Nanosecond laser pulse induced concentric surface structures on SiO2 layer Wei Sun,1,2 Hongji Qi,1,* Zhou Fang,1,2 Zhenkun Yu,1,2 Yi Liu,3 Kui Yi,1 and Jianda Shao1 1

Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China 2 University of Chinese Academy of Science, Beijing 10049, China 3 Laboratoire d’Optique Appliquée, ENSTA/CNRS/Ecole Polytechnique, 828, Boulevard des Maréchaux, Palaiseau F91762, France * [email protected]

Abstract: We report the periodic concentric surface structures on SiO2 layer induced by a single shot nanosecond laser pulse at 1.06 μm. The fringe period of the structures ranges from 7.0 μm to 26.8 μm, depending on the laser fluence and the distance from central defect precursor. The size and depth of the damage sites increase almost linearly with the laser fluence from 19.6 J/cm2 to 61 J/cm2. Plasma flash was clearly observed during the damage process. We attribute the formation mechanism of the structures to the interference between the reflected laser radiations at the air/shock-front and the shock-front/film interfaces. ©2014 Optical Society of America OCIS codes: (140.3330) Laser damage; (240.0310) Thin films; (260.3160) Interference; (310.6628) Subwavelength structures, nanostructures.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

H. M. van Driel, J. E. Sipe, and J. F. Young, “Laser-induced periodic surface-structure on solids - a universal phenomenon,” Phys. Rev. Lett. 49(26), 1955–1958 (1982). W. N. Han, L. Jiang, X. W. Li, P. J. Liu, L. Xu, and Y. F. Lu, “Continuous modulations of femtosecond laserinduced periodic surface structures and scanned line-widths on silicon by polarization changes,” Opt. Express 21(13), 15505–15513 (2013). Y. Liu, Y. Brelet, Z. B. He, L. W. Yu, B. Forestier, Y. K. Deng, H. B. Jiang, and A. Houard, “Laser-induced periodic annular surface structures on fused silica surface,” Appl. Phys. Lett. 102(25), 251103 (2013). A. Borowiec and H. K. Haugen, “Subwavelength ripple formation on the surfaces of compound semiconductors irradiated with femtosecond laser pulses,” Appl. Phys. Lett. 82(25), 4462–4464 (2003). J. E. Sipe, J. F. Young, J. S. Preston, and H. M. van Driel, “Laser-induced periodic surface structure. I. Theory,” Phys. Rev. B 27(2), 1141–1154 (1983). Q. H. Wu, Y. R. Ma, R. C. Fang, Y. Liao, Q. X. Yu, X. L. Chen, and K. Wang, “Femtosecond laser-induced periodic surface structure on diamond film,” Appl. Phys. Lett. 82(11), 1703–1705 (2003). L. Xue, J. J. Yang, Y. Yang, Y. S. Wang, and X. N. Zhu, “Creation of periodic subwavelength ripples on tungsten surface by ultra-short laser pulses,” Appl. Phys., A Mater. Sci. Process. 109(2), 357–365 (2012). E. L. Gurevich, “On the influence of surface plasmon-polariton waves on pattern formation upon laser ablation,” Appl. Surf. Sci. 278, 52–56 (2013). J. F. Young, J. S. Preston, H. M. van Driel, and J. E. Sipe, “Laser-induced periodic surface structure. II. Ecperiments on Ge, Si, Al, and brass,” Phys. Rev. B 27(2), 1155–1172 (1983). V. N. Tokarev and V. I. Konov, “Suppression of thermocapillary waves in laser melting of metals and semiconductors,” J. Appl. Phys. 76(2), 800–805 (1994). Y. F. Lu, W. K. Choi, Y. Aoyagi, A. Kinomura, and K. Fujii, “Controllable laser induced periodic structures at silicon–dioxide/silicon interface by excimer laser irradiation,” J. Appl. Phys. 80(12), 7052–7056 (1996). Y. F. Lu, J. J. Yu, and W. K. Choi, “Theoretical analysis of laser-induced periodic structures at silicon-dioxide/ silicon and silicon-dioxide/aluminum interfaces,” Appl. Phys. Lett. 71(23), 3439–3440 (1997). J. R. Serrano and D. G. Cahill, “Micron-scale buckling of SiO2 on Si,” J. Appl. Phys. 92(12), 7606–7610 (2002). Y. T. Pu, P. Ma, S. L. Chen, J. L. Zhu, G. Wang, F. Pan, P. Sun, X. H. Zhu, J. G. Zhu, and D. Q. Xiao, “Mechanism for atmosphere dependence of laser damage morphology in HfO2/SiO2 high reflective films,” J. Appl. Phys. 112(2), 023111 (2012). X. F. Liu, D. W. Li, Y. A. Zhao, and X. Li, “Further investigation of the characteristics of nodular defects,” Appl. Opt. 49(10), 1774–1779 (2010).

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Received 19 Nov 2013; revised 26 Dec 2013; accepted 14 Jan 2014; published 31 Jan 2014 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.002948 | OPTICS EXPRESS 2948

16. X. F. Liu, Y. A. Zhao, D. W. Li, G. H. Hu, Y. Q. Gao, Z. X. Fan, and J. D. Shao, “Characteristics of plasma scalds in multilayer dielectric films,” Appl. Opt. 50(21), 4226–4231 (2011). 17. R. Qiu, J. B. Wang, H. Ren, X. H. Li, P. C. Shi, H. Liu, and P. Ma, “Growth of laser-induced damage in fused silica under nanosecond laser irradiation,” High Power Laser Particle Beams 24(5), 1057–1062 (2012). 18. A. Heins and C. L. Guo, “Shock-induced concentric rings in femtosecond laser ablation of glass,” J. Appl. Phys. 113(22), 223506 (2013). 19. S. Papernov and A. W. Schmid, “Two mechanisms of crater formation in ultraviolet-pulsed-laser irradiated SiO2 thin films with artificial defects,” J. Appl. Phys. 97(11), 114906 (2005). 20. K. Sokolowski-Tinten, J. Bialkowski, A. Cavalleri, D. von der Linde, A. Oparin, J. Meyer-ter-Vehn, and S. I. Anisimov, “Transient states of matter during short pulse laser ablation,” Phys. Rev. Lett. 81(1), 224–227 (1998). 21. X. Zeng, X. L. Mao, R. Greif, and R. E. Russo, “Experimental investigation of ablation efficiency and plasma expansion during femtosecond and nanosecond laser ablation of silicon,” Appl. Phys., A Mater. Sci. Process. 80(2), 237–241 (2005). 22. S. Papernov and A. W. Schmid, “Testing asymmetry in plasma-ball growth seeded by a nanoscale absorbing defect embedded in a SiO2 thin-film matrix subjected to UV pulsed-laser radiation,” J. Appl. Phys. 104(6), 063101 (2008). 23. J. Grun, J. Stamper, C. Manka, J. Resnick, R. Burris, J. Crawford, and B. H. Ripin, “Instability of Taylor-Sedov blast waves propagating through a uniform gas,” Phys. Rev. Lett. 66(21), 2738–2741 (1991). 24. G. Taylor, “The formation of a blast wave by a very intense explosion. I. Theoretical discussion,” Proc. R. Soc. Lond. A Math. Phys. Sci. 201(1065), 159–174 (1950).

1. Introduction Laser induced periodic surface structures on the surface of solids have been observed as a universal phenomenon since the early days of laser [1]. The characteristics of the structures, also referred to as ripples, are depended on the material properties and the irradiation conditions, such as wavelength, laser polarization, laser fluence, pulse duration, incident angle and number of the laser pulse irradiation [2,3]. Many researchers have devoted themselves to observing and understanding the structures induced by femtosecond (fs) and nanosecond (ns) lasers. After irradiation with linearly polarized fs laser pulses, most of the ripples tend to be parallel with an equal space, and the period of the ripples is either smaller than or close to the wavelength of the incident laser [4]. The formation mechanism of the ripples has been satisfactorily explained in terms of the interference of the incident laser field with a few kinds of surface scattered waves [5], such as the excited electrons during laser interaction with materials [6], or the surface plasmon polaritonic wave [7]. Recently, E. L. Gurevich has reviewed the formation mechanism of fs induced surface structures in detail [8]. The pulsed ns laser induced surface structures have been observed on semiconductors [9–13], metals [9,10,12], and dielectric substrates [14]. The surface structure has close relationship with the thickness of the film and pulse duration, but is independent of the wavelength and fluence of the incident laser [12]. Under the pulsed laser irradiation, the intrinsic defects or impurities strongly absorb the energy of the incident laser, resulting in locally melting or bending deformation on the surface of substrates. The freezing of surface waves, which are generated on the molten layer, is considered to be the dominant mechanism of the periodic structures formation [11,12]. In this paper, we report the laser pulse induced concentric surface structures (LICSS) on the surface of dielectric SiO2 single layer deposited on the fused silica substrate. Under the irradiation of a single shot ns laser pulse, regular concentric circles together with plasma scald cone craters was generated on the surface of samples. A single defect is always found in the center of the concentric surface structures. The fringe spacing of this LICSS is much larger than the incident laser wavelength, depending on the laser fluence and the distance from the central defect. During the irradiation process, plasma flash was clearly observed by naked eye. The defect precursor on the surface of SiO2 thin film was surrounded by the trace of nonuniform shockwaves. We attribute the formation of this ns-LICSS to the interference of the reflected laser beams on the upper and lower surfaces of laser induced shock front on the surface of SiO2 thin film.

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Received 19 Nov 2013; revised 26 Dec 2013; accepted 14 Jan 2014; published 31 Jan 2014 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.002948 | OPTICS EXPRESS 2949

2. Experiments In our experiments, the fused silica substrates with 50 mm diameter and 5 mm thickness were ultrasonically cleaned before deposition. The single-layer coating was prepared by electronbeam deposition from SiO2 granular materials. The deposition temperature was 140 °C and the deposition rate was 0.6 nm/s. The physical thickness of the SiO2 single layer was about 720 nm, monitored by crystal oscillator system. The laser induced damage threshold (LIDT) testing was performed in the “1-on-1” regime according to the ISO standard 21254-1, using a pulsed Nd:YAG laser operating with a pulse duration of 12 ns at 1064 nm. The experimental setup was detailed in [15]. The mode of the Nd:YAG laser was TEM00 and the laser pulse was focused to an effective area of 0.12 mm2 at 1/e2 of the maximum intensity with a Gaussian beam profile. The sample was mounted on a 3-dimensional translation stage, which allowed movement to a fresh site after each shot. The laser beam irradiated the target plane normally and the separation of two nearby tested sites was 1.2 mm. Twenty sites on the sample surface were exposed to the same laser fluence in each step, and in situ damage detection was monitored with a CCD imaging system. The laser induced damage testing was carried out at several different laser fluences until the range of fluence was sufficiently broad to include the points of 0% damage probability and those of 100% damage probability. The surface morphologies of the damage sites were characterized with a Leica microscope. We also used a field emission scanning electron microscope (FE-SEM, Zeiss Auriga S40) to observe the details of the damage. In order to measure the depth and diameter of the damage points, an Optical Profiler (model: Wyko NT9100) with nanometer Z-height resolution and millimeter fields of view was used. A Veeco Dimension 3100 atomic force microscopy (AFM) with a horizontal and vertical nanometer resolution was also employed to characterize the periodic ripple of the damage sites. 3. Results and discussion 3.1 Morphologies of the damage sites Damage morphologies observed with optical microscope at different laser fluences are presented in Fig. 1. When the laser fluence was changed from 19.6 J/cm2 to 61 J/cm2, regularly concentric circles were always observed. The circles are sparser on both the central and the edge regions and are denser in the middle region of the damage sites. The damage size and depth increase almost linearly with the laser fluence, as shown in Fig. 2. The diameter of damage sites ranges from 207.3 μm to 520.1 μm, which is comparable to the beam size of the incident pulsed laser [Fig. 2(a)]. The corresponding depth of damage sites ranges from 38.1 nm to 79.8 nm, which is far less than the thickness of the film [Fig. 2(b)]. Apparently, the film is not delaminated in the irradiated spots. Furthermore, laser pulse with higher intensities resulted in larger and deeper damage pits, and more concentric circles can be seen under the same magnification of optical microscope.

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Received 19 Nov 2013; revised 26 Dec 2013; accepted 14 Jan 2014; published 31 Jan 2014 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.002948 | OPTICS EXPRESS 2950

Fig. 1. Optical microscopy images of damage sites (incidence angle 0°, 1-on-1 @1064 nm). Laser fluence: (a) 19.6 J/cm2; (b) 26.5 J/cm2; (c) 40.4 J/cm2; (d) 47.1 J/cm2; (e) 53.9 J/cm2; (f) 61 J/cm2. The threshold of the sample is measured to be around 10 J/cm2.

Fig. 2. Diameter (a) and depth (b) of the damage sites as a function of incident laser fluence.

The SEM image of a damage site at the fluence of 47.1 J/cm2 is shown in Fig. 3(a), and the magnified views of the marked rectangles (b)-(d) are shown in Figs. 3(b)–3(d). It should be pointed out that the circular rings exhibit elliptical shape because of the tilt of the sample during the SEM measurements. A molten pit about 5 μm in diameter is located at the center of damage site, implying that the local defect serves as the precursor of damage. With a higher magnification, many tiny pits were observed near the center of the pit. The plasma scald on the surface of SiO2 single layer was shown clearly in Figs. 3(b)–3(d), as observed previously in [16]. It suggests that plasma was formed during the laser irradiation. In addition, Fig. 4 shows the corresponding profile of the damage site in Fig. 3. The damage site with a diameter of 408.5 μm and a depth of 68.8 nm exhibits a concave shape. It should be noticed that the crater is very flat, with an apex angle of 179.96°, as shown in Fig. 3. In addition, the edge of damage site is a little bit higher compared with the surface of the undamaged film, due to the accumulation of the ejected materials.

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Received 19 Nov 2013; revised 26 Dec 2013; accepted 14 Jan 2014; published 31 Jan 2014 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.002948 | OPTICS EXPRESS 2951

Fig. 3. SEM microgram of the damage site at laser fluence of 47.1 J/cm2: (a) Full view of the damage site; (b), (c), and (d): Local magnified views of the marked rectangles (b)-(d) in (a).

Fig. 4. Profile of the damage site at laser fluence of 47.1 J/cm2: (a) diameter X = 408.553 μm; (b) depth Z = 68.8 nm.

3.2 Concentric circles induced by nanosecond laser pulse The spacing of the concentric circles can be determined more accurately with AFM, as shown in Fig. 5. The surface modulation is clearly shown on the plasma scald zone, and the average space of the LICSS is 11.78 μm in this case. The adjacent fringe spacing of damage sites is estimated to be 10.2 μm-19.7 μm for the fluence of 40.4 J/cm2, which corresponds to the damage site in Fig. 1(c).

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Received 19 Nov 2013; revised 26 Dec 2013; accepted 14 Jan 2014; published 31 Jan 2014 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.002948 | OPTICS EXPRESS 2952

Fig. 5. AFM images of the LICSS at laser fluence of 40.4 J/cm2, corresponding to Fig. 1(c) (scan area 50μm × 50μm).

The statistical spacing of the fringes is presented in Fig. 6, corresponding to the six damage sites in Fig. 1. The average spacing changes from 7.0 μm to 26.8 μm for laser fluence ranging from 19.6 J/cm2 to 61 J /cm2. It is apparent that the adjacent fringe spacing of nsLICSS of SiO2 single layer is depended on the intensity of the incident laser.

Fig. 6. Statistical spacing of the LICSS versus fluence of the incident laser.

Up to now, ns laser pulse induced concentric surface structures were once observed by Rong Qiu et al. on the uncoated fused silica. Expanded plasma erosion of the loose polishing layer was considered to be the formation mechanism, but no further investigation was performed [17]. While, recently, fs laser induced concentric rings induced by shockwave was reported by Alan Heins and Chunlei Guo in glass [18]. We herein consider that melting, followed by interference (laser induced shockwave involved) and subsequent re-solidification is excluded as a possible explanation, as we will explain below. As shown above, a molten pit appeared on the center of the damage site. Under the laser irradiation, local defects strongly absorb the energy of incident laser, and some materials may be ejected due to the thermalization of the laser energy, leaving the rest of damage site to a typical concave crater profile with rough surface, as described in [19]. The energy accumulated in the damage region leaves the plasma ablation region molten under such high temperature [20]. As seen from Fig. 3, the ablation region shows a different contrast compared with the undamaged region, and there are many tiny ablation holes in the ablation region. The plasma scalds are obviously involved in laser induced concentric surface structures. In fact, a plasma flash was generally observed during the damage processes of the samples in our experiments. Shadowgraphs of the ns laser induced plasma of silicon in air indicate that the plasma expanded in three dimensions at the similar velocities [21]. That is to say, plasma was generated around defects by the incident laser on the interface between the air and the sample surface, and thus facilitated the crater formation [22]. During the laser irradiation, an expanding plasma is generated due to high temperature, producing a shockwave [23] surrounding the defect precursor on the surface of SiO2 thin film. The refractive index of the shock front can be computed from the density based on the Taylor-Sedov model [24], which

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Received 19 Nov 2013; revised 26 Dec 2013; accepted 14 Jan 2014; published 31 Jan 2014 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.002948 | OPTICS EXPRESS 2953

is different from those of the air and the SiO2 thin film. The refractive index change leads to a high-quality optical interface suitable for producing interference fringes. Therefore, the incident laser will be reflected on the upper and lower surfaces of the shock front. The shock front layer acts as a plano-convex lens. When the incident beam passes through the shock front, the reflected laser radiations on two surfaces of shock front interferes with each other, leading to concentric circular laser intensity modulation. Since the molten of material is closely related with the laser intensity, the interference fringe pattern was recorded on the molten region during the cooling process after laser irradiation. 4. Conclusion In summary, laser induced periodic concentric surface structures were observed on SiO2 single layer after irradiation with single-shot ns laser with the laser fluence ranging from 19.6 J/cm2 to 61 J/cm2. The spatial space of LICSS was measured to be ~7.0 - 26.8 μm in our experiment, much larger than the incident wavelength of 1.06 μm. The plasma scald was clearly shown with SEM and AFM techniques. The interference between the reflected laser radiations at the air/shock-front interface and that at the shock-front/film interface contributes to the formation of the ns-LICSS, which records the laser induced shock wave process of SiO2 dielectric thin film with ns pulse laser.

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Received 19 Nov 2013; revised 26 Dec 2013; accepted 14 Jan 2014; published 31 Jan 2014 10 February 2014 | Vol. 22, No. 3 | DOI:10.1364/OE.22.002948 | OPTICS EXPRESS 2954

Nanosecond laser pulse induced concentric surface structures on SiO2 layer.

We report the periodic concentric surface structures on SiO2 layer induced by a single shot nanosecond laser pulse at 1.06 μm. The fringe period of th...
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