Optical properties and laser damage threshold of HfO(2)-SiO(2) mixed composite thin films.

Optical properties and laser damage threshold of HfO2–SiO2 mixed composite thin films Shuvendu Jena,* Raj Bahadur Tokas, Nitin M. Kamble, Sudhakar Thakur, and Naba Kishore Sahoo Atomic & Molecular Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India *Corresponding author: [email protected] Received 12 September 2013; revised 21 November 2013; accepted 13 December 2013; posted 20 December 2013 (Doc. ID 197626); published 5 February 2014

HfO2 –SiO2 mixed composite thin films have been deposited on fused silica substrate by co-evaporation of HfO2 and SiO2 through the reactive electron-beam evaporation technique. The composition-dependent refractive index and the absorption coefficient have been analyzed using different effective medium approximation (EMA) models in order to evaluate the suitability of these models for such mixed composite thin films. The discrepancies between experimentally determined and EMA-computed values are explained through microstructural and morphological evolutions observed in these mixed composite films. Finally, the dependence of the laser damage threshold as a function of silica content has been investigated, and the improved laser-induced damage threshold for films having more than 80% silica content has been explained through the defect-assisted multiphoton ionization process. © 2014 Optical Society of America OCIS codes: (160.4760) Optical properties; (160.4670) Optical materials; (310.6860) Thin films, optical properties; (310.3840) Materials and process characterization; (240.5770) Roughness; (140.3330) Laser damage. http://dx.doi.org/10.1364/AO.53.000850

1. Introduction

Research on mixed composite optical materials has been an area of intense study, because of their tunable refractive indices with the ability to produce continuous variation of the refractive index as well as a tunable optical bandgap with improved laser damage resistance [1–5]. With innovative multilayer designs, these mixed materials can be used to develop various types of graded index or inhomogeneous multilayer optical coatings with low mechanical stress, better tribological resistance, and a higher laser damage threshold than those of classical multilayer stacks due to the absence of significant interfaces [6–9]. Therefore, in order to achieve the desired performance of these gradient index optical coatings, it is a prerequisite to know the 1559-128X/14/050850-11$15.00/0 © 2014 Optical Society of America 850

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dependency of the optical dispersion curves as well as the laser damage threshold with the mixture composition. Moreover, the determination of the absorption edge as well as the absorption coefficient in the weak absorption region for different mixtures is necessary to define the transparent region, which can be useful for optical coatings to avoid absorption as well as radiation-induced damage. Hence, predicting the effective optical properties of any mixed oxide composite material is of the utmost importance, but to date there is no comprehensive and universally accepted theory to account for all of these aspects. Usually, the optical constants (refractive index and absorption coefficient) of such mixed composite films obtained through material mixing are tackled through different models based on analytical methods or effective medium approximations (EMAs), taking into account certain predefined sample geometries (grain structure) and microstructures [10]. The EMA models, such as the Bruggeman [11],

Maxwell Garnett [12], Lorentz–Lorentz [13], linear [13], Drude [14], and Looyenga models [15] and Bottcher’s generalized model [16] have been widely used to compute the compositional dependence of the refractive index as well as absorption coefficient for different mixed films [16–18]. Recently it has been observed that by adding foreign oxide materials, such as ZrO2, Al2 O3 , and SiO2 to pure HfO2 , the optical, mechanical properties and laser damage resistance of such mixed films (HfO2 –ZrO2 , HfO2 –Al2 O3 , HfO2 –SiO2 , etc.) have been dramatically improved [19,20]. But there is still a need to understand how laser damage thresholds can be increased by improving specific properties of such thin films using different optimized deposition techniques. Also, there is no systematic study on the appropriateness of EMA models for hafnia– silica mixed oxide thin films so that these models can be used while designing gradient index optical coatings using these mixtures. In the present study, HfO2 –SiO2 mixed composite films with a wide range of compositions produced by continuous mixing of HfO2 and SiO2 have been deposited on fused silica substrates by the electron-beam co-evaporation technique. The composition-dependent refractive index as well as the absorption coefficient around the strong absorption region for the mixed thin films has been analyzed using different EMA models. It has been observed that the effective refractive index of the composite films with silica content of more than 20% are very well modeled by the Drude model. But when the silica content is less than 20%, most composite films exhibit grain structure evolution leading to better and denser microstructure and morphology, which is confirmed by grazing incidence x-ray reflectivity (GIXR) and atomic force microscopy (AFM) analysis. All EMAs have failed to interpret such evolutions, which dominantly controlled the effective polarizability or dielectric constant. The absorption coefficient’s dependency on composition calculated by the Lorentz–Lorentz model shows fair approximation to the experimental data except for a lower silica content. The mixture films are found to show better surface roughness as compared to the pure hafnia, resulting in reduction of scattering loss. The laser-induced damage threshold (LIDT) measurement for nanosecond pulse duration (Nd:YAG laser, second harmonic) of such mixed films has been carried out, and the dependency of the damage threshold as a function of SiO2 content has been investigated.

deposition system. The fused silica substrates are properly cleaned using an ultrasonic and vapor degradation cleaning method before coating. The system is equipped with two 8 KW electron-beam guns with sweeps and automatic emission controls. The rate of deposition of an individual material is controlled and measured by independent quartz crystal monitors. The change in composition is achieved by changing the deposition rates of individual materials. The optical thickness of the composite films is measured by Leybold’s OMS-2000 optical thickness monitor. The total working pressure inside the chamber during codeposition was kept at 1 × 10−4 mbars, while the substrate temperature was kept at 300°C. The deposition process is controlled by software enabling simultaneous automatic measurements and acquisition of parameters (deposition rate, pressure inside the chamber, temperature, etc.). To suppress the unwanted fluctuation in rate of evaporation, the proportional, integration, and differential parameters of the thickness as well as the process control systems have been judiciously optimized. With such an accurate process control, the desired compositions of HfO2 –SiO2 mixed films have been achieved with 2% fluctuation in composition as measured by energy dispersive x-ray analysis [21]. The pure HfO2 shows a polycrystalline structure, while the mixed oxide films are nearly amorphous in nature. All the films studied here have an optical thickness (nd, where n is the refractive index and d is the geometrical film thickness) of 6–8 quarter-waves at a monitoring wavelength of 600 nm in order to have the appropriate numbers of interference fringes for spectrophotometric analysis. B. Optical Characterization

The transmission and reflection spectra of the films have been measured in the range 190–1100 nm at a close to normal angle of incidence using a Shimadzu UV-3101PC UV-VIS-NIR spectrophotometer. The relative uncertainty in the transmittance is 0.3%. In the measurement, air is used as a reference. Figure 1 shows the transmission as well as reflection

2. Experimental Details A.

Sample Preparation

The HfO2 –SiO2 composite thin films of various compositions in the xHfO2 :100-xSiO2 (x 0%, 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100%) systems have been deposited on fused silica substrates by the reactive electron-beam codeposition technique in a fully automatic thin-film

Fig. 1. Transmission and reflection of HfO2 –SiO2 mixed composite thin films. 10 February 2014 / Vol. 53, No. 5 / APPLIED OPTICS

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αfilm λ

Fig. 2. Experimental transmission with best-fit theoretical simulation of HfO2 90%-SiO2 10% mixed film.

spectra of some representative hafnia–silica mixtures in the range of 190–800 nm for the sake of clarity of the image. Figure 2 shows the experimental transmission spectra of a representative HfO2 –SiO2 (90%:10%) mixed composite film along with the best-fit theoretical spectra obtained using the inverse synthesis method described elsewhere [22–24]. The refractive index and film thickness have been derived as fitting parameters. The error in parameters derived from the transmission spectrum using the inverse synthesis method is negligible and is estimated to be 0.01 for the refractive index. The thickness parameter derived from the transmissions spectra is very close to the in situ measured film thickness as listed in Table 1, which indicates the reliability of the inverse synthesis method. Figure 3 shows the refractive index spectra of HfO2 –SiO2 composite films for whole compositions, while Fig. 4 shows the logarithm absorption coefficient of the film around the strong absorption region computed from the transmission (T) and reflection (R) spectra using the following formula [25]:

Table 1.

852

dfilm

ln

1−R ; T∕T sub

(1)

where dfilm is the film thickness, which is listed in Table 1, and T sub is the transmission of the bare substrate. The optical bandgap (Eg ) of the films has been determined form the obtained values of the absorption coefficient (α). Here it is assumed that the absorption coefficient of the mixed composite films (behaving like amorphous semiconductors) in the high absorption region is given according to Tauc’s relation for the allowed indirect transition [26] by the relation αhν1∕2 Khν − Eg , where K is a constant that depends on the transition probability and Eg is the optical bandgap, defined as the energy separation between the valence band and the conduction band and representing the onset of absorption related to interband transitions. The optical bandgap is determined by plotting αhν1∕2 as a function of the photon energy hν and extrapolating the linear curve progression to zero. For a mixture having more than 70% silica content, the optical bandgap is not derived, because the absorption edge is out of the spectrophotometric measurement limits. Figure 5 shows the Tauc plots for different compositions of HfO2 –SiO2 mixed composite films. C.

GIXR and AFM Characterization

The mass density of the mixed composite films has been measured by the GIXR technique carried out in an x-ray reflectometer (SOPRA, France). The measurements have been carried out with CuKα (1.54 Å) source with a grazing angle of incidence in the range of 0°–0.6° and with an angular resolution of 0.01°. Figure 6 shows the experimental GIXR spectra for pure HfO2 and a representative HfO2 –SiO2 (90%∶10%) mixed composite film along with the best-fit theoretical spectra obtained using the formalism described elsewhere [24,27]. The thicknesses of the films, derived from the optical transmission spectra, have been kept invariant during GIXR fitting. By fitting the x-ray reflectivity spectrum near

Parameters Obtained from Transmission, GIXR, AFM, and LIDT Measurement of HfO2 –SiO2 Films and Fused Silica Substrate

HfO2 %-SiO2 %) 100–0 90–10 80–20 70–30 60–40 50–50 40–60 30–70 20–80 10–90 0–100 Fused silica

1

Thickness Measured In Situ (nm)

Thickness Derived from Transmission Spectra (nm)

Bandgap (eV)

Film Density (g∕cm3 )

Roughness (nm) GIXR

Roughness (nm) AFM

Grain Size (nm)

LIDT F th J∕cm2

521 10 517 11 419 08 479 09 582 10 571 12 626 13 694 14 642 13 590 12 621 17 –

512 0.3 503 0.3 438 0.4 483 0.8 563 0.4 565 0.8 615 1.2 705 0.8 663 0.8 603 0.4 609 0.8 –

5.57 5.62 5.67 5.68 5.71 5.72 5.76 5.83 – – – 8.5Ref. 56

8.42 8.90 7.62 7.13 6.54 6.20 5.43 5.34 3.62 2.64 2.60 2.46

3.35 1.53 0.50 1.36 0.85 1.93 0.52 0.32 1.48 1.50 1.41 0.20

5.62 1.12 1.01 1.27 1.35 2.16 1.44 1.52 1.59 1.67 1.51 0.60

196 30 59 15 49 17 49 11 49 16 88 21 69 14 69 19 59 13 94 27 81 16 93

18.05 2.88 20.68 3.30 19.06 3.00 18.71 3.00 20.90 3.34 22.00 3.52 23.98 3.83 24.19 3.87 40.70 6.51 42.51 6.80 45.24 7.21 57.60 9.21


Fig. 3. Refractive index spectra of HfO2 –SiO2 mixed thin films.

Fig. 6. Experimental x-ray reflectivity with best-fit theoretical simulation for pure HfO2 and HfO2 90%-SiO2 10% mixed film.

Fig. 4. Dispersion of absorption coefficient for HfO2 –SiO2 mixed thin films.

Fig. 7. Surface morphology of representative HfO2 –SiO2 mixed composite thin films.

Fig. 5. Plot of αhν1∕2 versus hν for HfO2 –SiO2 mixed thin films.

the critical angle, the film density ρ and rms roughness have been estimated and are listed in Table 1. The error in density measurement is approximately 0.1 g∕cm3 due to the alignment of the sample position to the x-ray beam and the angular resolution of the instrument. AFM has been used to analyze the

surface morphology using NT-MDT’s Solver P47H multimode scanning probe system. AFM images were obtained in noncontact mode under ambient conditions using a silicon nitride cantilever, and 3D AFM images of a few representative films are shown in Fig. 7. The mean grain size and RMS roughness of the films have been determined from the topographic images and are listed in Table 1. The roughness obtained from the two techniques is different, which may be due to the scale at which measurements are performed. The roughness of all the mixed 10 February 2014 / Vol. 53, No. 5 / APPLIED OPTICS

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films is found to be much lower compared to the pure hafnia surface, resulting in less scattering loss. The possible explanation for the high hafnia surface roughness is microcrystalization of the material. In the present study, the desired film optical thickness and composition have been achieved by varying the deposition rate of constituent materials, keeping other process parameters fixed. It is well known that grain growth and its structure strongly depend on deposition conditions, such as deposition rate, substrate temperature, and deposition pressure. So for the mixed films, the varying deposition rate of constituent materials leads to a different growth structure of the films, and hence different grain size as well as different surface roughness, as observed for the hafnia–silica mixtures. D.

LIDT Measurement

The LIDTs of the HfO2 -SiO2 composite films have been measured in the “1-on-1” mode according to ISO-11254-1 [28]. The typical LIDT measurement setup is shown in Fig. 8. The second harmonic (532 nm) of a Q-switched Nd:YAG single-mode pulsed laser at a pulse width of 7 ns was used for the damage threshold measurement. The repetitive frequency of the laser was 10 Hz with the largest pulse energy of 600 mJ. The sample was set on a motorized goniometer to give precise 3D movements. The laser beam was focused on the test sample using a converging lens with a 75 mm focal length to generate high power density. The angle of incidence was slightly (2°–3°) off normal to avoid interference effects due to reflection from the substrate exit surface. The energy of the laser beam was controlled with a variable attenuator using cross polarizers, which maintains uniform pulse shape. The pulse energy was measured by an energy meter (FieldMaxII-P COHERENT) using a split-off portion of the beam (2%–4%) by a wedge plate. The beam profile was measured using a Spiricon beam profiler. The scattered radiation from the thin film to the CCD-based beam profilometer has been imaged by employing a CCD imaging system. Neutral density filters have been used to bring the intensity of the laser light

Fig. 8. Schematic diagram of laser-induced damage threshold (LIDT) measurement setup. 854


coming from the sample to the profilometer down so that the detector of the profilometer does not get damaged. The beam profile is top-hat in nature, and the spot diameter at the sample surface is 350 μm, as computed from the beam analysis. A fiber-optics-based spectrophotometer has been used to monitor the damage event. For the present experiment, the reflection and/or transmission measurement of the sample in pump–probe mode has been used to detect damage initiation. This method of damage detection is very sensitive for optical thinfilm coatings. Any physical change in the coating is reflected in the transmission/reflection spectrum. Damage onsets are verified through optical microscopy. The LIDT is defined as the pulse energy density when the damage occurs at 0% damage probability. The measurement repeatability has been verified by taking the LIDT measurement of mixed thin films deposited simultaneously on 2-fused silica substrate. The measured LIDT values for the films deposited on the 2-fused silica are very close to each other, which confirms the repeatability of the measurement. The total estimated error in the damage threshold measurement is approximately 16%, due to the uncertainties in the stability of the laser system, the beam energy measurements, and the beam profile measurements. 3. Results and Discussion

The composition parameter controls the optical properties, such as refractive index, absorption coefficient, and bandgap energy of the materials, which are observed for HfO2 –SiO2 mixed composite thin films as shown in Fig. 3, Fig. 4, and Table 1, respectively. Both the refractive index and absorption coefficient show normal dispersion and decrease with an increase in the SiO2 content, except for a certain range of mixtures which are discussed later. Since silica has a lower refractive index as well as being less absorptive than hafnia, it is expected that we will get a lower refractive index as well as a lower absorption coefficient for a mixture with a higher silica content. Again the refractive index is a strong function of material density, and silica is a less dense material compared to hafnia. From the GIXR measurement, it is clear that the film density decreases with increasing silica content, as listed in Table 1, as a result of which the refractive index decreases. But from the refractive index spectra, it is obvious that the refractive index of certain mixtures (90%∶10% and 80%∶20%) has a higher value in comparison with the pure HfO2 film. Thus this extended range of mixtures can be used for tailoring a high refractive index very conveniently for multilayer optical coating applications. Subsequently, the functional behaviors of the experimentally determined mixture refractive index and the absorption coefficient with respect to the volume fraction (f ) of SiO2 have been compared to those predicted by different EMA models. The EMA models describe the mixed composite film in terms of the spatially

homogeneous electromagnetic response. This also requires that the individual inclusions be far from each other to avoid inclusion clustering [29]. The following widely used EMA models are chosen to compare with our experimental data: Bruggeman model: f

ε1 − εeff ε − εeff 1 − f 2 0; ε1 2εeff ε2 2εeff

(2)

Maxwell Garnett model: εeff − ε2 ε − ε2 f 1 ; εeff 2ε2 ε1 2ε2

(3)

Lorentz–Lorentz model: εeff − 1 ε −1 ε −1 f 1 1 − f 2 ; εeff 2 ε1 2 ε2 2

(4)

Drude model: εeff f ε1 1 − f ε2 ;

(5)

Linear model: 1∕2 εeff f ε11∕2 1 − f ε21∕2 ;

(6)

Looyenga model: 1∕3 εeff f ε11∕3 1 − f ε21∕3 ;

(7)

where ε1 and ε2 are complex dielectric functions of the materials SiO2 and HfO2 , respectively, and f is the volume fraction of SiO2. εeff is the effective complex dielectric function of the mixed composite thin film. The refractive index and absorption coefficient for each hafnia–silica mixture have been calculated according to these EMA models using the optical constants of HfO2 and SiO2 determined from the transmission measurement of the pure materials and the volume fraction determined from the deposition rates. The experimentally determined refractive indices at 266 and 532 nm as a function of the volume fraction of SiO2 (f ) are shown in Fig. 9, together with theoretical values calculated using different EMA models. The results for the linear and Looyenga models are very close to those of the Maxwell Garnett and Bruggeman models; therefore, they are not plotted in Fig. 9 for the sake of clarity. The Drude model is found to be the most appropriate for finding the effective refractive index of the hafnia–silica mixtures, as shown in Fig. 9. This model suggests that the microstructure of the silica and hafnia phases is not point-like (spherically symmetric), and hence screening must be considered. The model depicts the situation where the induced dipoles were completely screened from one another, so that only the external applied field induced the dipole moments. As a result the effective polarization is just simply the volume

Fig. 9. Comparison of experimental and computed effective refractive index at 266 and 532 nm using different EMA models for HfO2 –SiO2 mixed thin films.

average of the polarization of the silica and hafnia phases, which is equivalent to the capacitors connected in parallel [30,31]. But for silica fractions CHfO2 16.7 JK −1 Kg−1 ). The computed normalized internal pressure using Eq. (8) for the different compositions of the mixed films is shown in Fig. 12. The thermal properties used for this calculation are taken from Ref. [42]. Figure 12 shows that the internal pressure or stress ΔP decreases with an increase in the SiO2 content. Hence the laser damage threshold due to mechanical stress increases with increasing silica content. Looking at the LIDT (F th ) versus optical bandgap plot for the mixtures of up to 70% silica content in the inset of Fig. 11, it is clear that the behavior of the LIDT is induced by another thin-film property and not by the optical bandgap energy [17]. But the optical bandgap energy is expected to play a crucial role in the sudden transition in LIDT value, as shown in Fig. 11. The LIDT for mixtures with a silica content equal to or more than 80% is approximately double that of the other mixtures, which may be due to the influence of the multiphoton absorption process in

the damage initiation. The rate of energy absorption from a multiphoton process is given by [43–45] dN e Nσ n I∕hνn ; dt

(9)

where N e is the electron density, N is the initial solid atom density, I is the laser intensity, n is the absorption order, and hν is the photon energy. Since the transmission edges of the films of higher silica content (more than 80% silica) do not fall at wavelength equal to or less than 190 nm, the optical bandgap of these films must have a value greater than 6.52 eV. Thus, for a 532 nm laser, three-photon absorption (3 × 2.33 eV 6.99 eV) is more probable. The intensity (I c ) necessary for three-photon absorption (n 3), which may occur in the mixed films, can be estimated by solving Eq. (9): s! Ne 3 Ic E; Nσ 3 τ

(10)

where σ 3 is the three-photon absorption cross section (≈10−81 cm6 s2 ) [46] and τ is the pulse duration. The electron density (N e ) necessary for rapid energy absorption would be at most the critical density (N c ) or at least 0.01N c depending on the size of the plasma [47]. Hence the lower and upper bounds of N e are 0.01N c and N c , respectively. For 532 nm laser radiation, the critical density N c 1000 nm∕λ2 × 1021 cm−3 1.88 × 1021 cm−3 [48]. Using Eq. (10) for 7 ns pulses, the lower and upper bounds of the LIDT are 294 and 1400 J∕cm2 , respectively. The observed LIDT ranges from 40 to 45 J∕cm2 for the films having more than 80% silica content. For very small spot sizes, damage threshold in excess of 280 J∕cm2 for a 1064 nm wavelength [49,50] up to 1760 J∕cm2 for a 532 nm wavelength [51] has been observed in fused silica for nanosecond laser, in which the pure multiphoton process is expected to dominate. In 10 February 2014 / Vol. 53, No. 5 / APPLIED OPTICS

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the present study, the laser beam diameter is 350 μm, which produces a large spot size indicating that pure multiphoton ionization cannot be the origin of damage initiation. Hence the pure multiphoton absorption process can’t be used to estimate the damage thresholds. Instead of the pure multiphoton process, the defect-assisted multiphoton mechanism [52] can be used to explain the observed damage thresholds for higher silica content films. The expected defects in the hafnia–silica mixed films may be localized absorbing inclusions, such as Hf inclusions, Si inclusions, and nonstoichiometric HfO2 or SiO2 inclusions, nonbridging oxygen hole centers [53–55], defects associated with boundaries and interfaces, and elementary point defects such as oxygen vacancies and interstitials [56]. These defects will be heated by absorbing the laser pulse and then transfer the heat to the host material through heat conduction, leading to a decrease in the bandgap of the host material, as observed by Saito and Ikushima for bulk SiO2 (decreasing bandgap from 8.5 to 7 eV when the temperature is increased from 4 to 1900 K) [57]. Since such defects and the temperature-dependent bandgap property of the host material can potentially lower the energy required to promote an electron to the conduction band, this reduces the order of the multiphoton process. Now, depending on the laser wavelength, two or three photons absorbed by a number of neighboring defects can lead to a sufficient population of conduction band electrons to start the localized cascade electron multiplication process responsible for damage initiation. This model accounts for the sharp transition in the LIDT value. For films having more than 80% silica, the expected bandgap is more than 6.5 eV, so the combination of two-photon absorption followed by defect state absorption by a single photon is expected to be responsible for the improved LIDT. Hence the density of defects is necessary to understand the damage mechanism and is determined by using this model. The overall damage thresholds as calculated for the pure multiphoton process are reduced, because the effective cross section for a multistep process is larger than that of a single step. For example, the cross section of a three-photon process is on the order of 10−81 cm6 s2, while the cross section for a combination of two-photon absorption followed by excited state absorption by a single photon is estimated to have an upper bound of 10−61 cm6 s2 [52]. Using this modified cross section in Eq. (10) with the observed damage threshold of 40.7 J∕cm2, a lower bound to the necessary defect density is estimated to be on the order of 104 cm−3. Localized defects of this order of magnitude are comparable to or less than the values reported by Krol et al. for a fused silica surface [58], Gallais et al. for SiO2 [59] and HfO2 [53] thin films, and Fu et al. for niobia–silica and zirconia–silica mixtures [49]. Moreover, a specific characteristic has been observed in the damage morphology of given represen858


Fig. 13. Laser-induced damage morphology of HfO2 –SiO2 mixed composite thin films.

tative samples, as shown in Fig. 13. In this figure, pure HfO2 film and films with a silica content up to 20% show an absorption-induced “melting and evaporation” type of damage, in which the center of the irradiated area has been simply removed while a melted area appears on the edges and the size of the lateral damage decreases with increasing silica content. For silica fractions above 30%, an absorption-induced “melting and re-crystallization” type of damage has been observed, in which with increasing silica content, small separated pits appear in the damage morphology. This could be due to absorbing inclusion defects embedded in the film [28]. These absorbing inclusion defects absorb a fraction of the incident energy and heat the surrounding host material, causing a collapse of the bandgap and plasma formation. Since this model predicts laser damage will be initiated at the sites of absorbing inclusion defects, it is reasonable to expect damage to occur at discrete localized centers, and not necessarily at the peak energy density of the laser. According to this defect-dominated damage model, for films with a lower silica content, the damage sites are centered on a single defect, while for films with a higher silica content, the damage sites form a connecting area composed of several defects [48]. The uncoated fused silica substrate shows a higher damage threshold value, and its damage morphology shows a very small single damage point, indicating localized defect absorption. From the damage morphology as well as the damage threshold values, it can be expected that the origin of the laser-induced damage mechanism may be different for certain compositions of mixtures compared to other compositions depending on the materials in the nanosecond regime. 4. Conclusion

HfO2 –SiO2 mixed composite films have been prepared by the electron-beam co-evaporation technique. The refractive index and absorption coefficient are found to be decreasing with increasing silica content, except the anomalous behavior of compositions with 10% and 20% silica, while the optical bandgap increases monotonically. The anomalous behavior has been clarified by GIXR

and AFM measurements. The experimentally observed composition-dependent refractive index and absorption coefficient have been compared with the theoretical computed values using different EMA models. It is observed that the Drude model is more appropriate to find the refractive index of the mixtures, while the Lorentz–Lorentz model computed values show fair agreement with the experimentally determined absorption coefficient values. The mixture films are found to have a better surface roughness with a smaller grain size compared to pure hafnia, resulting in a reduction in scattering loss. The LIDT measurement has been carried out. The improved LIDT for films having a more than 80% silica content has been explained through the defect-assisted multiphoton ionization process, which is the primary mechanism for damage initiation in such films. This study concludes that the LIDT values and the analyzed EMA models for optical dispersion with proper logistics can be useful when designing stable optical graded index or inhomogeneous multilayer interference coatings utilizing hafnia–silica mixtures. References 1. J. S. Chen, S. Chao, J. S. Kao, H. Niu, and C. H. Chen, “Mixed films of TiO2–SiO2 deposited by double electron-beam coevaporation,” Appl. Opt. 35, 90–96 (1996). 2. L. Yang, Y. Lai, J. S. Chen, P. H. Tsai, C. L. Chen, and C. J. Chang, “Compositional tailored sol-gel SiO2–TiO2 thin films: crystallization, chemical bonding configuration, and optical properties,” J. Mater. Res. 20, 3141–3149 (2005). 3. N. K. Sahoo, A. Thakur, R. B. Tokas, and N. M. Kamble, “Refractive-index tailoring and morphological evolutions in Gd2O3–SiO2 and ZrO2–SiO2 composite thin films,” Appl. Phys. A 89, 711–719 (2007). 4. D. Nguyen, L. A. Emmert, I. V. Cravetchi, M. Mero, W. Rudolph, M. Jupe, M. Lappschies, K. Starke, and D. Ristau, “TixSi1-x O2 optical coatings with tunable index and their response to intense subpicosecond laser pulse irradiation,” Appl. Phys. Lett. 93, 261903 (2008). 5. D. Rademacher, G. Brauer, B. Fritz, and M. Verghol, “Optical properties of silicon titanium oxide mixtures prepared by metallic mode reactive sputtering,” Appl. Opt. 51, 8047–8051 (2012). 6. V. Janicki, J. Sancho-Parramon, S. Yulin, M. Flemming, and A. Chuvilin, “Optical and structural properties of Nb2O5-SiO2 mixtures in thin films,” Surf. Coat. Technol. 206, 3650–3657 (2012). 7. B. J. Pond, J. I. DeBar, C. K. Carniglia, and T. Raj, “Stress reduction in ion beam sputtered mixed oxide films,” Appl. Opt. 28, 2800–2805 (1989). 8. R. Vernhes, O. Zabedia, J. E. Klemberg-Sapieha, and L. Martinu, “Single-material inhomogeneous optical filters based on microstructural gradients in plasma-deposited silicon nitride,” Appl. Opt. 43, 97–103 (2004). 9. J. Weber, H. Bartzch, and P. Frach, “Sputter deposition of silicon oxynitride gradient and multilayer coatings,” Appl. Opt. 47, C288–C292 (2008). 10. L. Gao and Z. Li, “Effective medium approximation for twocomponent nonlinear composites with shape distribution,” J. Phys. Condens. Matter 15, 4397–4409 (2003). 11. D. A. G. Bruggeman, “Berechnung verschiedener physikalischer Konstanten von heterogenen Substanzen,” Ann. Phys. 416, 636–664 (1935). 12. J. C. M. Garnett, “Colours in metal glasses and in metallic films,” Phil. Trans. R. Soc. A 203, 385–420 (1904). 13. L. Lorentz, “Über die Refractionsconstante,” Ann. Phys. 247, 70–103 (1880).

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Optical Properties of Nanoporous Germanium Thin Films.

Structural and optical properties of nanoscale Galinobisuitite thin films.

Optical and magneto-optical properties of metal phthalocyanine and metal porphyrin thin films.

SiO2 composite coatings.

Spectroscopic study and optical and electrical properties of Ti-doped ZnO thin films by spray pyrolysis.

Optical properties and electrical transport of thin films of terbium(III) bis(phthalocyanine) on cobalt.

Al2O3 thin films with high laser-induced damage thresholds.

Design, optical and antimicrobial properties of extremely thin alumina films colored with silver nanospecies.

Microstructure and optical properties of nanocrystalline Cu2O thin films prepared by electrodeposition.

Effect of oxygen vacancies on the laser-induced damage resistance of Y0.26Hf0.74Ox thin films.

Optical and thermal properties of azo derivatives of salicylic acid thin films.

Thin hydrogel films for optical biosensor applications.

Outstanding laser damage threshold in Li2MnGeS4 and tunable optical nonlinearity in diamond-like semiconductors.

Structural, optical and electrical properties of Zr-doped In₂O₃ thin films.

Semiconducting composite oxide Y2CuO4-5CuO thin films for investigation of photoelectrochemical properties.

Direct holographic imaging of ultrafast laser damage process in thin films.

Nanomechanical and optical properties of yttrium thin films by magnetron sputtering.

Annealing Effect on the Structural and Optical Properties of Sputter-Grown Bismuth Titanium Oxide Thin Films.

Temperature dependence of laser-induced damage threshold of optical coatings at different pulse widths.

Optical damage threshold of Au nanowires in strong femtosecond laser fields.

Optical properties and residual stress in Nb-Si composite films prepared by magnetron cosputtering.

All-optical helicity dependent magnetic switching in Tb-Fe thin films with a MHz laser oscillator.

Properties of plasmonic arrays produced by pulsed-laser nanostructuring of thin Au films.

Effect of Pt nanoparticles on the optical gas sensing properties of WO3 thin films.