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OPTICS LETTERS / Vol. 39, No. 22 / November 15, 2014
Effect of oxygen vacancies on the laser-induced damage resistance of Y0.26Hf0.74Ox thin films Xiaoying Chen,1,2,3 Lili Zhao,1 Xinjie Fu,1,2 Lijun You,1 Olaf Stenzel,3 Helena Kämmer,4 Felix Dreisow,4 Stefan Nolte,4 and Lixin Song1,* 1
Key Laboratory of Inorganic Coating Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China 2
3
University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China
Fraunhofer Institute for Applied Optics and Precision Engineering IOF, Albert-Einstein-Str. 7, 07745 Jena, Germany 4 Institute of Applied Physics, Friedrich-Schiller-University Jena, Albert-Einstein-Str. 15, 07745 Jena, Germany *Corresponding author: [email protected] Received September 12, 2014; revised October 14, 2014; accepted October 14, 2014; posted October 16, 2014 (Doc. ID 221941); published November 11, 2014 The influence of the oxygen sub-stoichiometry on the femtosecond laser-induced damage resistance of Y0.26 Hf 0.74 Ox films was investigated in this work. Photoluminescence (PL) spectroscopy was applied to analyze the various states of oxygen vacancies VO , and laser-induced damage measurement was performed using a single 500 fs pulse laser at a wavelength of 1030 nm. Based on the PL spectra, a probable band gap structure with different defect energy levels was obtained, which is significant in explaining an observed inverse correlation between the laser-induced damage threshold (LIDT) and the concentration of VO . The crystallization and the refractive index, which were previously found to be crucial factors in laser-induced damage resistance of optical thin films, are also closely dependent on the amount of induced oxygen vacancies. © 2014 Optical Society of America OCIS codes: (310.0310) Thin films; (320.0320) Ultrafast optics; (140.3330) Laser damage. http://dx.doi.org/10.1364/OL.39.006470
In order to improve the performance of optical coatings on devices in high-powered laser systems, the inducing factors of laser-induced damage on dielectric thin films have been widely investigated in past decades. Laser pulse duration, as well as wavelength and some microstructure flaws, such as nodular defects, strongly affected the laser damage resistance of the films [1,2]. Lately, however, attention is being increasingly paid to some intrinsic properties, such as the optical energy gap Eg and the refractive index of the films, especially when considering the interaction with an ultra-short pulse laser. As in this pulse regime, multi-photon absorption (MPA) and subsequent avalanche ionization leads to the final damage. A positive correlation between the LIDT and the Eg has been found in pure oxide layers, while the evolution of LIDT changes with the refractive index in mixed oxide layers [3,4]. In further investigations, a sub-stoichiometric effect, such as oxygen vacancy, cannot be avoided, since the preparation of optical thin-films usually requires high-vacuum environments. According to previous calculations, these oxygen vacancies could change some of the intrinsic properties of dielectric oxide [5,6]. The existence of VO will introduce some defect energy levels, which have trapped electrons between the valence band (VB) and the conduction band (CB). Thus, the energy for an electron excited from defect levels to the CB will be much lower compared with the Eg , leading to a higher possibility of photonionization in such oxide films. When interacting with an ultra-short pulse laser, these “trapped electrons” in defect levels could easily trigger the occurrence of lower-order MPA ionization, therefore lowering the LIDT. This encouraged us to carry out a research experiment on the influence of oxygen vacancy on the laser damage resistance of optical thin films. Our previous work [7] showed that the structural and optical properties of HfO2 , which is usually applied to 0146-9592/14/226470-04$15.00/0
optical coatings as a high-refractive-index material, could be altered by the incorporation of Y2 O3 . Therefore, we find it meaningful to investigate the laser-induced damage resistance of yttria-doped hafnia thin films. In our experiment, a total of five mixed-oxide single layers (130 nm), Y0.26 Hf 0.74 Ox , with different amounts of oxygen were prepared on fused silica substrates using electron beam evaporation technology. The evaporation source for the deposit was a combustion-synthesized binary oxide Y2 O3 0.13 HfO2 0.74 , which has been described in our previous work [7]. Instead of a post-annealing treatment, the gradient amount of oxygen vacancies was obtained by adjusting the flow rate of oxygen assistance while depositing, which could avoid the annealing effect on the optical properties and microstructure of as-deposited films that will be also investigated in this experiment. Different rates of assistant oxygen flow were set at 0, 15, 30, 45, and 60 sccm, and the samples are correspondingly denoted as YDH-1, YDH-2, YDH-3, YDH-4, and YDH-5, respectively. The substrate temperature was kept at 200°C, and the rate of deposit was controlled at 0.15 nm/s for all samples. The method to estimate the amount of sub-stoichiometry oxygen via x-ray photoelectron spectroscopy is not applicable, since the fully oxidized layer on the surface leads to an unrealistic result if we do not etch the surface with ion sputtering. The etching itself is an oxygenpreferential process and results in oxygen deficiencies in the films. Additionally, we conducted a PL measurement that has been recently used to analyze the defect states and energy band structures [8–11]. The amount of oxygen vacancies in the films was estimated by PL emission spectra, measured in the PL mode of a LabRam HR 800 micro-Raman spectroscope using an He–Cd laser excitation with a wavelength of 325 nm (equivalent to a photon energy of 3.825 eV) at room temperature. The © 2014 Optical Society of America
November 15, 2014 / Vol. 39, No. 22 / OPTICS LETTERS
laser damage test was performed under the ISO standard 11254-1[12] using a 500 fs pulse fiber laser at a wavelength of 1030 nm. The beam was focused on the target plane with a normal incident angle and the radius at 1∕e is 30 μm. The damage points on the sample were detected by a Nomarski microscope with a magnification of ×100 objective after irradiation. The crystallization of the films was demonstrated by x-ray diffraction (XRD) patterns. In XRD measurement, a grazing incidence angle of 1° was chosen to reduce the strong background noise from the fused silica substrate, and the grain size of the sample was estimated using the Scherrer equation. The refractive index and extinction coefficient of the films was measured by a variable angle spectroscopic ellipsometer following the Cauchy model. The band Eg of the samples was determined using the Tauc equation [13]. The extinction coefficients of our samples have an order of magnitude of 1 × 10−4 at the wavelength of 1030 nm, demonstrating that there is no linear absorption at this wavelength. The calculated Eg for all of our samples is about 6.24 eV; thus, in the later discussion of the band gap structure, the transmission energy from VB to CB is set at 6.24 eV. The PL emission spectra of the samples are given in Fig. 1(a). With the increasing amount of oxygen assistance, the intensity of these spectra gradually decreases from sample YDH-1 to YDH-5. For each sample, a broad emission band is present in the range from about 2.0 to 2.75 eV, while another two emission peaks are clearly noticed in the range from 2.75 to 3.6 eV. For more information, we performed a Gaussian decomposition on the PL spectra, which can be seen in Fig. 1(b). All these spectra decompose well into five Gaussian bands, centered at around 3.33, 3.09, 2.67, 2.33, and 1.96 eV. Based on the former experimental works concerning pure HfO2 and doped HfO2 thin films [8,10,14], the PL emission peaks
Fig. 1. (a) PL emission spectra of five YDH samples; (b) Gaussian decomposition of PL spectra, which was performed on YDH-1 as an example.
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in the range from 350 to 700 nm (3.55 to 1.78 eV) should be attributed to the defect levels of oxygen vacancies in the energy gap. The VO were introduced in two different ways in our experiment. First, trivalent Y3 substituting for tetravalent Hf 4 leads to the increase of VO . Second, the gradient amount of VO in different samples was achieved by changing the oxygen assistant flows while depositing the films in a high-vacuum environment. According to the theoretical calculation by Muñoz Ramo et al. [5,6], these defect levels correspond to the five different charge states of VO . As in [5], the neutral VO was denoted as V O 0 , which means that a neutral oxygen atom was extracted from the lattice and left two electrons in the vacancy site. Likewise, VO 2 , VO , VO − , and VO 2− also exist in the lattice matrix. According to the Gaussian decomposition and the theoretical calculation, a probable band gap structure with defect energy levels is represented in Fig. 2. For V O 2 , the corresponding defect level is not occupied by any electrons, so it should be at the highest defect level located between the CB and VB, at 1.96 eV below the CB. For VO and VO , a level containing one electron and a level containing two electrons were at 3.09 and 3.33 eV, respectively. For VO − and VO 2− , there also should be another two separate shallow levels at 0.42 and 0.76 eV below the CB, excluding the levels containing two electrons around 3.09 eV. The existence of shallow levels is estimated by the calculation work [5,15], but it was out of range in our PL spectra. Based on the analysis of the above oxygen vacancies, we now present the relationship between the LIDT and the oxygen vacancies of the samples. In Fig. 3, the total concentration of VO is obtained by summating the integral of five Gaussian-band curves corresponding to different VO states for each sample. The given value of LIDT is calculated in terms of “internal” LIDT [4], which takes the standing-wave electric field in the film and the incident electric field into consideration. The concentration of VO undergoes a gradual decrease from sample YDH-1 to YDH-5, resulting in a corresponding change of electron density in the defect levels. Concurrently, the value of the internal LIDT increased from 0.157 J∕cm2 for YDH-1 to 0.248 J∕cm2 for YDH-5, an increase of 75%. When interacting with the laser in the femtosecond regime, multi-photon ionization-induced
Fig. 2. VO -induced defect energy levels in the band gap, deduced by PL spectra.
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Fig. 3. The internal LIDT and the total concentration of the samples.
electrons initiate the avalanche. The LIDT has an inverse correlation to the VO concentration in the sample. This means the amount of defect level-trapped electrons affects the final laser damage resistance of the films. This could be explained as follows: for dielectric thin films with high band gap energy (as in our experiment), if an electron was expected to be excited from VB to CB Eg ≈ 6.24 eV with the laser wavelength of 1030 nm (1.21 eV), there should be at least a 5-photon absorption (5PA) process. When considering the oxygen vacancy defect in the films, the energy to excite electrons from defect levels of VO , VO , VO − , and VO 2− to CB only requires a 3PA process. From prior calculations of the nonlinear photon absorption coefficient βM and cross section σ M using a Z-scan theoretical analysis [16], the ratio of MPA coefficient β3 ∕β5 is about 1.4 × 1022 W∕cm2 2 and the ratio of cross section σ 3 ∕σ 5 is around 1.74 × 1059 photon∕cm2 · s2 . Thus, a 3PA will take priority over a 5PA process. Once these electrons are excited to the CB, they will serve as seed electrons for further avalanche ionization. Under the same exposure conditions, the density of seed electrons, which in our experiment depends on the concentration of oxygen vacancies, determines the probability of laser damage to the films. We also investigated the relationship between the concentration of VO , the refractive index, and the crystallization of the films. In a previous work it was reported that the refractive index is a much more direct factor affecting the anti-laser damage ability of mixed oxide layers, as compared with other intrinsic factors, such as Eg [4], and the larger grain size usually corresponding to a lower LIDT because of the grain boundary effect [17]. However, in our work, we found that they have a dependent relationship. As shown in Fig. 4(a), the refractive index is reduced when the concentration of VO is decreased in the films. Figure 4(b) gives the XRD patterns of different samples. By the estimation deduced from the Scherrer equation [18], YDH-1, which has the strongest diffraction peaks, has the largest grain sizes: the grain sizes in YDH-1 are 15 nm. In the other samples, the grain size gradually decreases to about 9 nm in YDH-5, demonstrating that the VO is acting as a promoter in the crystallization of the films. Thus, the refractive index, which is closely
Fig. 4. (a) The refractive index of YDH samples. Inset shows different samples of the refractive index at 1030 nm. (b) XRD patterns of YDH samples with the grain-sized representation of each sample in the inset.
dependent on the composition and microstructure of the film, could be regarded as an obvious manifestation that presents the strength of the interaction with the electromagnetic field. Additionally, the crystallization also correlates with the concentration of VO , especially for phase-transition materials like hafnia-based oxides. Higher concentrations of VO lead to a higher tendency of the high-pressure phase. Hence, oxygen vacancy is the key factor affecting not only the optical and structural properties, but also the laser-induced damage resistance of the dielectric films. In conclusion, we investigated the correlation between the concentration of oxygen vacancy and the laser damage resistance of dielectric thin films. Due to the existence of different types of VO , several defect levels acting as electron trapping center were ascertained in the energy gap. The inverse correlation between the concentration of VO and the internal LIDT reveals that the electrons trapped in the defect levels allow for a lower-order MPA process from defect level to the CB. The density of the trapped electrons, which serve as the seed electrons for an avalanche when excited to CB, will determine the laser damage resistance of the samples. Moreover, the refractive index and crystallization that have been reported influencing the laser damage resistance also strongly depend on the concentration of VO in the thin films, demonstrating that defects of oxygen vacancies are actually the triggering factor in laser-induced damage. We acknowledge the support of the German Federal Ministry of Education and Research under the Optical Technologies funding program (Contract No. 13N11691), as well as the support of the Key Laboratory of Science and Technology on High Energy Lasers, Chinese Academy of Engineering Physics (2012HCF01). References 1. B. C. Stuart, M. D. Feit, S. Herman, A. M. Rubenchik, B. W. Shore, and M. D. Perry, Phys. Rev. B 53, 1749 (1996). 2. L. Gallais, X. Cheng, and Z. Wang, Opt. Lett. 39, 1545 (2014). 3. M. Mero, J. Liu, W. Rudolph, D. Ristau, and K. Starke, Phys. Rev. B 71, 115109 (2005).
November 15, 2014 / Vol. 39, No. 22 / OPTICS LETTERS 4. B. Mangote, L. Gallais, M. Commandré, M. Mende, L. Jensen, H. Ehlers, M. Jupé, D. Ristau, A. Melninkaitis, J. Mirauskas, V. Sirutkaitis, S. Kičas, T. Tolenis, and R. Drazdys, Opt. Lett. 37, 1478 (2012). 5. D. Muñoz Ramo, J. L. Gavartin, A. L. Shluger, and G. Bersuker, Phys. Rev. B 75, 205336 (2007). 6. G. H. Chen, Z. F. Hou, X. G. Gong, and Q. Li, J. Appl. Phys. 104, 074101 (2008). 7. X. Chen, L. Song, L. You, and L. Zhao, Appl. Surf. Sci. 271, 248 (2013). 8. Y. Xiong, H. Tu, J. Du, M. Ji, X. Zhang, and L. Wang, Appl. Phys. Lett. 97, 012901 (2010). 9. S.-H. Chuang, H.-C. Lin, and C.-H. Chen, J. Alloys Compd. 534, 42 (2012). 10. J. Ni, Q. Zhou, Z. Li, and Z. Zhang, Appl. Phys. Lett. 93, 011905 (2008).
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11. S. A. Eliziario, L. S. Cavalcante, J. C. Sczancoski, P. S. Pizani, J. A. Varela, J. W. Espinosa, and E. Longo, Nanoscale Res. Lett. 4, 1371 (2009). 12. ISO Standard 11254-1, International Organization of Standardization 2000. 13. I. Kosacki, V. Petrovsky, and H. U. Anderson, Appl. Phys. Lett. 74, 341 (1999). 14. S. Chen, Z. Liu, L. Feng, T. Tan, and X. Zhao, Appl. Surf. Sci. 320, 699 (2014). 15. P. Broqvist and A. Pasquarello, Appl. Phys. Lett. 89, 262904 (2006). 16. D. S. Corrêa, L. De Boni, L. Misoguti, I. Cohanoschi, F. E. Hernandez, and C. R. Mendonça, Opt. Commun. 277, 440 (2007). 17. C. Xu, Q. Xiao, J. Ma, Y. Jin, J. Shao, and Z. Fan, Appl. Phys. Lett. 254, 6554 (2008). 18. A. Patterson, Phys. Rev. 56, 978 (1939).
OPTICS LETTERS / Vol. 39, No. 22 / November 15, 2014
Effect of oxygen vacancies on the laser-induced damage resistance of Y0.26Hf0.74Ox thin films Xiaoying Chen,1,2,3 Lili Zhao,1 Xinjie Fu,1,2 Lijun You,1 Olaf Stenzel,3 Helena Kämmer,4 Felix Dreisow,4 Stefan Nolte,4 and Lixin Song1,* 1
Key Laboratory of Inorganic Coating Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China 2
3
University of Chinese Academy of Sciences, 19 Yuquan Road, Beijing 100049, China
Fraunhofer Institute for Applied Optics and Precision Engineering IOF, Albert-Einstein-Str. 7, 07745 Jena, Germany 4 Institute of Applied Physics, Friedrich-Schiller-University Jena, Albert-Einstein-Str. 15, 07745 Jena, Germany *Corresponding author: [email protected] Received September 12, 2014; revised October 14, 2014; accepted October 14, 2014; posted October 16, 2014 (Doc. ID 221941); published November 11, 2014 The influence of the oxygen sub-stoichiometry on the femtosecond laser-induced damage resistance of Y0.26 Hf 0.74 Ox films was investigated in this work. Photoluminescence (PL) spectroscopy was applied to analyze the various states of oxygen vacancies VO , and laser-induced damage measurement was performed using a single 500 fs pulse laser at a wavelength of 1030 nm. Based on the PL spectra, a probable band gap structure with different defect energy levels was obtained, which is significant in explaining an observed inverse correlation between the laser-induced damage threshold (LIDT) and the concentration of VO . The crystallization and the refractive index, which were previously found to be crucial factors in laser-induced damage resistance of optical thin films, are also closely dependent on the amount of induced oxygen vacancies. © 2014 Optical Society of America OCIS codes: (310.0310) Thin films; (320.0320) Ultrafast optics; (140.3330) Laser damage. http://dx.doi.org/10.1364/OL.39.006470
In order to improve the performance of optical coatings on devices in high-powered laser systems, the inducing factors of laser-induced damage on dielectric thin films have been widely investigated in past decades. Laser pulse duration, as well as wavelength and some microstructure flaws, such as nodular defects, strongly affected the laser damage resistance of the films [1,2]. Lately, however, attention is being increasingly paid to some intrinsic properties, such as the optical energy gap Eg and the refractive index of the films, especially when considering the interaction with an ultra-short pulse laser. As in this pulse regime, multi-photon absorption (MPA) and subsequent avalanche ionization leads to the final damage. A positive correlation between the LIDT and the Eg has been found in pure oxide layers, while the evolution of LIDT changes with the refractive index in mixed oxide layers [3,4]. In further investigations, a sub-stoichiometric effect, such as oxygen vacancy, cannot be avoided, since the preparation of optical thin-films usually requires high-vacuum environments. According to previous calculations, these oxygen vacancies could change some of the intrinsic properties of dielectric oxide [5,6]. The existence of VO will introduce some defect energy levels, which have trapped electrons between the valence band (VB) and the conduction band (CB). Thus, the energy for an electron excited from defect levels to the CB will be much lower compared with the Eg , leading to a higher possibility of photonionization in such oxide films. When interacting with an ultra-short pulse laser, these “trapped electrons” in defect levels could easily trigger the occurrence of lower-order MPA ionization, therefore lowering the LIDT. This encouraged us to carry out a research experiment on the influence of oxygen vacancy on the laser damage resistance of optical thin films. Our previous work [7] showed that the structural and optical properties of HfO2 , which is usually applied to 0146-9592/14/226470-04$15.00/0
optical coatings as a high-refractive-index material, could be altered by the incorporation of Y2 O3 . Therefore, we find it meaningful to investigate the laser-induced damage resistance of yttria-doped hafnia thin films. In our experiment, a total of five mixed-oxide single layers (130 nm), Y0.26 Hf 0.74 Ox , with different amounts of oxygen were prepared on fused silica substrates using electron beam evaporation technology. The evaporation source for the deposit was a combustion-synthesized binary oxide Y2 O3 0.13 HfO2 0.74 , which has been described in our previous work [7]. Instead of a post-annealing treatment, the gradient amount of oxygen vacancies was obtained by adjusting the flow rate of oxygen assistance while depositing, which could avoid the annealing effect on the optical properties and microstructure of as-deposited films that will be also investigated in this experiment. Different rates of assistant oxygen flow were set at 0, 15, 30, 45, and 60 sccm, and the samples are correspondingly denoted as YDH-1, YDH-2, YDH-3, YDH-4, and YDH-5, respectively. The substrate temperature was kept at 200°C, and the rate of deposit was controlled at 0.15 nm/s for all samples. The method to estimate the amount of sub-stoichiometry oxygen via x-ray photoelectron spectroscopy is not applicable, since the fully oxidized layer on the surface leads to an unrealistic result if we do not etch the surface with ion sputtering. The etching itself is an oxygenpreferential process and results in oxygen deficiencies in the films. Additionally, we conducted a PL measurement that has been recently used to analyze the defect states and energy band structures [8–11]. The amount of oxygen vacancies in the films was estimated by PL emission spectra, measured in the PL mode of a LabRam HR 800 micro-Raman spectroscope using an He–Cd laser excitation with a wavelength of 325 nm (equivalent to a photon energy of 3.825 eV) at room temperature. The © 2014 Optical Society of America
November 15, 2014 / Vol. 39, No. 22 / OPTICS LETTERS
laser damage test was performed under the ISO standard 11254-1[12] using a 500 fs pulse fiber laser at a wavelength of 1030 nm. The beam was focused on the target plane with a normal incident angle and the radius at 1∕e is 30 μm. The damage points on the sample were detected by a Nomarski microscope with a magnification of ×100 objective after irradiation. The crystallization of the films was demonstrated by x-ray diffraction (XRD) patterns. In XRD measurement, a grazing incidence angle of 1° was chosen to reduce the strong background noise from the fused silica substrate, and the grain size of the sample was estimated using the Scherrer equation. The refractive index and extinction coefficient of the films was measured by a variable angle spectroscopic ellipsometer following the Cauchy model. The band Eg of the samples was determined using the Tauc equation [13]. The extinction coefficients of our samples have an order of magnitude of 1 × 10−4 at the wavelength of 1030 nm, demonstrating that there is no linear absorption at this wavelength. The calculated Eg for all of our samples is about 6.24 eV; thus, in the later discussion of the band gap structure, the transmission energy from VB to CB is set at 6.24 eV. The PL emission spectra of the samples are given in Fig. 1(a). With the increasing amount of oxygen assistance, the intensity of these spectra gradually decreases from sample YDH-1 to YDH-5. For each sample, a broad emission band is present in the range from about 2.0 to 2.75 eV, while another two emission peaks are clearly noticed in the range from 2.75 to 3.6 eV. For more information, we performed a Gaussian decomposition on the PL spectra, which can be seen in Fig. 1(b). All these spectra decompose well into five Gaussian bands, centered at around 3.33, 3.09, 2.67, 2.33, and 1.96 eV. Based on the former experimental works concerning pure HfO2 and doped HfO2 thin films [8,10,14], the PL emission peaks
Fig. 1. (a) PL emission spectra of five YDH samples; (b) Gaussian decomposition of PL spectra, which was performed on YDH-1 as an example.
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in the range from 350 to 700 nm (3.55 to 1.78 eV) should be attributed to the defect levels of oxygen vacancies in the energy gap. The VO were introduced in two different ways in our experiment. First, trivalent Y3 substituting for tetravalent Hf 4 leads to the increase of VO . Second, the gradient amount of VO in different samples was achieved by changing the oxygen assistant flows while depositing the films in a high-vacuum environment. According to the theoretical calculation by Muñoz Ramo et al. [5,6], these defect levels correspond to the five different charge states of VO . As in [5], the neutral VO was denoted as V O 0 , which means that a neutral oxygen atom was extracted from the lattice and left two electrons in the vacancy site. Likewise, VO 2 , VO , VO − , and VO 2− also exist in the lattice matrix. According to the Gaussian decomposition and the theoretical calculation, a probable band gap structure with defect energy levels is represented in Fig. 2. For V O 2 , the corresponding defect level is not occupied by any electrons, so it should be at the highest defect level located between the CB and VB, at 1.96 eV below the CB. For VO and VO , a level containing one electron and a level containing two electrons were at 3.09 and 3.33 eV, respectively. For VO − and VO 2− , there also should be another two separate shallow levels at 0.42 and 0.76 eV below the CB, excluding the levels containing two electrons around 3.09 eV. The existence of shallow levels is estimated by the calculation work [5,15], but it was out of range in our PL spectra. Based on the analysis of the above oxygen vacancies, we now present the relationship between the LIDT and the oxygen vacancies of the samples. In Fig. 3, the total concentration of VO is obtained by summating the integral of five Gaussian-band curves corresponding to different VO states for each sample. The given value of LIDT is calculated in terms of “internal” LIDT [4], which takes the standing-wave electric field in the film and the incident electric field into consideration. The concentration of VO undergoes a gradual decrease from sample YDH-1 to YDH-5, resulting in a corresponding change of electron density in the defect levels. Concurrently, the value of the internal LIDT increased from 0.157 J∕cm2 for YDH-1 to 0.248 J∕cm2 for YDH-5, an increase of 75%. When interacting with the laser in the femtosecond regime, multi-photon ionization-induced
Fig. 2. VO -induced defect energy levels in the band gap, deduced by PL spectra.
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OPTICS LETTERS / Vol. 39, No. 22 / November 15, 2014
Fig. 3. The internal LIDT and the total concentration of the samples.
electrons initiate the avalanche. The LIDT has an inverse correlation to the VO concentration in the sample. This means the amount of defect level-trapped electrons affects the final laser damage resistance of the films. This could be explained as follows: for dielectric thin films with high band gap energy (as in our experiment), if an electron was expected to be excited from VB to CB Eg ≈ 6.24 eV with the laser wavelength of 1030 nm (1.21 eV), there should be at least a 5-photon absorption (5PA) process. When considering the oxygen vacancy defect in the films, the energy to excite electrons from defect levels of VO , VO , VO − , and VO 2− to CB only requires a 3PA process. From prior calculations of the nonlinear photon absorption coefficient βM and cross section σ M using a Z-scan theoretical analysis [16], the ratio of MPA coefficient β3 ∕β5 is about 1.4 × 1022 W∕cm2 2 and the ratio of cross section σ 3 ∕σ 5 is around 1.74 × 1059 photon∕cm2 · s2 . Thus, a 3PA will take priority over a 5PA process. Once these electrons are excited to the CB, they will serve as seed electrons for further avalanche ionization. Under the same exposure conditions, the density of seed electrons, which in our experiment depends on the concentration of oxygen vacancies, determines the probability of laser damage to the films. We also investigated the relationship between the concentration of VO , the refractive index, and the crystallization of the films. In a previous work it was reported that the refractive index is a much more direct factor affecting the anti-laser damage ability of mixed oxide layers, as compared with other intrinsic factors, such as Eg [4], and the larger grain size usually corresponding to a lower LIDT because of the grain boundary effect [17]. However, in our work, we found that they have a dependent relationship. As shown in Fig. 4(a), the refractive index is reduced when the concentration of VO is decreased in the films. Figure 4(b) gives the XRD patterns of different samples. By the estimation deduced from the Scherrer equation [18], YDH-1, which has the strongest diffraction peaks, has the largest grain sizes: the grain sizes in YDH-1 are 15 nm. In the other samples, the grain size gradually decreases to about 9 nm in YDH-5, demonstrating that the VO is acting as a promoter in the crystallization of the films. Thus, the refractive index, which is closely
Fig. 4. (a) The refractive index of YDH samples. Inset shows different samples of the refractive index at 1030 nm. (b) XRD patterns of YDH samples with the grain-sized representation of each sample in the inset.
dependent on the composition and microstructure of the film, could be regarded as an obvious manifestation that presents the strength of the interaction with the electromagnetic field. Additionally, the crystallization also correlates with the concentration of VO , especially for phase-transition materials like hafnia-based oxides. Higher concentrations of VO lead to a higher tendency of the high-pressure phase. Hence, oxygen vacancy is the key factor affecting not only the optical and structural properties, but also the laser-induced damage resistance of the dielectric films. In conclusion, we investigated the correlation between the concentration of oxygen vacancy and the laser damage resistance of dielectric thin films. Due to the existence of different types of VO , several defect levels acting as electron trapping center were ascertained in the energy gap. The inverse correlation between the concentration of VO and the internal LIDT reveals that the electrons trapped in the defect levels allow for a lower-order MPA process from defect level to the CB. The density of the trapped electrons, which serve as the seed electrons for an avalanche when excited to CB, will determine the laser damage resistance of the samples. Moreover, the refractive index and crystallization that have been reported influencing the laser damage resistance also strongly depend on the concentration of VO in the thin films, demonstrating that defects of oxygen vacancies are actually the triggering factor in laser-induced damage. We acknowledge the support of the German Federal Ministry of Education and Research under the Optical Technologies funding program (Contract No. 13N11691), as well as the support of the Key Laboratory of Science and Technology on High Energy Lasers, Chinese Academy of Engineering Physics (2012HCF01). References 1. B. C. Stuart, M. D. Feit, S. Herman, A. M. Rubenchik, B. W. Shore, and M. D. Perry, Phys. Rev. B 53, 1749 (1996). 2. L. Gallais, X. Cheng, and Z. Wang, Opt. Lett. 39, 1545 (2014). 3. M. Mero, J. Liu, W. Rudolph, D. Ristau, and K. Starke, Phys. Rev. B 71, 115109 (2005).
November 15, 2014 / Vol. 39, No. 22 / OPTICS LETTERS 4. B. Mangote, L. Gallais, M. Commandré, M. Mende, L. Jensen, H. Ehlers, M. Jupé, D. Ristau, A. Melninkaitis, J. Mirauskas, V. Sirutkaitis, S. Kičas, T. Tolenis, and R. Drazdys, Opt. Lett. 37, 1478 (2012). 5. D. Muñoz Ramo, J. L. Gavartin, A. L. Shluger, and G. Bersuker, Phys. Rev. B 75, 205336 (2007). 6. G. H. Chen, Z. F. Hou, X. G. Gong, and Q. Li, J. Appl. Phys. 104, 074101 (2008). 7. X. Chen, L. Song, L. You, and L. Zhao, Appl. Surf. Sci. 271, 248 (2013). 8. Y. Xiong, H. Tu, J. Du, M. Ji, X. Zhang, and L. Wang, Appl. Phys. Lett. 97, 012901 (2010). 9. S.-H. Chuang, H.-C. Lin, and C.-H. Chen, J. Alloys Compd. 534, 42 (2012). 10. J. Ni, Q. Zhou, Z. Li, and Z. Zhang, Appl. Phys. Lett. 93, 011905 (2008).
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