Single-shot and multishot laser induced damage ... - SPIE Digital Library

Single-shot and multishot laser induced damage of HfO2/SiO2 multilayer at YAG third harmonic Yuanan Zhao*, Zhaosheng Tang, Jianda Shao, Zhengxiu Fan R&D Center for Optical Thin Film Coatings, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P. R. China 201800

ABSTRACT HfO2/SiO2 dielectric mirrors for 355 nm, prepared by conventional electron beam deposition, had been investigated with respect to their laser damage resistance. Two kinds of HfO2 with different purity were chosen as the high index material, whose impurity contents were evaluated by Glow Discharge Mass Spectrometer (GDMS) and X-ray Photoelectron Spectroscopy (XPS). Laser damage testing was performed both in the “1-on-1” and the “s-on-1” regime, using 355 nm pulsed laser with a pulse width of 8 ns. It was found that the laser induced damage threshold (LIDT) for single-shot was much higher than that for multishot. A phenomenon displayed that the impurity of zirconium was a critical hindrance in improving the LIDT in the single-shot process, but such an effect was not shown in the multishot process. The damage mechanism is different in the two manner of radiation, the main cause of the damage in single-shot is impurity absorption and that in the multishot is accumulation of structural defects. Optical microscopy and surface profiler was employed in mapping laser-induced damage morphology features after irradiation.

Keywords: HfO2/SiO2 dielectric mirror coatings, single-shot, multishot, laser-induced damage threshold, impurity, structural defects, accumulation effect 1.

INTRODUCTION

To understand of the origin of laser damage in optical coatings is important information for coating engineers to improve the coating process and laser damage resistance of coatings. Although there are general rules to produce dielectric coatings for high power laser applications concerning material choice, design and deposition technique [1, 2], specific properties of the coating can vary with manufacturer and even coating chamber resulting in different damage mechanism for each case. Furthermore, as a well known fact, damage mechanism varies with laser radiation manner [3, 4]. The well-known damage mechanism for single-shot laser radiation includes avalanche ionization [5], multi-photon ionization [6], impurity breakdown [7], and nodular defects breakdown [8]. Whereas, the damage mechanism of multishot radiation is much more complicate than that of single-shot radiation, and the damage is attributed for accumulation of pulse-to-pulse irreversible changes of optical properties of absorbing inclusions and ambient material matrix [3, 4]. Mechanisms of these changes have not been sufficiently studied to understand the accumulation effect which is obviously very important for practical use of optical system. The aim of this paper is to analyze the transformation of damage mechanism from single-shot to multishot radiation. Experimental details concerning the deposition process and the laser damage test apparatus will be given in Section 2. Section 3 of this article will be devoted to the experimental results and discussion in correlation with the results. The *

Corresponding author. R&D Center for Optical Thin Film Coatings, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, P. R. China 201800 Tel: +86-21-69918496 Fax: +86-21-69918028 Email: [email protected]

Laser-Induced Damage in Optical Materials: 2003, edited by G. J. Exarhos, A. H. Guenther, N. Kaiser, K. L. Lewis, M. J. Soileau, C. J. Stolz, Proceedings of SPIE Vol. 5273 (SPIE, Bellingham, WA, 2004) · 0277-786X/04/$15 · doi: 10.1117/12.522783

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conclusion will be presented in Section 4.

2. 2.1.

EXPERIMENTAL DETAILS

Sample preparation

The mirror coatings were prepared by conventional e-beam deposition, using the same deposition technology and in the same coating chamber. The film structure was (HL)^9H, where H denoted high index material HfO2 with one QWOT (Quarter Wavelength Optical Thickness) and L denoted low index material SiO2 with one QWOT, respectively, and the substrate was fused silica. In our experiments, two kinds of HfO2 with different purity were chosen as high index material. These two kinds of high index materials were denoted as M1 and M2 respectively and the mirror coatings made of these two kinds of HfO2 were denoted as S1 and S2. 2.2.

Purity analysis of material

The main impurity element in HfO2 is zirconium, which does not be taken into account in general analysis. Although the content of the impurity is very small, the dramatic absorbance in UV region can not be neglected. GDMS and XPS were employed to analysis the content of impurity elements before and after deposition respectively. 2.3.

Damage threshold determination He-Ne Laser

Beam Diagnostics Imaging system 2-D Stage

Attenuator

Len

Sample

HR@355, HT@1064&532

Nd:YAG+LBO+BBO

HR@355

HR@355, HT@1064&532 iMac

Fig. 1. Experimental setup of laser damage testing

Single-shot and multishot damage thresholds of the mirror coatings had been measured in the UV damage testing facility following ISO standard 11254-1.2. The experimental setup is shown in Fig.1. The flat-topped beam profile was achieved by imaging a circular aperture with a spherical lens of 250mm focal length. The sample was set upon a two-dimensional precision stage driven by a stepper motor. The angle of incidence was slightly (2º-3º) off normal to avoid interference effects due to reflection from the substrate exit surface. The laser energy that was used to damage the sample was obtained by adjustment of the attenuator, and the pulse energy was measured by an energy meter from a split-off portion of the beam. The He-Ne laser was used to help in monitoring the test. High sensitive on-line damage detection is performed by the help of a video microscopy system including digital image processing. The video micrographs of the site under test were acquired and stored before and after laser trigger; subsequent pixel-wise comparison of the digitized

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Proc. of SPIE Vol. 5273

images guaranteed an unbiased decision whether or not the respective laser pulse had caused damage. The LIDT was defined as the incident pulse’s energy density when the damage occurs at 0% damage possibility (joules per square centimeter). In the s-on-1 test, the same site of the sample was under 100 laser shots at the frequency of 1 Hz.

3. 3.1.

RESULTS AND DISCUSSION

Purity analysis results

The GDMS and XPS results are shown in Table 1 and Table 2 respectively. Before deposition, the content of zirconium element had much difference, nearly one order of magnitude, between two kinds of materials (see Table 1). The content difference did not change obviously after deposition process, from the XPS analysis (see Table 2), which is still about one order difference. Table 1 GDMS results of material

M1

Content of zirconium elements (ppm) 26126

M2

6445

Material

Table 2 XPS results of multilayer sample

Proportion of elements

Samples

3.2.

hafnium

zirconium

oxygen

S1

23.606

1.07

71.732

S2

21.027

0.141

68.935

Laser-induced damage threshold

LIDT of the samples is shown in Fig. 2. From the results, we can see, the LIDT of the 1-on-1 test is much higher than that of the 100-on-1 test for both S1 and S2 samples, and the LIDT of S2 sample is higher than that of S1 sample in the 1-on-1 test, but this kind of advantage is not clear in the 100-on-1 test.

10

1-ON-1 100-ON-1

LIDT (J/cm2)

8 6 4 2 0

S2

S1 Sample

Fig. 2. LIDT results of the samples


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3.3.

Damage morphology

Morphology observation permits precise views of damage characteristics. The surface morphology after laser radiation was accessed using optical microscopy and WYKO surface profiler. The photos of optical microscopy after 1-on-1 test are shown in Fig. 3. The depth information is shown in Fig. 4. For the S1 sample, the damage morphology appears irregularity [see Fig.3(a)], in some local areas the morphology was flat bottom pit, but in other areas, it shows the sharp crack. As the radiation energy increases further [see Fig.3(b)], the damage area and the sharp crack structure increases, but the flat bottom pit still appears. Inspection of the damage profile shows that the depth of flat bottom pits corresponded to the high/low index material interface [Fig.4(a) and 4(b)]. Because of the typical design of reflector, the maximum of electric field distribution locates at the high/low index interface and the interface is more prone to damage, which is the reason of the flat bottom pit. Compared with S1 sample, damage morphology of S2 sample shows the different characteristics. The damage zone is much regular than that of S1 sample, it shows the delamination structure at the relatively low radiation energy density [Fig.3(c)], and the depth information shows all the delamination position are located at the high/low index material interfaces [Fig.4(c)]. There also basically have no such sharp crack as in S2 sample, and as the laser energy increases, the morphology appears as the scalds [Fig.3(d)].

Fig. 3. Damage morphology of the samples under single-shot radiation

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3.4.

Discussions

The content of zirconium element of the two kinds of high index materials is dramatic different, and the extinction coefficient of zirconia is relatively high in UV region. The distribution of this kind of impurity is basically homogeneous. In addition, the same deposition chamber and deposition parameters were used during the fabrication process, so we can suppose that distribution of the structural defects in two kinds of samples is identical statistically. Both impurity and structural defects may influence the laser damage resistance of the samples. For the single-shot test, the LIDT of S1 is much lower than that of S2, which is inversed to the impurity content. Identical structural defects distribution causes the same possibility of entering the radiation area and the same possibility of being damaged, so the influence of structural defects on LIDT of the two kinds of samples is equal statistically. However, the difference of the impurity content will decide the difference of laser damage resistance. The higher content of impurity has, the lower the LIDT is. The impurity content of high index material determines the temperature rise difference under laser radiation between the high and low index layers [9, 10], which causes stress to press the adjacent layers and forms the crack structure. For S1 samples, there are relatively high content of impurity and the temperature rise in the high index layers during laser radiation is much higher, which promotes intense stress causing large number of sharp crack structures. As the laser fluence increases, the stress intensified and more crack structures appears. As for S2 sample, low content of impurity causes relatively weak stress and less crack structures. For the multishot test, the radiation fluence is much lower than that in single-shot test and the LIDT have no much difference between two kinds of samples. One possible explanation is that the temperature rise in the high index layers caused by impurity absorption is not very high under relatively lower fluence, but the accumulation of pulse-to-pulse irreversible changes of the structural defects can not be neglected. This kind of accumulation can be seen from experiment process, during laser radiation the scatter of the He-Ne laser enhanced step by step. Under the repetitive laser radiation, the damage process is as following: at first several shots, no visible change was observed, after that, there showed He-Ne laser scattering and the scattering light strengthened as the shot number increased, at last, the visible damage could be observed. In this process, the accumulation of the temperature rise in the multilayer does not exist at such a low repetitive frequency, and the damage must be caused by the accumulation of change of the structural defects. The difference of the impurity content does not influence in LIDT and the main factor influence the laser resistance is accumulation of change of structural defects. The analysis above shows that both the impurity and structural defects will play a role in damage the multilayer in single-shot laser radiation, whereas only structural defects take effects in multishot mode.

4.

Conclusion

For single-shot mode, both impurity and structural defects degrade the laser damage resistance, whereas, the influence of the impurity weakened for multishot radiation, the main damage mechanism is accumulation of changes caused by structural defects. ACKNOWLEDGMENTS The authors thank Ye Liu and Yonghao Jin for support of this study and for fruitful discussions.


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Fig. 4. Depth information of damage zone under single-shot radiation

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