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Laser-induced damage properties of antireflective porous glasses
Optics Communications 285 (2012) 5512–5518
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Laser-induced damage properties of antireflective porous glasses Ying Du a,b, Shijie Liu a, Hongbo He a,n, Yunxia Jin a, Fanyu Kong a,b, Heyuan Guan a,b a b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, No. 390 Qinghe Road, Jiading District, Shanghai, China Graduate School of Chinese Academy of Sciences, Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2012 Received in revised form 9 July 2012 Accepted 25 July 2012 Available online 30 August 2012
Porous nanostructures on BK7 glass manufactured by chemical treatment are able to function as antireflection (AR) components over a wide spectral range and have potential to achieve high laser damage resistance. The damage-resistant properties of antireflective porous glasses with nearly 100% transmittances exposed to pulse lasers with three different central wavelengths were investigated in this paper. The laser damage tests showed that the LIDTs under the irradiation of 12 ns 1064 nm pulses, 10 ns 532 nm pulses and 8 ns 355 nm pulses are 58 J/cm2, 20 J/cm2 and 12 J/cm2, respectively. These values are much higher than those of AR coated glasses, but are almost the same level of un-etched substrate. To understand possible damage mechanisms, the role of electric field distribution inside the porous structure during the laser radiation process was investigated by a three-dimensional finite difference time-domain method. The temperature distribution resulting from the internal electric field may be a main factor to induce the damage. In addition, the involved defects during the etching process are also responsible for the damage in the porous structure. Finally, some possible approaches to improve the LIDT are proposed. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Antireflection Subwavelength nanostructures Laser-induced damage threshold Finite difference time-domain
1. Introduction The development of high energy, or high peak power laser systems heavily rely on high damage threshold designs to maximize output efficiency and reduction of ghost reflections to avoid optical damage within the optical train [1]. Therefore, antireflection (AR) optics with laser resistance compatible to high output power is always indispensable in these laser systems. Conventional AR coatings are easily damaged at relatively low laser energy fluence and thereby severely limit the output power, lifetime, and reliability of the laser system. Additionally, many layers are required in order to widen the wavelength bandwidth, and the coating process is complicated and expensive. Instead, surface relief microstructures offer a possible alternative for high power laser systems [2], which can not only suppress light reflection but also enhance the laser-induced damage threshold (LIDT) of surfaces from an optic glass or window. In recent years, researchers have successfully prepared AR porous structures using chemical or physical methods, and these structures have higher laser damage threshold compared with conventional AR coatings. For high power laser applications, earlier work by Lowdermilk and Milam at LLNL [3], Cook et al. of Schott Glass [4,5] and Douglas S. Hobbs and Bruce D. MacLeod of TelAztec LLC [6–8], showed that AR nanostructures etched in the surface of various
n
Corresponding author. Tel.: þ86 021 6991 8895; fax: þ 86 021 6991 8028. E-mail address: [email protected] (H. He).
glasses had a potential to attain high LIDTs. In 2007, the LIDT tests of various anti-reflecting (AR) nanostructures built in fused silica and other glasses were shown to be up to three times greater than that of single-layer thin-film AR coatings, and at least five times greater than multiple-layer thin-film AR coatings [9]. Even though lots of experimental attempts have been devoted to improve the LIDT of porous glass to a certain extent, the laser damage-resistant mechanisms of AR porous glass, in our knowledge, have not been investigated before, which is essential to understand the damage process and find some other approaches to further improve the damage resistance. In this paper, LIDT tests of porous AR glasses have been conducted at three wavelengths ranging from the UV to NIR (1064, 532, and 355 nm), and compared to the LIDT of un-etched surface. The role of local electric fields (E-field) in the porous nanostructures under the irradiation of 355 nm, 532 nm and 1064 nm pulse laser was investigated using a three-dimensional finite-difference time-domain (3D-FDTD) method. The different laser-induced damage properties of etched glass and un-etched glass, including morphologies of damage sites and LIDT were discussed to reveal the possible damage mechanism.
2. Methodology 2.1. Sample preparation Three millimeter-thick BK7 glasses with a size of 30 30 mm2 were cleaned before being etched by acetone and ethanol under
0030-4018/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.07.120
Y. Du et al. / Optics Communications 285 (2012) 5512–5518
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sonication, which was followed by rinsing with deionized (DI) water. After drying in atmosphere, the glasses were immersed in solution containing Na2HPO4 and AlCl3 where the PH of solution was 7–8. Then the glasses were placed in a water bath (87 1C) for several hours, to obtain nanostructures with different depths on both sides of the glass. Finally, the glass substrates were removed and rinsed with DI water, and then dried in atmosphere. 2.2. Characterization of structures The morphology of the porous glass surface was determined using a Zeiss scanning electron microscope (SEM) (JSM-6360LA). 2.3. Spectra test Transmittance spectra of the glasses were measured using a Lambda 900 spectrophotometer (Perkin Elmer Company). The measurement accuracy of the spectrophotometer was 70.08%. 2.4. Laser-induced damage test LIDT testing has been conducted at three wavelengths ranging from the UV to NIR (1064, 532, and 355 nm). The experimental setup for laser damage is schematically shown in Fig. 1, in which the Nd:YAG laser system is operated at the TEM00 mode and the pulse width is 12 ns at 1064 nm (1o) with a near-Gaussian temporal profile. The LIDTs of the samples were tested in the ‘‘1-on-1’’ mode according to ISO 11254-1 [10]. The laser beam was focused on the target plane normally with 1 mm diameter spot (1/e2) by a nonspherical lens of 250 mm focal length. The laser energy that was used to irradiate the front surface of samples was obtained by the adjustment of the attenuator, and measured by an energy meter from a split-off portion of the beam. The sample was set upon a two-dimensional precision stage driven by a stepper motor. The He–Ne laser was used to help in monitoring the test. The 1o beam was frequency doubled (2o, 532 nm, 10 ns) and tripled (3o, 355 nm, 8 ns) using conversion crystals (LBO and BBO). The 532 nm and 355 nm laser were focused on the target plane normally with 450 mm and 200 mm diameter spot (1/e2), respectively. The damage morphologies were measured by Polarizing Light Microscopy (Leica DMR), and the surface and depth of the damage site were scanned by an atomic force microscope (AFM) (Dimension 3100, VEECO). 2.5. 3d-FDTD model for porous nanostructures In this work, we use 3D-FDTD simulations to investigate the effects of AR nanostructures on electric field (E-field) distribution of the front surface of the substrates. When an antireflective surface microstructure has a period much smaller than the light
Fig. 2. Scheme of the simulated structure (a) x–z plane view and (b) x–y plane view.
wavelength, i.e. in the quasi-static limit (the period-to-wavelength ratio L/l-0), zerothorder effective medium theory can be utilized to analyze its optical properties. The effective medium theory is formulated by making static approximations that entail assuming that electromagnetic fields do not vary spatially within a given material because of small L/l [11]. The reflectivity does not depend on the structure feature size. Then the periodicity of surface nanostructures would be less important to their performance. For these reasons, simulation of porous AR surface nanostructures can be done on the periodic nanostructures with similar feature sizes. In the work, nanostructures with periodic cylinders are simulated in order to investigate the role of E-field distribution inside nanostructures. Fig. 2 illustrates the model of nanostructures on single-side glass surface that we simulate. The E-field distribution for nanostructures on glass surface is calculated using the FDTD method [12]. For AR nanostructures integrated glass, the reflectance of front surface and back surface of the glass are small, which indicates that the E-field distribution of the front surface is hardly affected by that of the back surface. On the other hand, since the FDTD has finite analysis windows, an artificial boundary condition suppressing reflections at the analysis windows is required. In the FDTD simulation, absorbing boundary conditions are needed to truncate the computation domain without reflection [13]. Therefore, the simulation model is designed for singleside etched substrates. The models were conducted using a single hole as the unit cell, with the periodic boundary conditions applied to the x- and y-directions to describe an infinite square array and uniaxial perfectly matched layers along the z-direction [14]. The source of the input plane wave pulse was placed 70 nm above the top of nanoholes and polarized to the x-axis. To obtain the E-field distribution along the z-direction, the x–y plane monitors were placed at the interface between air and the porous structure. The monitors for the y–z and x–z planes were placed across the center of the nanohole. The E-field distribution was reported as the amplitude (9E9) normalized by the amplitude E0 of incident wave, i.e., [E(z)/E0], in W/m2.
3. Results 3.1. Morphology
Fig. 1. Experimental setup of laser damage testing.
Our previous works have focused on optical properties as functions of etching thickness, and structure geometries of porous glasses [15]. Our analysis on the morphology of nanostructures also showed that the density and height of nanostructures on the glass surface become larger with etching time. One of typical SEM images was shown in Fig. 3, and the feature size of nanostructures
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Fig. 3. SEM images of glasses etched for 26 h (a) Elevation view of porous structure and (b) cross-section view of porous structure [15].
Table 1 Samples for LIDT test. Sample
Etching time (h)
Etching thickness (nm)
Wavelength of max. transmittance (nm)
Max. transmittance (%)
Sample 1 Sample 2 Sample 3
12 20 25
50–60 150–180 230–270
395 543 1146
99.35 99.76 99.61
Fig. 4. Transmittance spectra of three samples.
could be extracted from the morphology of the etched glass surface by scanning electron microscope (SEM). As shown in Fig. 3(a), nano-sized pores are formed on both sides of the glass substrate, and the feature sizes of nanostructures vary from 30 nm to 60 nm. The cross-section view of nanostructures is also shown in Fig. 3(b), where the height of nanostructures is about 260 nm.
Fig. 5. Bar chart comparing the LIDT results for the three tested samples.
3.2. Optical performance The transmittance spectra of double-side nanostructures integrated glasses in the spectrum range from 300 nm to 1400 nm are shown in Fig. 4, where the etching times of three samples are 12 h, 20 h and 25 h, respectively. It is seen that more than 92% transmittances can be obtained in the measurement spectrum range and the transmittance curves show parabolic shapes when etching time is less than 20 h. When the etching time increases up to 25 h, more than two peaks resulting from the multiple interference of laser pulse can be observed in the transmittance curve. Moreover, the wavelength at the maximum transmittance of each curve is 395 nm, 543 nm and 1146 nm, respectively, which is approximately linearly proportional to the etching time or etching thickness, as shown in Table 1. These three samples were chosen for LIDT tests at 355 nm, 532 nm and 1064 nm, respectively, where the samples still have good antireflection performances, as shown in Fig. 4. 3.3. Laser-induced damage threshold and morphology Fig. 5 shows the LIDTs of three test samples under the irradiation of 8 ns 355 nm pulses, 10 ns 532 nm pulses and
12 ns 1064 nm pulses, respectively. The LIDTs of blank substrate and multilayer coatings are also shown in Fig. 5. As is seen, the LIDT of AR porous is 2 3 times higher than that of HfO2/SiO2 AR coatings [16], but almost the same level of un-etched BK7 substrate. The LIDTs for three kinds of samples tend to decrease with the reduction of wavelength from 1064 nm to 355 nm. Moreover, there was a remarkable difference of LIDT between etched glass and un-etched glass at 355 nm test. In comparison, the difference at 532 nm is smaller, and there is almost no difference of LIDT between etched glass and un-etched glass at 1064 nm test. The morphologies of most damage sites in porous glass are similar in 532 nm and 1064 nm test, and the size of damage sites is between 5 and 100 mm. The typical damage morphology and depth on etched glass surface observed with a VEECO Dimension 3100 atomic force microscope (AFM) is shown in Fig. 6. As is seen, an ellipse shape of damage site in about 100 mm diameter can be observed on the surface of porous glass. Its depth was 4 mm, as Fig. 6(b). Typical damage morphology on the un-etched glass substrate was also observed. In Fig. 7(a), the morphology of damage site
Y. Du et al. / Optics Communications 285 (2012) 5512–5518
was a pinpoint with many cracks around it. However, the measured depth as illustrated in Fig. 7(b): showed that the cracks were inside the bulk of substrate, and the depth of the hole was only 100 nm, while there were a lot of particles around the edge of the hole. 3.4. Simulation results of E-field distribution inside nanostructures The mechanisms which cause laser damage in optical materials are numerous and have been reviewed by other authors [17–19]. When laser irradiates on porous surface, local temperature of material raise and thermal stress distribution forms accordingly. As a result, the damage may happen because of local temperature exceeding melting point or local stress exceeding tensile strength [20]. The maximum temperature rise, DTmax, in a material due to laser irradiation is [21] 2AF
DT max ¼ constant pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
pK rC tp
Table 2 List of simulated parameters for sample 3. H¼height of the cylinders, D ¼diameter of the cylinders, L ¼ period of cylinders. Sample
Laser wavelength (nm)
H (nm)
D (nm)
L (nm)
Sample 3
1064
250
30–50 30–60 30–60 30–60
50 100 150 200
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where A¼absorption coefficient, F¼laser fluence, K¼thermal conductivity, r ¼density, C¼ specific heat, tp¼ FWHM pulse duration. The amount of energy absorbed to produce damage is determined by the intensity of light and hence by the E-field intensity [18]. It has been reported that the distribution of the temperature was similar to that of its electric field distribution [22]. Therefore, simulation of electric field is helpful to understand the weakest point in the porous nanostructure for LIDT tests. We simulated the E-field distribution of periodic nanostructures on the top of nanoholes (z¼0.1 mm) as a function of the structure diameter as well as the period at a wavelength of 1064 nm (normal incidence). The period of the cylinder was varied from 50 nm to 200 nm in steps of 50 nm and the calculation has been carried out with different diameter of cylinders (30–60 nm) as shown in Table 2. As shown in Fig. 8, the similar local E-field amplitude can be obtained with the different period and diameter of nanostructures, which verifies that when AR microstructures have a period much smaller than the light wavelength, the periodicity of surface nanostructures would be less important to their performance. Not only the period but also the diameter of nanostructures has little effect on the peak E-field amplitude. However, the results show the peak E-field amplitude will continue slowly to grow when the period of nanostructures becomes small. In order to understand the effect of the distribution of the nanostructures on the E-field distribution of porous glass surface, all the nanohole arrays were maintained the same diameter (50 nm). The calculation result of sample 2 (D ¼50 nm, L ¼50 nm, and H¼150 nm) at 532 nm wavelength, [E(z)/E0], was shown in Fig. 9.
Fig. 6. Typical damage morphologies of etched glass for 10 ns 532 nm pulses test. The incident pulse fluence is 25.2 J/cm2. (a) The surface of the damage site and (b) the depth of the damage site.
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Fig. 7. Typical damage morphologies of un-etched glass for 10 ns 532 nm pulses test. The incident pulse fluence is 25.2 J/cm2. (a) The surface of the damage site and (b) the depth of the damage site.
Fig. 9. E-field distribution of sample 2 when D ¼ 50 nm, L ¼50 nm at x–y plane view (z ¼0.1 mm).
Fig. 8. Normalized peak E-field amplitude variation of three samples with 50–200 nm period.
From Fig. 9, one can observe that the strong E-field can be generated inside the holes, and the stronger electric fields are always located at the edge of the nanostructures in the air for all depth and period investigated. To verify the trends of LIDT results
of three samples (Fig. 5), all the nanohole arrays were maintained the same diameter (50 nm) while the period was varies from 50 to 200 nm (Table 3), and the etching depth of sample 1 is determined to be 50 nm, and that of sample 2 and sample 3 is 150 nm, 250 nm, respectively. The peak E-fields at the x–y plane on top of nanoholes (z ¼0.1 mm) in three typical sample depths and four typical period (Table 3) are plotted in Fig. 10 for etched glass, and compared with that of un-etched glass.
Y. Du et al. / Optics Communications 285 (2012) 5512–5518
4. Discussion To test the LIDT at three wavelengths ranging from the UV to NIR (1064, 532, and 355 nm) of porous surface, three samples were chosen with high transmittance at respective laser wavelength, and high damage resistance of glasses was observed from Fig. 5. As expected, the LIDT decreases when decreasing the wavelength. This is mainly due to the decrease of extinction coefficient with the increase of wavelength, thus the absorption of laser decreases and the LIDT increases [23]. Meanwhile, the LIDT of AR porous glass is much higher than that of AR coatings, because the LIDTs of AR coatings are limited by the interface effect inside two layers, such as thermal transformation, stress mismatching. Moreover, there was a remarkable difference of LIDT between etched glass and un-etched glass at 355 nm test, and at 532 nm the difference is smaller. In comparison, there is almost no difference of LIDT between etched glass and un-etched
Table 3 List of simulated parameters for samples. H ¼height of the cylinders, D ¼diameter of the cylinders, L ¼period of cylinders. Sample
Laser wavelength (nm)
H (nm)
D (nm)
L (nm)
Sample 1 Sample 2 Sample 3
355 532 1064
50 150 250
50
50–200
Fig. 10. Normalized E-field amplitude variation of three samples with 50–200 nm period.
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glass at 1064 nm test. This indicates that the inherent characteristics of materials become more noticeable after the etching process. The intrinsic absorption plays a dominant role in damage mechanism in short wavelength, and impurity defect becomes the major cause of damage in long wavelength [23]. From the simulation results in Fig. 10, the maximum local E-field amplitude on un-etched substrate surfaces were always lower than that of etched glass. This agrees well with the values predicted by the LIDT result from Fig. 5. In Fig. 6(a), the laser damage morphology of a damage site on the porous surface shows an ellipse shape, which displays that some defect points appear in the damage areas, and the damage enlarges from these points. Fig. 6(b) shows the damage morphology of un-etched glass. The shapes of the damage sites on etched glass and un-etched glass show the intrinsic morphology of materials have changed after the etching process, and indicate homogeneous distribution of intrinsic defects within laser spot on etched glass in contrary to the un-etched surface that includes single defects initiating the damage. However, the depth of laser damage site was much higher than that of etched nanostructure, which illustrated the subsurface defect and impurities introduced during the etching process had a notably significant effect on the LIDT. The cracks observed on the un-etched substrate are associated with internal thermal stress induced by partially irradiation of a laser beam. The simulation results in Fig. 10 show that the peak E-field amplitudes of un-etched glass at 1064 nm, 532 nm, 355 nm tests are 0.799, 0.8, 0.801, respectively. When the period of nanostructures is 50 nm, 100 nm and on un-etched glass, as the wavelength is decreased from 1064 nm, 532 nm to 355 nm, the peak local E-fields increase and the difference between the etched glass and un-etched glass is larger; this has an agreement with the LIDT result as Fig. 5. In particular, the simulation data for periods 150 nm and 200 nm shows that the peak local E-fields at 532 nm test is the maximum, which contradicts with the experimental data. A fundamental reason is that the etching can result in multiple defects inside the porous layer which can reduce the damage threshold. Moreover, Al3 þ was verified to be introduced during the etching process by our previous work, shown in Fig. 11, which supported the conclusion. Possible ways to improve the LIDT: a) Decreasing subsurface on substrate by HF etching is in favor of improving LIDT. b) The methods to improve LIDT mainly focus on how to reduce the defect types and density. These kinds of methods rely on
Fig. 11. EDS spectrum of glass surface. (a) Elements in glass surface before etching and (b) elements in glass surface after etching.
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substrate materials, preparation equipment and preparation technology. Thus, reducing multiple defects inside the porous layer during etching process can improve the LIDT. c) Laser conditioning can be useful for AR porous surface’s LIDT improvement [23]. d) Annealing of etched glass is favorable to diminish the point defects introduced by etching [23].
5. Conclusion Surface nanostructures etched directly into the bulk BK7 substrate show a LIDT at least two times greater than frequently-used multilayer thin-film AR coatings, but are almost the same level of un-etched substrate at three wavelengths ranging from the UV to NIR (1064, 532, and 355 nm). The results show that the LIDT increases with the wavelength increasing. The shapes of the damage sites indicate homogeneous distribution of intrinsic defects within laser spot on etched glass in contrary to the un-etched surface that includes single defects initiating the damage. From the simulation results of various nanostructures, one can find out that changing the period and diameter of nanostructures on glass surface has little effect on the peak E-field when microstructure has a period much smaller than the light wavelength. The maximum local E-field amplitude on unetched substrate surfaces was always lower than that of etched glass. This agrees well with the values predicted by the LIDT result. There is almost no difference of maximum E-field amplitude at wavelength 1064 and 355 nm, which contradicts with the LIDT, and this indicates that impurities or defects are introduced after etching. It is possible to provide the proper preparation technology to reduce defects for LIDT improvement purpose. We plan to keep fabricating and testing AR surfaces on various materials, such as fused silica and crystal materials, for use at different wavelengths.
Acknowledgments The authors gratefully acknowledge Yun Cui and Qilin Xiao for their measurements of optical properties and morphology, and
Shunli Chen and Fanyu Kong for LIDT test. This work is supported by the National Natural Science Foundation of China (Grant no. 11104293).
References [1] J.H. Bohn, et al., in: S. Dodorico (Ed.), Optical Astronomical Instrumentation, Parts 1 and 2, Proceedings of the Society of Photo-optical Instrumentation Engineers (SPIE), vol. 3355, 1998, 962 pp. [2] D.S. Hobbs, B.D. MacLeod, in: R.W. Tustison (Ed.), Window and Dome Technologies and Materials IX, Proceedings of the Society of Photo-optical Instrumentation Engineers (SPIE), 5786, 2005, pp. 349–364. [3] W.H. Lowdermilk, D. Milam, Applied Physics Letters 36 (1980) 891. [4] L.M. Cook, et al., Journal of the American Ceramic Society 65 (1982) C152. [5] L.M. Cook, et al., Applied Optics 21 (1982) 1482. [6] D.S. Hobbs, B.D. MacLeod, in: Exarhos, GJ; Guenther, AH; Lewis, KL (Ed.), Laser-Induced Damage in Optical Materials; 2007 (6720), 2008 L7200. [7] D.S. Hobbs, et al., in: R.W. Tustison (Ed.), Window and Dome Technologies and Materials X 6545 (2007) Y5450. [8] D.S. Hobbs, et al., in: R.W. Tustison (Ed.), Proceedings of the SPIE—The International Society for Optical Engineering 8016 (2011) 01. [9] D.S. Hobbs, Laser-Induced Damage in Optical Materials: 2008 7132 (2008) 71321K1. [10] ISO 11254-1:2000, Lasers and Laser-Related Equipment-Determination of Laser-Induced Damage Threshold of Optical Surfaces—Part 1: 1-on-1 test. [11] X.F. Jing, et al., Optics Express 17 (2009) 16119. [12] D.M. Sullivan, IEEE Microwave Theory and Techniques Society, IEEE Press, New York, 2000. [13] C.J. Ting, et al., Journal of Micromechanics and Microengineering 18 (2008) 075001 (9 pp). [14] J.J. Xu, et al., Optics Express 19 (2011) 20493. [15] Y. Du, et al., Optik - International Journal for Light and Electron Optics, submitted for publication. [16] Y.N. Zhao, et al., Applied Surface Science 245 (2005) 335. [17] B.E. Yoldas, et al., Journal of Applied Physics 57 (1985) 2455. [18] M.L. Protopapa, et al., Alt’99 International Conference on Advanced Laser Technologies 4070 (2000) 380. [19] B. Cho, et al., in: Exarhos, GJ; Gruzdev, VE; Menapace, JA (Ed.). Laser-Induced Damage in Optical Materials: 2010, Proceedings of SPIE–The International Society for Optical Engineering, vol. 7842, (2010) 78420B. [20] Z. Xia, et al., Optics Communications 285 (2012) 70. [21] B. Cho, et al., in: G. J.G.V.E.M.J.A.R.D.S.M.J. Exarhos (Eds.). Laser -Induced Damage in Optical Materials: 2010, Proceedings of the SPIE–The International Society for Optical Engineering, vol. 7842, 2010. [22] C.J. Stolz, et al., Effect of SiO2 overcoat thickness on laser damage morphology of HfO2/SiO2 Brewster’s angle polarizers at 1064 nm, vol. 2966, 1997. [23] C. Xu, et al., Chinese Physics Letters 27 (2010) 114205.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Laser-induced damage properties of antireflective porous glasses Ying Du a,b, Shijie Liu a, Hongbo He a,n, Yunxia Jin a, Fanyu Kong a,b, Heyuan Guan a,b a b
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, No. 390 Qinghe Road, Jiading District, Shanghai, China Graduate School of Chinese Academy of Sciences, Beijing, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 March 2012 Received in revised form 9 July 2012 Accepted 25 July 2012 Available online 30 August 2012
Porous nanostructures on BK7 glass manufactured by chemical treatment are able to function as antireflection (AR) components over a wide spectral range and have potential to achieve high laser damage resistance. The damage-resistant properties of antireflective porous glasses with nearly 100% transmittances exposed to pulse lasers with three different central wavelengths were investigated in this paper. The laser damage tests showed that the LIDTs under the irradiation of 12 ns 1064 nm pulses, 10 ns 532 nm pulses and 8 ns 355 nm pulses are 58 J/cm2, 20 J/cm2 and 12 J/cm2, respectively. These values are much higher than those of AR coated glasses, but are almost the same level of un-etched substrate. To understand possible damage mechanisms, the role of electric field distribution inside the porous structure during the laser radiation process was investigated by a three-dimensional finite difference time-domain method. The temperature distribution resulting from the internal electric field may be a main factor to induce the damage. In addition, the involved defects during the etching process are also responsible for the damage in the porous structure. Finally, some possible approaches to improve the LIDT are proposed. Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved.
Keywords: Antireflection Subwavelength nanostructures Laser-induced damage threshold Finite difference time-domain
1. Introduction The development of high energy, or high peak power laser systems heavily rely on high damage threshold designs to maximize output efficiency and reduction of ghost reflections to avoid optical damage within the optical train [1]. Therefore, antireflection (AR) optics with laser resistance compatible to high output power is always indispensable in these laser systems. Conventional AR coatings are easily damaged at relatively low laser energy fluence and thereby severely limit the output power, lifetime, and reliability of the laser system. Additionally, many layers are required in order to widen the wavelength bandwidth, and the coating process is complicated and expensive. Instead, surface relief microstructures offer a possible alternative for high power laser systems [2], which can not only suppress light reflection but also enhance the laser-induced damage threshold (LIDT) of surfaces from an optic glass or window. In recent years, researchers have successfully prepared AR porous structures using chemical or physical methods, and these structures have higher laser damage threshold compared with conventional AR coatings. For high power laser applications, earlier work by Lowdermilk and Milam at LLNL [3], Cook et al. of Schott Glass [4,5] and Douglas S. Hobbs and Bruce D. MacLeod of TelAztec LLC [6–8], showed that AR nanostructures etched in the surface of various
n
Corresponding author. Tel.: þ86 021 6991 8895; fax: þ 86 021 6991 8028. E-mail address: [email protected] (H. He).
glasses had a potential to attain high LIDTs. In 2007, the LIDT tests of various anti-reflecting (AR) nanostructures built in fused silica and other glasses were shown to be up to three times greater than that of single-layer thin-film AR coatings, and at least five times greater than multiple-layer thin-film AR coatings [9]. Even though lots of experimental attempts have been devoted to improve the LIDT of porous glass to a certain extent, the laser damage-resistant mechanisms of AR porous glass, in our knowledge, have not been investigated before, which is essential to understand the damage process and find some other approaches to further improve the damage resistance. In this paper, LIDT tests of porous AR glasses have been conducted at three wavelengths ranging from the UV to NIR (1064, 532, and 355 nm), and compared to the LIDT of un-etched surface. The role of local electric fields (E-field) in the porous nanostructures under the irradiation of 355 nm, 532 nm and 1064 nm pulse laser was investigated using a three-dimensional finite-difference time-domain (3D-FDTD) method. The different laser-induced damage properties of etched glass and un-etched glass, including morphologies of damage sites and LIDT were discussed to reveal the possible damage mechanism.
2. Methodology 2.1. Sample preparation Three millimeter-thick BK7 glasses with a size of 30 30 mm2 were cleaned before being etched by acetone and ethanol under
0030-4018/$ - see front matter Crown Copyright & 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.07.120
Y. Du et al. / Optics Communications 285 (2012) 5512–5518
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sonication, which was followed by rinsing with deionized (DI) water. After drying in atmosphere, the glasses were immersed in solution containing Na2HPO4 and AlCl3 where the PH of solution was 7–8. Then the glasses were placed in a water bath (87 1C) for several hours, to obtain nanostructures with different depths on both sides of the glass. Finally, the glass substrates were removed and rinsed with DI water, and then dried in atmosphere. 2.2. Characterization of structures The morphology of the porous glass surface was determined using a Zeiss scanning electron microscope (SEM) (JSM-6360LA). 2.3. Spectra test Transmittance spectra of the glasses were measured using a Lambda 900 spectrophotometer (Perkin Elmer Company). The measurement accuracy of the spectrophotometer was 70.08%. 2.4. Laser-induced damage test LIDT testing has been conducted at three wavelengths ranging from the UV to NIR (1064, 532, and 355 nm). The experimental setup for laser damage is schematically shown in Fig. 1, in which the Nd:YAG laser system is operated at the TEM00 mode and the pulse width is 12 ns at 1064 nm (1o) with a near-Gaussian temporal profile. The LIDTs of the samples were tested in the ‘‘1-on-1’’ mode according to ISO 11254-1 [10]. The laser beam was focused on the target plane normally with 1 mm diameter spot (1/e2) by a nonspherical lens of 250 mm focal length. The laser energy that was used to irradiate the front surface of samples was obtained by the adjustment of the attenuator, and measured by an energy meter from a split-off portion of the beam. The sample was set upon a two-dimensional precision stage driven by a stepper motor. The He–Ne laser was used to help in monitoring the test. The 1o beam was frequency doubled (2o, 532 nm, 10 ns) and tripled (3o, 355 nm, 8 ns) using conversion crystals (LBO and BBO). The 532 nm and 355 nm laser were focused on the target plane normally with 450 mm and 200 mm diameter spot (1/e2), respectively. The damage morphologies were measured by Polarizing Light Microscopy (Leica DMR), and the surface and depth of the damage site were scanned by an atomic force microscope (AFM) (Dimension 3100, VEECO). 2.5. 3d-FDTD model for porous nanostructures In this work, we use 3D-FDTD simulations to investigate the effects of AR nanostructures on electric field (E-field) distribution of the front surface of the substrates. When an antireflective surface microstructure has a period much smaller than the light
Fig. 2. Scheme of the simulated structure (a) x–z plane view and (b) x–y plane view.
wavelength, i.e. in the quasi-static limit (the period-to-wavelength ratio L/l-0), zerothorder effective medium theory can be utilized to analyze its optical properties. The effective medium theory is formulated by making static approximations that entail assuming that electromagnetic fields do not vary spatially within a given material because of small L/l [11]. The reflectivity does not depend on the structure feature size. Then the periodicity of surface nanostructures would be less important to their performance. For these reasons, simulation of porous AR surface nanostructures can be done on the periodic nanostructures with similar feature sizes. In the work, nanostructures with periodic cylinders are simulated in order to investigate the role of E-field distribution inside nanostructures. Fig. 2 illustrates the model of nanostructures on single-side glass surface that we simulate. The E-field distribution for nanostructures on glass surface is calculated using the FDTD method [12]. For AR nanostructures integrated glass, the reflectance of front surface and back surface of the glass are small, which indicates that the E-field distribution of the front surface is hardly affected by that of the back surface. On the other hand, since the FDTD has finite analysis windows, an artificial boundary condition suppressing reflections at the analysis windows is required. In the FDTD simulation, absorbing boundary conditions are needed to truncate the computation domain without reflection [13]. Therefore, the simulation model is designed for singleside etched substrates. The models were conducted using a single hole as the unit cell, with the periodic boundary conditions applied to the x- and y-directions to describe an infinite square array and uniaxial perfectly matched layers along the z-direction [14]. The source of the input plane wave pulse was placed 70 nm above the top of nanoholes and polarized to the x-axis. To obtain the E-field distribution along the z-direction, the x–y plane monitors were placed at the interface between air and the porous structure. The monitors for the y–z and x–z planes were placed across the center of the nanohole. The E-field distribution was reported as the amplitude (9E9) normalized by the amplitude E0 of incident wave, i.e., [E(z)/E0], in W/m2.
3. Results 3.1. Morphology
Fig. 1. Experimental setup of laser damage testing.
Our previous works have focused on optical properties as functions of etching thickness, and structure geometries of porous glasses [15]. Our analysis on the morphology of nanostructures also showed that the density and height of nanostructures on the glass surface become larger with etching time. One of typical SEM images was shown in Fig. 3, and the feature size of nanostructures
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Fig. 3. SEM images of glasses etched for 26 h (a) Elevation view of porous structure and (b) cross-section view of porous structure [15].
Table 1 Samples for LIDT test. Sample
Etching time (h)
Etching thickness (nm)
Wavelength of max. transmittance (nm)
Max. transmittance (%)
Sample 1 Sample 2 Sample 3
12 20 25
50–60 150–180 230–270
395 543 1146
99.35 99.76 99.61
Fig. 4. Transmittance spectra of three samples.
could be extracted from the morphology of the etched glass surface by scanning electron microscope (SEM). As shown in Fig. 3(a), nano-sized pores are formed on both sides of the glass substrate, and the feature sizes of nanostructures vary from 30 nm to 60 nm. The cross-section view of nanostructures is also shown in Fig. 3(b), where the height of nanostructures is about 260 nm.
Fig. 5. Bar chart comparing the LIDT results for the three tested samples.
3.2. Optical performance The transmittance spectra of double-side nanostructures integrated glasses in the spectrum range from 300 nm to 1400 nm are shown in Fig. 4, where the etching times of three samples are 12 h, 20 h and 25 h, respectively. It is seen that more than 92% transmittances can be obtained in the measurement spectrum range and the transmittance curves show parabolic shapes when etching time is less than 20 h. When the etching time increases up to 25 h, more than two peaks resulting from the multiple interference of laser pulse can be observed in the transmittance curve. Moreover, the wavelength at the maximum transmittance of each curve is 395 nm, 543 nm and 1146 nm, respectively, which is approximately linearly proportional to the etching time or etching thickness, as shown in Table 1. These three samples were chosen for LIDT tests at 355 nm, 532 nm and 1064 nm, respectively, where the samples still have good antireflection performances, as shown in Fig. 4. 3.3. Laser-induced damage threshold and morphology Fig. 5 shows the LIDTs of three test samples under the irradiation of 8 ns 355 nm pulses, 10 ns 532 nm pulses and
12 ns 1064 nm pulses, respectively. The LIDTs of blank substrate and multilayer coatings are also shown in Fig. 5. As is seen, the LIDT of AR porous is 2 3 times higher than that of HfO2/SiO2 AR coatings [16], but almost the same level of un-etched BK7 substrate. The LIDTs for three kinds of samples tend to decrease with the reduction of wavelength from 1064 nm to 355 nm. Moreover, there was a remarkable difference of LIDT between etched glass and un-etched glass at 355 nm test. In comparison, the difference at 532 nm is smaller, and there is almost no difference of LIDT between etched glass and un-etched glass at 1064 nm test. The morphologies of most damage sites in porous glass are similar in 532 nm and 1064 nm test, and the size of damage sites is between 5 and 100 mm. The typical damage morphology and depth on etched glass surface observed with a VEECO Dimension 3100 atomic force microscope (AFM) is shown in Fig. 6. As is seen, an ellipse shape of damage site in about 100 mm diameter can be observed on the surface of porous glass. Its depth was 4 mm, as Fig. 6(b). Typical damage morphology on the un-etched glass substrate was also observed. In Fig. 7(a), the morphology of damage site
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was a pinpoint with many cracks around it. However, the measured depth as illustrated in Fig. 7(b): showed that the cracks were inside the bulk of substrate, and the depth of the hole was only 100 nm, while there were a lot of particles around the edge of the hole. 3.4. Simulation results of E-field distribution inside nanostructures The mechanisms which cause laser damage in optical materials are numerous and have been reviewed by other authors [17–19]. When laser irradiates on porous surface, local temperature of material raise and thermal stress distribution forms accordingly. As a result, the damage may happen because of local temperature exceeding melting point or local stress exceeding tensile strength [20]. The maximum temperature rise, DTmax, in a material due to laser irradiation is [21] 2AF
DT max ¼ constant pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
pK rC tp
Table 2 List of simulated parameters for sample 3. H¼height of the cylinders, D ¼diameter of the cylinders, L ¼ period of cylinders. Sample
Laser wavelength (nm)
H (nm)
D (nm)
L (nm)
Sample 3
1064
250
30–50 30–60 30–60 30–60
50 100 150 200
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where A¼absorption coefficient, F¼laser fluence, K¼thermal conductivity, r ¼density, C¼ specific heat, tp¼ FWHM pulse duration. The amount of energy absorbed to produce damage is determined by the intensity of light and hence by the E-field intensity [18]. It has been reported that the distribution of the temperature was similar to that of its electric field distribution [22]. Therefore, simulation of electric field is helpful to understand the weakest point in the porous nanostructure for LIDT tests. We simulated the E-field distribution of periodic nanostructures on the top of nanoholes (z¼0.1 mm) as a function of the structure diameter as well as the period at a wavelength of 1064 nm (normal incidence). The period of the cylinder was varied from 50 nm to 200 nm in steps of 50 nm and the calculation has been carried out with different diameter of cylinders (30–60 nm) as shown in Table 2. As shown in Fig. 8, the similar local E-field amplitude can be obtained with the different period and diameter of nanostructures, which verifies that when AR microstructures have a period much smaller than the light wavelength, the periodicity of surface nanostructures would be less important to their performance. Not only the period but also the diameter of nanostructures has little effect on the peak E-field amplitude. However, the results show the peak E-field amplitude will continue slowly to grow when the period of nanostructures becomes small. In order to understand the effect of the distribution of the nanostructures on the E-field distribution of porous glass surface, all the nanohole arrays were maintained the same diameter (50 nm). The calculation result of sample 2 (D ¼50 nm, L ¼50 nm, and H¼150 nm) at 532 nm wavelength, [E(z)/E0], was shown in Fig. 9.
Fig. 6. Typical damage morphologies of etched glass for 10 ns 532 nm pulses test. The incident pulse fluence is 25.2 J/cm2. (a) The surface of the damage site and (b) the depth of the damage site.
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Fig. 7. Typical damage morphologies of un-etched glass for 10 ns 532 nm pulses test. The incident pulse fluence is 25.2 J/cm2. (a) The surface of the damage site and (b) the depth of the damage site.
Fig. 9. E-field distribution of sample 2 when D ¼ 50 nm, L ¼50 nm at x–y plane view (z ¼0.1 mm).
Fig. 8. Normalized peak E-field amplitude variation of three samples with 50–200 nm period.
From Fig. 9, one can observe that the strong E-field can be generated inside the holes, and the stronger electric fields are always located at the edge of the nanostructures in the air for all depth and period investigated. To verify the trends of LIDT results
of three samples (Fig. 5), all the nanohole arrays were maintained the same diameter (50 nm) while the period was varies from 50 to 200 nm (Table 3), and the etching depth of sample 1 is determined to be 50 nm, and that of sample 2 and sample 3 is 150 nm, 250 nm, respectively. The peak E-fields at the x–y plane on top of nanoholes (z ¼0.1 mm) in three typical sample depths and four typical period (Table 3) are plotted in Fig. 10 for etched glass, and compared with that of un-etched glass.
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4. Discussion To test the LIDT at three wavelengths ranging from the UV to NIR (1064, 532, and 355 nm) of porous surface, three samples were chosen with high transmittance at respective laser wavelength, and high damage resistance of glasses was observed from Fig. 5. As expected, the LIDT decreases when decreasing the wavelength. This is mainly due to the decrease of extinction coefficient with the increase of wavelength, thus the absorption of laser decreases and the LIDT increases [23]. Meanwhile, the LIDT of AR porous glass is much higher than that of AR coatings, because the LIDTs of AR coatings are limited by the interface effect inside two layers, such as thermal transformation, stress mismatching. Moreover, there was a remarkable difference of LIDT between etched glass and un-etched glass at 355 nm test, and at 532 nm the difference is smaller. In comparison, there is almost no difference of LIDT between etched glass and un-etched
Table 3 List of simulated parameters for samples. H ¼height of the cylinders, D ¼diameter of the cylinders, L ¼period of cylinders. Sample
Laser wavelength (nm)
H (nm)
D (nm)
L (nm)
Sample 1 Sample 2 Sample 3
355 532 1064
50 150 250
50
50–200
Fig. 10. Normalized E-field amplitude variation of three samples with 50–200 nm period.
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glass at 1064 nm test. This indicates that the inherent characteristics of materials become more noticeable after the etching process. The intrinsic absorption plays a dominant role in damage mechanism in short wavelength, and impurity defect becomes the major cause of damage in long wavelength [23]. From the simulation results in Fig. 10, the maximum local E-field amplitude on un-etched substrate surfaces were always lower than that of etched glass. This agrees well with the values predicted by the LIDT result from Fig. 5. In Fig. 6(a), the laser damage morphology of a damage site on the porous surface shows an ellipse shape, which displays that some defect points appear in the damage areas, and the damage enlarges from these points. Fig. 6(b) shows the damage morphology of un-etched glass. The shapes of the damage sites on etched glass and un-etched glass show the intrinsic morphology of materials have changed after the etching process, and indicate homogeneous distribution of intrinsic defects within laser spot on etched glass in contrary to the un-etched surface that includes single defects initiating the damage. However, the depth of laser damage site was much higher than that of etched nanostructure, which illustrated the subsurface defect and impurities introduced during the etching process had a notably significant effect on the LIDT. The cracks observed on the un-etched substrate are associated with internal thermal stress induced by partially irradiation of a laser beam. The simulation results in Fig. 10 show that the peak E-field amplitudes of un-etched glass at 1064 nm, 532 nm, 355 nm tests are 0.799, 0.8, 0.801, respectively. When the period of nanostructures is 50 nm, 100 nm and on un-etched glass, as the wavelength is decreased from 1064 nm, 532 nm to 355 nm, the peak local E-fields increase and the difference between the etched glass and un-etched glass is larger; this has an agreement with the LIDT result as Fig. 5. In particular, the simulation data for periods 150 nm and 200 nm shows that the peak local E-fields at 532 nm test is the maximum, which contradicts with the experimental data. A fundamental reason is that the etching can result in multiple defects inside the porous layer which can reduce the damage threshold. Moreover, Al3 þ was verified to be introduced during the etching process by our previous work, shown in Fig. 11, which supported the conclusion. Possible ways to improve the LIDT: a) Decreasing subsurface on substrate by HF etching is in favor of improving LIDT. b) The methods to improve LIDT mainly focus on how to reduce the defect types and density. These kinds of methods rely on
Fig. 11. EDS spectrum of glass surface. (a) Elements in glass surface before etching and (b) elements in glass surface after etching.
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substrate materials, preparation equipment and preparation technology. Thus, reducing multiple defects inside the porous layer during etching process can improve the LIDT. c) Laser conditioning can be useful for AR porous surface’s LIDT improvement [23]. d) Annealing of etched glass is favorable to diminish the point defects introduced by etching [23].
5. Conclusion Surface nanostructures etched directly into the bulk BK7 substrate show a LIDT at least two times greater than frequently-used multilayer thin-film AR coatings, but are almost the same level of un-etched substrate at three wavelengths ranging from the UV to NIR (1064, 532, and 355 nm). The results show that the LIDT increases with the wavelength increasing. The shapes of the damage sites indicate homogeneous distribution of intrinsic defects within laser spot on etched glass in contrary to the un-etched surface that includes single defects initiating the damage. From the simulation results of various nanostructures, one can find out that changing the period and diameter of nanostructures on glass surface has little effect on the peak E-field when microstructure has a period much smaller than the light wavelength. The maximum local E-field amplitude on unetched substrate surfaces was always lower than that of etched glass. This agrees well with the values predicted by the LIDT result. There is almost no difference of maximum E-field amplitude at wavelength 1064 and 355 nm, which contradicts with the LIDT, and this indicates that impurities or defects are introduced after etching. It is possible to provide the proper preparation technology to reduce defects for LIDT improvement purpose. We plan to keep fabricating and testing AR surfaces on various materials, such as fused silica and crystal materials, for use at different wavelengths.
Acknowledgments The authors gratefully acknowledge Yun Cui and Qilin Xiao for their measurements of optical properties and morphology, and
Shunli Chen and Fanyu Kong for LIDT test. This work is supported by the National Natural Science Foundation of China (Grant no. 11104293).
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