.3330, 160.6030, 160.3900.

1. Introduction

In the context of large high-power laser facilitiessuch as the Laser Megajoule (LMJ) [1], the lifetimeof optical components is a major concern [2]. A maincause of lifetime reduction is laser-induced damage.Many studies were carried out to understand thisprocess and so to avoid it [3]. Among the causes ofsurface damage, onemaymentionmaterial imperfec-tions [4], self focusing [5], and particle contamination[6,7]. This last contribution can be emitted duringlaser operation by human activities, material aging,or degradation [8,9]. Moreover, in the case of LMJ,target explosion will generate debris [10]. But pollu-tion is difficult to control, and so it is not easy todistinguish its impact from other kinds of damageprecursor. A way to study the effect of particle con-tamination on optics under laser irradiation is to

use artificial contaminants on optical components[11–15]. In preliminary work on this subject, we hadobserved the effect of irradiation on metallic dots,which are typical of metallic pollution likely to bepresent on components of the LMJ chain [16,17].Then, we compared and contrasted the resultsobtained for aluminum dots of different sizes [18].In this paper we report a more complete study in-cluding the results of a simulation, which permitsus to understand the results obtained with the largerdots.

One surprising aspect of our investigation was theobservation of a remarkable “cleaning” effect, inwhich, by an appropriate choice of fluence, contami-nants initially present on the surface can be re-moved. This could be achieved without damagingthe silica surface, and after removal, the surfaceshowed considerable resistance to subsequent lasershots with fluences up to 40 J=cm2. This observationmay have important consequences for controlling

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1164 APPLIED OPTICS / Vol. 47, No. 8 / 10 March 2008

damage in high-power lasers, and we consider it to bea major aspect of our work.Section 2 describes the experimental procedure.

The comparison of behavior for different size dotsunder laser irradiation is presented in Section 3,and photothermal measurements are discussed inSection 4. Numerical simulations and an interpreta-tion of some of our results are given in Section 5.Then the laser cleaning effect mentioned above isanalyzed in Section 6. Finally, we give our conclu-sions in Section 7.

2. Experimental Procedure

For this study, we used superpolished 25mm dia-meter and 3mm thick silica substrates. The contami-nation studied here is typical of that found in thelaser chain (both in chemical composition and in size)[9]. Aluminum is an appropriate choice, since it is themajor component in the metallic structure support-ing the laser. Aluminum particles were placed onthe substrates by a photolithography process. Weobtained a regular pattern with 36 dots, as shownin Fig. 1(a). We worked with particles of two distinctsizes, 50 μm× 50 μm × 1 μm and 5 μm × 5 μm× 1 μm.The distance between adjoining dots is 3mm. Eachparticle was observed by optical microscopy, and aprofilometer was used to measure the topographyof each dot surface and its neighborhood. The pro-filometer in this experiment had a vertical and lat-eral resolution of 3nm and 360nm, respectively. Atypical image of a 50 μm × 50 μm× 1 μm dot is shownin Fig. 1(b), and the corresponding profile along a cutas measured by profilometry is given in Fig. 1(c).After characterization, the samples were exposed

to a laser beam that illuminated each dot indepen-dently. This beam was delivered by a Nd:YAG laserwith a pulse length of 6:5ns at 1064nm. The beamwas focused by a 5m focal length lens to get a Gaus-sian spatial profile with a diameter of 0:6mm at 1=eon the sample surface. This beam size permitted usto perform dot-by-dot irradiation. The silica sub-

strate was placed vertically on a mobile sampleholder. Since the probability of damage occurrenceunder laser irradiation is greater on the outputsurface of a silica substrate [19], the substrate wasoriented so as to place the dots on the output face.In the case of the small dots (5 μm), a He–Ne(633nm) laser beam and a portable inspection micro-scope were used to help in locating the dot.

A preliminary test was performed on bare silica tomeasure the damage threshold that turned out to be60 J=cm2. Then the tests were carried out first with asingle shot on a subset of dots at one of the followingfluences: 1, 3, 5, 10, 15, 20, 30, 35, and 40 J=cm2.Next, some of the remaining dots were irradiatedby either 10, 100, or 1000 successive shots at a fre-quency of 1Hz. The He–Ne laser beam and the por-table inspection microscope allowed the observationin situ of the dot modification under irradiation. Ifand when substantial damage did occur, the se-quence was stopped in order to avoid catastrophicdamage to other dots on the sample.

After irradiation, each particle was observed againby optical microscopy, and its height profile was re-measured by profilometry. In addition, changes inthe local absorption of the silica substrate after irra-diation were measured at the dot location with aphotothermal microscope. The principle of this mi-croscope is based on photothermal deflection of atransmitted probe beam, which is focused at thesame location as a pump beam (1064nmwavelength)[20,21]. With this device, we obtained absorptionmappings of the silica surface with lateral resolutionless than 1 μm.

3. Comparison of Behavior of Two Different Size Dots

After irradiation of the 50 μm wide dots, we hadpreviously observed distinct behaviors for the dot[17]: no modification for a fluence of 1 J=cm2, smallablation at a fluence of 3 J=cm2, strong ablation atan intermediate fluence (5 J=cm2), and finally com-plete ejection at fluences greater than or equal to10 J=cm2. When aluminum was ejected, we observedan excavation in the silica substrate with the sameshape as the dot. The higher the fluence, the deeperand wider was the excavation. At 5 J=cm2, a shallowdepression a few nanometers deep was observed inthe silica under the removed dot, and it was sur-rounded by tiny hills about 25nm in height.

In the case of the 5 μm wide dots [18], again with athickness of 1 μm, similar behaviors were observedfor fluences less than or equal to 5 J=cm2. For an ir-radiation at fluences greater than 5 J=cm2, while alu-minum was also completely removed, its ejection lefta crater on the silica surface that was larger than theinitial dot. A similar effect was observed for the bigdots only at fluences greater than or equal to15 J=cm2. This morphological difference is illustratedin Fig. 2 for both sizes of dot irradiated at 15 J=cm2.Profilometer images are shown on the left, and cor-responding profiles along dotted lines are givenon the right. The observation by optical microscopy

Fig. 1. (a) Schematic of a sample containing 50 μm square dots,with an example of (b) an optical image and (c) a correspondingprofile.

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showed that in both cases aluminum was removedand left a print in the silica. For the 50 μm widedot, the latter is characterized by the dot shapeand size, and in the case of the 5 μm wide dot, thesilica substrate seems to bear an image of the alumi-num dot. All around these prints, we distinguished acircular shape. The height profile of the 50 μm widedot showed a 250nm deep excavation with a largerwidth than the initial dot. To quantify the size andshape of the excavations seen in the silica, we intro-duce three parameters that we call crater diameter,crater depth, and central depth. While these aredefined somewhat arbitrarily, they represent, respec-tively, the maximal size of the excavation, the depthof the excavation in the silica region outside the dot,and the depth of the hole directly underneath thedot. Using these variables we can attempt to under-stand the similarities and differences between thetwo sizes of dots.First, we consider the evolution of the circular cra-

ters with an increase in the fluence. Figure 3 shows acomparison between 50 and 5 μm wide dots for thecrater depth and the crater diameter measured afterirradiation at different fluences. For the 50 μm widedots the circular crater appeared at a fluence of15 J=cm2, for both dot sizes, its parameters (depthand diameter) increased with the fluence. The crater

depth has an evolution that is independent of the dotsize. At 15 J=cm2 the diameter of the crater for thebigger dot is much greater (120 μm) than for the5 μm wide dot (20 μm). However, after irradiationgreater than 30 J=cm2, the values obtained for alarge and small dot were close, although there wasa factor of 10 difference between their dimensions.Thus for this high fluence the crater diameter doesnot depend on the dot size.

Second, we examine the dependence on the fluenceof the central crater observed in Fig. 2. Figure 4shows the profiles obtained for 5 μm and 50 μm widedots irradiated at 10, 15, and 20 J=cm2. For the 50 μmwide dot, the print in the silica is tightly correlatedwith the dot shape and size. Just as for the largercircular crater, the central depth grows with the flu-ence. For the 5 μm wide dot, the excavation profile islike a spike at 10 J=cm2. At 15 J=cm2, the main exca-vation is 3 μmwide with two narrow spikes next to it.At 20 J=cm2, the excavation looks like the one ob-tained after irradiation of the bigger dots: indeed,the width is approximately the same as the dotwidth. To analyze the dependence on the dot sizeof the silica excavation under the dot, Fig. 5 showsthe central depth measured under the dot after irra-diation at difference fluences.

Fig. 2. Topographical images of the 50 and 5 μm wide dots after laser irradiation at 15J=cm2 and their height profiles.

Fig. 3. Crater depth (left) and crater diameter (right) of the hollow obtained around the dots after one irradiation at the indicatedfluences.


The central depth of the excavation in the silicasubstrate induced by the 5 and the 50 μm wide dotsunder irradiation increases with the fluence. Forboth, at 5 J=cm2, the excavation is very small. Fora fluence from 5 to 15 J=cm2, the central depth ofthe 50 μm wide dot grows quasi proportionally withthe fluence until 200nm, then its increase is slowerto reach 250nm at 35 J=cm2. After the irradiation ofa 5 μm wide dot at 10 J=cm2, the central depth valueis already 350nm (greater than the maximum for thelarge dot). Then a slow increase that is quasi linearwith the fluence is observed. In that case the excava-tion is not necessarily of the same size as the dot. Themain difference between the behaviors of small andlarge dots under irradiation is the existence of athreshold fluence necessary to create an excavationunder the dot for only the smaller dot.This study of the comparison of behaviors between

two dots size reveals that an important differenceappeared on the silica substrate when they were

irradiated at a fluence greater than 5J=cm2. Thishighlights a difference in the physics involved forthe two different dot sizes.

4. Photothermal Analysis of the Absorption Propertiesof Dots

The presence of absorbing centers in the silica maybe precursors of catastrophic laser-induced damage[22]. Our results exhibit a significant modificationof the silica surface after one laser irradiation withfluence greater than or equal to 5 J=cm2. A criticalpoint for the modified silica we observe is the pre-sence or absence of absorbing centers. A useful toolfor studying this local absorption is photothermalmicroscopy.

We have noticed that the morphology of the irra-diated sites depends strongly on the size of the dots,and this observation is reinforced by the photother-mal absorption measurements shown in Fig. 6. Theabsorption is measured by using a laser beam of dia-meter 1 μm. To construct the maps the sample is dis-placed by steps of 4 μm for the 50 × 50 μm square dotsand by steps of 2 μm for the 5 μm× 5 μm square dots.Rather different results are observed for the two dotsizes. For the 50 μmwide dot, the shot at 5J=cm2 pro-duces a zone of strong absorption whose size isslightly greater than the original dot. In contrast,at the higher fluences, there is little or no absorption,which suggests that all the metal has been removedfrom the neighborhood of the dot. Quite differentbehavior is seen for the 5 μm wide dots. At 5 J=cm2

the result is not unlike that for the 50 μm widedot, with comparable absorption levels over a regioncomparable in size with the initial dot. However, at10 J=cm2, the size of the absorbing zone is muchgreater than the size of the dot, while the absorptionlevel remains about the same as at 5 J=cm2. At15 J=cm2 both the level of absorption and the sizeof the absorbing zone are reduced, compared with10 J=cm2, while at 20 J=cm2 no absorption abovebackground is observed.

Taking into account the information contained inFigs. 5 and 6, we propose the following hypothesis:for the 50 μm wide dots, once the threshold near5 J=cm2 is exceeded, the silica situated under thedot begins to be evacuated, and the dot itself is va-porized, leaving behind little or no absorbing residue.In contrast, the silica situated below the 5 μm widedots is only partially evacuated, and some of the alu-minum remains distributed, generally over a surfacemuch larger than the dot itself. This suggests that forthe smaller dots the heating mechanism is not uni-form over the size of the dots. A possible explanationcould be the escape of heat at the edges of the dots,which would be relatively more important in the caseof the 5 μm wide dots. These photothermal measure-ments yield support to our claim that the mechan-isms of dot removal are somewhat different for thetwo different sizes of dots.

Fig. 4. Profiles of the (a) 50 μmand (b) 5 μmwide dots for differentfluences of irradiation.

Fig. 5. Central depth measured at the dot location after irra-diation.


5. Interpretation by Complementary Study

From the experimental study, our claim rests on dif-ferences inmorphology and differences in the absorp-tion properties in the region from which the dotswere removed. Since the 50 μm wide dots containabout 50 times as much aluminum as the 5 μm widedots before irradiation, it is not surprising that theabsorbing domains are much greater in size, butthe contrasting results at 10 J=cm2 suggest thatquite different mechanisms are involved in dotremoval.

A. Numerical Simulations

We used a one-dimensional Lagrangian hydrody-namics code called ESTHER, which is an upgradedversion of the code DELPOR [23,24]. This code pre-dicts the evolution of matter from the solid to theplasma state under the influence of energy depositedby the laser. The space–time evolution of the internalenergy is obtained by solving the Helmholtz waveequation. Since ESTHER is a one-dimensional code,the aluminum dot is represented by a 1 μm thicklayer on the silica surface.The numerical simulations show that the energy

deposited in the aluminum vaporizes it and thatthe nearby silica reaches a temperature where ittoo is vaporized. The resulting plasma then absorbsstill more energy from the laser, leading to the abla-tion of the silica. When the fluence increases, thetime needed to reach this highly absorbing state de-creases, and a larger fraction of the incident pulse isabsorbed by the plasma. In this way extremely hightemperatures are reached at high fluence. In Fig. 7,we show a comparison between the experimental va-lue for the maximum depth of the crater (craterdepth plus central depth) and the calculated thick-ness of ablated silica. The simulation results showa progressive increase of the ablated thickness silica

with the fluence. For the 50 μm wide dots, there isreasonable agreement between the simulated resultsand the experimental ones for both size dots. For the5 μmwide dots, the agreement is not very good essen-tially because the point at a fluence of 10 J=cm2 isquite different from the simulation and from thetrend seen at higher fluences. We are unable to de-cide whether this is due to an anomalous measure-ment (the measurement was based on two dots) orwhether it represents a real disagreement with thesimulation. Only further experimental work can dis-tinguish between these alternative hypotheses.

B. Successive Shots

Even though the photothermal microscopy does notindicate strong absorption, it remains of interest tosee how the absorption signal varies with the num-ber of successive shots on the site. In the case of a5 μm wide dot, the silica surface modifications after

Fig. 6. Photothermal absorption maps for 50 μm and 5 μm wide dots at fluences of 5, 10, 15, and 20J=cm2.

Fig. 7. Numerical simulations versus experimental results forthe maximum depth.


one shot seem to be more consistent from one dot toanother. In Fig. 8 we show twomappings obtained for5 μm wide dots after 1 and 1000 pulses, respectively,at 15 J=cm2. It is clear that after 1000 pulses, theweakly absorbing centers have disappeared. Themapping obtained after a 15 J=cm2 irradiation showsa maximum absorption of 2:10 × 10−3 where the dotwas. This absorbing area is about 10 μm wide. Undersubsequent irradiations at the same fluence, nocatastrophic damage is observed, and this high ab-sorption decreases. It reaches the noise level of thephotothermal microscope. We interpret this ab-sorption as being due to the presence of residualaluminum particles at the dot location and theredeposition of small aluminum particles aroundand especially in the crater area. The absorption de-crease is due to the aluminum vaporization underseveral shots. This result shows that the absorptionsignal is caused by the presence of very fine alumi-num particles.

6. Cleaning Effect

The aim of this work is to study the effects of a sur-face pollutant and, if possible, to neutralize it. Wehave shown that for aluminum dots, at the fluencesused, vaporization occurs for dots of both sizes. Inaddition, our simulation shows that at high enoughfluence, not only the aluminum, but also the under-lying silica, is vaporized.To advance beyond these results we performed

additional irradiations using a modified experimen-tal protocol. New samples with the dots were studiedby using a single irradiation at different fluences at1064nm, 6:5nm, followed by a series of shots at40 J=cm2. This represents the highest fluence wecould use in single-shot mode without causing majordamage [18]. Our aim was to see whether successivehigh-fluence laser shots could be resisted by the sitesfrom which the dots had been removed. Some of ourresults are displayed in Fig. 9. For the 50 μm widedots, after ten shots at 40 J=cm2, there is a cata-strophic damage. The first shot removes the particle,but its fluence is so high that the silica is affected. Sodamage appears during the following shots. How-ever, in Figs. 9(b) and 9(c), the results obtained afterone shot at 5 J=cm2 or 15 J=cm2 followed by 100 shotsat 40 J=cm2 show no catastrophic damage. Theyshow a conditioning effect [25,26]. Nonetheless they

are different. In the first case, thanks to photother-mal microscopy measurement, we had observed thata fluence as low as 5 J=cm2 is not enough to vaporizeall the aluminum. Because of residual aluminum,silica is heated and transformed by the successiveshots at 40 J=cm2 but remains stable. In the secondcase, a fluence of 15 J=cm2 totally vaporizes the alu-minum dot at the first shot and creates an excavationin the heated silica. For the 5 μmwide dots, two shotsinduce catastrophic damage [Fig. 9(a)]. In Fig. 9(e), adark footprint (three times the dot width) appearsafter one shot at 5 J=cm2 and subsequent shots at40 J=cm2. By comparison with Fig. 9(b), we show thatthe contribution of residual contaminant is moreimportant in the case of the 5 μm wide dot. For thedots of both sizes, after one shot at 15 J=cm2, themodified silica behaves like uncontaminated silica,and it can resist more than 100 shots at 40 J=cm2.

Thus we find that a procedure exists that allowsdot removal and yet leaves the site capable of resist-ing high fluence. Its efficiency depends on the para-meters of the first shot. We showed that a weakfluence is not enough to remove aluminum particlesproperly . To optimize particle removal without af-fecting the substrate, an intermediate fluence isnecessary.

7. Conclusion

To study the effect of laser irradiation on a contami-nated silica substrate, we deposited artificial alumi-num square dots on a silica surface. Particles werepositioned on the output face of the silica substrateand irradiated at 1064nm, 6:5ns with variousfluences. We showed that below 40 J=cm2 thereis no catastrophic damage but silica modificationsoccurred. An excavation is created whose shapeand size depend on the initial particle size. Aroundthe initial dot, depending on the fluence, silicais ablated, thereby creating a crater. Subsequentirradiations on this modified silica do not producecatastrophic damage, and the silica behaves likean uncontaminated substrate. Even if the aluminumis not completely vaporized after one shot, the follow-

Fig. 8. Photothermal mapping of two 5 μm wide dots, after either1 or 1000 irradiations at fluences of 15J=cm2.

Fig. 9. Images of three 50 μm wide dots after irradiation; (a) 10shots at 40J=cm2, (b) 1 shot at 5 J=cm2 and 100 shots at 40J=cm2,(c) 1 shot at 15J=cm2 and 100 shots at 40J=cm2. Also shown areimages of three 5 μm wide dots after irradiation; (d) 2 shots at40J=cm2, (e) 1 shot at 5J=cm2 and 100 shots at 40J=cm2, (f) 1 shotat 15J=cm2 and 100 shots at 40J=cm2.


ing shots at 40 J=cm2 cause a dark print in the silica.Thus this effect does not shorten the lifetime (∼100shots) of the optical component to be used at a fluencebelow 40 J=cm2.The only concern with such large defects on an

optical surface is to determine whether by perturbingthe propagation in a high power laser they couldcause hot spots on other optical components orwhether they could modify the focal spot size atthe target. These problems have been studied theo-retically and experimentally with silica dots depos-ited on a LMJ-type optical component [27,28]. Forboth cases, the sizes needed to affect either the opti-cal component or the focal spot sizes are of the orderof a millimeter. This seems to be much greaterthan the defects we could make by laser cleaningof metallic particles.

The authors are grateful to T. Donval, L.Lamaignère, and M. Loiseau for experimental facil-itation and to A. Roques and P. Combis for their con-tribution in simulations. Discussions with J. T.Donohue are also appreciated. This work is sup-ported by the laser science program of the Commis-sariat à l’Énergie Atomique.

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Effect of laser irradiation on silica substrate contaminated by aluminum particles