Open Access
Add to BibSonomyAbstract
The damage effect of the combined irradiation of 1ω and 3ω in multilayer films was investigated. The experiments were held in both the Laser Induced Damage Threshold (LIDT) mode and the damage probability mode. Moreover, the effect of the laser pre-conditioning was also discussed. It was found that with two wavelengths illumination simultaneously, the number of the sensitive defects still govern the damage probability of the samples, and the energy absorption of the defects to pulse laser is a basic process in causing damage. Additionally, correlative theory models were built to explain the experimental results.
©2009 Optical Society of America
1. Introduction
With the development of high power laser systems, the UV beam has been widely applied in scientific research. This wavelength is obtained by using frequency conversion techniques that are based on the nonlinear optical material such as KDP/DKDP crystal, and is separated from the frequency mixing lights by beam splitter or prism [1]. It is well known that the short wavelength is easier to cause damage to the optical material than long wavelength beam [2–5]. As two wavelengths present simultaneously, the damage initiation is more complex. At present, many laboratories have investigated the damage growth of optical materials under exposure of multiple wavelengths [6–10], such as crystal, fused silica, and splitter mirror etc. Some previous papers have investigated on the KDP crystal [6–8], P. DeMange has built a model to evaluate the contribution of IR beam by the concept of equivalent damage effect, they consider that the damage performance of dual-wavelength excitation can be predicted from the damage performance at each wavelength separately. Other work is about the fused silica [9,10], Mary and Lamaignère suggest that the contribution of IR beam can be related to a fraction, under certain conditions the IR beam fluence does not contribute to the growth.
In this paper, the effect of multiple wavelengths on laser induced damage in multilayer mirrors was investigated. The thin film beam splitters for third harmonic separation were prepared by conventional e-beam evaporation, and these samples were used to separate IR and UV beam. The previous studies about crystals and fused silica are all based on the multi-shot irradiation of the material under multiple wavelengths. However, we focused on the “1-on-1” regime according to ISO 11254-1. Firstly, we concentrated on the LIDT of samples tested by 3ω combined different 1ω fluence, the effect of 1ω pre-exposure was also discussed. Moreover, correlative criterion was built to evaluate the contribution of external 1ω. In addition, we represented the damage probability with different fluence combined of two wavelengths directly, and this method can demonstrate the effect of the external 1ω clearly. In the final part of the paper, small absorbing precursors and defect statistical models were given to explain the experimental phenomena.
2. Experimental setup and sample preparation
The illumination of the experiment equipment is shown in Fig. 1 . The testing was performed by using the fundamental, second, third harmonics of a pulsed Nd:YAG laser with pulse duration of 12 ns, 10ns, 8ns, respectively. He–Ne laser was used to monitor the test; pulse energy of each harmonic beam was controlled by a half-wave plate and polarizer combination, and was recorded by energy meter separately. The two beams have obtained a good overlap of the energy peaks on the target plane by adjusting the reflective mirrors, and through changing the path of the 3ω beam in Fig. 1, the two wavelengths can arrive at the sample at the same time. The beams diameters (1/e2) were focused to a 1/e2 height of 241, 552μm and widths of 424, 685μm for 3ω and 1ω with cylindrical lenses (f = 450mm).
The samples were prepared by conventional e-beam evaporation with film structure (HL)133.3H2A1.6L, where H denoted high index material HfO2 with one QWOT (Quarter Wavelength Optical Thickness with monitoring wavelength 351nm) and L denoted low index material SiO2 with one QWOT, A denoted Al2O3 with one QWOT. The substrate was K9 glass with the radius of 50mm. The transmittance of samples for 1ω is 95.6% and the reflection for 3ω is 99.9%.
3. Experimental results
3.1 The LIDT of the samples tested by 1-on-1 mode
The LIDT of the samples which irradiated by 3ω combined different fluence of 1ω are shown in Fig. 2 . The 1ω fluences chosen were 0, 1.9, 4.0, 5.4 and 6.5 J/cm2, and were lower than the LIDT tested by 1ω only (9.0 J/cm2). The LIDT of the samples S1, S2, S3, S4 tested by 3ω only are 6.8, 7.0, 7.1 and 7.0 J/cm2. With external different 1ω fluence, the LIDT are 5.3, 4.8, 4.0 and 1.9 J/cm2. From the chart we can find that the higher 1ω fluence, the more the sample damage threshold declines. However, the drop of the LIDT varies with 1ω fluence, therefore it is difficult to evaluate the effect of 1ω. In order to present the 1ω contribution, R 1ω is defined as the contribution fraction of 1ω:
Where Φ 3ω + 1ω is the LIDT of the samples for two wavelengths combined irradiations, Φ 3ω the LIDT of the samples tested by 3ω only, and Φ 1ω the corresponding 1ω fluence in combined lights. R 1ω gives the ratio of the decrease of the LIDT with the corresponding 1ω fluence, which represents the contribution of 1ω. When 1ω fluence was held at 1.9, 4.0, 5.4 and 6.5 J/cm2, the corresponding R 1ω are 0.7, 0.4, 0.5 and 0.9. It appears that the 1ω contribution varies with the fluence of 1ω, and the higher 1ω fluence is, the more important role it plays in the damage of the sample.
Fig. 2 The LIDT of the samples which were tested by 3ω only, 3ω with 1ω and 3ω with 1ω after 1ω pre-condition.
It is well known that laser conditioning, pre-exposure to less than damaging laser fluence, has been shown to improve the damage resistance of the multilayer films [11,12]. The previous works have shown that pre-exposure to 1ω have a significant effect in improving the LIDT of the sample tested by 1ω and have little effect to 3ω [13]. In this part of work, we concentrate on the LIDT of the samples for two wavelengths combined irradiations after 1ω pre-exposure. The samples have been scanned by 1ω 5.0J/cm2 with spot size 500μm, and the scan spacing is 250μm for single step mode. Figure 2 shows the LIDT of the samples tested by 3ω with 1ω after 1ω pre-exposure, and the LIDT are 6.0, 5.8, 5.2 and 3.2 J/cm2. It is found that all the LIDT of the samples have been improved after pre-exposure to 1ω.
3.2 The damage probability method to evaluate the damage effect of the combined lights
Damage initiation is localized and probabilistic when laser damage is linked to defects; the LIDT of the optical material is determined by the damage probability [14,15]. In this section, we obtained the damage probability of the samples which irradiated by different combined fluence of 1ω and 3ω; the use of the laser damage probability representation can identify the effect of 1ω clearly. According to the damage effect of 3ω only, the 3ω fluence were divided into three classifications: 5.5 J/cm2 is called the lower LIDT, which cannot cause any pits of the sample. 7.3 J/cm2 is the near LIDT, in this 3ω fluence, the damage probability is 10%. The 3ω fluence 8.0 J/cm2 with the damage probability 40% is the higher LIDT.
For the damage probability mode, 20 shots at each fluences for the combined pulses were tested. Figure 3 gives the damage probability of the sample with corresponding combination fluence of two beams, where 1ω fluence were 0, 0.5, 1.0, 1.9, 2.9, 4.0, 5.4, 6.5 J/cm2. In Fig. 3, X-coordinate indicates the 1ω fluence, Y-coordinate shows the damage probability, and the error bar is defined as 5%. In all of the 3ω fluence level, the slope of damage probability curve growth along with the increase of the 1ω fluence. However, when 1ω fluence varies from 0 to 6.5 J/cm2, the growth of damage probability in the 3ω fluence levels is 0.3, 0.6, and 0.4, respectively. It appears that when 3ω at near LIDT level, the damage probability has the most obvious growth in all 3ω fluence levels. Furthermore, all the 3ω level evolution curve of the damage probability shows that when 1ω fluence is from 4.0 to 6.5 J/cm2, the damage probability increased significantly, and this also suggests that the higher 1ω fluence contributes more in reducing the LIDT of the sample.
3.3 surface morphology of samples
Figure 4 show the surface morphologies of samples when 3ω at the higher LIDT, the near LIDT and the lower LIDT, respectively. The photos were accessed by leica optical microscopy. All of the morphologies clearly evidence an initiation process by small defects. When 3ω at the higher LIDT, compare Fig. 4 (d), (f) we can find that the area of the crafter increased, and some pits are joined with adjacent pits. When 3ω was held at the near LIDT, the number of the crafters increases significantly with 1ω 4.0J/cm2, the effect of 1ω is very obvious. However, when 3ω at the lower LIDT, only adding the highest fluence of 1ω (6.5 J/cm2) can cause the damage clearly.
Fig. 4 The surface morphology of samples which were irradiated by different fluence combined of 1ω and 3ω
4. Discussions
4.1 The small absorbing particle model
In nanosecond pulse width region, the number of the sensitivity defects determines the damage probability and the LIDT of the samples [16,17]. In 1ω and 3ω present simultaneously case, 1ω fluence in combined beams is lower than the LIDT of the sample, and the surface morphology of the sample is similar to 3ω only case. When 3ω at the lower LIDT, lower 1ω fluence has little effect in raising the damage probability(as shown in Fig. 3); in 3ω at the near LIDT or the higher LIDT case, a small quantity of 1ω can raising the damage probability significantly. This result indicates us that the total energy absorbing of the defect to the two lights is still a basic process in laser damage. Assuming there is only one defect under the laser irradiation [18], the power absorbed by a spherical absorbing particle is Q i = 3σ i I/4πa 3 (i = 1ω, 3ω), where σ is the absorption cross section and I the intensity, a the radial of the spherical defect, the absorption cross section was usually calculated with mie theory. For simplicity, we just consider the integration of the absorption of the half-spherical surface on the propagation direction [19,20]. When the light irradiates the surface of a spherical particle, one part of the light was reflected, another part was absorbed by the spherical particle, and we have:
where R is the reflectivity of the spherical particle to the light which determined by Classical electromagnetic theory [21], k the extinction coefficient of defect, λ the wavelength of the incidence light. The temperature evolution in the spherical defect can be obtained by solving the equation of heat conduction: The boundary condition is: Where r is the distance from the center of the defect, t the lasting time of the pulse and t<τ (the pulse duration); T p, T f present the temperature evolution in the spherical defect and film, and D p, D f, C p, C f are the thermal diffusivity and thermal conductivity in the defect and film separately. In case of hafnium inclusions in hafnium, the defect radius of 150nm which chosen as a parameter is not sensitivity for 3ω [22]. Damage is assumed to take place when the temperature of the inclusion reaches a critical T c, and this critical value is defined as the fusing point of the hafnium inclusions 2300K [23,24]. The temperature rise is plotted as a function of distance from the center of the defect on Fig. 5 . It is found that when 3ω at the near LIDT, the highest temperature in the defect is above the critical temperature T c. However, when 3ω was held at the lower LIDT, the highest temperature is below the T c. The experimental results suggest that as two wavelengths present simultaneously, the effect of 1ω vary with 1ω fluence, and the total energy absorbing Q = Q 1ω·R 1ω + Q 3ω [25], where R 1ω is the contribution fraction of 1ω which can be obtained by experiment. If consider the energy absorption of 1ω, then the total temperature rise have exceeded the critical T c, as shown in Fig. 5. That is to say, as to the insensitive defects for 3ω, the energy absorption is enhanced by external 1ω, then the temperature of these defects rise to the critical value.Fig. 5 The temperature rise as a function of the distance from the center of defect, where k 1ω = 3.1, k 3ω = 2.58, a = 150 nm, D p = 9.9 × 10−6 m2/s, D f = 6.8 × 10−7 m2/s, C p = 18.4 W/m·k, C f = 1.67 W/m·k, τ 1ω = 12ns, τ 3ω = 8ns [22]
4.2 Defect statistical model fit to experimental data
In the above section, we have presented the numerical simulation of temperature evolution in single spherical defect for two wavelengths combined irradiation simultaneously. It is well known that defects have different size distribution [26,27], those parts of defects which cannot reach the critical temperature with the irradiation of 3ω only will become the sensitive defects with external 1ω, and then the damage probability increased. As to the collection of isolated surface defects under illumination of the laser, each defect has its own defect threshold T, P (F) is the probability of the defect which receives more fluence than its critical fluence, and this probability can be expressed as [28]:
where N (F) is the number of defects under the laser spot which can induce damage at the fluence F. Under simultaneous irradiation of 1ω and 3ω pulse, N (F) = N 1ω (F) + N 3ω (F), which N 3ω (F) are the number of sensitive defects under laser spot of 3ω pulse, N 1ω (F) is the part of sensitive defects with the contribution of 1ω. In case of surface precursors [29,30], one have: S(F) is the part of the spot size with fluence F greater than the precursor threshold T, S(F) = (πμ 2/2)ln(F/T), μ is the laser spot radius on the surface of sample at 3ω waist. g i(T) gives the number of defects per unit area that damage at fluence between T and T + dT, a Gaussian distribution of defect thresholds was considered [30], the ensemble function is given by: Where ΔT is the threshold standard deviation, T 0 is the threshold mean value; d i is the sensitive defect density.Figure 6 gives the experiment data for the irradiations of 3ω only, 3ω with 1ω (6.5J/cm2) and 3ω with 1ω after 1ω pre-condition (described in section 3.1), and the LIDT are 7.0, 1.9, 3.2 J/cm2, respectively. With the model mentioned above, we have obtained a good agreement between the experimental data and the numerical simulation within the error. We use the following parameter d 3ω = 5.5 × 105 defects/mm2, d 1ω = 1.6 × 105 defects/mm2, d 1ω′ = 8.2 × 104 defects/mm2; which d 3ω, d 1ω, d 1ω′denote the contribution of 3ω, 1ω and 1ω after 1ω pre-condition, respectively. In the above discussion, we have proposed that the increasing number of the sensitive defects reduce the LIDT of the sample for two wavelengths combined irradiation, these sizes of defects which are not sensitive for 3ω have become the damage induced factor with 1ω. The numerical analysis result demonstrates that as two lights present simultaneously, the number of the sensitive defects caused by 1ω is less than 3ω. After pre-exposure to 1ω, parts of the sensitive defects for 1ω were cleared, and then the effect of 1ω reduced.
Fig. 6 Numerical analysis for the irradiations of 3ω only, 3ω with 1ω, and 3ω with 1ω after 1ω pre-condition, respectively, where T 0 = 9.0J/cm2, ΔT = 3.0J/cm2, μ 3ω = 380μm
5. Conclusions
We have presented a study of multiple wavelength laser damage in the thin film beam splitters for third harmonic separation. There is no corresponding criterion for two wavelengths combined irradiation on coating in ISO11254-1. In this article, the test results have extended the coverage of the standard and evaluative criterion of the 1ω effect has been built. For the LIDT of the sample, the contribution of 1ω depends on the 1ω fluence, the higher 1ω fluence contributes more in reducing the LIDT. The damage probability mode illustrates that when 3ω was held at the near LIDT, the damage is more prone to occur with external 1ω. The experimental results demonstrate that the energy absorption of defect to laser pulse is still the origin of the damage to multilayer films as two wavelengths present simultaneously, and reducing the number of sensitive defects is an effective method to improve the LIDT of the samples.
References and links
1. J. J. De Yoreo, A. K. Burnham, and P. K. Whitman, “Developing KH2PO4 and KD2PO4 crystals for the world’s most powerful laser,” Int. Mater. Rev. 47(3), 113–152 (2002). [CrossRef]
2. C. W. Carr, H. B. Radousky, A. M. Rubenchik, M. D. Feit, and S. G. Demos, “Localized dynamics during laser-induced damage in optical materials,” Phys. Rev. Lett. 92(8), 087401 (2004). [CrossRef] [PubMed]
3. C. W. Carr, H. B. Radousky, and S. G. Demos, “Wavelength dependence of laser-induced damage: determining the damage initiation mechanisms,” Phys. Rev. Lett. 91(12), 127402 (2003). [CrossRef] [PubMed]
4. H. Kouta, “Wavelength Dependence of Repetitive-Pulse Laser-Induced Damage Threshold in β-BaB(2)O(4).,” Appl. Opt. 38(3), 545–547 (1999). [CrossRef]
5. C. J. Stolz, S. Hafeman, and T. V. Pistor, “Light intensification modeling of coating inclusions irradiated at 351 and 1053 nm,” Appl. Opt. 47(13), C162–C166 (2008). [CrossRef] [PubMed]
6. P. DeMange, R. A. Negres, A. M. Rubenchik, H. B. Radousky, M. D. Feit, and S. G. Demos, “Understanding and predicting the damage performance of KDxH2−xPO4 crystals under simultaneous exposure to 532 and 355-nm pulses,” Appl. Phys. Lett. 89(18), 181922–181923 (2006). [CrossRef]
7. P. DeMange, R. A. Negres, A. M. Rubenchik, H. B. Radousky, M. D. Feit, and S. G. Demos, “The energy coupling efficiency of multi-wavelength laser pulses to damage initiating defects in deuterated KH2PO4 nonlinear crystals,” J. Appl. Phys. 103(8), 083122–083128 (2008). [CrossRef]
8. S. Reyné, M. Loiseau, G. Duchateau, J. Y. Natoli, and L. Lamaignère, “Towards a better understanding of multi-wavelength effects on KDP crystals,” Proc. SPIE 7361, 73610 (2009). [CrossRef]
9. L. Lamaignère, S. Reyne, M. Loiseau, J. C. Poncetta, and H. Bercegol, “Effect of wavelengths combination on initiation and growth of laser-induced surface damage in SiO2,” Proc. SPIE 6720, 67200 (2007). [CrossRef]
10. A. N. Mary and E. D. Eugene, “Laser damage growth in fused silica with simultaneous 351nm and 1053nm irradiation,” Proc. SPIE 7132, 71321 (2008).
11. C. J. Stolz, L. M. Sheehan, S. M. Maricle, and S. Schwartz, “A study of laser conditioning methods of hafnia silica multilayer mirrors,” Proc. SPIE 3578, 144–153 (1998). [CrossRef]
12. C. J. Stolz, L. M. Sheehan, K. Gunten, R. P. Bevis, and D. J. Smith, “The advantages of evaporation of Hafnium in a reactive environment to manufacture high damage threshold multilayer coatings by electron-beam deposition,” Proc. SPIE 3738, 318–324 (1999). [CrossRef]
13. P. DeMange, C. W. Carr, R. A. Negres, H. B. Radousky, and S. G. Demos, “Multiwavelength investigation of laser-damage performance in potassium dihydrogen phosphate after laser annealing,” Opt. Lett. 30(3), 221–223 (2005). [CrossRef] [PubMed]
14. C. Y. Wei, J. D. Shao, H. B. He, K. Yi, and Z. X. Fan, “Mechanism initiated by nanoabsorber for UV nanosecond-pulse-driven damage of dielectric coatings,” Opt. Express 16(5), 3376–3382 (2008). [CrossRef] [PubMed]
15. H. Krol, L. Gallais, C. Grezesbesset, J. Natoli, and M. Commandré, “Investigation of nanoprecursors threshold distribution in laser-damage testing,” Opt. Commun. 256(1–3), 184–189 (2005). [CrossRef]
16. J. O. Porteus and S. C. Seitel, “Absolute onset of optical surface damage using distributed defect ensembles,” Appl. Opt. 23(21), 3796–3805 (1984). [CrossRef] [PubMed]
17. D. Milam, R. A. Bradbury, and M. Bass, “Laser damage threshold for dielectric coating as determined by inclusions,” Appl. Phys. Lett. 23(12), 654–657 (1973). [CrossRef]
18. G. Duchateau and A. Dyan, “Coupling statistics and heat transfer to study laser-induced crystal damage by nanosecond pulses,” Opt. Express 15(8), 4557–4576 (2007). [CrossRef] [PubMed]
19. M. R. Lange and J. K. McIver, “Laser damage threshold predictions based on the effects of thermal and optical properties employing a spherical impurity model,” Proc. SPIE 688, 454 (1985).
20. M. R. Lange and J. K. McIver, “Anomalous absorption in optical coatings,” Proc. SPIE 746, 515 (1987).
21. H. A. Macleod, Thin Film Optical Filters, third edition, 2001
22. L. Gallais, J. Capoulade, J. Y. Natoli, and M. Commandré, “Investigation of nano-defect properties in optical coatings by coupling measured and simulated laser damage statistics,” J. Appl. Phys. 104(5), 053120 (2008). [CrossRef]
23. J. Y. Natoli, L. Gallais, B. Bertussi, A. During, M. Commandré, J. L. Rullier, F. Bonneau, and P. Combis, “Localized pulsed laser interaction with submicronic gold particles embedded in silica: a method for investigating laser damage initiation,” Opt. Express 11(7), 824–829 (2003). [CrossRef] [PubMed]
24. J. Dijon, G. Ravel, and B. André, “Thermomechanical model of mirror laser damage at 1.06pm. Part 2: flat bottom pits formation,” Proc. SPIE 3578, 398–407 (1998). [CrossRef]
25. M. D. Feit, A. M. Rubenchik, and J. B. Trenholme, “Simple model of laser damage initiation and conditioning in frequency conversion crystals,” Proc. SPIE 5991, 59910 (2005). [CrossRef]
26. M. D. Feit and A. M. Rubenchik, “Implication of nanoabsorber initiators for damage probability curves, pulse length scaling and laser conditioning,” Proc. SPIE 5273, 74–82 (2004). [CrossRef]
27. Z. L. Xia, Z. X. Fan, and J. D. Shao, “A New theory for evaluating the number density of inclusions in films,” Appl. Surf. Sci. 252(23), 8235–8238 (2006). [CrossRef]
28. L. Gallais, J. Y. Natoli, and C. Amra, “Statistical study of single and multiple pulse laser-induced damage in glasses,” Opt. Express 25, 1465–1474 (2002).
29. M. Zhou, J. D. Shao, Z. X. Fan, G. H. Hu, and Y. G. Shan, “Damage performance of thin-film beam splitter for third harmonic separation under simultaneous exposure to 1ω and 3ω pulses,” Opt. Commun. 282(15), 3132–3135 (2009). [CrossRef]
30. J. Capoulade, L. Gallais, J. Y. Natoli, and M. Commandré, “Multiscale analysis of the laser-induced damage threshold in optical coatings,” Appl. Opt. 47(29), 5272–5280 (2008). [CrossRef] [PubMed]
